human brain essay

Essay on Human Brain

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100 Words Essay on Human Brain

The human brain: an overview.

The human brain is a complex organ, responsible for all our thoughts, feelings, and actions. It’s made up of billions of nerve cells, or neurons, which communicate through electrical signals.

Parts of the Brain

Brain’s functionality.

The brain is always active, even during sleep. It processes information from our senses, helps us understand the world around us, and makes decisions. It’s truly a remarkable organ!

250 Words Essay on Human Brain

Introduction.

The human brain, a marvel of biological engineering, is the most complex organ in the human body. It is the epicenter of human consciousness, responsible for our thoughts, emotions, and actions.

Structure and Function

Neuroplasticity.

A remarkable feature of the brain is its neuroplasticity, the ability to form and reorganize synaptic connections in response to learning, experience, or injury. This adaptability underscores the brain’s capacity for lifelong learning and recovery.

Cognitive Abilities

Cognitive abilities such as memory, attention, and problem-solving are facilitated by the brain’s intricate network of neurons. These abilities enable us to navigate and interpret the world around us, engage in social interactions, and make decisions.

Brain and Technology

Advancements in technology have led to breakthroughs in understanding the brain. Techniques like fMRI and EEG provide detailed insights into brain activity, paving the way for treatments of neurological disorders.

The human brain, with its intricate structure and impressive capabilities, continues to be a subject of fascination and study. Its complexity and adaptability underscore the limitless potential of human cognition, making it a cornerstone of our identity as a species.

500 Words Essay on Human Brain

Introduction to the human brain.

The human brain, a product of millions of years of evolutionary progression, is a marvel of biological engineering. It is a complex organ, responsible for controlling all the functions of the human body, processing sensory information, and coordinating responses. The brain is an intricate network of billions of neurons, which communicate and work together to generate our thoughts, feelings, and actions.

Structural Complexity of the Brain

The cerebellum, located beneath the cerebrum, coordinates motor control, balance, and coordination. The brainstem, connecting the brain to the spinal cord, controls automatic functions vital for survival, such as heartbeat, breathing, and digestion.

Neurons: The Building Blocks

Neurons, the fundamental units of the brain, transmit information through electrical and chemical signals. They consist of a cell body, dendrites, and an axon. The dendrites receive signals from other neurons, which are then passed through the cell body and down the axon to the next neuron. This communication forms neural networks, the basis for all brain activity.

Brain Plasticity

One of the most fascinating aspects of the human brain is its plasticity, the ability to reorganize itself by forming new neural connections throughout life. This adaptability allows us to learn new skills, adapt to changes, and recover from brain injuries. Neuroplasticity underscores the brain’s remarkable capacity for resilience and growth.

The Brain and Consciousness

Future research directions.

Despite significant advances in neuroscience, much about the brain remains a mystery. Key questions about consciousness, memory formation, and the nature of intelligence are yet to be fully answered. The development of advanced neuroimaging techniques and computational models offers exciting possibilities for future research.

In conclusion, the human brain, with its intricate architecture and dynamic functionality, is a testament to the complexity and beauty of human life. As we continue to unravel its mysteries, we deepen our understanding of what it means to be human, highlighting the importance of continued research in this fascinating field.

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Introduction: The Human Brain

By Helen Phillips

4 September 2006

A false-colour Magnetic Resonance Image (MRI) of a mid-sagittal section through the head of a normal 42 year-old woman, showing structures of the brain, spine and facial tissues

(Image: Mehau Kulyk / Science Photo Library)

The brain is the most complex organ in the human body. It produces our every thought, action , memory , feeling and experience of the world. This jelly-like mass of tissue, weighing in at around 1.4 kilograms, contains a staggering one hundred billion nerve cells, or neurons .

The complexity of the connectivity between these cells is mind-boggling. Each neuron can make contact with thousands or even tens of thousands of others, via tiny structures called synapses . Our brains form a million new connections for every second of our lives. The pattern and strength of the connections is constantly changing and no two brains are alike.

It is in these changing connections that memories are stored, habits learned and personalities shaped , by reinforcing certain patterns of brain activity, and losing others.

Grey matter

While people often speak of their “ grey matter “, the brain also contains white matter . The grey matter is the cell bodies of the neurons, while the white matter is the branching network of thread-like tendrils – called dendrites and axons – that spread out from the cell bodies to connect to other neurons.

But the brain also has another, even more numerous type of cell, called glial cells . These outnumber neurons ten times over. Once thought to be support cells, they are now known to amplify neural signals and to be as important as neurons in mental calculations. There are many different types of neuron, only one of which is unique to humans and the other great apes, the so called spindle cells .

Brain structure is shaped partly by genes , but largely by experience . Only relatively recently it was discovered that new brain cells are being born throughout our lives – a process called neurogenesis . The brain has bursts of growth and then periods of consolidation , when excess connections are pruned. The most notable bursts are in the first two or three years of life, during puberty , and also a final burst in young adulthood.

How a brain ages also depends on genes and lifestyle too. Exercising the brain and giving it the right diet can be just as important as it is for the rest of the body.

Chemical messengers

The neurons in our brains communicate in a variety of ways. Signals pass between them by the release and capture of neurotransmitter and neuromodulator chemicals, such as glutamate , dopamine , acetylcholine , noradrenalin , serotonin and endorphins .

Some neurochemicals work in the synapse , passing specific messages from release sites to collection sites, called receptors. Others also spread their influence more widely, like a radio signal , making whole brain regions more or less sensitive.

These neurochemicals are so important that deficiencies in them are linked to certain diseases. For example, a loss of dopamine in the basal ganglia, which control movements, leads to Parkinson’s disease . It can also increase susceptibility to addiction because it mediates our sensations of reward and pleasure.

Similarly, a deficiency in serotonin , used by regions involved in emotion, can be linked to depression or mood disorders, and the loss of acetylcholine in the cerebral cortex is characteristic of Alzheimer’s disease .

Brain scanning

Within individual neurons, signals are formed by electrochemical pulses. Collectively, this electrical activity can be detected outside the scalp by an electroencephalogram (EEG).

These signals have wave-like patterns , which scientists classify from alpha (common while we are relaxing or sleeping ), through to gamma (active thought). When this activity goes awry, it is called a seizure . Some researchers think that synchronising the activity in different brain regions is important in perception .

Other ways of imaging brain activity are indirect. Functional magnetic resonance imaging ( fMRI ) or positron emission tomography ( PET ) monitor blood flow. MRI scans, computed tomography ( CT ) scans and diffusion tensor images (DTI) use the magnetic signatures of different tissues, X-ray absorption, or the movement of water molecules in those tissues, to image the brain.

These scanning techniques have revealed which parts of the brain are associated with which functions . Examples include activity related to sensations , movement, libido , choices , regrets , motivations and even racism . However, some experts argue that we put too much trust in these results and that they raise privacy issues .

Before scanning techniques were common, researchers relied on patients with brain damage caused by strokes , head injuries or illnesses, to determine which brain areas are required for certain functions . This approach exposed the regions connected to emotions , dreams , memory , language and perception and to even more enigmatic events, such as religious or “ paranormal ” experiences.

One famous example was the case of Phineas Gage , a 19 th century railroad worker who lost part of the front of his brain when a 1-metre-long iron pole was blasted through his head during an explosion. He recovered physically, but was left with permanent changes to his personality , showing for the first time that specific brain regions are linked to different processes.

Structure in mind

The most obvious anatomical feature of our brains is the undulating surfac of the cerebrum – the deep clefts are known as sulci and its folds are gyri. The cerebrum is the largest part of our brain and is largely made up of the two cerebral hemispheres . It is the most evolutionarily recent brain structure, dealing with more complex cognitive brain activities.

It is often said that the right hemisphere is more creative and emotional and the left deals with logic, but the reality is more complex . Nonetheless, the sides do have some specialisations , with the left dealing with speech and language , the right with spatial and body awareness.

See our Interactive Graphic for more on brain structure

Further anatomical divisions of the cerebral hemispheres are the occipital lobe at the back, devoted to vision , and the parietal lobe above that, dealing with movement , position, orientation and calculation .

Behind the ears and temples lie the temporal lobes , dealing with sound and speech comprehension and some aspects of memory . And to the fore are the frontal and prefrontal lobes , often considered the most highly developed and most “human” of regions, dealing with the most complex thought, decision making , planning, conceptualising, attention control and working memory. They also deal with complex social emotions such as regret , morality and empathy .

Another way to classify the regions is as sensory cortex and motor cortex , controlling incoming information, and outgoing behaviour respectively.

Below the cerebral hemispheres, but still referred to as part of the forebrain, is the cingulate cortex , which deals with directing behaviour and pain . And beneath this lies the corpus callosum , which connects the two sides of the brain. Other important areas of the forebrain are the basal ganglia , responsible for movement , motivation and reward.

Urges and appetites

Beneath the forebrain lie more primitive brain regions. The limbic system , common to all mammals, deals with urges and appetites. Emotions are most closely linked with structures called the amygdala , caudate nucleus and putamen . Also in the limbic brain are the hippocampus – vital for forming new memories; the thalamus – a kind of sensory relay station; and the hypothalamus , which regulates bodily functions via hormone release from the pituitary gland .

The back of the brain has a highly convoluted and folded swelling called the cerebellum , which stores patterns of movement, habits and repeated tasks – things we can do without thinking about them.

The most primitive parts, the midbrain and brain stem , control the bodily functions we have no conscious control of, such as breathing , heart rate, blood pressure, sleep patterns , and so on. They also control signals that pass between the brain and the rest of the body, through the spinal cord.

Though we have discovered an enormous amount about the brain, huge and crucial mysteries remain. One of the most important is how does the brain produces our conscious experiences ?

The vast majority of the brain’s activity is subconscious . But our conscious thoughts, sensations and perceptions – what define us as humans – cannot yet be explained in terms of brain activity.

• psychology /

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Brain Anatomy and How the Brain Works

What is the brain.

The brain is a complex organ that controls thought, memory, emotion, touch, motor skills, vision, breathing, temperature, hunger and every process that regulates our body. Together, the brain and spinal cord that extends from it make up the central nervous system, or CNS.

What is the brain made of?

Weighing about 3 pounds in the average adult, the brain is about 60% fat. The remaining 40% is a combination of water, protein, carbohydrates and salts. The brain itself is a not a muscle. It contains blood vessels and nerves, including neurons and glial cells.

What is the gray matter and white matter?

Gray and white matter are two different regions of the central nervous system. In the brain, gray matter refers to the darker, outer portion, while white matter describes the lighter, inner section underneath. In the spinal cord, this order is reversed: The white matter is on the outside, and the gray matter sits within.

Gray matter is primarily composed of neuron somas (the round central cell bodies), and white matter is mostly made of axons (the long stems that connects neurons together) wrapped in myelin (a protective coating). The different composition of neuron parts is why the two appear as separate shades on certain scans.

Each region serves a different role. Gray matter is primarily responsible for processing and interpreting information, while white matter transmits that information to other parts of the nervous system.

How does the brain work?

The brain sends and receives chemical and electrical signals throughout the body. Different signals control different processes, and your brain interprets each. Some make you feel tired, for example, while others make you feel pain.

Some messages are kept within the brain, while others are relayed through the spine and across the body’s vast network of nerves to distant extremities. To do this, the central nervous system relies on billions of neurons (nerve cells).

Main Parts of the Brain and Their Functions

At a high level, the brain can be divided into the cerebrum, brainstem and cerebellum.

The cerebrum (front of brain) comprises gray matter (the cerebral cortex) and white matter at its center. The largest part of the brain, the cerebrum initiates and coordinates movement and regulates temperature. Other areas of the cerebrum enable speech, judgment, thinking and reasoning, problem-solving, emotions and learning. Other functions relate to vision, hearing, touch and other senses.

Cerebral Cortex

Cortex is Latin for “bark,” and describes the outer gray matter covering of the cerebrum. The cortex has a large surface area due to its folds, and comprises about half of the brain’s weight.

The cerebral cortex is divided into two halves, or hemispheres. It is covered with ridges (gyri) and folds (sulci). The two halves join at a large, deep sulcus (the interhemispheric fissure, AKA the medial longitudinal fissure) that runs from the front of the head to the back. The right hemisphere controls the left side of the body, and the left half controls the right side of the body. The two halves communicate with one another through a large, C-shaped structure of white matter and nerve pathways called the corpus callosum. The corpus callosum is in the center of the cerebrum.

The brainstem (middle of brain) connects the cerebrum with the spinal cord. The brainstem includes the midbrain, the pons and the medulla.

• Midbrain. The midbrain (or mesencephalon) is a very complex structure with a range of different neuron clusters (nuclei and colliculi), neural pathways and other structures. These features facilitate various functions, from hearing and movement to calculating responses and environmental changes. The midbrain also contains the substantia nigra, an area affected by Parkinson’s disease that is rich in dopamine neurons and part of the basal ganglia, which enables movement and coordination.
• Pons. The pons is the origin for four of the 12 cranial nerves, which enable a range of activities such as tear production, chewing, blinking, focusing vision, balance, hearing and facial expression. Named for the Latin word for “bridge,” the pons is the connection between the midbrain and the medulla.
• Medulla. At the bottom of the brainstem, the medulla is where the brain meets the spinal cord. The medulla is essential to survival. Functions of the medulla regulate many bodily activities, including heart rhythm, breathing, blood flow, and oxygen and carbon dioxide levels. The medulla produces reflexive activities such as sneezing, vomiting, coughing and swallowing.

The spinal cord extends from the bottom of the medulla and through a large opening in the bottom of the skull. Supported by the vertebrae, the spinal cord carries messages to and from the brain and the rest of the body.

The cerebellum (“little brain”) is a fist-sized portion of the brain located at the back of the head, below the temporal and occipital lobes and above the brainstem. Like the cerebral cortex, it has two hemispheres. The outer portion contains neurons, and the inner area communicates with the cerebral cortex. Its function is to coordinate voluntary muscle movements and to maintain posture, balance and equilibrium. New studies are exploring the cerebellum’s roles in thought, emotions and social behavior, as well as its possible involvement in addiction, autism and schizophrenia.

Brain Coverings: Meninges

Three layers of protective covering called meninges surround the brain and the spinal cord.

• The outermost layer, the dura mater , is thick and tough. It includes two layers: The periosteal layer of the dura mater lines the inner dome of the skull (cranium) and the meningeal layer is below that. Spaces between the layers allow for the passage of veins and arteries that supply blood flow to the brain.
• The arachnoid mater is a thin, weblike layer of connective tissue that does not contain nerves or blood vessels. Below the arachnoid mater is the cerebrospinal fluid, or CSF. This fluid cushions the entire central nervous system (brain and spinal cord) and continually circulates around these structures to remove impurities.
• The pia mater is a thin membrane that hugs the surface of the brain and follows its contours. The pia mater is rich with veins and arteries.

Lobes of the Brain and What They Control

Each brain hemisphere (parts of the cerebrum) has four sections, called lobes: frontal, parietal, temporal and occipital. Each lobe controls specific functions.

