1. A paper presented
On
Brain
Submitted by:
Akhilesh Kumar MAury
University of Petroleum & Energy Studies
Dehradun

2. Brain
The brain is the center of the nervous system in all
vertebrate, and most invertebrate, animals. Some
primitive animals such as jellyfish and starfish have a
decentralized nervous system without a brain, while
sponges lack any nervous system at all. In vertebrates, the
brain is located in the head, protected by the skull and
close to the primary sensory apparatus of vision, hearing,
balance, taste, and smell.
Brains can be extremely complex. The cerebral cortex of the human brain contains roughly 15–
33 billion neurons depending on gender and age, linked with up to 10,000 synaptic connections
each. Each cubic millimeter of cerebral cortex contains roughly one billion synapses. These
neurons communicate with one another by means of long protoplasmic fibers called axons,
which carry trains of signal pulses called action potentials to distant parts of the brain or body
and target them to specific recipient cells.
The most important biological function of the brain is to manage and control the functions and
actions of an animal. Brains control behavior either by activating muscles, or by causing
secretion of chemicals such as hormones. Even single-celled organisms may be capable of
extracting information from the environment and acting in response to it. Sponges, which lack a
central nervous system, are capable of coordinated body contractions and even locomotion. In
vertebrates, the spinal cord by itself contains neural circuitry capable of generating reflex
responses as well as simple motor patterns such as swimming or walking. However,
sophisticated control of behavior on the basis of complex sensory input requires the information-
integrating capabilities of a centralized brain.
Despite rapid scientific progress, much about how brains work remains a mystery. The
operations of individual neurons and synapses are now understood in considerable detail, but the
way they cooperate in ensembles of thousands or millions has been very difficult to decipher.
Methods of observation such as EEG recording and functional brain imaging tell us that brain
operations are highly organized, but these methods do not have the resolution to reveal the
activity of individual neurons.

3. Structure
The brain is the most complex biological structure known, and comparing the brains of different
species on the basis of appearance is often difficult. Nevertheless, there are common principles
of brain architecture that apply across a wide range of species. These are revealed mainly by
three approaches. The evolutionary approach means comparing brain structures of different
species, and using the principle that features found in all branches that have descended from a
given ancient form were probably present in the ancestor as well. The developmental approach
means examining how the form of the brain changes during the progression from embryonic to
adult stages. The genetic approach means analyzing gene expression in various parts of the brain
across a range of species. Each approach complements and informs the other two.
The cerebral cortex is a part of the brain
that most strongly distinguishes
mammals from other vertebrates,
primates from other mammals, and
humans from other primates.
The relationship between brain size, body
size and other variables has been studied
across a wide range of species. Brain size
increases with body size but not
proportionally.

4. Invertebrates
For invertebrates (e.g., insects, molluscs, worms, etc.) the components of the brain differ so
greatly from the vertebrate pattern that it is hard to make meaningful comparisons except on the
basis of genetics. Two groups of invertebrates have notably complex brains: arthropods (insects,
crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs).
The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend
through the body of the animal. Arthropods have a central brain with three divisions and large
optical lobes behind each eye for visual processing. Cephalopods have the largest brains of any
invertebrates. The brain of the octopus in particular is highly developed, comparable in
complexity to the brains of some vertebrates.
There are a few invertebrates whose brains have been studied intensively. The large sea slug
Aplysia was chosen by Nobel Prize-winning neurophysiologist Eric Kandel, because of the
simplicity and accessibility of its nervous system, as a model for studying the cellular basis of
learning and memory, and subjected to hundreds of experiments. The most thoroughly studied
invertebrate brains, however, belong to the fruit fly Drosophila and the tiny roundworm
Caenorhabditis elegans (C. elegans).

