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Brain Structure
1. Introduction
The human brain has many parts and each has a specific function. Each of our
brains is unique and is ever changing and sensitive to its environment. Its
modules are interdependent and interactive and their functions are not totally
fixed. This system is so complex, that it may never succeed in comprehending
itself, yet it continues to try.
Occupying the skull cavity (cranium), the adult human brain normally weighs
from 21/4 to 31/4 lb (1–1.5 kg). Differences in weight and size do not correlate
with differences in mental ability.
The brain is the portion of the vertebrate central nervous system that is
enclosed within the cranium, continuous with the spinal cord, and composed
of gray matter and white matter. It is the primary center for the regulation and
control of bodily activities, receiving and interpreting sensory impulses, and
transmitting information to the muscles and body organs. It is also the seat of
consciousness, though and, memory.
The cerebrum is the largest and most developmentally advanced portion of the
brain. It controls a number of higher functions, including speech, emotion, the
integration of sensory stimuli, initiation of the final common pathways for
movement, and fine control of movement. Each half of the cerebrum is divided
into four parts, the Frontal lobe, Parietal lobe, Occipital lobe and Temporal
lobe.
The cerebellum is the second largest area of the brain. It controls reflexes,
balance and certain aspects of movement and coordination.
The brain stem is responsible for a variety of automatic functions that are
critical to life, such as breathing, digestion and heart beat – as well as
alertness and arousal (the state of being awake).Various clumps of cells in the
brain stem determine the brain's general level of alertness and regulate the
vegetative processes of the body such as breathing and heartbeat.
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2. Anatomy of the brain
A. Cerebral Cortex
The outermost and top layer of the brain is the cerebral cortex. The cerebral
cortex is the most recently evolved and most complex part of the brain. As
one moves lower into the brain, the parts have increasingly primitive and
basic functions and are less likely to require conscious control. The cortex
plays a central role in many complex brain functions including memory,
attention, perceptual awareness, "thinking", language and consciousness.
The cerebral cortex is involved in higher functions such as sensory
perception, generation of motor commands, spatial reasoning, conscious
thought, and in humans, language. The cortex is also thought to be
responsible for higher level cognitive functions, such as learning, memory,
and complex thought. The cortex constitutes about 85% of the human
brain’s total mass. So, the cerebral cortex is the largest part of the brain
(the outermost layer of the cerebral hemisphere which is composed of grey
matter). The cerebral hemispheres are thus able to analyze sensory data,
perform memory functions, learn new information, form thoughts and
make decisions.
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1. right cerebral cortex
2. longitudinal fissure
3. cerebellum
4. frontal lobe
5. central sulcus
6. parietal lobe
The cerebral cortex is a brain structure in vertebrates; the outermost layers of the
cerebrum has a grey color, hence the name "grey matter".
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Grey matter is formed by neurons and their unmyelinated fibers while the white
matter below the grey matter of the cortex is formed predominantly by
myelinated axons interconnecting different regions of the central nervous system.
The human cerebral cortex is 2-4 mm (0.08-0.16 inches) thick.
The surface of the cerebral cortex is folded in large mammals like humans, where
more than two thirds of the cortical surface is buried in the grooves, called
"sulci". The phylogenetically more ancient part of the cerebral cortex, the
hippocampus, is differentiated in five layers, while the more recent neo-cortex is
differentiated in six basic layers. A relative variation in thickness or cell type
(among other parametres) allows us to distinguish among different neocortical
architectonic fields. The geometry of these fields seems to be related to the
anatomy of the cortical folds and, for example, layers in the upper part of the
cortical grooves (called gyri) are more clearly differentiated than in its deeper
parts (called sulcal "fundi").
Cerebral Cortex is responsible for:
• thinking and learning
• creativity
• five senses
• memory and emotion
• problem-solving
• decisions
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This area of the brain is thought to be involved in planning complex cognitive behaviours
and in the expression of personality and appropriate social behaviour. The prefrontal
cortex is found in the cortical regions of the frontal lobe which are anterior to the
primary and association motor cortices. It is divided into the dorso-lateral, orbito-frontal
(also called the limbic frontal lobe) and mesial prefrontal areas.