• Frontal lobe. The largest lobe of the brain, located in the front of the head, the frontal lobe is involved in personality characteristics, decision-making and movement. Recognition of smell usually involves parts of the frontal lobe. The frontal lobe contains Broca’s area, which is associated with speech ability.
• Parietal lobe. The middle part of the brain, the parietal lobe helps a person identify objects and understand spatial relationships (where one’s body is compared with objects around the person). The parietal lobe is also involved in interpreting pain and touch in the body. The parietal lobe houses Wernicke’s area, which helps the brain understand spoken language.
• Occipital lobe. The occipital lobe is the back part of the brain that is involved with vision.
• Temporal lobe. The sides of the brain, temporal lobes are involved in short-term memory, speech, musical rhythm and some degree of smell recognition.

Deeper Structures Within the Brain

Pituitary gland.

Sometimes called the “master gland,” the pituitary gland is a pea-sized structure found deep in the brain behind the bridge of the nose. The pituitary gland governs the function of other glands in the body, regulating the flow of hormones from the thyroid, adrenals, ovaries and testicles. It receives chemical signals from the hypothalamus through its stalk and blood supply.

Hypothalamus

The hypothalamus is located above the pituitary gland and sends it chemical messages that control its function. It regulates body temperature, synchronizes sleep patterns, controls hunger and thirst and also plays a role in some aspects of memory and emotion.

Small, almond-shaped structures, an amygdala is located under each half (hemisphere) of the brain. Included in the limbic system, the amygdalae regulate emotion and memory and are associated with the brain’s reward system, stress, and the “fight or flight” response when someone perceives a threat.

Hippocampus

A curved seahorse-shaped organ on the underside of each temporal lobe, the hippocampus is part of a larger structure called the hippocampal formation. It supports memory, learning, navigation and perception of space. It receives information from the cerebral cortex and may play a role in Alzheimer’s disease.

Pineal Gland

The pineal gland is located deep in the brain and attached by a stalk to the top of the third ventricle. The pineal gland responds to light and dark and secretes melatonin, which regulates circadian rhythms and the sleep-wake cycle.

Ventricles and Cerebrospinal Fluid

Deep in the brain are four open areas with passageways between them. They also open into the central spinal canal and the area beneath arachnoid layer of the meninges.

The ventricles manufacture cerebrospinal fluid , or CSF, a watery fluid that circulates in and around the ventricles and the spinal cord, and between the meninges. CSF surrounds and cushions the spinal cord and brain, washes out waste and impurities, and delivers nutrients.

Blood Supply to the Brain

Two sets of blood vessels supply blood and oxygen to the brain: the vertebral arteries and the carotid arteries.

The external carotid arteries extend up the sides of your neck, and are where you can feel your pulse when you touch the area with your fingertips. The internal carotid arteries branch into the skull and circulate blood to the front part of the brain.

The vertebral arteries follow the spinal column into the skull, where they join together at the brainstem and form the basilar artery , which supplies blood to the rear portions of the brain.

The circle of Willis , a loop of blood vessels near the bottom of the brain that connects major arteries, circulates blood from the front of the brain to the back and helps the arterial systems communicate with one another.

Cranial Nerves

Inside the cranium (the dome of the skull), there are 12 nerves, called cranial nerves:

• Cranial nerve 1: The first is the olfactory nerve, which allows for your sense of smell.
• Cranial nerve 2: The optic nerve governs eyesight.
• Cranial nerve 3: The oculomotor nerve controls pupil response and other motions of the eye, and branches out from the area in the brainstem where the midbrain meets the pons.
• Cranial nerve 4: The trochlear nerve controls muscles in the eye. It emerges from the back of the midbrain part of the brainstem.
• Cranial nerve 5: The trigeminal nerve is the largest and most complex of the cranial nerves, with both sensory and motor function. It originates from the pons and conveys sensation from the scalp, teeth, jaw, sinuses, parts of the mouth and face to the brain, allows the function of chewing muscles, and much more.
• Cranial nerve 6: The abducens nerve innervates some of the muscles in the eye.
• Cranial nerve 7: The facial nerve supports face movement, taste, glandular and other functions.
• Cranial nerve 8: The vestibulocochlear nerve facilitates balance and hearing.
• Cranial nerve 9: The glossopharyngeal nerve allows taste, ear and throat movement, and has many more functions.
• Cranial nerve 10: The vagus nerve allows sensation around the ear and the digestive system and controls motor activity in the heart, throat and digestive system.
• Cranial nerve 11: The accessory nerve innervates specific muscles in the head, neck and shoulder.
• Cranial nerve 12: The hypoglossal nerve supplies motor activity to the tongue.

The first two nerves originate in the cerebrum, and the remaining 10 cranial nerves emerge from the brainstem, which has three parts: the midbrain, the pons and the medulla.

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Introductory essay

Written by the educators who created Mapping and Manipulating the Brain, a brief look at the key facts, tough questions and big ideas in their field. Begin this TED Study with a fascinating read that gives context and clarity to the material.

Here is this mass of jelly, three-pound mass of jelly you can hold in the palm of your hand, and it can contemplate the vastness of interstellar space. It can contemplate the meaning of infinity and it can contemplate itself contemplating on the meaning of infinity. VS Ramachandran

The brain may well be our body's most mysterious organ. Unbelievably complex, utterly fascinating, and notoriously difficult to study, we're left wondering: What exactly does the brain do and how does it do it?

Despite two centuries of intensive research, supported in recent decades by impressive technological advances, answers to many of our questions about the brain are still distant. The reason is easy to appreciate: the brain contains more than ten billion cells — a number equivalent to the total human population on Earth — interacting with each other through about 1,000 times as many connections. Imagine that what's going on in your brain is like a shrunk-down version of the global human population interacting through the Internet. The Internet is hard enough to understand even though we created it; now imagine trying to understand a process of similar complexity without the benefit of knowing how it was generated!

As you listen to these TEDTalks and expand your study of neuroscience through other sources, remember that although we might now know a great deal more about the brain than we did at the start of the 19th century, it's a tiny fraction of what there is to know. Bear in mind that many current ideas may prove wrong. Indeed, it's the excitement of generating and testing, and trying to prove or disprove ideas that might explain the great unknown inside our heads that motivates many research neuroscientists around the world.

A brief history of brain science

The Egyptians wrote the first known descriptions of the brain and its anatomy about 3700 years ago, but another 1200 years elapsed before Greek philosophers of the Hippocratic School identified the brain as the organ responsible for our cognitive functions. Around 400 B.C., Hippocrates declared, "Men ought to know that from the brain, and from the brain only, arise our pleasures, joy, laughter and jests, as well as our sorrows, pains, griefs, and tears." However, not everyone agreed: although Plato and Hippocrates thought that the brain was responsible for sensation, intelligence and mental processes, Aristotle believed it was the heart.

Over the next 2500 years, the work of great European intellectuals including Galen of Bergama, Leonardo da Vinci and Rene Descartes improved our understanding of the brain. By the start of the 19th century, the brain's importance as the organ of perception and higher mental function was beyond doubt.

In the early 1800s, scientists made an important conceptual breakthrough when they hypothesized that different brain functions are carried out in specific and distinct brain regions. Brain regionalization, a concept central to several of the TEDTalks we'll watch, remains an important though controversial component of modern neuroscience.

Some of the initial models of brain regionalization were severely misguided, mainly because they were built on little or no evidence. For example, the Viennese physician Franz Joseph Gall (1758-1828) became convinced for the flimsiest of reasons that each of mankind's mental faculties, including our moral and intellectual capabilities, are each controlled by a separate "organ" within the cerebral hemispheres of the brain. The pseudo-science of phrenology that grew out of Gall's claims gained an enormous popular following in the 19th century; advocates believed that skilled practitioners could feel the lumps and bumps on an individual's skull to gain information about the underlying "organs" and thus fully describe the individual's personality and mental abilities.

Although phrenology is now discredited, the fundamental idea that different functions are localized to different areas of the brain turned out to have merit — even if Gall got the details wrong. The story of phrenology also provides a salutary lesson on the dangers of accepting popular beliefs about aspects of brain function and dysfunction that are difficult to critically evaluate through scientific experimentation. Even today, it's common to find that people think they know more than it's currently possible to know about how and why brains work or go wrong; for example, the causes and cures for various types of mental illness, which may contribute to the social stigma that surrounds these conditions.

Through the late 19th and early 20th centuries, scientists including Pierre Paul Broca, Carl Wernicke, Korbinian Brodmann and Wilder Penfield found credible scientific evidence supporting the subdivision of the brain into discrete areas with different specific functions. Their work was based on studies of patients with localized lesions of the brain, of the anatomical differences between different parts of the brain and of the effects of stimulating discrete brain regions on bodily actions. Together, scientists such as these laid the foundations of modern neuroscience. As you watch the TEDTalks in Mapping and Manipulating the Brain , notice how the speakers reference some of the same approaches used by Broca, Wernicke, Brodmann and Penfield, and how they apply the concepts of brain regionalization and localization of function . Bear in mind, however, that although these concepts are useful, they're also controversial -- more on this below.

How brains are built

Spanish scientist Santiago Ramón Y Cajal (1852-1934) is often thought of as the father of modern neuroscience. Through his extensive and beautiful studies of the microscopic structure of the brain, he discovered that the neuron is the fundamental unit of the nervous system. Since Ramón Y Cajal's breakthrough, scientists have sought to understand how the billions of neurons in the brain are organized to support so many complex functions.

This daunting task would likely be easier if we could follow the process by which the brain is generated, but following brain development is very difficult to do in humans. Thus, we often have to infer how the human brain develops by studying the developing brains of other species, so-called "model organisms" selected for their particular advantages in certain experimental procedures. Aside from helping us to work out how the adult brain functions, research on brain development is a major area in neuroscience for other reasons as well. For example, many conditions like schizophrenia and autism can be traced back to abnormalities in earlier brain development.

The great molecular, structural and functional diversity of brain cells, along with their specializations and precise interactions, are acquired in an organized way through processes that build on differences between the relatively small numbers of cells in the early embryo. As more and more cells are generated in a growing organism, new cells diversify in specific ways as a result of interactions with pre-existing cells, continually adding to the organism's complexity in a highly regulated manner. To understand how brains develop we need to know how their cells develop in specific and reproducible ways as a result of their own internal mechanisms interacting with an expanding array of stimuli from outside the cell.

Since, as discussed above, regionalization is a prominent organizing feature in mature brains, when and how is it established during brain development? Some of the most exciting research on brain development in recent years has focused on this question.

For neurons to develop regional identities, they must possess or acquire information on where they are located within the brain so that they can take on the appropriate specializations. How neurons gain positional information has been one of the most prominent themes in developmental neuroscience in the last 50 years or so, as indeed it has in the broader field of developmental biology (positional identity is required not only by brain cells).

The model that has dominated current thinking was famously elaborated in the 1960s by Lewis Wolpert in his French flag analogy. Here, a signal produced by a group of organizer cells diffuses from its source through a surrounding field of cells. In so doing, it forms a concentration gradient with more of the signal present in areas closer to the source. Cells respond to the concentration of this signal. In Wolpert's French Flag analogy, they become blue, white or red (in reality, they would become cells of different types, not different colors). Close to the source, cells receive signals above the highest threshold (to become blue, or type 1). Beyond this, cells respond to a lower dose (to become white, or type 2) while farther still cells do not receive enough of the signal to respond (and become red, or type 3). Here the model is expressed in terms of three outcomes, but there might be a different number of outcomes depending on the locations and/ or stages of development. The important point is that cells can work out where they are based on the level of signal they receive and they respond accordingly by developing different attributes.

Beyond Wolpert's basic model, the issue of how brain regionalization develops is an important question and we have relatively few answers. Regional specification is a prerequisite for the development of the connections that must link each region of the brain in a stereotypical and highly precise way (but allowing room for plasticity at a fine level). How these trillions of connections are made is another of life's great mysteries.

The connectome and connectionism

Since Ramón Y Cajal's first description of the neuron, scientists have vastly expanded our understanding of the structure and function of these individual building blocks of the brain. However, as Tim Berners-Lee comments, this is just the first step in understanding how our brains really work: "There are billions of neurons in our brains, but what are neurons? Just cells. The brain has no knowledge until connections are made between neurons. All that we know, all that we are, comes from the way our neurons are connected."

You'll hear about the "connectome" in Sebastian Seung's TEDTalk. The suffix "–ome" is used with increasing frequency to indicate a complete collection of whatever units are specified in the first part of the word, such as genes (hence genome), proteins (proteome) or connections (connectome). The connectome of the human brain is bewildering in its complexity, but the development of new brain imaging methods has catalyzed the first serious attempts to map it in living brains. At present, the resolution of imaging methods that can be applied to living brains isn't sufficient to follow individual connections (called axons). In these TEDTalks you'll hear about an attempt to come at the problem from the other direction, using very high resolution imaging of non-living brain tissue to reconstruct the ultramicroscopic anatomy of connections around individual cells. The extent to which these approaches are likely to succeed remains controversial.

The theory known as connectionism addresses a somewhat different matter within the field of brain organization: the relationship between connectivity and function. Essentially, the idea is that higher mental processes such as object recognition, memory and language result from the activity of the connections between areas of the brain rather than the activity of specific discrete regions. Whereas connectionists would agree that primary sensory and motor functions (i.e. responses to sensory stimuli and the activation of movements) are strongly localized to defined areas within the brain, they argue that this applies less clearly at higher cognitive levels. The theory emphasizes the relationship between connected brain areas and the function of the brain as a whole, with all parts having the potential to contribute to cognitive function. You should appreciate, therefore, that there is as yet no accepted view of the extent to which our higher mental functions are localized to particular parts of the brain. It is worth remembering this as you listen to the TEDTalks; keep an open mind on these truly fascinating issues.

Ways of studying brain function

In these TEDTalks, you're going to hear about some of the ways in which we can work out what the human brain does and how it does it. One longstanding approach is to examine what happens when people suffer brain lesions. Phineas Gage, a Vermont railroad worker, provides one spectacular historical example from 1848. Gage was packing gunpowder into a hole when it exploded, blowing the tamping rod through the front of his brain. Astonishingly, he survived and recovered, but those closest to him claimed that he had a very different personality. From this example, scientists hypothesized that elements of human personality are localized to the frontal lobes.

In Jill Bolte Taylor's TEDTalk, you'll hear how Taylor's own stroke provides further evidence for localization of brain function. A few words of caution, however: when we study the effects of a lesion on the brain, we're really learning about what the rest of the brain does without the damaged part, which is not quite the same as what the damaged structure itself does. Maybe this seems rather subtle, but in some cases it becomes important, for example if a lesion causes other parts of the brain to alter what they do.

You'll also hear about powerful techniques for observing the activity of living brains, for example using functional magnetic resonance imaging (FMRI; see the TEDTalk by Oliver Sacks). And you'll hear about methods for looking at the fine structure of neurons in post-mortem material, as in Sebastian Seung's TEDTalk. All have advantages and limitations, but together they give ever- increasing insight into the workings of the human mind.

Let's begin the TEDTalks with neuroanatomist Jill Bolte Taylor, who provides a basic overview of the brain and describes what she learned firsthand about its structure and function when at age 37 she suffered a massive hemorrhage in the left hemisphere of her brain.