5. Because of the large array of techniques available for studying their genetics, fruit flies have
been a natural subject for studying the role of genes in brain development. Remarkably, many
aspects of Drosophila neurogenetics have turned out to be relevant to humans. The first
biological clock genes, for example, were identified by examining Drosophila mutants that
showed disrupted daily activity cycles. A search in the genomes of vertebrates turned up a set of
analogous genes, which were found to play similar roles in the mouse biological clock—and
therefore almost certainly in the human biological clock as well.
Like Drosophila, C. elegans has been studied largely because of its importance in genetics. In
the early 1970s, Sydney Brenner chose it as a model system for studying the way that genes
control development. One of the advantages of working with this worm is that the body plan is
very stereotyped: the nervous system of the hermaphrodite morph contains exactly 302 neurons,
always in the same places, making identical synaptic connections in every worm. In a heroic
project, Brenner's team sliced worms into thousands of ultrathin sections and photographed every
section under an electron microscope, then visually matched fibers from section to section, in
order to map out every neuron and synapse in the entire body. Nothing approaching this level of
detail is available for any other organism, and the information has been used to enable a
multitude of studies that would not have been possible without it.
Vertebrates
The brains of vertebrates are made of very soft tissue, with a texture that has been compared to
Jello. Living brain tissue is pinkish on the outside and mostly white on the inside, with subtle
variations in color. Vertebrate brains are surrounded by a system of connective tissue membranes
called meninges that separate the skull from the brain. This three-layered covering is composed
of (from the outside in) the dura mater ("hard mother"), arachnoid mater ("spidery mother"), and
pia mater ("soft mother"). The arachnoid and pia are physically connected and thus often
considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space
which contains cerebrospinal fluid (CSF), which circulates in the narrow spaces between cells
and through cavities called ventricles, and serves to nourish, support, and protect the brain tissue.
Blood vessels enter the central nervous system through the perivascular space above the pia
mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier
which protects the brain from toxins that might enter through the blood.
The first vertebrates appeared over 500 million years ago (mya), during the Cambrian period,
and may have somewhat resembled the modern hagfish in form. Sharks appeared about
450 mya, amphibians about 400 mya, reptiles about 350 mya, and mammals about 200 mya. No
modern species should be described as more "primitive" than others, since all have an equally
long evolutionary history, but the brains of modern hagfishes, lampreys, sharks, amphibians,
reptiles, and mammals show a gradient of size and complexity that roughly follows the
evolutionary sequence. All of these brains contain the same set of basic anatomical components,
but many are rudimentary in hagfishes, whereas in mammals the foremost parts are greatly
elaborated and expanded.

6. All vertebrate brains share a common underlying form, which can most easily be appreciated by
examining how they develop. The first appearance of the nervous system is as a thin strip of
tissue running along the back of the embryo. This strip thickens and then folds up to form a
hollow tube. The front end of the tube develops into the brain. In its earliest form, the brain
appears as three swellings, which eventually become the forebrain, midbrain, and hindbrain. In
many classes of vertebrates these three parts remain similar in size in the adult, but in mammals
the forebrain becomes much larger than the other parts and the midbrain quite small.
Neuroanatomists usually consider the brain to consist of six main regions: the telencephalon
(cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain),
cerebellum, pons, and medulla oblongata. Each of these areas in turn has a complex internal
structure. Some areas, such as the cortex and cerebellum, consist of layers, folded or convoluted
to fit within the available space. Other areas consist of clusters of many small nuclei. If fine
distinctions are made on the basis of neural structure, chemistry, and connectivity, thousands of
distinguishable areas can be identified within the vertebrate brain.
Some branches of vertebrate evolution have led to substantial changes in brain shape, especially
in the forebrain. The brain of a shark shows the basic components in a straightforward way, but
in teleost fishes (the great majority of modern species), the forebrain has become "everted", like
a sock turned inside out. In birds, also, there are major changes in shape. One of the main
structures in the avian forebrain, the dorsal ventricular ridge, was long thought to correspond to
the basal ganglia of mammals, but is now thought to be more closely related to the neocortex.
The medulla, along with the spinal cord, contains many small nuclei involved in a wide
variety of sensory and motor functions.
The hypothalamus is a small region at the base of the forebrain, whose complexity and
importance belies its size. It is composed of numerous small nuclei, each with distinct
connections and distinct neurochemistry. The hypothalamus is the central control station
for sleep/wake cycles, control of eating and drinking, control of hormone release, and
many other critical biological functions.
Like the hypothalamus, the thalamus is a collection of nuclei with diverse functions.
Some of them are involved in relaying information to and from the cerebral hemispheres.
Others are involved in motivation. The subthalamic area (zona incerta) seems to contain
action-generating systems for several types of "consummatory" behaviors, including
eating, drinking, defecation, and copulation.
The cerebellum modulates the outputs of other brain systems to make them more precise.
Removal of the cerebellum does not prevent an animal from doing anything in particular,
but it makes actions hesitant and clumsy. This precision is not built-in, but learned by
trial and error. Learning how to ride a bicycle is an example of a type of neural plasticity
that may take place largely within the cerebellum.
The tectum, often called "optic tectum", allows actions to be directed toward points in
space. In mammals it is called the "superior colliculus", and its best studied function is to
direct eye movements. It also directs reaching movements, though. It gets strong visual

