The Human Brain
The human brain is the center of the human nervous system and is a highly complex organ. Enclosed in the cranium,
it has the same general structure as the brains of other mammals, but is over three times as large as the brain of
a typical mammal with an equivalent body size.[1] Most of the expansion comes from the cerebral cortex, a
convoluted layer of neural tissue that covers the surface of the forebrain. Especially expanded are the frontal
lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract
thought. The portion of the brain devoted to vision is also greatly enlarged in human beings.
Brain evolution, from the earliest shrewlike mammals through primates to hominids, is marked by a steady increase
in encephalization, or the ratio of brain to body size. The human brain has been estimated to contain 50–100
billion (1011) neurons, of which about 10 billion (1010) are cortical pyramidal cells. These cells pass signals to
each other via as many as 1000 trillion (1015) synaptic connections.[2]
The brain monitors and regulates the body's actions and reactions. It continuously receives sensory information,
and rapidly analyzes this data and then responds, controlling bodily actions and functions. The brainstem controls
breathing, heart rate, and other autonomic processes. The neocortex is the center of higher-order thinking,
learning, and memory. The cerebellum is responsible for the body's balance, posture, and the coordination of
movement.
In spite of the fact that it is protected by the thick bones of the skull, suspended in cerebrospinal fluid, and
isolated from the bloodstream by the blood-brain barrier, the delicate nature of the human brain makes it
susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries
such as a blow to the head, a stroke, or poisoning by a wide variety of chemicals that can act as neurotoxins.
Infection of the brain is rare because of the barriers that protect it, but is very serious when it occurs. The
human brain is also susceptible to degenerative disorders, such as Parkinson's disease, multiple sclerosis, and
Alzheimer's disease. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought
to be caused at least partially by brain dysfunctions, although the nature of such brain anomalies is not well
understood.
Structure
Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter[3]
The adult human brain weighs on average about 3 lb (1.5 kg)[4] with a size (volume) of around 1130 cubic
centimetres (cm3) in women and 1260 cm3 in men, although there is substantial individual variation.[5] Men with the
same body height and body surface area as women have on average 100g heavier brains,[6] although these differences
do not correlate in any simple way with gray matter neuron counts or with overall measures of cognitive
performance.[7] Neanderthals had larger brains at adulthood than present-day humans.[8] The brain is very soft,
having a consistency similar to soft gelatin or firm tofu. Despite being referred to as "grey matter", the live
cortex is pinkish-beige in color and slightly off-white in the interior. At the age of 20, a man has around 176,000
km and a woman, about 149,000 km of myelinated axons in their brains.[9]
General features
Drawing of the human brain, showing several important structures
The cerebral hemispheres form the largest part of the human brain and are situated above most other brain
structures. They are covered with a cortical layer with a convoluted topography.[10] Underneath the cerebrum lies
the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the brain, beneath the cerebrum
and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look
different from any other brain area. The same structures are present in other mammals, although the cerebellum is
not so large relative to the rest of the brain. As a rule, the smaller the cerebrum, the less convoluted the
cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other
hand, is more convoluted than the cortex of a human.
The dominant feature of the human brain is corticalization. The cerebral cortex in humans is so large that it
overshadows every other part of the brain. A few subcortical structures show alterations reflecting this trend. The
cerebellum, for example, has a medial zone connected mainly to subcortical motor areas, and a lateral zone
connected primarily to the cortex. In humans the lateral zone takes up a much larger fraction of the cerebellum
than in most other mammalian species. Corticalization is reflected in function as well as structure. In a rat,
surgical removal of the entire cerebral cortex leaves an animal that is still capable of walking around and
interacting with the environment.[11] In a human, comparable cerebral cortex damage produces a permanent state of
coma. The amount of association cortex, relative to the other two categories, increase dramatically as one goes
from simpler mammals, such as the rat and the cat, to more complex ones, such as the chimpanzee and the human.[12]
Major gyri and sulci on the lateral surface of the cortex
The cerebral cortex is essentially a sheet of neural tissue, folded in a way that allows a large surface area to
fit within the confines of the skull. Each cerebral hemisphere, in fact, has a total surface area of about 1.3
square feet.[13] Anatomists call each cortical fold a sulcus, and the smooth area between folds a gyrus. Most human
brains show a similar pattern of folding, but there are enough variations in the shape and placement of folds to
make every brain unique. Nevertheless, the pattern is consistent enough for each major fold to have a name, for
example, the "superior frontal gyrus", "postcentral sulcus", or "trans-occipital sulcus". Deep folding features in
brain such as the inter-hemispheric and lateral fissure, and the insular cortex are present in almost all normal
subjects.