B. The Lobes of the brain
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Two sulci - the central sulcus and the lateral sulcus - are used to divide each
hemisphere into four sections known as lobes: the Frontal lobe, Parietal
lobe, Temporal lobe, and Occipital lobe.
The FRONTAL LOBE has a major role in the planning and execution of
movements. It contains the pre-frontal, pre-motor and motor areas, listed
from front to back.The Pre-frontal cortex is particularly associated with
higher level thought, decision-making and planning. It has a significant
inhibitory role over impulses and actions. The pre-motor and motor
cortices process and transmit information regarding body movement. There
are two pathways connecting the motor cortex and the basal ganglia to
coordinate movement.
FRONTAL LOBE
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The Parietal lobe is separated from the frontal cortex by the central sulcus. It
lies posterior to (behind) the frontal lobe and superior to (above) the temporal
lobe. The parietal lobe contains the primary sensory cortex through which
sensations, such as touch and pressure, are felt. In addition, it has a key role
in spatial orientation and information processing.
PARIETAL LOBE
The Temporal lobe is located inferior to (below) the frontal and parietal
lobes. It is primarily involved with auditory processing and memory.
TEMPORAL LOBE
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The Occipital lobe is located posterior to (behind) the temporal lobe and is
the visual center of the brain. Visual information from the eyes is processed
here.
OCCIPITAL LOBE
C. The Cerebellum
The Cerebellum is a distinct brain structure connected to the brainstem behind
the cerebral hemispheres. It has a very convoluted cortex. Learning and
executing coordinated movements are thought to be major functions of the
cerebellum. It may also have cognitive roles.
The cerebellum is located in the posterior fossa of the skull, dorsal to the pons
and medulla from which it is separated by the Aqueduct of Sylvius and the
fourth ventricle. Like the cerebrum, the cerebellum is covered by cortex and
consists of two hemispheres, each of which is divided into lobes. The
hemispheres are separated from one another by a thin structure called the
vermis, or "worm."
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The cerebellum is involved in a feedback loop for muscle movement. When
the cortex sends a message for motor movement to the lower motor neurons in
the brain stem and spinal cord, it also sends a copy of this message to the
cerebellum. This is conveyed from pyramidal fibers in the cortex on the
cortico-pontinecerebeller tract to the cerebellum. In addition, information gets
to the cerebellum from muscle spindles, joints and tendons. This information
(proprioception and kinesthesia) lets the cerebellum know about the
movements that have been executed, so that it can determine how well motor
commands coming from the cortex are being carried out. This has ben called
its comparator function.
The cerebellum plays a major role in the coordination of muscle activity for
the production of smooth movement through its connections with the
pyramidal and extrapyramidal systems and the descending reticular formation.
Due to its role in the coordination of fine motor movements, the cerebellum
makes important contributions to the control of rapid, alternating muscle
movements necessary for speech and swallowing.
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The cerebellar cortex has a well-defined cellular architecture
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- The Purkinje cell
Role of the the Purkinje cell
The cerebellar cortex is organised into three layers: the molecular layer (outer;
top in the diagram), the Purkinje cell layer, and the granule cell layer. The
central white matter contains afferent and efferent axons. Only two significant
inputs reach the cerebellar cortex: mossy fibres and climbing fibres. Mossy fibres
are in the majority (4:1) and carry a wealth of sensory and contextual information
of multiple modalities. They make specialised excitatory synapses in structures
called 'glomeruli' with the dendrites of the very numerous granule cells. This
synapse constitutes an extremely important filter of the incoming information
and is a focus of our experimental and theoretical investigations. Granule cell
axons form parallel fibres that run transversely in the molecular layer, making
excitatory en passant synapses with several cell types, but in particular with
Purkinje cells.
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Each Purkinje cell receives huge numbers of parallel fibre synapses (some
150000). These synapses are thought to be a major storage site for the
information acquired during motor learning. The Purkinje cell axon provides the
only output from the cerebellar cortex, via the deep cerebellar nuclei. Each
Purkinje cell receives just one climbing fibre input, but this input is very
powerful because it involves several hundreds of synaptic contacts. The climbing
fibre is thought to have a role in 'instructing' learning in the cerebellum. Several
other cell types, collectively called interneurones, are important in organising the
activity of the cerebellar cortex.