7. inputs, but also inputs from other senses that are useful in directing actions, such as
auditory input in owls, input from the thermosensitive pit organs in snakes, etc. In some
fishes, such as lampreys, it is the largest part of the brain.
The pallium is a layer of gray matter that lies on the surface of the forebrain. In reptiles
and mammals it is called cortex instead. The pallium is involved in multiple functions,
including olfaction and spatial memory. In mammals, where it comes to dominate the
brain, it subsumes functions from many subcortical areas.
The hippocampus, strictly speaking, is found only in mammals. However, the area it
derives from, the medial pallium, has counterparts in all vertebrates. There is evidence
that this part of the brain is involved in spatial memory and navigation in fishes, birds,
reptiles, and mammals.
The basal ganglia are a group of interconnected structures in the forebrain, of which our
understanding has increased enormously over the last few years. The primary function of
the basal ganglia seems to be action selection. They send inhibitory signals to all parts of
the brain that can generate actions, and in the right circumstances can release the
inhbition, so that the action-generating systems are able to execute their actions. Rewards
and punishments exert their most important neural effects within the basal ganglia.
The olfactory bulb is a special structure that processes olfactory sensory signals, and
sends its output to the olfactory part of the pallium. It is a major brain component in
many vertebrates, but much reduced in primates.
Bilateria
Nervous system of a bilaterian animal is in the form of a nerve cord with segmental
enlargements, and a "brain" at the front.
With the exception of a few primitive forms such as sponges and jellyfish, all living animals are
bilateria, meaning animals with a bilaterally symmetric body shape (that is, left and right sides
that are approximate mirror images of each other)
Mammals
The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but
dramatic differences appear in the forebrain, which is not only greatly enlarged, but also altered
in structure. In mammals, the surface of the cerebral hemispheres is mostly covered with 6-
layered isocortex, more complex than the 3-layered pallium (neuroanatomy) seen in most
vertebrates. Also the hippocampus of mammals has a distinctive structure.

8. Unfortunately, the evolutionary history of these mammalian features, especially the 6-layered
cortex, is difficult to work out.[41] This is largely because of a missing link problem. The
ancestors of mammals, called synapsids, split off from the ancestors of modern reptiles and birds
about 350 million years ago. However, the most recent branching that has left living results
within the mammals was the split between monotremes (the platypus and echidna), marsupials
(opossum, kangaroo, etc.) and placentals (most living mammals), which took place about 120
million years ago. The brains of monotremes and marsupials are distinctive from those of
placentals in some ways, but they have fully mammalian cortical and hippocampal structures.
Thus, these structures must have evolved between 350 and 120 million years ago, a period that
has left no evidence except fossils, which do not preserve tissue as soft as brain
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. 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 involved in
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[citation needed], of
which about 10 billion (1010) are
cortical pyramidal cells.[citation needed]
These cells pass signals to each other
via approximately 100 trillion
(1014)[citation needed] synaptic connections.

9. 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. More common are
genetically based diseases, such as Parkinson's disease, multiple sclerosis, and many others. 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 of human brain
The adult human brain weighs on average about 3 lb (1.5 kg) with a size of around 1130 cubic
centimetres (cm3) in women and 1260 cm3 in men, although there is substantial individual
variation. The brain is very soft, having a consistency similar to tofu. When alive, it is tan-gray
on the outside and mostly yellow-white on the inside, with subtle variations in color.
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.

10. Situated at the top and covered with a convoluted cortex, the cerebral hemispheres form the
largest part of the human brain. 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 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.

11. 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 sub cortical 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. In a human, comparable cerebral cortex damage produces a
permanent state of coma.
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. 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".
Topography
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. 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. 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

12. 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
Lateralization
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. 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.

13. 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.

14. Sources of information
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). 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).[9] This technique has the
same temporal resolution as EEG but much better spatial resolution, although not as good as
fMRI. 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
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.[10]
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.

15. Effects of brain damage
A key source of information about the function of brain regions is the effects of damage to
them.[11] 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
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.
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.
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.

16. Pathology
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, 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.