Cortical divisions
The four lobes of the cerebral cortex
The bones of the human skull
Four lobes
Outwardly, the cerebral cortex is nearly symmetrical, with left and right hemispheres. Anatomists conventionally
divide each hemisphere into four "lobes", the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. This
categorization does not actually arise from the structure of the cortex itself: the lobes are named after the bones
of the skull that overlie them. There is one exception: the border between the frontal and parietal lobes is
shifted backward to the central sulcus, a deep fold that marks the line where the primary somatosensory cortex and
primary motor cortex come together.
Functional divisions
Researchers who study the functions of the cortex divide it into three functional categories of regions, or areas.
One consists of the primary sensory areas, which receive signals from the sensory nerves and tracts by way of relay
nuclei in the thalamus. Primary sensory areas include the visual area of the occipital lobe, the auditory area in
parts of the temporal lobe and insular cortex, and the somatosensory area in the parietal lobe. A second category
is the primary motor area, which sends axons down to motor neurons in the brainstem and spinal cord. This area
occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The third category
consists of the remaining parts of the cortex, which are called the association areas. These areas receive input
from the sensory areas and lower parts of the brain and are involved in the complex process that we call
perception, thought, and decision making.
Brodmann's classification of areas of the cortex
Different parts of the cerebral cortex are involved in different cognitive and behavioral functions. The
differences show up in a number of ways: the effects of localized brain damage, regional activity patterns exposed
when the brain is examined using functional imaging techniques, connectivity with subcortical areas, and regional
differences in the cellular architecture of the cortex. Anatomists describe most of the cortex—the part they call
isocortex—as having six layers, but not all layers are apparent in all areas, and even when a layer is present, its
thickness and cellular organization may vary. Several anatomists have constructed maps of cortical areas on the
basis of variations in the appearance of the layers as seen with a microscope. One of the most widely used schemes
came from Brodmann, who split the cortex into 51 different areas and assigned each a number (anatomists have since
subdivided many of the Brodmann areas[citation needed]). For example, Brodmann area 1 is the primary somatosensory
cortex, Brodmann area 17 is the primary visual cortex, and Brodmann area 25 is the anterior cingulate cortex.
Topography
Topography of the primary motor cortex, showing which body part is controlled by each zone
Many of the brain areas Brodmann defined have their own complex internal structures. In a number of cases, brain
areas are organized into "topographic maps", where adjoining bits of the cortex correspond to adjoining parts of
the body, or of some more abstract entity. A simple example of this type of correspondence is the primary motor
cortex, a strip of tissue running along the anterior edge of the central sulcus, shown in the image to the right.
Motor areas innervating each part of the body arise from a distinct zone, with neighboring body parts represented
by neighboring zones. Electrical stimulation of the cortex at any point causes a muscle-contraction in the
represented body part. This "somatotopic" representation is not evenly distributed, however. The head, for example,
is represented by a region about three times as large as the zone for the entire back and trunk. The size of a zone
correlates to the precision of motor control and sensory discrimination possible[citation needed]. The areas for
the lips, fingers, and tongue are particularly large, considering the proportional size of their represented body
parts.
In visual areas, the maps are retinotopic—that is, they reflect the topography of the retina, the layer of light-
activated neurons lining the back of the eye. In this case too the representation is uneven: the fovea—the area at
the center of the visual field—is greatly overrepresented compared to the periphery. The visual circuitry in the
human cerebral cortex contains several dozen distinct retinotopic maps, each devoted to analyzing the visual input
stream in a particular way [citation needed]. The primary visual cortex (Brodmann area 17), which is the main
recipient of direct input from the visual part of the thalamus, contains many neurons that are most easily
activated by edges with a particular orientation moving across a particular point in the visual field. Visual areas
farther downstream extract features such as color, motion, and shape.
In auditory areas, the primary map is tonotopic. Sounds are parsed according to frequency (i.e., high pitch vs. low
pitch) by subcortical auditory areas, and this parsing is reflected by the primary auditory zone of the cortex. As
with the visual system, there are a number of tonotopic cortical maps, each devoted to analyzing sound in a
particular way.
Within a topographic map there can sometimes be finer levels of spatial structure. In the primary visual cortex,
for example, where the main organization is retinotopic and the main responses are to moving edges, cells that
respond to different edge-orientations are spatially segregated from one another[citation needed].
Lateralization
Main article: Lateralization of brain function
Routing of neural signals from the two eyes to the brain
Each hemisphere of the brain interacts primarily with one half of the body, but for reasons that are unclear, the
connections are crossed: the left side of the brain interacts with the right side of the body, and vice versa.