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D. Cerebral Hemispheres
The outermost layer of the cerebral hemisphere is composed of grey matter
(cortices are asymmetrical). Both hemispheres are able to analyze sensory
data, perform memory functions, learn new information, form thoughts
and make decisions.
1. Left Hemisphere  Sequential Analysis:
- Systematic, logical interpretation of information,
- interpretation and production of symbolic information;
o
o
o
o
language,
mathematics,
abstraction and reasoning,
memory stored in a language format.
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2. Right Hemisphere  Holistic Functioning:
- processing multi-sensory input simultaneously to provide "holistic"
picture of one's environment;
o Visual spatial skills.
o Holistic functions such as dancing and gymnastics are
coordinated by the right hemisphere.
o Memory is stored in auditory, visual and spatial modalities.
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E. The Corpus Callosum
The Corpus callosum connects right and left hemisphere to allow for
communication between the hemispheres (forms roof of the lateral and third
ventricles).
The corpus callosum is a bridge of neural fibers that connects the two
hemispheres of the cerebrum and acts as a switching center for signals
traveling between them.
The corpus callosum:
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is a band of over 200 million nerve fibers connecting the two sides
(hemispheres) of the brain
provides the main route for the transfer and integration of information
between the two hemispheres of the brain
typically begins to develop around the 10-11th week of pregnancy
continues to mature throughout pregnancy, childhood, and adolescence
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3. The Brain stem
The brain stem is comprised of the medulla, the pons and other neural cell
nuclei. This part of the brain controls many vital autonomic functions such as
heart rate, breathing, and blood pressure. The brain stem plays a vital role in
basic attention, arousal, and consciousness. All information to, and from our
body passes through the brain stem on the way to or from the brain.
The brain stem is the lower part of the brain, adjoining and structurally
continuous with the spinal cord. The upper segment of the human brain stem,
the pons, contains nerve fibers that connect the two halves of the cerebellum.
It is vital in coordinating movements involving right and left sides of the body.
Below the pons and continuous with the spinal cord is the medulla, which
transmits ascending and descending nerve fibers between the spinal cord and
the brain.
The medulla also directly controls many involuntary muscular and
glandular activities, including breathing, heart contraction, artery
dilation, salivation, vomiting, and probably laughing. The medulla
controls also the perceptual functions such as the primary aspects of
sound localization. The nuclei of some of the nerves that originate in the
brain are also located in the brain stem. Nerve fibers in the brain stem do not
readily regenerate; hence injury may result in permanent loss of function.
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The lower part of the brain stem is the medulla oblongata, grossly comprising
the medullary pyramids and the olivary bodies or olives. The pons is a structure
above the medulla. The reticular activating system is situated in between the
medulla and metencephalon and is considered to be at the "core."
Differentiation of the brain stem from the cerebrum is complex, both
anatomically and taxonomically. The adult human brainstem emerges from
parts of all three vesicles in the neural tube.
The brain stem is the stalk of the brain below the cerebral hemispheres. It is the major route for
communication between the forebrain, the spinal cord, and peripheral nerves. It controls various
autonomic functions such as respiration and the regulation of heart rhythms. Afferent stimulus
from the body passes from the spinal cord, through the pons, which routes the stimulus to the
opposite side of the brain for processing. So, if a person suffers a right-side brain stroke, the left
side of the body is normally impacted due to the cross-over of information in the pons.
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4. The function of the brain
The brain is the core of the nervous system where thought, memories,
sensation, motor coordination and all metabolic control is accomplished. The
brain can be divided into three general sections: the forebrain, the hindbrain
and the midbrain.
The Forebrain, which contains the cerebral cortex, is the region where most of
the higher processes such as memory, logical thought, consciousness, and
reasoning are carried out. The cerebral cortex and cerebrum are divided into
right and left hemispheres. These hemispheres, which are separated by a band
of transverse nerve fibers called the Corpus Callosum, have their own level of
specialization. The forebrain also contains important switching centres and
metabolic regulatory centres like the hypothalamus and pituitary gland.