Jill Bolte Taylor

My stroke of insight, relevant talks.

VS Ramachandran

3 clues to understanding your brain.

Oliver Sacks

What hallucination reveals about our minds.

Sebastian Seung

I am my connectome.

Christopher deCharms

A look inside the brain in real time.

A light switch for neurons

Rebecca Saxe

How we read each other's minds.

The human brain, explained

Learn about the most complex organ in the human body, from its structure to its most common disorders.

Here’s something to wrap your mind around: The human brain is more complex than any other known structure in the universe . Weighing in at three pounds, on average, this spongy mass of fat and protein is made up of two overarching types of cells—called glia and neurons—and it contains many billions of each. Neurons are notable for their branch-like projections called axons and dendrites, which gather and transmit electrochemical signals. Different types of glial cells provide physical protection to neurons and help keep them, and the brain, healthy.

Together, this complex network of cells gives rise to every aspect of our shared humanity. We could not breathe, play, love, or remember without the brain.

Anatomy of the brain

The cerebrum is the largest part of the brain , accounting for 85 percent of the organ's weight. The distinctive, deeply wrinkled outer surface is the cerebral cortex. It's the cerebrum that makes the human brain—and therefore humans—so formidable. Animals such as elephants, dolphins, and whales actually have larger brains, but humans have the most developed cerebrum. It's packed to capacity inside our skulls, with deep folds that cleverly maximize the total surface area of the cortex .

The cerebrum has two halves, or hemispheres, that are further divided into four regions, or lobes. The frontal lobes, located behind the forehead, are involved with speech, thought, learning, emotion, and movement. Behind them are the parietal lobes, which process sensory information such as touch, temperature, and pain. At the rear of the brain are the occipital lobes, dealing with vision. Lastly, there are the temporal lobes, near the temples, which are involved with hearing and memory.

The second-largest part of the brain is the cerebellum , which sits beneath the back of the cerebrum. It plays an important role in coordinating movement, posture, and balance.

The third-largest part is the diencephalon, located in the core of the brain. A complex of structures roughly the size of an apricot, its two major sections are the thalamus and hypothalamus. The thalamus acts as a relay station for incoming nerve impulses from around the body that are then forwarded to the appropriate brain region for processing. The hypothalamus controls hormone secretions from the nearby pituitary gland. These hormones govern growth and instinctual behaviors, such as when a new mother starts to lactate. The hypothalamus is also important for keeping bodily processes like temperature, hunger, and thirst balanced.

Seated at the organ's base, the brain stem controls reflexes and basic life functions such as heart rate, breathing, and blood pressure. It also regulates when you feel sleepy or awake and connects the cerebrum and cerebellum to the spinal cord.

The brain is extremely sensitive and delicate, and so it requires maximum protection, which is provided by the hard bone of the skull and three tough membranes called meninges. The spaces between these membranes are filled with fluid that cushions the brain and keeps it from being damaged by contact with the inside of the skull.

Blood-brain barrier

Want more proof that the brain is extraordinary? Look no further than the blood-brain barrier. The discovery of this unique feature dates to the 19th century, when various experiments revealed that dye, when injected into the bloodstream, colored all of the body’s organs except the brain and spinal cord. The same dye, when injected into the spinal fluid, tinted only the brain and spinal cord.

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This led scientists to learn that the brain has an ingenious, protective layer. Called the blood-brain barrier, it’s made up of special, tightly bound cells that together function as a kind of semi-permeable gate throughout most of the organ . It keeps the brain environment safe and stable by preventing some toxins, pathogens, and other harmful substances from entering the brain through the bloodstream, while simultaneously allowing oxygen and vital nutrients to pass through.

Health conditions of the brain

Of course, when a machine as finely calibrated and complex as the brain gets injured or malfunctions, problems arise. One in five Americans suffers from some form of neurological damage , a wide-ranging list that includes stroke, epilepsy, and cerebral palsy, as well as dementia.

Alzheimer’s disease , which is characterized in part by a gradual progression of short-term memory loss, disorientation, and mood swings, is the most common cause of dementia . It is the sixth leading cause of death in the United States, and the number of people diagnosed with it is growing. Worldwide, some 50 million people suffer from Alzheimer’s or some form of dementia. While there are a handful of drugs available to mitigate Alzheimer’s symptoms, there is no cure. Researchers across the globe continue to develop treatments that one day might put an end to the disease’s devasting effects.

Far more common than neurological disorders, however, are conditions that fall under a broad category called mental illness . Unfortunately, negative attitudes toward people who suffer from mental illness are widespread. The stigma attached to mental illness can create feelings of shame, embarrassment, and rejection, causing many people to suffer in silence. In the United States, where anxiety disorders are the most common forms of mental illness, only about 40 percent of sufferers receive treatment. Anxiety disorders often stem from abnormalities in the brain’s hippocampus and prefrontal cortex.

Attention-deficit/hyperactivity disorder, or ADHD , is a mental health condition that also affects adults but is far more often diagnosed in children. ADHD is characterized by hyperactivity and an inability to stay focused. While the exact cause of ADHD has not yet been determined, scientists believe that it may be linked to several factors, among them genetics or brain injury. Treatment for ADHD may include psychotherapy as well as medications. The latter can help by increasing the brain chemicals dopamine and norepinephrine, which are vital to thinking and focusing.

Depression is another common mental health condition. It is the leading cause of disability worldwide and is often accompanied by anxiety. Depression can be marked by an array of symptoms, including persistent sadness, irritability, and changes in appetite. The good news is that in general, anxiety and depression are highly treatable through various medications—which help the brain use certain chemicals more efficiently—and through forms of therapy.

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CONSTRUCTION OF THE HUMAN FOREBRAIN

Terry l. jernigan.

University of California, San Diego

Joan Stiles

The adult human brain is arguably the most complex of biological systems. It contains 86 billion neurons (the information processing cells of the brain) and many more support cells. The neurons, with the assistance of the support cells, form trillions of connections creating complex, interconnected neural networks that support all human thought, feeling, and action. A challenge for modern neuroscience is to provide a model that accounts for this exquisitely complex and dynamic system. One fundamental part of this model is an account of how the human brain develops. This essay describes two important aspects of this developmental story. The first part of the story focuses on the remarkable and dynamic set of events that unfold during the prenatal period to give rise to cell lineage that form the essential substance of the brain, particularly the structures of the cerebral hemispheres. The second part of the story focuses on the formation of the major brain pathways of the cerebrum, the intricate fiber bundles that connect different populations of neurons to form the information processing systems that support all human thought and action. These two aspects of early brain development provide an essential foundation for understanding how the structure, organization, and functioning of the human brain emerge.

INTRODUCTION

Evolution has selected for a developmental process by which the exquisite structure of the human brain appears to unfold miraculously. These events seem to be precisely orchestrated by an invisible hand of a master biological artist assembling an intricately designed model. However, as we uncover the molecular and cellular interactions that drive these developmental processes, we recognize that the system is evolving over time via countless signals exchanged between cells and groups of cells in many local communities throughout the growing brain. These developmental events resemble the kinds of interactions that occur among people as they come and go from a rapidly growing and constantly changing frontier town. Structures are built, expanded, repurposed or torn down. Neighborhoods are added to meet functional demands, and modified or even abandoned in favor of more efficient systems as both stationary and migrating populations dynamically interact with each other and with their changing environments.

This essay considers the series of remarkable structural changes that, over time, give rise to the complex architecture of the mature human brain. It examines, first, the development of the cell lines that make up neural structures, and then the processes by which the circuits, pathways, and information processing networks of the human brain emerge.

THE MAJOR CELL TYPES OF THE HUMAN BRAIN AND THEIR FUNCTIONS

Mature human brains are made of two main types of cells: neurons and glial cells. Neurons define the information processing circuits of the brain, and fall into two broad categories: Excitatory projection neurons and inhibitory interneurons (see Figures 1 and ​ and2), 2 ), which play complementary roles in the regulation of brain signaling. Glial cells play a variety of roles in both the development and later functioning of these circuits. There are several macroglial cell populations: oligodentrocytes and oligodendrocyte precursor cells (OPCs), astrocytes, ependymal cells, and an important population of microglial cells (see Figure 3 ). The name given to this class of cells derives from a word meaning “glue”, which reflects the lack of early appreciation of the significance of these cells. Glial cell populations have historically been considered to be the “handymen and housekeepers” of the brain, supporting a variety of neural functions, repairing systems, and clearing away debris. However, recent evidence suggests much more complex roles for glial cell populations in the formation of neural circuitry. Finally, during development, another critically important class of cells is neural progenitor cells . These are cells that have the potential to generate many cell types, and are the source of all neurons and macroglial cells in the brain.

Diagram of a prototypical projection neuron illustrating the cell body, and the neural processes, specifically the axon which can extend over substantial distances to make contact with other neurons and the dendrites which are a major site of contact with the incoming axons of other neurons. Blausen.com staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine . DOI:10.15347/wjm/2014.010. ISSN 20018762. Creative Commons Attribution 2.5 Generic license.

Coronal section of the neocortex containing projection neurons (visualized by green GFP staining) and interneurons (visualized by antibody staining in red). Scale bar: 100 μm. 22 , Figure 6f , slightly altered (plus scalebar, minus letter “f”). Creative Commons Attribution 2.5 Generic

The four major classes of glial cells. The macroglial cells are the oligodendrocytes which form myelin sheaths around the axons in the brain, astrocytes which perform a range of functions in the formation and maintenance of neural pathways as well as housekeeping functions and the ependymal cells which play an important role in the production of cerebral spinal fluid (CSF). Macroglial cells are derived from the neural progenitor cell line. Microglia are phagocytes that serve to clear waste but they also play important roles in the maturation of neural circuits. Blausen.com staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine . Creative Commons Attribution 2.5 Generic license. DOI:10.15347/wjm/2014.010. ISSN 20018762. Blausen gallery 2014.

Neural progenitor cells

Neural progenitor cells are the first to be specified in brain development. During the third week after conception, the two-layered, flattened human embryo acquires a three-layered structure. The cells located along the midline of the upper layer receive molecular signals from other cells that induce them to differentiate into neural progenitor cells, the cells that will subsequently produce nearly all of the cells that make up the brain. By the end of the fourth week, the embryo undergoes dramatic transformation and begins to assume the familiar 3-dimensional shape of the emerging fetus. A major part of this transformation is the formation of the “neural tube” which is the first real neural structure (see Darnell and Gilbert, Neuroembrology , WIREs Dev Biology , and Power and Schlaggar, Neural plasticity across the lifespan , WIREs Dev Biology, also in the collection How We Develop ). The neural progenitor cells line the inside wall of this tube establishing what will become “ventricular and subventricular zones,” the places where many neurons and neural support cells are born.

Neural progenitor cells give rise to most cells of the brain. The generation of the wide variety of cell types comes about via molecular signaling that promotes change in the fate of dividing cells. Following initial specification of the neural progenitor cells, those cells begin to divide symmetrically , producing two “daughter cells” that are identical copies of the original cell. Thus symmetrical cell division in the progenitor population serves to increase the size of the pool of neural progenitors.

Beginning at about 6 weeks gestational age in humans, some of the progenitors change their mode of cell division and begin to divide asymmetrically . During symmetrical cell division, both daughter cells receive identical molecular signals that preserve the characteristics of the parent cell in both offspring. However, during asymmetrical cell division, critical molecular signals are distributed asymmetrically within the dividing progenitor cell such that the two daughter cells receive different signals that promote divergent developmental paths and cell fates. In particular, one daughter cell receives molecular signals that preserve its fate as a progenitor cell, while the other receives different, “proneural” signals that result in its differentiation into a neuron. Thus, this form of asymmetrical cell division results in the preservation of one progenitor cell that will continue to divide and produce more cells, and one neuron that is no longer capable of cell division. Asymmetric cell division can also give rise to diversity within the progenitor population, by creating progenitors that can produce different kinds of daughter cells (e.g., different neuron types, glial cells, etc). This divergence in progenitor cell lineages is driven by extrinsically and intrinsically generated molecular signals. 1 The distinctions among cell types appear to be related in an important way to the timing and location of the birth of the gradually diversifying progenitor cell lines, creating a cascade of changes in the progenitor population that gives rise to the increasing diversity of cell types in the developing brain.

One important type of molecular signal is morphogenic signaling. This type of signaling is crucial to the spatial patterning of the brain’s structure, that is, the specification of which neural cells will assume different roles and functions and migrate to different positions in the brain. Morphogens are signaling molecules that are secreted by cells and diffuse through tissue to create concentration gradients. The particular morphogenic signal a progenitor cell receives differs depending on the local concentrations of the morphogens. Morphogenic signals trigger the expression or repression of transcription factors within the cell receiving the signals, and this can affect its state and alter the specific types of cells it produces.

Immediately after closure of the neural tube, the neural progenitors multiply rapidly throughout the proliferative zones. As illustrated in Figure 4 , there are two important proliferative zones in the brain, a more dorsally located zone in the higher regions of the brain, and a ventral zone located deep in the brain. The progenitors for excitatory projection neurons of the cerebral cortex are born in the dorsal proliferative zone, while excitatory neurons of the deep subcortical structures of the brain, as well as many of the brain’s inhibitory interneurons, are produced from precursors in the ventral proliferative zone.

Migration patterns from the two primary neural proliferative zones. A. The Dorsal Proliferative System consists of the ventricular (VZ) and subventricular zones. Radial glial cells in the VZ are a major population of neural progenitor cells; they also extend a cellular process from the VZ to the cortical surface that serves as a kind of scaffold for migrating neurons. The ganglionic eminences (GE) make up the Ventral Proliferative zone. Interneurons (green) migrating into the cerebral wall from the GE interact with radial glia (red) and can exhibit changes in direction of migration after contacting radial glia. Interneurons can use radial glia as a scaffold upon which to migrate as they ascend to the cortical plate (CP) or descend in the direction of the ventricular zone (VZ). Particular orientation and morphological dynamics of migration may be associated with particular subsets of interneurons 23 Creative Commons Attribution 2.5 Generic. B. Most of the projection neurons in the brain are produced in the VZ and an adjacent proliferative region called the SVZ (not shown). Radial glia progenitors produce neurons that then migrate to the neocortex via the radial glial scaffold. Malatesta and Gotz 24 .

One of the most important information processing structures in the brain is the neocortex. The neocortex is a thin layer of cells (approximately 2–5 mm thick) that covers the surface of the brain, much like the rind of an orange (see Figure 5 ). The neocortex comprises six layers, each containing a unique complement of neurons and support cells. The cortical layers emerge in an orderly fashion during prenatal development. The projection neurons are “born” in the dorsal proliferative zone and migrate to their respective layers in an “inside-out” pattern, with the neurons of the deepest (6 th ) layer arriving first, followed by migration of neurons bound for the more superficial layers (see Figure 6 ). This orderly vertical pattern of neuron migration creates what has been called the “laminar”, or layered, structure of the neocortex. The cortical inhibitory interneurons, migrating tangentially toward their cortical targets from the ventral proliferative zones (see Figure 4 ), appear to require signals from the earlier arriving projection neurons to enter the cortex and assume their laminar positions 2 .