[citation needed] Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord
to the brain, both cross the midline at brainstem levels. Visual input follows a more complex rule: the optic
nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve
split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to
the left side of the brain, whereas connections from the right half of the retina go to the right side of the
brain. Because each half of the retina receives light coming from the opposite half of the visual field, the
functional consequence is that visual input from the left side of the world goes to the right side of the brain,
and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and
visual input from the left side of the visual field—an arrangement that presumably is helpful for visuomotor
coordination.
The corpus callosum, a nerve bundle connecting the two cerebral hemispheres, with the lateral ventricles directly
below
The two cerebral hemispheres are connected by a very large nerve bundle called the corpus callosum, which crosses
the midline above the level of the thalamus. There are also two much smaller connections, the anterior commisure
and hippocampal commisure, as well as many subcortical connections that cross the midline. The corpus callosum is
the main avenue of communication between the two hemispheres, though. It connects each point on the cortex to the
mirror-image point in the opposite hemisphere, and also connects to functionally related points in different
cortical areas.
In most respects, the left and right sides of the brain are symmetrical in terms of function. For example, the
counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling
the left hand. There are, however, several very important exceptions, involving language and spatial cognition. In
most people, the left hemisphere is "dominant" for language: a stroke that damages a key language area in the left
hemisphere can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere
would cause only minor impairment to language skills.
A substantial part of our current understanding of the interactions between the two hemispheres has come from the
study of "split-brain patients"—people who underwent surgical transection of the corpus callosum in an attempt to
reduce the severity of epileptic seizures. These patients do not show unusual behavior that is immediately obvious,
but in some cases can behave almost like two different people in the same body, with the right hand taking an
action and then the left hand undoing it. Most such patients, when briefly shown a picture on the right side of the
point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable
to describe it, but may be able to give an indication with the left hand of the nature of the object shown.
It should be noted that the differences between left and right hemispheres are greatly overblown in much of the
popular literature on this topic. The existence of differences has been solidly established, but many popular books
go far beyond the evidence in attributing features of personality or intelligence to the left or right hemisphere
dominance.[citation needed]
Development
Main article: Neural development in humans
During the first 3 weeks of gestation, the human embryo's ectoderm forms a thickened strip called the neural plate.
The neural plate then folds and closes to form the neural tube. This tube flexes as it grows, forming the cerebral
hemispheres at the head, and the cerebellum and pons towards the tail.
Brain of human embryo at 4.5 weeks, showing interior of forebrain
Brain interior at 5 weeks
Brain viewed at midline at 3 months
Sources of information
Neuroscientists, along with researchers from allied disciplines, study how the human brain works. Such research has
expanded considerably in recent decades. The "Decade of the Brain", an initiative of the United States Government
in the 1990s, is considered to have marked much of this increase in research.[14]
Information about the structure and function of the human brain comes from a variety of experimental methods. Most
information about the cellular components of the brain and how they work comes from studies of animal subjects,
using techniques described in the brain article. Some techniques, however, are used mainly in humans, and therefore
are described here.
Computed tomography of human brain, from base of the skull to top, taken with intravenous contrast medium
EEG
By placing electrodes on the scalp it is possible to record the summed electrical activity of the cortex, in a
technique known as electroencephalography (EEG).[15] EEG measures mass changes in population synaptic activity from
the cerebral cortex, but can only detect changes over large areas of the brain, with very little sensitivity for
sub-cortical activity. EEG recordings can detect events lasting only a few thousandths of a second. EEG recordings
have good temporal resolution, but poor spatial resolution.
MEG
Apart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a
technique known as magnetoencephalography (MEG).[16] This technique has the same temporal resolution as EEG but
much better spatial resolution, although not as good as MRI. The greatest disadvantage of MEG is that, because the
magnetic fields generated by neural activity are very weak, the method is only capable of picking up signals from
near the surface of the cortex, and even then, only neurons located in the depths of cortical folds (sulci) have
dendrites oriented in a way that gives rise to detectable magnetic fields outside the skull.
Structural and functional imaging
Main article: Neuroimaging
A scan of the brain using fMRI
There are several methods for detecting brain activity changes by three-dimensional imaging of local changes in
blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the
bloodstream. The newest method, functional magnetic resonance imaging (fMRI), has considerably better spatial
resolution and involves no radioactivity.[17] Using the most powerful magnets currently available, fMRI can
localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal
resolution is poor: when brain activity increases, the blood flow response is delayed by 1–5 seconds and lasts for
at least 10 seconds. Thus, fMRI is a very useful tool for learning which brain regions are involved in a given
behavior, but gives little information about the temporal dynamics of their responses. A major advantage for fMRI
is that, because it is non-invasive, it can readily be used on human subjects.