The Midbrain includes the general region at the apex of the brain stem where
the forebrain and hindbrain are joined. It also acts as a switching and relay
center between the two.
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The Hindbrain, which is separated from the forebrain by the midbrain, is
located at the junction of the spinal cord and cerebrum. This region contains the
more primitive structures of the brain from an evolutionary standpoint, but they
function to regulate some of the most vital processes in the body. Breathing,
heart rate, and muscle coordination are controlled here and sensory impulses are
filtered and routed at this point.
The Cerebral cortex is the outermost layer of the cerebrum and houses centers
for higher brain functions like learning, reasoning, deduction, and emotion.The
Spinal cord is an extension of the central nervous system that routes the somatic
and autonomic nerves to and from the body and brain. The Occipital lobe
receives and processes visual information and maps it onto the cerebral cortex in
a complex network. The Parietal lobe, which includes the somatosensory area,
is responsible for processing and storing sensory information from the body.
The temporal lobes are responsible for auditory processing as well as some
language processing in the left temporal lobe.
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The Frontal lobe, which includes the motor cortex, is the centre of personality
and emotion, and performs tasks that involve logic and reasoning. Broca's area
plays a role in speech synthesis. Damage to this area can result in aphasia.
Wernicke's area is involved in the correlation of syntax and diction in speech.
Any damage to this area can result in agnosia.
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5. Specifications
Medulla oblongata
A. The Medulla Oblongata is a centre for the reflexive control of vital functions
like heart rate, breathing, swallowing, etc.
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B. The Amygdala is part of the limbic system that controls some of the more
basic drives such as aggression and sexuality.
C. The Cerebellum is the main switching centre that interprets movement
directives from the motor cortex and is responsible for maintaining posture,
muscle tone, and storing some types of memory.
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The Fornix
D. The Fornix is a series of nerve fibers that connect the hypothalamus to the
hippocampus.
E. The Reticular Formation (RF) modifies outgoing signals for movement,
posture, and muscle tone and prioritizes incoming signals coming from the body.
The RF also contains reflex arcs for breathing, sneezing, coughing, and vomiting
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F. The Thalamus is an important nexus in the brain that receives and filters all
incoming signals (except auditory signals) from the body one last time before
they are sent to the cerebral cortex.
G. The Hypothalamus functions as the primary center for emotion and drives
and controls sex, anger, temperature control, hormone release, eating, drinking,
sleep and pleasure pathways.
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H. The Hippocampus is at the core of the temporal lobes and controls the more
primitive pleasure stimuli and aversion stimuli pathways and associations. Some
long-term memory is stored here as well.
I. The Hippocampus is particularly involved with memory phenomena,
especially with the formation of long-term memory (the one that, sometimes,
lasts forever). When both hippocampi (right and left) are destroyed, nothing can
be retained in the memory. The subject quickly forgets any recently received
message.
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The pituitary gland secretes hormones (e.g. somatotrophic = growth
hormone) and controls the action of other glands like the thyroid gland,
adrenal glands, ovaries, and testes.
J. The Pituitary gland is located at the base of the brain; the pituitary is
functionally linked to the hypothalamus. It is divided into two lobes: the anterior
or front lobe (adenohypophysis) and the posterior or rear lobe
(neurohypophysis).
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Posterior pituitary (neurohypophysis)
The posterior lobe is connected to a part of the brain called the
hypothalamus via the infundibulum (or stalk), giving rise to the
tuberoinfundibular pathway. Hormones are made in nerve cell bodies
positioned in the hypothalamus, and these hormones are then transported
down the nerve cell's axons to the posterior pituitary. Hypothalamic
neurons fire such hormones, releasing them into the capillaries of the
pituitary gland.
 The hormones secreted by the posterior pituitary are
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
Oxytocin comes from the paraventricular nucleus in the
Hypothalamus
Antidiuretic hormone (ADH - also known as vasopressin),
comes from the supraoptic nucleus in the Hypothalamus
Anterior pituitary (Adenohypophysis)
The anterior lobe is derived from the oral ectoderm and is composed of
glandular epithelium. The anterior pituitary is functionally linked to the
hypothalamus via the hypophysial-portal vascular connection in the
pituitary stalk. Through this vascular connection, the hypothalamus
integrates stimulatory and inhibitory central and peripheral signals to the
five phenotypically distinct pituitary cell types. Women with an
overdeveloped pituitary may develop masculine outlook upon life.