A. Coronal section (see reference of slice location on whole brain insert) of an adult human brain showing the gray and white matter compartments as well as the fluid filled ventricular cavities. The neocortex is the thin layer of cells covering the surface of the brain. White matter pathways run beneath the cortical surface. Deep gray matter nuclei serve as relay stations. John A Beal, PhD Dep’t. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport. Creative Commons Attribution 2.5 Generic license.

Cortical Plate Formation. A schematic representation of human neocortical development is shown. A. The VZ prior to the onset of neuronal migration contains symmetrically dividing progenitor cells. B. The first neurons to leave the VZ form a sparse preplate layer at the edge of the VZ (GW<7). C. Axons of the preplate neurons along with the axons projecting from subcortical areas, form the intermediate zone. D. The preplate is split into an outer MZ and a deeper subplate zone by the developing cortical plate (beginning GW7–8). E. In the mature cortex, only the cortical layers and the underlying white matter pathways are evident. VZ=ventricular zone, MZ=marginal zone, PP=preplate, SP=subplate, SZ=subventricular zone.

Once neurons arrive at and assume their positions in the developing neocortex, they begin to extend processes (axons and dendrites) that allow them to form connections (synapses) and communicate with other neurons, eventually establishing interconnected pathways and information processing networks. In some cases, axonal processes form short local connections to nearby neurons. In other cases, axons can extend over much greater distances, some as far as a meter or more. Neurons generate electrochemical signals that are transmitted along the axonal processes (acting much like a telephone wire) enabling communication among distinct and sometimes quite distant neuronal populations. The development of these signaling pathways is protracted, in come cases extending well into adolescence (see below).

In addition to this vertical, laminar organization, the neocortex is also organized in the horizontal dimension into discrete functional areas that are distinguished by the types of neurons they contain, the specific pattern of connections they make, and the functions they carry out (See Figure 7 3 ). During neurogenesis, complex patterns of molecular signaling within the dorsal ventricular zone determine the initial “areal fate” of the cortical projection neurons (e.g., whether they will migrate anteriorly to brain areas that control motor functions and become motor neurons, posteriorly to visual areas to become visual neurons, or more laterally to auditory areas to become auditory neurons). This early specification of areal fate in neocortical neurons sets the stage for subsequent developmental changes that will eventually give rise to functionally distinct cortical regions. However, this very early fate specification is primitive and malleable. The full specification of a neuron as a motor or visual neuron depends upon many intervening developmental events that include both molecular signaling and input from the environment.

This drawing shows the regions of the human cerebral cortex as delineated by Korvinian Brodmann on the basis of cytoarchitecture. Creative Commons Attribution 2.5 Generic license, modified by User:Looie496, original artist unknown but probably Brodmann - This image was made by modifying a scan of p 288 of the book “Anatomy of the Nervous System”, by Stephen Walter Ranson, W. B. Saunders, 1920.

Most of the neurons that make up the brain are generated prenatally. Exceptions include local interneurons of the cerebellum 4 , and a specialized class of large projection neuron located in the fronto-insular cortex and the anterior cingulate called von Economo neurons 5 . Both of these classes of neurons are produced early in the prenatal period. In addition two types of neurons continue to be produced through the life span. These include neurons of the olfactory bulb, a structure involved in smell, and the dentate gyrus of the hippocampus, an important memory structure. With these exceptions, neuron production is complete by midgestation. The completion of neurogenesis frees the neural progenitor population to produce other types of cells that are essential for brain organization and function, specifically, the macroglial cell populations.

Glial cell populations

There are three main types of macroglial cells - oligodendrocyes, astrocytes, and ependymal cells - all of which are derived from the neural progenitor cell population. In addition there is a population of microglial cells that derive from cells in the bone marrow but that colonize the embryonic brain very early. Each class of cells plays important roles in both the development of the brain and in its ongoing functioning.

Oligodendrocytes

Oligodendrocytes are important cells that, when mature, will extend “myelin sheaths” that wrap around neuronal axons. Myelin is a fatty substance that serves to insulate axonal fibers, dramatically improving conduction of electrochemical signals along them. The process of myelination unfolds over a long period of time. It begins before birth and extends well into the postnatal period. While myelin is evident in most brain regions by the end of the second year of life, there is evidence that myelination continues into the sixth decade of life 6 , 7 . The progression of myelination is well documented. It begins in the basic sensory-motor pathways and progresses to pathways connecting multimodal brain regions.

At the end of cortical neurogenesis, the neural precursor cells stop generating neurons and begin to produce “intermediate glial precursor cells”. Some intermediate glial precursors are called oligodendrocyte precursor cells (OPCs). These cells will generate oligodendrocytes (others will produce astrocytes). Recent evidence suggests that OPCs born at different times may arise from different proliferative regions, with the first ones born in the subcortical (more ventral) zones, but the last precursors, born in the perinatal and postnatal periods, emanating primarily from the dorsal “cortical” proliferative zones 8 . OPC’s continue proliferating as they migrate to all parts of the developing brain, a process that extends for many years postnatally in humans. By birth, OPCs are distributed widely throughout the brain where they begin to differentiate into oligodendrocytes, and then rapidly begin to mature and myelinate axons in sensorimotor fiber tracts. Critically, however, the brain’s connecting fiber tracts are not fully myelinated for several decades, suggesting that OPCs may continue to mature into myelinating oligodendrocytes over a very protracted developmental time course in humans.

Astrocytes are the most abundant macroglial cell type in the brain, substantially outnumbering neurons. Traditionally, the role of astrocytes was thought to be limited to structural support and basic housekeeping functions that serve to optimize the neuronal environment (e.g., maintaining ion and pH balance, clearing waste, delivering oxygen and glucose). More recently, the role of astrocytes in the dynamic regulation of neuron production, neural network organization, and modulation of neural activity has elevated their importance in understanding the development and mature functioning of the brain 9 , 10 . An appreciation of the more varied role of astrocytes in neural structure and function has brought about an increased understanding of the dynamic nature of both brain development and later functioning. Neurons and astrocytes serving as “housekeepers” alone could not account for the complex processes that are observed. As Nedergaard has noted, “astrocytes provide not so much the glue of the neuronal network of the brain as its dynamic, self-organizing and auto-regenerative scaffold… Simply stated, astrocytes tell neurons what to do, besides just cleaning up their mess” 10 . The critical functions of astrocytes begin early in development and extend through the lifespan.

As was the case with OPCs, the astrocyte precursor cells begin to emerge at the end of cortical neurogenesis as a specific line of intermediate glial precursor cells. Astrocyte numbers increase several-fold immediately after birth 11 . These cells continue to elaborate and contribute to dynamic processes of neural circuit development, influencing both synapse formation and elimination as well as the morphology of dendritic spines 12 . The astrocyte precursors continue proliferating as they migrate radially and tangentially to all parts of the developing brain, and, as was the case with OPCs, this process continues throughout late gestation and for many years postnatally in humans. Indeed, given the role of astrocytes in basic neural functions, astrocyte production likely extends throughout the lifespan.

Ependymal cells

Ependymal cells are a specialized class of cells located in the walls of the brain’s ventricular system. The ventricular system is an interconnected network of hollow cavities and tubes within the brain and spinal cord. The ventricular system originates in the hollow opening in the embryonic neural tube, and becomes elaborated across the course of fetal development. The ventricles are filled with a constantly recycled supply of cerebrospinal fluid (CSF). CSF provides buoyancy and cushions the brain from injury; it maintains chemical stability and serves to clear waste from the brain. The ependymal cells and capillary beds in the walls of the ventricular system comprise the choroid plexus, the source of much of the brain’s CSF. The ependymal cells also have small motile hair cells that move CSF through the system. Ependymal cells are derived from neural progenitor cells undergoing their final round of cell division during late embryogenesis.

In addition to these three major classes of macroglial cells, which are all derived from neural progenitors, there are also resident microglia in the brain that are not intrinsic neural cells but are of a different lineage derived from bone marrow. The routes and exact timing of the colonization of the brain by microglia are still poorly understood; it is clear, however, that microglia are present in widespread areas of the brain during the proliferation, differentiation, and migration of all classes of neural cells, and they have been observed to interact with the developing neural cells directly. Microglia appear to stake out surveillance territories throughout the nervous system in close interaction with the developing neural cells. Recent evidence suggests that microglia may play a critical role in maturation of neural circuits, for instance, by pruning excessive and unnecessary cells and synapses, as they have been observed to engulf and remove neuronal processes and newborn neural cells 13 .

Birth and death of cells and connections

We end this section on cells with a word about a surprising but important fact: Nearly half of all the billions of cells and trillions of connections that are produced in the developing brain are systematically eliminated, either by a regulated process of cell death (called apoptosis) or by pruning of connections. Early in the development of the human brain, there is an initial dramatic overproduction of brain cells of many types followed by apoptosis of a large portion of those cells. The surviving cells also establish many more connections than will survive, and these connections are pruned back over an extended postnatal period as synapses and dendritic and axonal arbors are thinned 14 (see review by Low and Cheng, 2006). These regressive processes are still not fully understood but appear to involve competition for trophic factors (substances that promote cell growth and survival) and activity-dependent selection of connections.

Apoptosis occurs in the prenatal period and is observed within both neuronal and neural precursor populations. A number of functions have been ascribed to the cell death phenomena. These range from a form of error correction, to the elimination of transient cell populations that serve a specific but time-delimited role in development, to, most importantly, the regulation of cell numbers within a neural circuit that optimizes the pattern of connectivity between neurons, their efferent targets and their afferent inputs 15 . Synaptic exuberance and subsequent pruning are largely postnatal events that extend through childhood into adolescence. There is considerable evidence that neurons initially receive widespread synaptic inputs, and that synapse elimination serves to sharpen and stabilize neural circuitry. Both activity-dependent and activity-independent factors contribute to these dynamic processes (see Power and Schlaggar, Neural plasticity across the lifespan , WIREs Dev Biology, also in the collection How We Develop ) (see Schlagger these essays). Beyond these early regressive events, recent evidence suggests that well into adolescence, dynamic changes continue in the patterns of connectivity between excitatory and inhibitory neurons, and in their interactions with other brain cells 16 . These processes continuously remodel the neural circuitry, reflecting the dynamic, protracted and activity-dependent nature of brain development.

IMAGING THE DEVELOPING HUMAN BRAIN

Since information about the construction of the brain at the cellular level comes mostly from animal models, we still have limited understanding of the nature and time course of maturation and remodeling of connectivity in the brains of children (note: we use the term maturation to refer to the emerging stability of dynamically and adaptively developing neural systems, rather than the kinds of deterministic constructs associated with the term in older biological models). However, noninvasive neuroimaging techniques have provided a window on the developing brain and have in some cases yielded surprising new information. Furthermore, new technologies continue to emerge that allow us to measure change in brain architecture and tissue biology with greater sensitivity across childhood and adolescence.

At a very general level the brain contains gray matter, white matter, and fluid compartments. Gray matter compartments contain concentrations of neurons along with other support cells and appear gray in brain sections (see Figure 5 ). White matter contains the myelinated fiber pathways that connect groups of neurons and appear white in brain sections. The ventricular system in the brain contains cerebral spinal fluid. The signals recorded during Magnetic Resonance Imaging (MRI) differ markedly depending on tissue type, thus allowing for the differentiation of the brain’s gray matter, white matter, and fluid. With development, the distribution of signals from these three types of tissue changes providing insight into patterns of developmental change in the brain’s architecture.

MRI studies reveal dramatic changes in the tissues of the developing brain during the postnatal brain growth spurt. These changes presumably reflect many of the postnatal processes outlined above: the continuing proliferation of progenitors and maturation of oligodendrocytes and astrocytes, as well as increases in the brain’s microglial population; the overproduction of connections and synapses followed by selective pruning; and the ongoing myelination of axons. MR imaging provides information about the timing and anatomical distribution of these processes, especially the deposition of myelin by the maturing oligodendrocytes, since myelination has strong effects on tissue contrast in MR images 17 . Just after birth, MRI evidence of early myelination (specific signal change in the tissue) first appears in the sensorimotor pathways that connect the sense organs to their primary targets in the cerebral cortex, and in the fiber tracts that connect the two cerebral hemispheres to each other. Later these changes gradually spread throughout the white matter, as other fiber tracts from deep gray matter structures to the cortex and tracts connecting different areas of the cortex to each other become fully myelinated.

The earliest MRI morphometric studies (i.e., measuring tissue volumes and brain shape) comparing children and adults revealed that gray matter volumes in the cerebral cortex and subcortical nuclei appeared considerably larger in school-aged children than in young adults 18 . MRI measurements indicated that brain volume increases dramatically in the first decade after birth but very little thereafter. This leveling off reflects the net effects of waning progressive changes that are associated with continuing maturation of cell populations and opposing regressive changes, perhaps associated with “pruning” of neuronal processes. These observations are consistent with histological (i.e., microscopic study of tissue) evidence of ongoing myelination across this period, and evidence of reduction of synaptic density in cortex during childhood. Nonetheless, it remains unclear to what extent these factors, or other tissue changes that occur concurrently, contribute to the changing morphology observed with MRI.

Methods for examining the changing large-scale architecture of the cortex in developing children have advanced in recent years. In the largest MRI study of developing children using these methods, over 1000 typically developing individuals between 3 and 20 years of age were examined in the Pediatric Imaging, Neurocognition, and Genetics (PING) study 19 . Data from the PING study suggest that the time course of change in cortical surface area and cortical thickness are very distinct. Age-related change in cortical surface area and thickness are illustrated in the maps of annualized rate of change shown in Figures 8 and ​ and9. 9 . As Figure 8 shows, there is significant expansion of cortical surface area during preschool ages and early school age years. By 4 years of age, the greatest changes in surface area are occurring within cortical regions responsible for high-level cortical functions such as prefrontal cortex (e.g., planning, language) and temporoparietal association areas (e.g., visuospatial processing, language); still increasing but to a lesser extent are surface areas of primary sensory (visual, auditory) and sensorimotor cortex. By the 10th year, some cortical regions begin to show decreases in surface area, especially within occipital and superior parietal lobes; however, continued cortical area expansion still occurs in other regions. From 10 to 16 years, the balance between contracting and late expanding areas shifts further until cortical surface area contraction is present throughout almost the entire cortex. These data show clearly that the peak of total cortical surface area at around 10 years represents the net effect of waning expansion in some regions and early contraction in others.

Annualized rate of cortical expansion at different ages. Maps above show different views of the cerebral cortex and the color codes for the estimated rate of expansion (warm colors) or contraction (blue). Note that in some areas of the cortex of 4–6 year olds, the cortical surface is expanding at a rate above 3.5% per year. In older children expansion decelerates and gives way to modest levels of surface area contraction. Measured in the PING sample and described in 25 .

Annualized rate of apparent thinning of the cortex on MRI. Maps above show different views of the cerebral cortex and the color codes for the estimated rate of thinning. Note that in some areas of the cortex of 4–10 year olds, the cortex appears to think at a rate above 1.5% per year. In older children age-related thinning continues but at a more modest rate. Measured in the PING sample and described in Jernigan et al., 2015 26 .