Effects of brain damage
Main article: Neuropsychology
A key source of information about the function of brain regions is the effects of damage to them.[18] In humans,
strokes have long provided a "natural laboratory" for studying the effects of brain damage. Most strokes result
from a blood clot lodging in the brain and blocking the local blood supply, causing damage or destruction of nearby
brain tissue: the range of possible blockages is very wide, leading to a great diversity of stroke symptoms.
Analysis of strokes is limited by the fact that damage often crosses into multiple regions of the brain, not along
clear-cut borders, making it difficult to draw firm conclusions.
Language
Location of two brain areas that play a critical role in language, Broca's area and Wernicke's area
In human beings, it is the left hemisphere that usually contains the specialized language areas. While this holds
true for 97% of right-handed people, about 19% of left-handed people have their language areas in the right
hemisphere and as many as 68% of them have some language abilities in both the left and the right hemisphere.
[citation needed] The two hemispheres are thought to contribute to the processing and understanding of language:
the left hemisphere processes the linguistic meaning of prosody (or, the rhythm, stress, and intonation of
connected speech), while the right hemisphere processes the emotions conveyed by prosody.[19] Studies of children
have shown that if a child has damage to the left hemisphere, the child may develop language in the right
hemisphere instead. The younger the child, the better the recovery. So, although the "natural" tendency is for
language to develop on the left, human brains are capable of adapting to difficult circumstances, if the damage
occurs early enough.
The first language area within the left hemisphere to be discovered is Broca's area, named after Paul Broca, who
discovered the area while studying patients with aphasia, a language disorder. Broca's area doesn't just handle
getting language out in a motor sense, though. It seems to be more generally involved in the ability to process
grammar itself, at least the more complex aspects of grammar. For example, it handles distinguishing a sentence in
passive form from a simpler subject-verb-object sentence — the difference between "The boy was hit by the girl" and
"The girl hit the boy."
The second language area to be discovered is called Wernicke's area, after Carl Wernicke, a German neurologist who
discovered the area while studying patients who had similar symptoms to Broca's area patients but damage to a
different part of their brain. Wernicke's aphasia is the term for the disorder occurring upon damage to a patient's
Wernicke's area.
Wernicke's aphasia does not only affect speech comprehension. People with Wernicke's aphasia also have difficulty
recalling the names of objects, often responding with words that sound similar, or the names of related things, as
if they are having a hard time recalling word associations[citation needed].
Pathology
A human brain showing frontotemporal lobar degeneration causing frontotemporal dementia
Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to
affect large areas of the organ, sometimes causing major deficits in intelligence, memory, personality, and
movement. Head trauma caused, for example, by vehicle or industrial accidents, is a leading cause of death in youth
and middle age. In many cases, more damage is caused by resultant edema than by the impact itself. Stroke, caused
by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.
Other problems in the brain can be more accurately classified as diseases than as injuries. Neurodegenerative
diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease are
caused by the gradual death of individual neurons, leading to diminution in movement control, memory, and
cognition.
Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder
may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic
function. These disorders may be treated by psychotherapy, psychiatric medication or social intervention and
personal recovery work; the underlying issues and associated prognosis vary significantly between individuals.
Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the meninges, the
membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as "mad cow
disease") is deadly in cattle and humans and is linked to prions. Kuru is a similar prion-borne degenerative brain
disease affecting humans. Both are linked to the ingestion of neural tissue, and may explain the tendency in human
and some non-human species to avoid cannibalism. Viral or bacterial causes have been reported in multiple sclerosis
and Parkinson's disease, and are established causes of encephalopathy, and encephalomyelitis.
Many brain disorders are congenital, occurring during development. Tay-Sachs disease, fragile X syndrome, and Down
syndrome are all linked to genetic and chromosomal errors. Many other syndromes, such as the intrinsic circadian
rhythm disorders, are suspected to be congenital as well. Normal development of the brain can be altered by genetic
factors, drug use, nutritional deficiencies, and infectious diseases during pregnancy.
Certain brain disorders are treated by neurosurgeons, while others are treated by neurologists and psychiatrists.
Visualization of a diffusion tensor imaging (DTI) measurement of a human brain. Depicted are reconstructed axon
tracts that run through the mid-sagittal plane. Especially prominent are the U-shaped fibers that connect the two
hemispheres through the corpus callosum (the fibers come out of the image plane and consequently bend towards the
top) and the fiber tracts that descend toward the spine (blue, within the image plane).