 The anterior pituitary produces and secretes:
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Growth hormone (GH)
o human growth hormone : somatropin and somatotropin
Prolactin
o Prolactin has many effects, the most important of which is
to stimulate the mammary glands to produce milk
(lactation); peptide hormone
Follicle-stimulating hormone (FSH)
o (male/female) FSH stimulates the maturation of germ cells
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
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Luteinizing hormone (LH)
o (male/female) essential for reproduction (testosterone –
menstruation)
Thyroid-stimulating hormone (TSH or thyrotropin)
o regulates the endocrine function of the thyroid gland
Adrenocorticotropic hormone (ACTH or corticotropin)
o ACTH stimulates the cortex
Endorphins
o Produce analgesia; biochemical compounds responsible
for sense of well-being.
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The anterior pituitary produces and secretes and it does this in response to
releasing hormones produced by the hypothalamus. These travel to the
anterior lobe by way of a special capillary system, called the hypothalamichypophyseal portal system.
These hypothalamic signalling hormones include:

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TRH (thyrotropin-releasing hormone)
o Tripeptide hormone that stimulates the release of thyroidstimulating hormone and prolactin by the anterior pituitary.
CRH (corticotropin-releasing hormone)
o Stimulates corticotropes to secrete corticotropin (ACTH) and
other biologically active substances (for example β-endorphin).
DA (dopamine, "prolactin inhibiting factor"/PIF)
o function as a neurotransmitter, responsible for: Movement,
Cognition, Regulating Prolactin secretion, Motivation and
pleasure !
GnRH (gonadotropin-releasing hormone)
o GNRH1 activity is very low during childhood and is activated at
and during the puberty.
GHRH (growth hormone releasing hormone)
o stimulates growth hormone (GH) secretion
The pituitary is remarkable gland. It lies at the base of the brain, attached
to it by a stalk and enclosed in a tiny box. It is made up of two lobes, which
have different origin and different function. The anterior lobe develops from
the embryonic mouth cavity and grows upwards to join the post lobe, which
grows downwards from the brain to meet it. It is no bigger than pea and yet
it produces more hormones than any other endocrine glands. The pituitary
manufactures a growth hormone and a number of these hormones, two of
these acts on the thyroid and suprarenal cortex and has been named
respectively, the thyrotropic and Adrenotrophic hormones. It affects the sex
glands by two gonadotropic hormones. It produces a hormone called
prolactin, which causes the mammary glands to secrete milk, and there is
evidence that a parathyrotropic hormone controls the parathyroids. Finally,
the hormone is present which raises the sugar content of the blood, thus
controlling the action of insulin, which may be called the diabetogenic
hormone. The pituitary gland gives the tune to all other glands, which are
dependent upon it.
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The pineal gland secretes melatonin, which regulates the body's circadian
rhythms and, to an extent, its metabolism.
K. The pineal gland is a tiny cone shaped body in the middle of the head behind
and just above the pituitary. It contains pigment similar to that found in the eyes
and is connected by two nerve cords. With the optic thalamic, it is said to control
the action of light upon the body and for this reason, scientists have suggested
that it is the remnant of the third eye.
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The limbic system is a whole system of interconnected structures in the forebrain that
are closely associated with emotional response.
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L. The modules that lie beneath the Corpus callosum are known as the Limbic
system. This area is older then the cortex in evolutionary terms and is known as
the mammalian brain because it is thought to have first emerged in mammals.
This part of the brain, and even that below it, is unconscious, and yet has a
profound affect on our experience because it is densely connected to the
conscious cortex above it and constantly feeds information upwards.
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M. The basal ganglia are a group of large nuclei that partially surround the
thalamus. These nuclei are important in the control of movement. The red
nucleus and substantia nigra of the midbrain have connections with the basal
ganglia.
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Lymbic System
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