Cortical thickness, unlike cortical surface area, shows no developmental increase at any point across this age range. In fact, apparent cortical thickness (as measured with MRI) decreases continuously throughout the cortex from age 3 into young adulthood ( Figure 9 ), with the rate of thinning appearing to slow slightly with age.

Unfortunately, it is still difficult to link these protracted developmental changes in cortical architecture to any specific postnatal cellular processes, either those described above or others not yet discovered. One important question is how the changes in surface area and thickness of the cortex relate to ongoing myelination of axons in fiber tracts. The cortex may appear to thin on MRI because of increased myelination in the white matter tracts coursing within and near the deepest layer of cortex. Myelin production, by increasing the volume of the brain, may contribute to expansion of cortical surface area. Changes in dendritic and synaptic density may also contribute to these measurable developmental changes in the cortex, or to similar changes in size and shape of subcortical structures. Continuing development of new imaging methods is shedding some light on these questions.

Although myelination is not directly measurable, a specialized MRI technique called diffusion weighted imaging (DWI) reveals ongoing maturation of fiber tracts 20 . DWI measures the diffusivity, or movement, of protons in water molecules in brain tissue. Several measures derived with this method exhibit strong age dependence during postnatal development, because age-related changes in tissue microstructure systematically alter the behavior of diffusing protons. A common use of DWI involves measuring the degree to which diffusion is occurring freely within the tissue (diffusivity) and the degree to which the diffusion occurs disproportionately in a particular direction (anisotropy). Researchers refer to this method as diffusion tensor imaging (DTI) (see Figure 10 ). The method reveals that in fluid-filled areas in the brain, not surprisingly, diffusivity of water molecules is high and shows little directionality, that is, the protons diffuse rapidly and randomly in the fluid, like a drop of dye in water ( Figure 10A ). Diffusivity in gray matter is lower because cellular structures impede the movement of the diffusing protons, but diffusion is still relatively isotropic; that is, there is little net directionality to the diffusion, like a drop of dye in oatmeal ( Figure 10B ).

Diffusion Tensors: A: Illustration of tensor from region with high isotropic diffusivity, as in cerebrospinal fluid. B: Tensor exhibiting isotropic, but lower diffusivity, as in gray matter. C: Elongated tensor exhibiting anisotropy, as in fiber tracts. D. Illustration of tractography of the superior longitudinal fasciculus (a major fiber tract connecting posterior with frontal parts of the cortex) shown in red.

Although in mature white matter diffusivity is also low, the rate of diffusion is higher along the long axis of fiber bundles because the highly linear structure of the coherently oriented axons leads to less restriction of the diffusing protons along this long axis than in other directions, like a drop of dye on a pile of cooked spaghetti. This asymmetrical restriction of movement produces what is called diffusion anisotropy . Fractional anisotropy (FA) is a measure of this kind of directionality. High FA is illustrated in the elongated structure shown in Figure 10C . Analyses of these measures of FA throughout the brain, reflecting changes in the spatial orientation of diffusion, have been used to trace the course of fiber tracts. These methods, referred to as tractography, allow investigators to locate known fiber tracts in the brain so that they can be measured. They have also been used to estimate the degree of apparent structural connection between regions. Figure 10D provides an example of pathway mapping using tractography in the data from one individual. Red streamlines show regions within the scan denoting the superior longitudinal fasciculus (SLF), a white matter fiber tract that connects posterior temporal and parietal brain regions to frontal regions.

Postnatally, diffusivity declines dramatically in the brain in a widespread anatomical distribution that includes both gray and white matter structures. Diffusivity in white matter of human newborns is high, and exhibits low directionality (FA). As the fiber tracts mature, and myelination proceeds, diffusivity declines, and FA increases, with diffusivity increasingly oriented parallel to the fiber bundles. Thus this kind of MR imaging allows us to monitor developmental change in the biology of the brain’s connecting pathways noninvasively in children. The denser packing of cells and their axons, in part associated with growth of tightly wrapped myelin sheaths and increasing axon diameters, reduces the fluid-filled extracellular spaces, which contributes to the observed decline in diffusivity. Since water molecules diffuse more randomly in these extracellular spaces, reducing this water also reveals more clearly the linearly oriented fiber bundles.

Changes in diffusion parameters continue throughout childhood and adolescence in a regionally varying pattern. For example, FA reaches adult levels earlier in long projection fibers (the major fiber pathways that connect cortical and subcortical regions of the brain) and commissural fibers (the large fiber bundles connecting the two brain hemispheres) than in association fibers (the fiber pathways that interconnect cortical regions, also called cortico-cortical pathways). Some cortico-cortical tracts continue to exhibit age-related FA increases well into the third decade. Although less often a focus of developmental studies than changes in fiber tracts, age-related decreases in diffusivity and increases in FA are also measurable in most deep gray matter structures. The biological mechanisms that underlie these gray matter changes in diffusivity are not well understood, but investigators have speculated that changing cell density or cell connections might play a role.

Importantly, although we cannot measure specific cellular mechanisms directly with these developmental MRI signals, there is strong accumulating evidence that some of the individual differences that we observe among children in their developing functional abilities are associated with differences in these MRI parameters, and, further, that these associations appear to be specific to the neural circuitry that mediates the functions 21 . These associations are likely to reflect functionally relevant differences among children in the neural connectivity in their brains, differences in the pace of biological maturation of neural circuits, or a combination of these factors. Similarly, the associations may reflect genetic factors, effects of experience on the biology of the brain, or both. A better explanation for these relationships is the goal of much ongoing work with noninvasive imaging in children.

In summary, developmental neurobiology has provided much exciting information about the progression of brain development, but we still have not defined the nature and timelines of the processes by which the full complement of brain cells assume the structure and organization of maturely functioning neural circuits in the human brain. Thus, many critically important questions remain about the structural development of the human brain. More definitive information about the cellular alterations that underlie the developmental signals now measurable with noninvasive imaging would be extremely useful. Such links between changes that can be monitored in the living human brain and details of the underlying neural circuitry would provide a firmer basis for new mechanistic models to account for the concurrent neurophysiological and behavioral changes we observe in developing children. Deeper understanding of the means by which the human nervous system extracts meaning from and adapts to its experiences may reveal new strategies for intervening when development goes awry.

CONCLUDING POINTS

• A pool of neural progenitor cells that appears only 3 weeks after conception will ultimately produce all of the other cell types in the brain (except the microglia), but these different cell lines emerge in response to different molecular signals, from different regions in the proliferative zone, and at different times.
• During development a general principle is that cells, and later connections between cells, are produced in much greater numbers than the numbers of surviving cell populations and connections, and the pruning back is essential for normal function of the neural circuits they form.
• Because most of what we know about how brains are constructed comes from the study of other mammals (most often rodents), this leaves many questions about the time course of the events occurring during postnatal brain development in children.
• Nevertheless, noninvasive imaging provides a compelling view of the developing brain, and this view reveals that the human brain, and its connectivity in particular, continues to mature well into adulthood.
• Human developmental neuroscience reveals that behavioral differences among developing children often map onto differences measurable with brain imaging.

Implications

Brain development is an incredibly complex process that involves many elements and processes that interact over time contributing to progressive change at all levels of the emerging neural system. This is a view of neural development anchored in the process of development itself, with each step influenced by myriad cues arising from multiple levels of the expanding neural system. None of these factors acts in isolation to determine developmental outcome. Rather, each contributes to the many complex and multifaceted processes that underlie the ongoing progress of brain development. This model has important implications for our understanding of both typical and atypical development. As we more closely map brain development to behavior, we will learn when brain developmental events are biasing functional development toward adverse outcomes, and ultimately how we might best intervene, either to prevent these events or to adapt the environment of the child to avert the negative outcomes.

Acknowledgments

This work was supported by grants from the National Institute on Drug Abuse and the Eunice Kennedy Shriver National Institute for Child Health and Human Development: RC2DA029475 and R01HD061414, and the Lundbeck Foundation: R32-A3161. The author would also like to acknowledge the support of the UCSD Kavli Institute for Brain and Mind.

Contributor Information

Terry L. Jernigan, University of California, San Diego.

Joan Stiles, University of California, San Diego.

BIBLIOGRAPHY

Further reading.

• Jernigan TL, Baaré WF, Stiles J, Madsen KS. Postnatal brain development: structural imaging of dynamic neurodevelopmental processes. In: Braddick O, Atkinson J, Innocenti GM, editors. Progress in Brain Research, volume 189, Gene Expression to Neurobiology and Behavior: Human Brain Development and Developmental Disorders. Burlington: Academic Press; 2011. pp. 77–92. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Nadarajah B, Parnavelas JG. Modes of neuronal migration in the developing cerebral cortex. Nat Rev Neurosci. 2002; 3 (6):423–432. [ PubMed ] [ Google Scholar ]
• Valiente M, Marin O. Neuronal migration mechanisms in development and disease. Curr Opin Neurobiol. 2010; 20 (1):68–78. [ PubMed ] [ Google Scholar ]

The Functions of the Human Brain Essay

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The brain forms the control center and coordinates all functions and other organs within the body. The brain executes its functions by sending and receiving signals in nerve impulses through the neurons. Though the neurons interconnect the whole brain, it is divided into compartments that perform different functions. The main purpose of the compartmental divisions of the brain is to promote brain activity by division of roles per compartment (Ahanger et al., 2021). Each section executes a specific function and controls some selected body parts. For effective action, the brain cannot work as a single unit due to the risk of poor coordination of impulses.

The cerebrum brain plays a role in maintaining the body’s balance and posture. This function is executed by making postural adjustments. The brain signals the vestibular receptors and proprioceptors and commands the change in position and muscle weight through the motor neurons to ensure that balance is achieved. The cerebrum is another part of the human brain located in the uppermost section. Its main role in the body is to coordinate the function and processing of the sensory functions. These include the main body senses, such as vision, hearing, and touch necessary for the body’s normal functioning. This function aids in controlling body movement and intellectual development, which enables studying, memory, and emotion processes. The brainstem is the most bottom portion of the brain. It is the section responsible for performing all subconscious functions. Such tasks include inhalation and exhalation, and sustaining heart rate.

The frontal lobe of the cerebrum forms the section covered by the frontal bones, as the name suggests. It constitutes two pairs, the right and left frontal cortex. The lobe plays a significant role, especially in mental functioning. It aids in planning, individual memory management, and decision-making. Other essential functions include speech and language coordination through the Broca’s region (Baker et al., 2018). The area helps in constructing words and arranging them chronologically to produce a coherent speech.

The frontal lobe is responsible for motor skills learning. Mastery of the coordinated functions, including voluntary walking and running, are enhanced. A person can also differentiate and categorize various regions by using the frontal lobe of the cerebrum. Individual personalities are developed in the region as it controls impulse responses. The interplay of signals that define one’s characteristics is in the frontal lobe. It also serves to manage the attention of someone adequately.

The functioning of the frontal lobe can be adversely affected in case of damage from traumatic injury or disease. Such a condition is attention deficit hyperactivity disorder, abbreviated as ADHD. It results in differences in the mental development of a person and affects brain activity and attention levels. The common sign of ADHD includes self-focused behavior, impatience, emotional turmoil, interruptive behavior, fidgeting, and lack of focus. Most of them will also have several unfinished tasks, and they noisily conduct their activities (Danielson et al., 2018). They talk and move excessively and have challenges sitting still in one location for a prolonged period. If this condition occurs, the frontal lobe will be damaged, and its ability to perform its functions will be impaired. In addition to the effect on attention ability, the person may have other secondary problems related to the inability of the lobe to carry out other duties adequately.

Medical or therapeutic interventions are recommended to manage attention deficit hyperactivity disorder. However, it is advisable to initiate both management regimes for efficiency and quick recovery of the lobe. Medically, the recommended drugs of choice that can be used under strict prescription include guanfacine, atomoxetine, lisdexamfetamine, and methylphenidate (Danielson et al., 2018). Various programs, such as psychoeducation, behavioral therapy, group training, cognitive behavior, and social skills training are encouraged in therapy.

Ahanger, S. H., Delgado, R. N., Gil, E., Cole, M. A., Zhao, J., Hong, S. J., Kriegstein, A, R., Nowakowski, T, J., Pollen, A, A., & Lim, D. A. (2021). Distinct nuclear compartment-associated genome architecture in the developing mammalian brain. Nature Neuroscience , 24 (9), 1235-1242. Web.

Baker, C. M., Burks, J. D., Briggs, R. G., Stafford, J., Conner, A. K., Glenn, C. A., Sali, G., McCoy, T. M., Battiste, J. D., O’Donoghue, D. L., & Sughrue, M. E. (2018). A Connectomic Atlas of the Human Cerebrum—Chapter 4: The Medial Frontal Lobe, Anterior Cingulate Gyrus, and Orbitofrontal Cortex. Operative Neurosurgery , 15 (1), S122-S174. Web.

Danielson, M. L., Bitsko, R. H., Ghandour, R. M., Holbrook, J. R., Kogan, M. D., & Blumberg, S. J. (2018). Prevalence of parent-reported ADHD diagnosis and associated treatment among US children and adolescents, 2016. Journal of Clinical Child & Adolescent Psychology , 47 (2), 199-212. Web.

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Essays About Human Brain

Brief description of human brain.

The human brain is the most complex organ in the body, responsible for controlling our thoughts, emotions, and actions. It plays a crucial role in our everyday functioning, from simple tasks like breathing and walking to complex activities like problem-solving and decision-making.

Importance of Writing Essays on ... Read More Brief Description of Human Brain

Importance of writing essays on this topic.

Essays on the human brain are essential for understanding the intricate workings of the brain and its impact on human behavior, cognition, and mental health. They also provide a platform for exploring the latest research and advancements in neuroscience, psychology, and related fields.

Tips on Choosing a Good Topic

• Focus on current issues and debates in neuroscience and psychology.
• Explore the relationship between the brain and behavior in specific populations, such as children, the elderly, or individuals with neurological disorders.
• Consider interdisciplinary approaches that integrate neuroscience with other fields, such as philosophy, ethics, or artificial intelligence.

Essay Topics

• The impact of stress on the brain and mental health.
• The role of neurotransmitters in regulating mood and behavior.
• Exploring the link between brain injuries and cognitive impairment.
• The ethical implications of brain-computer interfaces.
• The influence of genetics on brain development and functioning.
• The neurobiology of addiction and substance abuse.
• The neurological basis of memory and learning.
• The effects of meditation and mindfulness on brain function.
• The relationship between sleep patterns and brain health.
• The future of artificial intelligence and its impact on the human brain.

Concluding Thought

Writing essays on the human brain offers a unique opportunity to delve into the complexities of the mind and gain a deeper understanding of what makes us human. By exploring diverse topics within this field, we can contribute to the collective knowledge and foster critical thinking about the brain and its profound influence on our lives.

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• What is brain health...

What is brain health and why is it important?

Read our brain health collection.