Metabolism
Brain metabolism normally is completely dependent upon blood glucose as an energy source, since fatty acids do not
cross the blood-brain barrier.[20] During times of low glucose (such as fasting), the brain will instead use ketone
bodies for fuel. The brain does not store any glucose in the form of glycogen, in contrast, for example, to
skeletal muscle.
it has the same general structure as the brains of other mammals, but is over three times as large as the brain of
a typical mammal with an equivalent body size.[1] Most of the expansion comes from the cerebral cortex, a
convoluted layer of neural tissue that covers the surface of the forebrain. Especially expanded are the frontal
lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract
thought. The portion of the brain devoted to vision is also greatly enlarged in human beings.
Brain evolution, from the earliest shrewlike mammals through primates to hominids, is marked by a steady increase
in encephalization, or the ratio of brain to body size. The human brain has been estimated to contain 50–100
billion (1011) neurons, of which about 10 billion (1010) are cortical pyramidal cells. These cells pass signals to
each other via as many as 1000 trillion (1015) synaptic connections.[2]
The brain monitors and regulates the body's actions and reactions. It continuously receives sensory information,
and rapidly analyzes this data and then responds, controlling bodily actions and functions. The brainstem controls
breathing, heart rate, and other autonomic processes. The neocortex is the center of higher-order thinking,
learning, and memory. The cerebellum is responsible for the body's balance, posture, and the coordination of
movement.
In spite of the fact that it is protected by the thick bones of the skull, suspended in cerebrospinal fluid, and
isolated from the bloodstream by the blood-brain barrier, the delicate nature of the human brain makes it
susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries
such as a blow to the head, a stroke, or poisoning by a wide variety of chemicals that can act as neurotoxins.
Infection of the brain is rare because of the barriers that protect it, but is very serious when it occurs. The
human brain is also susceptible to degenerative disorders, such as Parkinson's disease, multiple sclerosis, and
Alzheimer's disease. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought
to be caused at least partially by brain dysfunctions, although the nature of such brain anomalies is not well
understood.
Structure
Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter[3]
The adult human brain weighs on average about 3 lb (1.5 kg)[4] with a size (volume) of around 1130 cubic
centimetres (cm3) in women and 1260 cm3 in men, although there is substantial individual variation.[5] Men with the
same body height and body surface area as women have on average 100g heavier brains,[6] although these differences
do not correlate in any simple way with gray matter neuron counts or with overall measures of cognitive
performance.[7] Neanderthals had larger brains at adulthood than present-day humans.[8] The brain is very soft,
having a consistency similar to soft gelatin or firm tofu. Despite being referred to as "grey matter", the live
cortex is pinkish-beige in color and slightly off-white in the interior. At the age of 20, a man has around 176,000
km and a woman, about 149,000 km of myelinated axons in their brains.[9]
General features
Drawing of the human brain, showing several important structures
The cerebral hemispheres form the largest part of the human brain and are situated above most other brain
structures. They are covered with a cortical layer with a convoluted topography.[10] Underneath the cerebrum lies
the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the brain, beneath the cerebrum
and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look
different from any other brain area. The same structures are present in other mammals, although the cerebellum is
not so large relative to the rest of the brain. As a rule, the smaller the cerebrum, the less convoluted the
cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other
hand, is more convoluted than the cortex of a human.
The dominant feature of the human brain is corticalization. The cerebral cortex in humans is so large that it
overshadows every other part of the brain. A few subcortical structures show alterations reflecting this trend. The
cerebellum, for example, has a medial zone connected mainly to subcortical motor areas, and a lateral zone
connected primarily to the cortex. In humans the lateral zone takes up a much larger fraction of the cerebellum
than in most other mammalian species. Corticalization is reflected in function as well as structure. In a rat,
surgical removal of the entire cerebral cortex leaves an animal that is still capable of walking around and
interacting with the environment.[11] In a human, comparable cerebral cortex damage produces a permanent state of
coma. The amount of association cortex, relative to the other two categories, increase dramatically as one goes
from simpler mammals, such as the rat and the cat, to more complex ones, such as the chimpanzee and the human.[12]
Major gyri and sulci on the lateral surface of the cortex
The cerebral cortex is essentially a sheet of neural tissue, folded in a way that allows a large surface area to
fit within the confines of the skull. Each cerebral hemisphere, in fact, has a total surface area of about 1.3
square feet.[13] Anatomists call each cortical fold a sulcus, and the smooth area between folds a gyrus. Most human
brains show a similar pattern of folding, but there are enough variations in the shape and placement of folds to
make every brain unique. Nevertheless, the pattern is consistent enough for each major fold to have a name, for
example, the "superior frontal gyrus", "postcentral sulcus", or "trans-occipital sulcus". Deep folding features in
brain such as the inter-hemispheric and lateral fissure, and the insular cortex are present in almost all normal
subjects.
Cortical divisions
The four lobes of the cerebral cortex
The bones of the human skull
Four lobes
Outwardly, the cerebral cortex is nearly symmetrical, with left and right hemispheres. Anatomists conventionally
divide each hemisphere into four "lobes", the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. This
categorization does not actually arise from the structure of the cortex itself: the lobes are named after the bones
of the skull that overlie them. There is one exception: the border between the frontal and parietal lobes is
shifted backward to the central sulcus, a deep fold that marks the line where the primary somatosensory cortex and
primary motor cortex come together.
Functional divisions
Researchers who study the functions of the cortex divide it into three functional categories of regions, or areas.
One consists of the primary sensory areas, which receive signals from the sensory nerves and tracts by way of relay
nuclei in the thalamus. Primary sensory areas include the visual area of the occipital lobe, the auditory area in
parts of the temporal lobe and insular cortex, and the somatosensory area in the parietal lobe. A second category
is the primary motor area, which sends axons down to motor neurons in the brainstem and spinal cord. This area
occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The third category
consists of the remaining parts of the cortex, which are called the association areas. These areas receive input
from the sensory areas and lower parts of the brain and are involved in the complex process that we call
perception, thought, and decision making.
Brodmann's classification of areas of the cortex
Different parts of the cerebral cortex are involved in different cognitive and behavioral functions. The
differences show up in a number of ways: the effects of localized brain damage, regional activity patterns exposed
when the brain is examined using functional imaging techniques, connectivity with subcortical areas, and regional
differences in the cellular architecture of the cortex. Anatomists describe most of the cortex—the part they call
isocortex—as having six layers, but not all layers are apparent in all areas, and even when a layer is present, its
thickness and cellular organization may vary. Several anatomists have constructed maps of cortical areas on the
basis of variations in the appearance of the layers as seen with a microscope. One of the most widely used schemes
came from Brodmann, who split the cortex into 51 different areas and assigned each a number (anatomists have since
subdivided many of the Brodmann areas[citation needed]). For example, Brodmann area 1 is the primary somatosensory
cortex, Brodmann area 17 is the primary visual cortex, and Brodmann area 25 is the anterior cingulate cortex.
Topography
Topography of the primary motor cortex, showing which body part is controlled by each zone
Many of the brain areas Brodmann defined have their own complex internal structures. In a number of cases, brain
areas are organized into "topographic maps", where adjoining bits of the cortex correspond to adjoining parts of
the body, or of some more abstract entity. A simple example of this type of correspondence is the primary motor
cortex, a strip of tissue running along the anterior edge of the central sulcus, shown in the image to the right.
Motor areas innervating each part of the body arise from a distinct zone, with neighboring body parts represented
by neighboring zones. Electrical stimulation of the cortex at any point causes a muscle-contraction in the
represented body part. This "somatotopic" representation is not evenly distributed, however. The head, for example,
is represented by a region about three times as large as the zone for the entire back and trunk. The size of a zone
correlates to the precision of motor control and sensory discrimination possible[citation needed]. The areas for
the lips, fingers, and tongue are particularly large, considering the proportional size of their represented body
parts.
In visual areas, the maps are retinotopic—that is, they reflect the topography of the retina, the layer of light-
activated neurons lining the back of the eye. In this case too the representation is uneven: the fovea—the area at
the center of the visual field—is greatly overrepresented compared to the periphery. The visual circuitry in the
human cerebral cortex contains several dozen distinct retinotopic maps, each devoted to analyzing the visual input
stream in a particular way [citation needed]. The primary visual cortex (Brodmann area 17), which is the main
recipient of direct input from the visual part of the thalamus, contains many neurons that are most easily
activated by edges with a particular orientation moving across a particular point in the visual field. Visual areas
farther downstream extract features such as color, motion, and shape.
In auditory areas, the primary map is tonotopic. Sounds are parsed according to frequency (i.e., high pitch vs. low
pitch) by subcortical auditory areas, and this parsing is reflected by the primary auditory zone of the cortex. As
with the visual system, there are a number of tonotopic cortical maps, each devoted to analyzing sound in a
particular way.
Within a topographic map there can sometimes be finer levels of spatial structure. In the primary visual cortex,
for example, where the main organization is retinotopic and the main responses are to moving edges, cells that
respond to different edge-orientations are spatially segregated from one another[citation needed].
Lateralization
Main article: Lateralization of brain function
Routing of neural signals from the two eyes to the brain
Each hemisphere of the brain interacts primarily with one half of the body, but for reasons that are unclear, the
connections are crossed: the left side of the brain interacts with the right side of the body, and vice versa.