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• Yongjun Wang , professor 1 2 ,
• Yuesong Pan , associate professor 1 2 ,
• Hao Li , professor 1 2
• 1 Department of Neurology, Beijing Tiantan Hospital, Capital Medical University, Beijing, China
• 2 China National Clinical Research Center for Neurological Diseases, Beijing, China
• Correspondence to: Y Wang yongjunwang{at}ncrcnd.org.cn

Yongjun Wang and colleagues discuss the definition of brain health and the opportunities and challenges of future research

The human brain is the command centre for the nervous system and enables thoughts, memory, movement, and emotions by a complex function that is the highest product of biological evolution. Maintaining a healthy brain during one’s life is the uppermost goal in pursuing health and longevity. As the population ages, the burden of neurological disorders and challenges for the preservation of brain health increase. It is therefore vital to understand what brain health is and why it is important. This article is the first in a series that aims to define brain health, analyse the effect of major neurological disorders on brain health, and discuss how these disorders might be treated and prevented.

Definition of brain health

Currently, there is no universally recognised definition of brain health. Most existing definitions have only a general description of normal brain function or emphasise one or two dimensions of brain health. The US Centers for Disease Control and Prevention defined brain health as an ability to perform all the mental processes of cognition, including the ability to learn and judge, use language, and remember. 1 The American Heart Association/American Stroke Association (AHA/ASA) presidential advisory defined optimal brain health as “average performance levels among all people at that age who are free of known brain or other organ system diseases in terms of decline from function levels, or as adequacy to perform all activities that the individual wishes to undertake.” 2

The brain is a complex organ and has at least three levels of functions that affect all aspects of our daily lives: interpretation of senses and control of movement; maintenance of cognitive, mental, and emotional processes; and maintenance of normal behaviour and social cognition. Brain health may therefore be defined as the preservation of optimal brain integrity and mental and cognitive function at a given age in the absence of overt brain diseases that affect normal brain function.

Effect of major neurological disorders on brain health

Several neurological disorders may disrupt brain function and affect humans’ health. Medically, neurological disorders that cause brain dysfunction can be classified into three groups:

Brain diseases with overt damage to brain structures, such as cerebrovascular diseases, traumatic brain injury, brain tumours, meningitis, and communication and sensory disorders

Functional brain disorders with detectable destruction of brain connections or networks, such as neurodegenerative diseases (eg, Parkinson’s disease, Alzheimer’s disease, and other dementias) and mental disorders (eg, schizophrenia, depression, bipolar disorder, alcoholism, and drug abuse)

Other brain disorders without detectable structural or functional impairment, such as migraine and sleep disorders.

These neurological disorders may have different or common effects on brain health and function. For instance, Alzheimer’s disease is the main type of dementia, with a decline in different domains of cognitive function. Mood disorders may cause dysfunction in execution, reward processing, and emotional regulations. In addition to physical disability, aphasia, gait and balance problems, and cerebrovascular diseases may lead to cognitive impairment and dementia, which are neglected by both patients and physicians.

Ageing and burden of neurological disorders

Human ageing is mainly reflected in the aspects of brain ageing and degradation of brain function. The number of people aged 60 years and over worldwide was around 900 million in 2015 and is expected to grow to two billion by 2050. 3 With the increases in population ageing and growth, the burden of neurological disorders and challenges to the preservation of brain health steeply increase. People with neurological disorders will have physical disability, cognitive or mental disorders, and social dysfunction and be a large economic burden.

Globally, neurological disorders were the leading cause of disability adjusted life years (276 million) and the second leading cause of death (9 million) in 2016, according to the Global Burden of Diseases study. 4 Stroke, migraine, Alzheimer’s disease and other dementias, and meningitis are the largest contributors to neurological disability adjusted life years. 4 About one in four adults will have a stroke in their lifetime, from the age of 25 years onwards. 5 Roughly 50 million people worldwide were living with dementia in 2018, and the number will more than triple to 152 million by 2050. 6 In the following decades, governments will face increasing demand for treatment, rehabilitation, and support services for neurological disorders.

Opportunities and challenges of future research on brain health

Opportunities and challenges exist in the assessment of brain health, the mechanism of brain function and dysfunction, and approaches to promote brain health ( box 1 ).

Lack of metrics or tools to comprehensively assess or quantify brain health

Little knowledge about the mechanisms of brain function and dysfunction

Few effective approaches to prevent and treat brain dysfunction in some major neurological disorders, such as dementia

Need to precisely preserve brain functions for people with neurosurgical diseases

Defining and promoting optimal brain health require the scientific evaluation of brain health. However, it is difficult to comprehensively evaluate or quantify brain health through one metric owing to the multidimensional aspects of brain health. Many structured or semistructured questionnaires have been developed to test brain health by self-assessments or close family member assessments of daily function or abilities. In recent decades new structural and functional neuroimaging techniques have been applied to evaluate brain network integrity and functional connectivity. 7 However, these subjective or objective measures have both strengths and weaknesses. For instance, scales such as the mini-mental state examination and Montreal cognitive assessment are simple and easy to implement but are used only as global screening tools for cognitive impairment; tests such as the digit span, Rey-Osterrieth complex figure test, trail making A and B, Stroop task, verbal fluency test, Boston naming test, and clock drawing test are used mainly to assess one or two specific domains of memory, language, visuospatial, attention, and executive function; and neuroimaging techniques, although non-invasive and objective, still have disadvantages of test contraindications, insufficient temporal or spatial resolution, motion artefact, and high false discovery rates, which limit their clinical transformation.

Another difficulty in measuring brain health is that age, culture, ethnicity, and geography specific variations exist in the perception of optimal brain health. Patient centred assessment of brain function, such as self-perception of cognitive function and quality of life, should also be considered when measuring brain health. 8 Universal acceptable, age appropriate, multidimensional, multidisciplinary, and sensitive metrics or tools are required to comprehensively measure and monitor brain function and brain health.

To promote optimal brain health, we need a better understanding of the mechanisms of brain function and dysfunction. Unfortunately, little is known about the working mechanism of the brain. Although we have made considerable developments in neuroscience in recent decades, we still cannot totally decipher the relations between spatiotemporal patterns of activity across the interconnected networks of neurons and thoughts or the cognitive and mental state of a person. 9 Recent progress in brain simulation and artificial intelligence provides a vital tool to understand biological brains, and vice versa. 10 11 The development of brain inspired computation, brain simulation, and intelligent machines was emphasised in the European Union and China Brain Project. 9 12

Meanwhile, the mechanisms behind the brain dysfunction in some neurological disorders are still not well understood, especially for mental and neurodegenerative disorders. Further investigation of the mechanisms of brain diseases may indicate approaches to treatment and improve brain function. Brain imaging based cognitive neuroscience may unravel the underlying brain mechanism of cognitive dysfunction and provide an avenue to develop a biological framework for precision biomarkers of mood disorders. 13

Most common neurological diseases, such as cerebrovascular diseases and Alzheimer’s disease, have complex aetiopathologies, typically involving spatial-temporal interactions of genetic and environmental factors. However, a single genetic factor could account for the disease progression of monogenic neurological disorders. These diseases could be more readily investigated by simplified cross species modelling, leading to better understanding of their mechanisms and greater efficiency in testing innovative therapies. Such research may provide a window to promote the investigation of common neurological disorders and general brain health, as discussed by Chen and colleagues elsewhere in this series. 14

Few effective approaches are available to prevent and treat brain dysfunction in some major neurological disorders, such as dementia. Neurons are not renewable, and brain dysfunction is always irreversible. Recent trials targeting amyloid clearance and the selective inhibition of tau protein aggregation failed to improve cognition or modify disease progression in patients with mild Alzheimer’s disease. 15 16 More attention has focused on other potential therapeutic targets, such as vascular dysfunction, inflammation, and the gut microbiome, as discussed by Shi and colleagues. 17 In particular, recent studies showed that the early impairment of cognition was induced by the disruption of neurovascular unit integrity, which may cause hypoperfusion and the breakdown of the blood-brain barrier and subsequent impairment in the clearance of proteins in the brain. 18 19 Physical activity, mental exercise, a healthy diet and nutrition, social interaction, ample sleep and relaxation, and control of vascular risk factors are considered six pillars of brain health. The AHA/ASA presidential advisory recommended the AHA’s Life’s Simple 7 (non-smoking, physical activity, healthy diet, appropriate body mass index, blood pressure, total cholesterol, and blood glucose) to maintain optimal brain health. 2 Pan and colleagues discuss how this may indicate a new dawn of preventing some cognitive impairment and brain dysfunction by preventing vascular risk factors or cerebrovascular diseases. 20

For other neurological disorders with potential therapeutic approaches, the main aim is to preserve brain function. Impaired brain function due to anatomical structural damage is underestimated in patients with neurosurgical diseases such as brain tumours, trauma, and epilepsy. In recent years, treatment targets for neurosurgical diseases have changed from focusing on survival or life expectancy to balancing brain structures and functions. Precise preservation of brain function requires an understanding of the exquisite relation between brain structure and function and advanced technologies to visualise brain structure-function relations. 21

Another example of the predicament associated with protection of brain function is uncertainty in the treatment response in epilepsy management. Current standard care for epilepsy relies on a trial and error approach of sequential regimens of antiseizure medications. The time delay due to this treatment approach means that such treatments may be less effective and irreversible damage may occur. Chen and colleagues 22 describe how recent advances in personalised epilepsy management based on artificial intelligence, genomics, and patient derived stem cells are bringing some hope to overcome this predicament in epilepsy management and promise a more effective strategy. 23 24

Brain health is the maintenance of multidimensional aspects of brain function. However, several neurological disorders may affect brain health in one or more aspects of brain function. Deciphering and promoting the function and health of the brain, the most mysterious organ in the human body, will have a dramatic impact on science, medicine, and society. 25 In the past seven years, a number of large scale brain health initiatives have been launched in several countries to promote the development of neuroscience, brain simulation, and brain protection. 9 However, further challenges are raised by the different key research directions of brain projects in different countries. In the face of these challenges, Liu and colleagues argue that collaboration on brain health research is urgently needed. 26 As the other articles in this series describe, coordinated research has enormous potential to improve the prognosis of brain disorders.

Key messages

Brain health is the preservation of optimal brain integrity and mental and cognitive function and the absence of overt neurological disorders

Human ageing increases the burden of brain dysfunction and neurological diseases and the demands for medical resources

Further studies are required to assess brain health, understand the mechanism of brain function and dysfunction, and explore effective approaches to promote brain health.

Contributors and sources: YW proposed the idea for this series on brain health. YW and YP drafted the first manuscript. All the authors critically reviewed and revised the manuscript. YP and HL expertise is in the area of clinical research methods and clinical research on stroke. YW is an expert in clinical research on stroke and neurological diseases. YW is the guarantor.

Competing interests We have read and understood BMJ policy on declaration of interests and declare that the study was supported by grants from the National Science and Technology Major Project (2017ZX09304018), National Key R&D Program of China (2018YFC1312903, 2017YFC1310902, 2018YFC1311700, and 2018YFC1311706), National Natural Science Foundation of China (81971091), Beijing Hospitals Authority Youth Programme (QML20190501), and Beijing Municipal Science and Technology Commission (D171100003017002).

Provenance and peer review: Commissioned; externally peer reviewed.

This article is part of a series launched at the Chinese Stroke Association annual conference on 10 October 2020, Beijing, China. Open access fees were funded by the National Science and Technology Major Project. The BMJ peer reviewed, edited, and made the decision to publish these articles.

This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/ .

• ↵ Centers for Disease Control and Prevention. Healthy aging. What is a healthy brain? New research explores perceptions of cognitive health among diverse older adults. https://www.cdc.gov/aging/pdf/perceptions_of_cog_hlth_factsheet.pdf
• Gorelick PB ,
• Iadecola C ,
• American Heart Association/American Stroke Association
• ↵ WHO Global Health Ethics team. Ageing. https://www.who.int/ethics/topics/ageing/en/ . 2019
• GBD 2016 Neurology Collaborators.
• Feigin VL ,
• GBD 2016 Lifetime Risk of Stroke Collaborators
• ↵ Alzheimer’s Disease International. World Alzheimer report 2018. The state of the art of dementia research: new frontiers. https://www.alz.co.uk/research/world-report-2018
• Gordon MF ,
• Lenderking WR ,
• Patient-Reported Outcome Consortium’s Cognition Working Group
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• Sabbagh MN ,
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Essay on Human Brain: Structure and Function

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The nervous system of man and other group of vertebrates is divided into three main parts:

1. Central nervous system (CNS) comprising brain and spinal cord.

2. Peripheral nervous system (PNS) consisting of cranial and spinal nerves.

3. Autonomic nervous system (ANS) including sympathetic and parasympathetic nerves.

Central Nervous System (C.N.S.) (Coloured Plate-I and II):

It is the most complicated and highly specialized organ of the body. An adult human brain weighs about 1400 gms. (In a new born baby it is about 400 gms and becomes double after one year) and has a volume of about 1500 c.c. It is enclosed in a bony case called cranium which protects brain against external injury.

(a) Meninges. The brain is being surrounded by three membranes called meninges (sing-meninx).

The meninges are:

(i) Outer duramater is a tough white membrane present below the cranium.

(ii) Middle arachnoid layer having a net work of Fibres.

(iii) Inner piamater is a thin, transparent, and vascular membrane close to the surface of brain and spinal cord. At two places the piameter fuses with the thin, dorsal surface of the brain to form choroid plexus. The meninges protect the brain from external shock.

The space between duramater and arachnoid is the subdural space and the space between arachnoid and piamater is known as subarachnoid space (Fig. 1.3). These spaces surrounding the brain as well as the cavities within the brain are filled with a lymphatic fluid, called cerebrospinal fluid (CSF). The cerebrospinal fluid is also present within the spinal cord.

The cerebrospinal fluid protects the central nervous system from external shock; helps in exchange of nutrients and waste products between the nervous tissue and blood and maintains a constant pressure in and around the brain. The brain is formed of two types of nervous tissue, Grey matter on the outer side and White matter on the inner side. The former is made of non-medullated nerve cells whereas the latter is formed of medullated nerve cells. The brain or encephalon is a white, bilaterally symmetrical structure.

It is divided into three main parts: (Fig. 1.4 and Fig. 1.5)

1. Fore brain or Prosencephalon

2. Mid brain or Mesencephalon

3. Hind brain or Rhombencephalon

Fore Brain:

It is the anterior part of the brain and largest among the three parts.

It consists of three parts:

a. Olfactory lobes,

b. Cerebral hemispheres and

c. Diencephalon.

(a) Olfactory Lobes:

The anterior most part of the fore brain is a pair of olfactory lobes. They consist of an anterior club shaped olfactory bulb and posterior olfactory tract. The cavity of the olfactory lobes or rhinocoel is not well marked. (Fig. 1.5) The olfactory lobes of human being are not significantly developed as seen in lower vertebrates. The olfactory lobes are the centers of smell.

(b) Cerebral Hemispheres:

The cerebral hemispheres or cerebrum is the largest part of the brain occupying about two-third of the entire brain. It is divided into right and left cerebral hemispheres by a deep longitudinal median groove (Fig. 1.6 and Fig. 1.7).