[citation needed] Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord
to the brain, both cross the midline at brainstem levels. Visual input follows a more complex rule: the optic
nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve
split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to
the left side of the brain, whereas connections from the right half of the retina go to the right side of the
brain. Because each half of the retina receives light coming from the opposite half of the visual field, the
functional consequence is that visual input from the left side of the world goes to the right side of the brain,
and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and
visual input from the left side of the visual field—an arrangement that presumably is helpful for visuomotor
coordination.
The corpus callosum, a nerve bundle connecting the two cerebral hemispheres, with the lateral ventricles directly
below
The two cerebral hemispheres are connected by a very large nerve bundle called the corpus callosum, which crosses
the midline above the level of the thalamus. There are also two much smaller connections, the anterior commisure
and hippocampal commisure, as well as many subcortical connections that cross the midline. The corpus callosum is
the main avenue of communication between the two hemispheres, though. It connects each point on the cortex to the
mirror-image point in the opposite hemisphere, and also connects to functionally related points in different
cortical areas.
In most respects, the left and right sides of the brain are symmetrical in terms of function. For example, the
counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling
the left hand. There are, however, several very important exceptions, involving language and spatial cognition. In
most people, the left hemisphere is "dominant" for language: a stroke that damages a key language area in the left
hemisphere can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere
would cause only minor impairment to language skills.
A substantial part of our current understanding of the interactions between the two hemispheres has come from the
study of "split-brain patients"—people who underwent surgical transection of the corpus callosum in an attempt to
reduce the severity of epileptic seizures. These patients do not show unusual behavior that is immediately obvious,
but in some cases can behave almost like two different people in the same body, with the right hand taking an
action and then the left hand undoing it. Most such patients, when briefly shown a picture on the right side of the
point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable
to describe it, but may be able to give an indication with the left hand of the nature of the object shown.
It should be noted that the differences between left and right hemispheres are greatly overblown in much of the
popular literature on this topic. The existence of differences has been solidly established, but many popular books
go far beyond the evidence in attributing features of personality or intelligence to the left or right hemisphere
dominance.[citation needed]
Development
Main article: Neural development in humans
During the first 3 weeks of gestation, the human embryo's ectoderm forms a thickened strip called the neural plate.
The neural plate then folds and closes to form the neural tube. This tube flexes as it grows, forming the cerebral
hemispheres at the head, and the cerebellum and pons towards the tail.
Brain of human embryo at 4.5 weeks, showing interior of forebrain
Brain interior at 5 weeks
Brain viewed at midline at 3 months
Sources of information
Neuroscientists, along with researchers from allied disciplines, study how the human brain works. Such research has
expanded considerably in recent decades. The "Decade of the Brain", an initiative of the United States Government
in the 1990s, is considered to have marked much of this increase in research.[14]
Information about the structure and function of the human brain comes from a variety of experimental methods. Most
information about the cellular components of the brain and how they work comes from studies of animal subjects,
using techniques described in the brain article. Some techniques, however, are used mainly in humans, and therefore
are described here.
Computed tomography of human brain, from base of the skull to top, taken with intravenous contrast medium
EEG
By placing electrodes on the scalp it is possible to record the summed electrical activity of the cortex, in a
technique known as electroencephalography (EEG).[15] EEG measures mass changes in population synaptic activity from
the cerebral cortex, but can only detect changes over large areas of the brain, with very little sensitivity for
sub-cortical activity. EEG recordings can detect events lasting only a few thousandths of a second. EEG recordings
have good temporal resolution, but poor spatial resolution.
MEG
Apart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a
technique known as magnetoencephalography (MEG).[16] This technique has the same temporal resolution as EEG but
much better spatial resolution, although not as good as MRI. The greatest disadvantage of MEG is that, because the
magnetic fields generated by neural activity are very weak, the method is only capable of picking up signals from
near the surface of the cortex, and even then, only neurons located in the depths of cortical folds (sulci) have
dendrites oriented in a way that gives rise to detectable magnetic fields outside the skull.
Structural and functional imaging
Main article: Neuroimaging
A scan of the brain using fMRI
There are several methods for detecting brain activity changes by three-dimensional imaging of local changes in
blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the
bloodstream. The newest method, functional magnetic resonance imaging (fMRI), has considerably better spatial
resolution and involves no radioactivity.[17] Using the most powerful magnets currently available, fMRI can
localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal
resolution is poor: when brain activity increases, the blood flow response is delayed by 1–5 seconds and lasts for
at least 10 seconds. Thus, fMRI is a very useful tool for learning which brain regions are involved in a given
behavior, but gives little information about the temporal dynamics of their responses. A major advantage for fMRI
is that, because it is non-invasive, it can readily be used on human subjects.