These two cerebral hemispheres are connected by a transverse sheet of nerve fibres, called corpus callosum (Fig. 1.12 & 1.13). The anterior part of corpus callosum is bent slightly downward to form genu while the posterior part is raised upward to form splenium. The cerebral hemispheres are divided by 3 deep fissures into 4 lobes viz. frontal, parietal, temporal and occipital lobes (Fig. 1.8).

Each lobe has its own specific function.

The frontal lobe has two areas:

(a) Motor area, controls the voluntary movements of the muscles,

(b) Premotor area, controls the involuntary movements of muscles and the same of the autonomic nervous system.

The parietal lobe or the somaestic area is the centre for the perceptions of sensations like pain, touch and temperature. The occipital lobe has two areas viz., the visual area for visual sensations and auditory area for hearing sensations. On the ventral side there is a longitudinal fissure called rhinal fissure which separates the hippocampal lobe or pyriform lobe from anterior lobe in each cerebral hemisphere. Insula or Island of Reil is a small lobe of cerebrum, covered over by parietal frontal, and temporal lobes. It co-ordinates the action of different regions of cerebral hemispheres.

The outer layer of cerebral hemispheres is called cerebral cortex or neopallium. It is formed of grey matter and contains millions of neurons. Grey matter also forms islands on the white matter. These are called cerebral nuclei. The cerebral cortex is highly convulated in order to increase its surface area.

The ridges of these convultions are called gyri and depressions between them as sulci. The cavities within the cerebral hemispheres are called lateral ventricles (First and second ventricles) which open to third ventricle (cavity of diencephalon) by a foramen of Monro (Fig. 1.9, 1.10 and 1.11).

The cerebral hemispheres have the following functions:

(i) They govern the mental abilities like learning, memory, intelligence, thinking etc.

(ii) Cerebrum is the seat of consciousness and controls reflexes like laughing and weeping,

(iii) Cerebrum responds to pain, touch, cold etc. The centres of reception of these are located in the sensory areas called soma esthetic area present in the parietal lobe.

(iv) It controls voluntary and spontaneous actions of the animal

(v) It interprets various sensations or stimuli.

(c) Diencephalon:

It is the last part of the forebrain and is almost covered dorsally by cerebral hemispheres. On its dorsal surface it bears a pineal stalk with a rounded pineal body at its top. The dorsal vascular wall forms anterior choroid plexus. Its cavity is called diocoel or third ventricle. On the ventral side hypothalamus forms the floor of the diocoel. It consists of number of scattered masses of the grey matter in the white matter. The pituitary hangs below the hypothalamus by a stalk called infundibulum. Below pituitary is mammillary body. Two optic nerves cross each other to form optic chiasma in front of the pituitary (Fig. 1.12).

Diencephalon regulates manifestations of emotions and recognizes sensations like heat, cold and pain. Hypothalamus contains nerve centres for temperature regulations, hunger, thirst and emotions. It also produces various neurohormones that control the secretions of anterior pituitary. Hormones of posterior pituitary are produced in hypothalamus and later transported to pituitary. Hypothalamus contains higher centres of autonomic nervous system and controls carbohydrate, fat metabolism, blood pressure and water balance.

The Pineal body or epiphysis is an endocrine gland producing the hormone melatonin which controls pigmentation in certain animals. Recently it has been claimed to function as a Biological clock regulating day and night periodicity.

The mid brain connects cerebral hemispheres with cerebellum. It consists of optic lobes and crura cerebri. There are four, solid optic lobes, which arise from the dorsolateral side of the mid brain. They are collectively called as corpora quadrigemina. (In case of frog there are two hollow optic lobes which are together known as corpora bigemina).

The anterior pair of optic lobes are called Superior colliculi and the posterior optic lobes are called Inferior colliculi (both are formed of grey matter). The former acts as the centre for visual reflex and the latter acts as centre for auditory reflex. On the ventral side of mid brain in its floor there are two large longitudinal bands of nerve fibres called crura cerebri (sing, crus cerebrum) which connect the medulla oblongata with the cerebral hemisphere.

The cavity of the mid brain is in the form of a narrow canal called iter which connects third ventricle with the fourth ventricle. The optic lobes are the centres of visual and auditory reflexes. The crura cerebri controls the activities of the eye muscles.

Hind Brain:

The hind brain consists of cerebellum and brain stem (pons varoli and medulla oblongata).

Cerebellum is the second largest part of the brain. Like cerebral hemispheres its upper surface is formed of grey matter and forms cerebellar cortex and the deeper central part is medulla formed of white matter.

Cerebellar nuclei of gray matter are scattered in the white matter.

Cerebellum is partially divided into three lobes:

central vermis, two lateral lobes and outer floccular lobes.

The grey matter of cerebellum exhibits tree-like structure called vitae (Fig. 1.13).

Cerebellum maintains equilibrium, and controls posture. It modulates and moderates voluntary movements initiated in cerebrum. It makes the movement of the body smooth, steady and coordinated. The brain stem consists of pons varoli in front of cerebellum and medulla oblongata behind, the latter continues behind as spinal cord. Pons varoli is composed of thick bundles of transverse nerve fibres which connect two sides of the cerebellum. It coordinates muscle movements on the two sides of the body.

Medulla oblongata is the posterior most part of the brain. Its cavity is called fourth ventricle or metacoel. The roof of the medulla is thin and vascular forming posterior choroid plexus, the latter secretes cerebrospinal fluid. Medulla oblongata controls all the involuntary actions of the body. It has cardiac centre (to regulate heart beat), respiratory centre (to control rate and depth of breathing), gastric centre (controls flow of gastric juices), reflex centres (controls act of swallowing, vomiting, salivation, choking etc.). It acts as a pathway for conducting impulses from spinal nerves to spinal cord and then to brain.

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The Weight of Wonder: how Much does the Human Brain Weigh?

This essay about the weight of the human brain explores its average mass of 1.3 to 1.4 kilograms and its composition of water, fats, proteins, and more. It discusses how the brain’s weight varies throughout life stages, from infancy through adulthood, and emphasizes its dense structure packed with neurons and glial cells. Beyond its physical properties, the essay underscores the brain’s critical role in cognition, consciousness, and adaptability. It highlights ongoing research that continues to unravel the brain’s complexities and its profound impact on human experience and identity.

How it works

Have you ever wondered about the weight of the human brain? It’s a question that sparks curiosity, often accompanied by myths and misconceptions. The brain, our body’s command center, is a marvel of nature, but its actual weight might surprise you. Let’s delve into the fascinating world of brain anatomy and physiology to uncover the truth.

The human brain, on average, weighs about 1.3 to 1.4 kilograms (2.87 to 3.09 pounds) in adults. This weight can vary slightly based on factors such as age, sex, and individual differences in brain size.

Despite its relatively small size compared to the rest of the body, which typically weighs around 60 kilograms (132 pounds) in adults, the brain plays an outsized role in controlling our thoughts, emotions, movements, and bodily functions.

When exploring the weight of the brain, it’s essential to understand its composition. The brain is primarily composed of water (about 75%), which contributes significantly to its weight. The rest is a complex mix of fats, proteins, carbohydrates, and salts. Structurally, the brain consists of different regions, each responsible for specific functions such as vision, movement, language, and memory. These regions, interconnected through a vast network of neurons and synapses, enable the brain to process information and regulate the body’s activities seamlessly.

Interestingly, the weight of the brain is not static throughout life. It undergoes changes from birth through adulthood. At birth, a baby’s brain weighs approximately 350-400 grams (0.77-0.88 pounds). During childhood and adolescence, the brain experiences rapid growth and development, reaching nearly its full adult size by the age of 6. However, it continues to undergo subtle changes throughout adulthood, influenced by factors such as learning, experiences, and aging.

To put the brain’s weight into perspective, consider its density. The brain is one of the most densely packed organs in the body, containing billions of neurons (nerve cells) and even more glial cells that support and protect these neurons. Despite its small size relative to other organs, the brain’s intricate structure allows it to process vast amounts of information simultaneously, enabling complex cognitive functions and behaviors.

Beyond its physical weight, the brain’s importance cannot be overstated. It is the seat of consciousness, enabling us to perceive the world, form memories, solve problems, and interact with others. Moreover, research continues to uncover the brain’s remarkable adaptability and plasticity, demonstrating its ability to reorganize and form new connections in response to experiences and injuries.

In conclusion, while the human brain may weigh only about 1.3 to 1.4 kilograms, its significance and complexity are immeasurable. Understanding its weight is just a glimpse into the intricate workings of this extraordinary organ. As neuroscience advances, so too does our appreciation for the brain’s role in shaping who we are and how we experience life. So next time you ponder the weight of the human brain, remember, its true measure lies not in pounds or kilograms, but in the profound impact it has on our existence.

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The Human Brain's Complexity Verges on The Brink of Chaos, Physicists Say

The human brain is said to be the most complex object in the known Universe. Its 89 billion neurons each have around 7,000 connections on average, and the physical structure of all those entities may be balanced precariously on a knife's edge, according to a new study.

Two physicists at Northwestern University in the US – Helen Ansell and István Kovács – have now used statistical physics to explain the complexity seen in a highly detailed 3D map of not just part of a human brain, but part of a mouse's and fruit fly's brains as well.

At a cellular level, their framework suggests the high-level hardware encased in our skulls is at a structural sweet spot closely approaching a phase transition.

"An everyday example of this is when ice melts into water. It's still water molecules, but they are undergoing a transition from solid to liquid," explains Ansell.

"We certainly are not saying that the brain is near melting. In fact, we don't have a way of knowing what two phases the brain could be transitioning between. Because if it were on either side of the critical point, it wouldn't be a brain."

In the past, some scientists have suspected that phase transitions play an important role in biological systems. The membrane that surrounds cells is a good example. This lipid bilayer fluctuates between gel and liquid states to let proteins and liquid in and out.

By contrast, however, the central nervous system may teeter on a critical point of transition, while never actually becoming something else.

A common feature of this critical point is the branch-like structure of neurons, known as fractal patterns . Fractals, like those seen in snowflakes, molecules , or the distribution of galaxies , emerge in the most complex of systems . In physics, the fractal dimension is a "critical exponent" that sits on the edge of chaos , between order and disorder.

Ansell and Kovác now argue that the nanoscale presence of fractals in 3D brain reconstructions is a sign of this 'criticality'.

Because of data limitations, the duo were only able to analyze a single partial brain region of a human, a mouse, and a fruit fly. Yet even with this limited picture, the team found matching fractal-like patterns that looked similar regardless of whether they zoomed in or out.

The relative size of various neuron segments and their diversity seem to be maintained across scales and species. Neither too organized nor too random, the brain's systems are juuuust right, balancing the costs of neural 'wiring' with the requirements of long-distance connections.

This 'Goldilocks effect' could very well be a universal, governing principle of all animal brains, Ansell and Kovács argue, although proving that will require far more research.

"Initially, these structures look quite different – a whole fly brain is roughly the size of a small human neuron," says Ansell. "But then we found emerging properties that are surprisingly similar."

Further studies are now needed to determine if that shared criticality exists across the full scale of the animal brain and among various species.

While previous studies have analyzed brain criticality when it comes to neuron dynamics , it hasn't been possible until recently to analyze and compare the structure of animal brains at a cellular level.

Data limitations still exist, of course, but there is currently a large-scale effort in neuroscience to map the anatomy and connections of the brain in as much detail as possible .

A single cubic millimeter of a human brain was recently reconstructed, and last year, we got the first-ever complete map of a fruit fly brain, as well as a cellular map of the mouse brain .

"[The structural level] has been a missing piece for how we think about the brain's complexity," says physicist István Kovács from Northwestern.

"Unlike in a computer where any software can run on the same hardware, in the brain the dynamics and the hardware are strongly related."

Ansell says the team's findings "open the way" to a simple physical model that can describe statistical patterns of the brain. One day, such a feat could be used to improve brain research and to train artificial intelligence systems.

The study was published in Communications Physics .

Human Brain Essays

The brain: a marvel of complex functions, popular essay topics.

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Do We Need Language to Think?

A group of neuroscientists argue that our words are primarily for communicating, not for reasoning.

By Carl Zimmer

For thousands of years, philosophers have argued about the purpose of language. Plato believed it was essential for thinking. Thought “is a silent inner conversation of the soul with itself,” he wrote.

Many modern scholars have advanced similar views. Starting in the 1960s, Noam Chomsky, a linguist at M.I.T., argued that we use language for reasoning and other forms of thought. “If there is a severe deficit of language, there will be severe deficit of thought,” he wrote .

As an undergraduate, Evelina Fedorenko took Dr. Chomsky’s class and heard him describe his theory. “I really liked the idea,” she recalled. But she was puzzled by the lack of evidence. “A lot of things he was saying were just stated as if they were facts — the truth,” she said.

Dr. Fedorenko went on to become a cognitive neuroscientist at M.I.T., using brain scanning to investigate how the brain produces language. And after 15 years, her research has led her to a startling conclusion: We don’t need language to think.

“When you start evaluating it, you just don’t find support for this role of language in thinking,” she said.

When Dr. Fedorenko began this work in 2009, studies had found that the same brain regions required for language were also active when people reasoned or carried out arithmetic.

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May 15, 2024

A Cubic Millimeter of a Human Brain Has Been Mapped in Spectacular Detail

Google scientists have modeled all 150 million connections of a fragment of the human brain at nanoscale resolution

By Carissa Wong & Nature magazine

Neurons in a fragment of brain cortex.

Daniel Berger, Lichtman Lab, Harvard University

Researchers have mapped a tiny piece of the human brain in astonishing detail. The resulting cell atlas, which was described today in Science 1 and is available online , reveals new patterns of connections between brain cells called neurons, as well as cells that wrap around themselves to form knots, and pairs of neurons that are almost mirror images of each other.

The 3D map covers a volume of about one cubic millimetre, one-millionth of a whole brain, and contains roughly 57,000 cells and 150 million synapses — the connections between neurons. It incorporates a colossal 1.4 petabytes of data. “It’s a little bit humbling,” says Viren Jain, a neuroscientist at Google in Mountain View, California, and a co-author of the paper. “How are we ever going to really come to terms with all this complexity?”

Slivers of brain

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The brain fragment was taken from a 45-year-old woman when she underwent surgery to treat her epilepsy. It came from the cortex, a part of the brain involved in learning, problem-solving and processing sensory signals. The sample was immersed in preservatives and stained with heavy metals to make the cells easier to see. Neuroscientist Jeff Lichtman at Harvard University in Cambridge, Massachusetts, and his colleagues then cut the sample into around 5,000 slices — each just 34 nanometres thick — that could be imaged using electron microscopes.

Jain’s team then built artificial-intelligence models that were able to stitch the microscope images together to reconstruct the whole sample in 3D. “I remember this moment, going into the map and looking at one individual synapse from this woman’s brain, and then zooming out into these other millions of pixels,” says Jain. “It felt sort of spiritual.”

When examining the model in detail, the researchers discovered unconventional neurons, including some that made up to 50 connections with each other. “In general, you would find a couple of connections at most between two neurons,” says Jain. Elsewhere, the model showed neurons with tendrils that formed knots around themselves. “Nobody had seen anything like this before,” Jain adds.