Effects of brain damage
Main article: Neuropsychology
A key source of information about the function of brain regions is the effects of damage to them.[18] In humans,
strokes have long provided a "natural laboratory" for studying the effects of brain damage. Most strokes result
from a blood clot lodging in the brain and blocking the local blood supply, causing damage or destruction of nearby
brain tissue: the range of possible blockages is very wide, leading to a great diversity of stroke symptoms.
Analysis of strokes is limited by the fact that damage often crosses into multiple regions of the brain, not along
clear-cut borders, making it difficult to draw firm conclusions.
Language
Location of two brain areas that play a critical role in language, Broca's area and Wernicke's area
In human beings, it is the left hemisphere that usually contains the specialized language areas. While this holds
true for 97% of right-handed people, about 19% of left-handed people have their language areas in the right
hemisphere and as many as 68% of them have some language abilities in both the left and the right hemisphere.
[citation needed] The two hemispheres are thought to contribute to the processing and understanding of language:
the left hemisphere processes the linguistic meaning of prosody (or, the rhythm, stress, and intonation of
connected speech), while the right hemisphere processes the emotions conveyed by prosody.[19] Studies of children
have shown that if a child has damage to the left hemisphere, the child may develop language in the right
hemisphere instead. The younger the child, the better the recovery. So, although the "natural" tendency is for
language to develop on the left, human brains are capable of adapting to difficult circumstances, if the damage
occurs early enough.
The first language area within the left hemisphere to be discovered is Broca's area, named after Paul Broca, who
discovered the area while studying patients with aphasia, a language disorder. Broca's area doesn't just handle
getting language out in a motor sense, though. It seems to be more generally involved in the ability to process
grammar itself, at least the more complex aspects of grammar. For example, it handles distinguishing a sentence in
passive form from a simpler subject-verb-object sentence — the difference between "The boy was hit by the girl" and
"The girl hit the boy."
The second language area to be discovered is called Wernicke's area, after Carl Wernicke, a German neurologist who
discovered the area while studying patients who had similar symptoms to Broca's area patients but damage to a
different part of their brain. Wernicke's aphasia is the term for the disorder occurring upon damage to a patient's
Wernicke's area.
Wernicke's aphasia does not only affect speech comprehension. People with Wernicke's aphasia also have difficulty
recalling the names of objects, often responding with words that sound similar, or the names of related things, as
if they are having a hard time recalling word associations[citation needed].
Pathology
A human brain showing frontotemporal lobar degeneration causing frontotemporal dementia
Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to
affect large areas of the organ, sometimes causing major deficits in intelligence, memory, personality, and
movement. Head trauma caused, for example, by vehicle or industrial accidents, is a leading cause of death in youth
and middle age. In many cases, more damage is caused by resultant edema than by the impact itself. Stroke, caused
by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.
Other problems in the brain can be more accurately classified as diseases than as injuries. Neurodegenerative
diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease are
caused by the gradual death of individual neurons, leading to diminution in movement control, memory, and
cognition.
Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder
may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic
function. These disorders may be treated by psychotherapy, psychiatric medication or social intervention and
personal recovery work; the underlying issues and associated prognosis vary significantly between individuals.
Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the meninges, the
membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as "mad cow
disease") is deadly in cattle and humans and is linked to prions. Kuru is a similar prion-borne degenerative brain
disease affecting humans. Both are linked to the ingestion of neural tissue, and may explain the tendency in human
and some non-human species to avoid cannibalism. Viral or bacterial causes have been reported in multiple sclerosis
and Parkinson's disease, and are established causes of encephalopathy, and encephalomyelitis.
Many brain disorders are congenital, occurring during development. Tay-Sachs disease, fragile X syndrome, and Down
syndrome are all linked to genetic and chromosomal errors. Many other syndromes, such as the intrinsic circadian
rhythm disorders, are suspected to be congenital as well. Normal development of the brain can be altered by genetic
factors, drug use, nutritional deficiencies, and infectious diseases during pregnancy.
Certain brain disorders are treated by neurosurgeons, while others are treated by neurologists and psychiatrists.
Visualization of a diffusion tensor imaging (DTI) measurement of a human brain. Depicted are reconstructed axon
tracts that run through the mid-sagittal plane. Especially prominent are the U-shaped fibers that connect the two
hemispheres through the corpus callosum (the fibers come out of the image plane and consequently bend towards the
top) and the fiber tracts that descend toward the spine (blue, within the image plane).
Metabolism
Brain metabolism normally is completely dependent upon blood glucose as an energy source, since fatty acids do not
cross the blood-brain barrier.[20] During times of low glucose (such as fasting), the brain will instead use ketone
bodies for fuel. The brain does not store any glucose in the form of glycogen, in contrast, for example, to
skeletal muscle.
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