The team also found pairs of neurons that were near-perfect mirror images of each other. “We found two groups that would send their dendrites in two different directions, and sometimes there was a kind of mirror symmetry,” Jain says. It is unclear what role these features have in the brain.

Proofreaders needed

The map is so large that most of it has yet to be manually checked, and it could still contain errors created by the process of stitching so many images together. “Hundreds of cells have been ‘proofread’, but that’s obviously a few per cent of the 50,000 cells in there,” says Jain. He hopes that others will help to proofread parts of the map they are interested in. The team plans to produce similar maps of brain samples from other people — but a map of the entire brain is unlikely in the next few decades, he says.

“This paper is really the tour de force creation of a human cortex data set,” says Hongkui Zeng, director of the Allen Institute for Brain Science in Seattle. The vast amount of data that has been made freely accessible will “allow the community to look deeper into the micro-circuitry in the human cortex”, she adds.

Gaining a deeper understanding of how the cortex works could offer clues about how to treat some psychiatric and neurodegenerative diseases. “This map provides unprecedented details that can unveil new rules of neural connections and help to decipher the inner working of the human brain,” says Yongsoo Kim, a neuroscientist at Pennsylvania State University in Hershey.

This article is reproduced with permission and was first published on May 9, 2024 .

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• 19 June 2024

Human neuroscience is entering a new era — it mustn’t forget its human dimension

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Studies involving people who are awake during brain surgery are helping to explain how the brain produces and perceives speech. Credit: BSIP/Universal Images Group/Getty

In neuroscience, ‘Broca’s area’ is a well-known part of the brain that is crucial for speech production. It is named after the nineteenth-century physician-researcher who discovered it — Paul Broca. Less well known, however, is the person whose brain enabled Broca to do so. His name was Louis Victor Leborgne and he had lost his ability to speak at age 30.

Leborgne’s story reminds us why we must never ignore the people involved, assume they’ve consented or fail to acknowledge them appropriately — especially in an age when a lot of neuroscientific research involves humans.

This week’s issue of Nature includes several studies devoted to human neuroscience. They highlight the opportunities researchers have to study the human brain in never-before-seen detail. For example, single-neuron recordings of people who are awake while undergoing brain surgery are helping to explain how the brain produces and perceives speech . Similarly, atlases of brain-cell types, neural circuits and gene-expression maps have the potential to revolutionize our understanding of the cellular and molecular processes that underline behaviour and cognition .

Read the paper: Language is primarily a tool for communication rather than thought

These technologies are helping researchers to explore what sets the human brain apart from those of other species, and how its cognitive abilities have evolved. For example, the role of non-invasive imaging in learning about cognitive abilities is discussed in a Perspective article by Feline Lindhout at the Medical Research Council’s Laboratory of Molecular Biology in Cambridge, UK, and her colleagues 1 . In another article, Evelina Fedorenko at the Massachusetts Institute of Technology in Cambridge and her colleagues also draw on this literature to argue that, in humans, language probably serves mainly as a communication tool rather than as a means for thinking or reasoning 2 — and that language is not a prerequisite for complex thought.

One desirable outcome for human neuroscience would be to develop personalized treatments for neurological and psychiatric disorders, because translating the results of studies in animals has not proved successful or sufficient for generating effective therapies at scale. But in grasping these opportunities, researchers must keep in mind that the brain is different from other organs — it’s the seat of people’s memory, experiences and personality. When using the human brain — whether in small cubes removed during neurosurgery, or through 3D organoids made from stem cells and grown in cultures to resemble parts of the developing human brain — for research, scientists must consider the dignity and respect owed to the individuals concerned.

Read the paper: A molecular and cellular perspective on human brain evolution and tempo

The 1964 Declaration of Helsinki is the basis of research ethics for studies involving humans. Participants are asked to complete a consent form before the start of a study. Researchers have to ensure participants are fully informed about the study’s goals and whether and how they will benefit from the research. Sources of funding should also be declared and a participant must be able to withdraw at any time. According to neuroethicist Judy Illes at the University of British Columbia in Vancouver, Canada, ideally, consent should not be something that is done only once. It should be revisited during a study, so that participants can make informed decisions at different stages 3 . This is especially important for studies involving vulnerable people, because their circumstances might change during a study.

In another Perspective article, Tomasz Nowakowski at the University of California, San Francisco, and a team of neurosurgeons, neurologists and neuroscientists 4 call on the neuroscience community to revisit these standards of ethical practice. A key challenge they identify is how to handle the ramifications of advances in machine learning and artificial intelligence (AI).

Read the paper: Large-scale neurophysiology and single-cell profiling in human neuroscience

Researchers who use cell atlases, single-cell technologies and spatial-genomic analyses benefit hugely from AI and machine-learning algorithms when analysing large data sets. Yet, AI technologies have the potential to re-identify anonymized information by analysing vast data sets and finding patterns that trace back to individuals. AI models that analyse large data sets can also make predictions related to features of peoples’ behaviour and their cognitive abilities. This has the potential to cause harm, for example, through biased or erroneous profiling of people on the basis of their neurological data, says neuroethicist Karen Rommelfanger, founder of the Institute of Neuroethics, who is based in Atlanta, Georgia.

Nowakowski and his colleagues propose that researchers use controlled archives, access to which requires approval, and that they restrict data use to the conditions specified in consent forms. To implement such changes will require conversations between study participants, academic researchers and the companies that have a considerable role in the current AI advances. Informed-consent information will also need to change, to account for the risks of researchers’ increased reliance on AI tools.

The team is right to stress the need for improved standards in data ethics and sharing that are jointly created by scientists, private partners and the research participants. Without a doubt, human neuroscience is entering a new and important era. However, it can fulfil its goals of improving human experiences only when study participants are involved in discussions about the future of such research.

Nature 630 , 530 (2024)

doi: https://doi.org/10.1038/d41586-024-02022-3

Lindhout, F. W., Krienen, F. M., Pollard, K. S. & Lancaster, M. A. Nature 630 , 596–608 (2024).

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Fedorenko, E., Piantadosi, S. T. & Gibson, E. A. F. Nature 630 , 575–586 (2024).

Van der Loos, K. I., Longstaff, H., Virani, A. & Illes, J. J. Law Biosc. 2 , 69–78 (2014).

Lee, A. T., Chang, E. F., Paredes, M. F. & Nowakowski, T. J. Nature 630 , 587–595 (2024).

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The brain has a waste removal system and scientists are figuring out how it works

Jon Hamilton

The brain needs to flush out waste products to stay healthy and fend off conditions like Alzheimer's disease. Scientists are beginning to understand how the the brain's waste removal system works.

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Technologies enable 3D imaging of whole human brain hemispheres at subcellular resolution

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Observing anything and everything within the human brain, no matter how large or small, while it is fully intact has been an out-of-reach dream of neuroscience for decades. But in a new study in Science,  an MIT-based team describes a technology pipeline that enabled them to finely process, richly label, and sharply image full hemispheres of the brains of two donors — one with Alzheimer’s disease and one without — at high resolution and speed.

“We performed holistic imaging of human brain tissues at multiple resolutions, from single synapses to whole brain hemispheres, and we have made that data available,” says senior and corresponding author Kwanghun Chung, associate professor the MIT departments of Chemical Engineering and Brain and Cognitive Sciences and member of The Picower Institute for Learning and Memory and the Institute for Medical Engin­­­­eering and Science. “This technology pipeline really enables us to analyze the human brain at multiple scales. Potentially this pipeline can be used for fully mapping human brains.”

The new study does not present a comprehensive map or atlas of the entire brain, in which every cell, circuit, and protein is identified and analyzed. But with full hemispheric imaging, it demonstrates an integrated suite of three technologies to enable that and other long-sought neuroscience investigations. The research provides a “proof of concept” by showing numerous examples of what the pipeline makes possible, including sweeping landscapes of thousands of neurons within whole brain regions; diverse forests of cells, each in individual detail; and tufts of subcellular structures nestled among extracellular molecules. The researchers also present a rich variety of quantitative analytical comparisons focused on a chosen region within the Alzheimer’s and non-Alzheimer’s hemispheres.

The importance of being able to image whole hemispheres of human brains intact and down to the resolution of individual synapses (the teeny connections that neurons forge to make circuits) is two-fold for understanding the human brain in health and disease, Chung says.

Superior samples

On one hand, it will enable scientists to conduct integrated explorations of questions using the same brain, rather than having to (for example) observe different phenomena in different brains, which can vary significantly, and then try to construct a composite picture of the whole system. A key feature of the new technology pipeline is that analysis doesn’t degrade the tissue. On the contrary, it makes the tissues extremely durable and repeatedly re-labelable to highlight different cells or molecules as needed for new studies for potentially years on end. In the paper, Chung’s team demonstrates using 20 different antibody labels to highlight different cells and proteins, but they are already expanding that to a hundred or more.

“We need to be able to see all these different functional components — cells, their morphology and their connectivity, subcellular architectures, and their individual synaptic connections — ideally within the same brain, considering the high individual variabilities in the human brain and considering the precious nature of human brain samples,” Chung says. “This technology pipeline really enables us to extract all these important features from the same brain in a fully integrated manner.”

On the other hand, the pipeline’s relatively high scalability and throughput (imaging a whole brain hemisphere once it is prepared takes 100 hours, rather than many months) means that it is possible to create many samples to represent different sexes, ages, disease states, and other factors that can enable robust comparisons with increased statistical power. Chung says he envisions creating a brain bank of fully imaged brains that researchers could analyze and re-label as needed for new studies to make more of the kinds of comparisons he and co-authors made with the Alzheimer’s and non-Alzheimer’s hemispheres in the new paper.

Three key innovations

Chung says the biggest challenge he faced in achieving the advances described in the paper was building a team at MIT that included three especially talented young scientists, each a co-lead author of the paper because of their key roles in producing the three major innovations. Ji Wang, a mechanical engineer and former postdoc, developed the “Megatome,” a device for slicing intact human brain hemispheres so finely that there is no damage to them. Juhyuk Park, a materials engineer and former postdoc, developed the chemistry that makes each brain slice clear, flexible, durable, expandable, and quickly, evenly, and repeatedly labelable — a technology called “mELAST.” Webster Guan, a former MIT chemical engineering graduate student with a knack for software development, created a computational system called “UNSLICE” that can seamlessly reunify the slabs to reconstruct each hemisphere in full 3D, down to the precise alignment of individual blood vessels and neural axons (the long strands they extend to forge connections with other neurons).

No technology allows for imaging whole human brain anatomy at subcellular resolution without first slicing it, because it is very thick (it’s 3,000 times the volume of a mouse brain) and opaque. But in the Megatome, tissue remains undamaged because Wang, who is now at a company Chung founded called LifeCanvas Technologies, engineered its blade to vibrate side-to-side faster, and yet sweep wider, than previous vibratome slicers. Meanwhile she also crafted the instrument to stay perfectly within its plane, Chung says. The result are slices that don’t lose anatomical information at their separation or anywhere else. And because the vibratome cuts relatively quickly and can cut thicker (and therefore fewer) slabs of tissue, a whole hemisphere can be sliced in a day, rather than months.

A major reason why slabs in the pipeline can be thicker comes from mELAST. Park engineered the hydrogel that infuses the brain sample to make it optically clear, virtually indestructible, and compressible and expandable. Combined with other chemical engineering technologies developed in recent years in Chung’s lab, the samples can then be evenly and quickly infused with the antibody labels that highlight cells and proteins of interest. Using a light sheet microscope the lab customized, a whole hemisphere can be imaged down to individual synapses in about 100 hours, the authors report in the study. Park is now an assistant professor at Seoul National University in South Korea.

“This advanced polymeric network, which fine-tunes the physicochemical properties of tissues, enabled multiplexed multiscale imaging of the intact human brains,” Park says.

After each slab has been imaged, the task is then to restore an intact picture of the whole hemisphere computationally. Guan’s UNSLICE does this at multiple scales. For instance, at the middle, or “meso” scale, it algorithmically traces blood vessels coming into one layer from adjacent layers and matches them. But it also takes an even finer approach. To further register the slabs, the team purposely labeled neighboring neural axons in different colors (like the wires in an electrical fixture). That enabled UNSLICE to match layers up based on tracing the axons, Chung says. Guan is also now at LifeCanvas.

In the study, the researchers present a litany of examples of what the pipeline can do. The very first figure demonstrates that the imaging allows one to richly label a whole hemisphere and then zoom in from the wide scale of brainwide structures to the level of circuits, then individual cells, and then subcellular components, such as synapses. Other images and videos demonstrate how diverse the labeling can be, revealing long axonal connections and the abundance and shape of different cell types including not only neurons but also astrocytes and microglia.

Exploring Alzheimer’s

For years, Chung has collaborated with co-author Matthew Frosch, an Alzheimer’s researcher and director of the brain bank at Massachusetts General Hospital, to image and understand Alzheimer’s disease brains. With the new pipeline established they began an open-ended exploration, first noticing where within a slab of tissue they saw the greatest loss of neurons in the disease sample compared to the control. From there, they followed their curiosity — as the technology allowed them to do — ultimately producing a series of detailed investigations described in the paper.

“We didn’t lay out all these experiments in advance,” Chung says. “We just started by saying, ‘OK, let’s image this slab and see what we see.’ We identified brain regions with substantial neuronal loss so let’s see what’s happening there. ‘Let’s dive deeper.’ So we used many different markers to characterize and see the relationships between pathogenic factors and different cell types.

“This pipeline allows us to have almost unlimited access to the tissue,” Chung says. “We can always go back and look at something new.”

They focused most of their analysis in the orbitofrontal cortex within each hemisphere. One of the many observations they made was that synapse loss was concentrated in areas where there was direct overlap with amyloid plaques. Outside of areas of plaques the synapse density was as high in the brain with Alzheimer’s as in the one without the disease.

With just two samples, Chung says, the team is not offering any conclusions about the nature of Alzheimer’s disease, of course, but the point of the study is that the capability now exists to fully image and deeply analyze whole human brain hemispheres to enable exactly that kind of research.

Notably, the technology applies equally well to many other tissues in the body, not just brains.

“We envision that this scalable technology platform will advance our understanding of the human organ functions and disease mechanisms to spur development of new therapies,” the authors conclude.

In addition to Park, Wang, Guan, Chung, and Frosch, the paper’s other authors are Lars A. Gjesteby, Dylan Pollack, Lee Kamentsky, Nicholas B. Evans, Jeff Stirman, Xinyi Gu, Chuanxi Zhao, Slayton Marx, Minyoung E. Kim, Seo Woo Choi, Michael Snyder, David Chavez, Clover Su-Arcaro, Yuxuan Tian, Chang Sin Park, Qiangge Zhang, Dae Hee Yun, Mira Moukheiber, Guoping Feng, X. William Yang, C. Dirk Keene, Patrick R. Hof, Satrajit S. Ghosh, and Laura J. Brattain.

The main funding for the work came from the National Institutes of Health, The Picower Institute for Learning and Memory, The JPB Foundation, and the NCSOFT Cultural Foundation.

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