Brain Structures
The brain, the body's "control central," is one of the largest of adult organs, consisting
of over 100 billion neurons and weighing about 3 pounds. It is typically divided as
follows:
Major division
Subdivision
Telencephalon
Forebrain
Diencephalon
Midbrain
Hindbrain
Mesencephalon
Metencephalon
Myelencephalon
Principal Structures
Cerebral cortex
Basal ganglia
Limbic system
Thalamus
hypothalamus
Tectum
Tegmentum
Cerebellum
Pons
Medulla oblongata
The Hindbrain
The hindbrain consists of two major divisions – the myelencephalon and the
metencephalon.
Myelencephalon:
The myelencephalon contains the medulla oblongata and a part of the reticular
formation.
The medulla controls vital functions such as regulation of the
1
cardiovascular system, respiration, and skeletal muscle tonus. In fact, so vital is the
medulla to survival that diseases or injuries affecting it often prove fatal.
Metencephalon:
The myelencephalon consists of the pons and the cerebellum.
The pons contains a part of the reticular formation. The pons seems to play an
important role in sleep and arousal. It also relays information from the cerebral cortex
to the cerebellum
The cerebellum has two hemispheres and resembles a miniature version of the
cerebrum. It is covered by a cerebellar cortex and has a set of deep cerebellar nuclei –
the core. The core recieve projections from the cortex and send projections out of the
the cerebellum to other parts of the brain. Each projection is attached to the pons by
by bundles of axons.
Damage to the cerebellum impairs standing, walking, or performance of coordinated
movements. The cerebellum receives visual, auditory, vestibular, and somatosensory
information, and it also receives information about individual muscle movements
being directed by the brain. The cerebellum integrates this information and modifies
the motor outflow, exerting a coordinating and smoothing effect on the movements.
Cerebellar damage results in jerky, poorly coordinated movements; extensive
cerebellar damage makes it impossible even to stand.
The Midbrain
The midbrain, also called the mesencephalon consists of the tegmentum and the
tectum.
The tegmentum consists of a large portion of the reticular formation. The reticular
formation is characterized by a diffuse, interconnected network of neurons with
complex dendritic and axonal processes. It occupies the core of the brain stem, from
the lower border of the medulla to the upper border of the midbrain. The reticular
formation receives sensory information by means of various pathways and projects
axons to the cerebral cortex, thalamus, and spinal cord. It plays a role in sleep,
arousal, attention, muscle tonus, movement, and various vital reflexes. It also plays a
role in the ability to focus attention on a particular stimuli.
The tegmentum also contains the periaqueductal gray matter and the substantia nigra.
The periaqueductal gray matter is involved in species-typical behaviour such as
fighting and mating.
The tectum consists of the superior colliculi and the inferior colliculi. The superior
colliculi are part of the visual system and involved in visual reflexes and reactions to
moving stimuli. The inferior colliculi are part of the auditory system. Together, they
2
help to orient head and eyes towards what is seen and heard and play a vital role in
survival.
The Forebrain
The forebrain consists of the diencephalon and the telencephelon.
Diencephalon:
The most important structures in the diencephalon are the thalamus and the
hypothalamus.
The hypothalamus lies at the base of the brain below the thalamus. It controls the
autonomic nervous system and the endocrine system. Moreover, it organizes
behaviours related to survival of the species – fighting, feeding, fleeing, and mating.
The pituitary gland is attached to the base of the hypothalamus via the pituitary stalk.
A special system of blood vessels directly connects the hypothalamus to the anterior
pituitary gland. The hypothalamus hormones secreted into the stalk stimulate the
pituitary gland to stimulate its hormones.
The pituitary gland is called the master gland. The anterior pituitary (when
stimulated by the hypothalamus) secretes hormones that play roles in reproductive
physiology and behaviour, and control the endocrine glands. The posterior pituitary
gland (also when stimulated by the hypothalamus) secrete hormones that cause
uterine contractions during childbirth, hormones that stimulate milk production
during lactation, and hormones that regulate urine output by the kidneys.
The thalamus has two lobes connected by a bridge of gray matter. The thalamus has
projection fibres (axons that arise from cell bodies located in one region of the brain
and synapse on neurons located within another region). The thalamus is divided into
several nuclei (groups of neurons of similar shape). Some receive sensory information
from the sensory systems and relay them to specific areas in the cerebral cortex.
Other thalamic nuclei relay motor information from areas of the cerebral cortex.
Some thalamic nuclei are involved in controlling the general excitability of the
cerebral cortex. Because the general function of the thalamus is to relay information
to and from the cerebral cortex, it is sometimes known as the great relay exchange.
Telencephalon:
The telencephalon includes most of the two symmetrical cerebral hemispheres that
make up the cerebrum. The cerebral hemispheres are covered by the cerebral cortex
and contain the limbic system and the basal ganglia.
The basal ganglia are a collection of subcortical nuclei that are involved in the control
of movement. Parkinson’s disease (characterized by weakness, tremors, rigidity of
3
limbs, poor balance, difficulty in initiating movement)is caused by the degeneration of
certain neurons located in the midbrain that send axons to the basal ganglia.
The limbic system consists of the limbic cortex, the hippocampus, the amygdala, the
cingulated gyrus, the fornix and mammillary bodies. The limbic cortex seems to have
the primary function of motivation and emotion. The hippocampus seems to be
involved in memory and learning. The amygdala and some regions of the limbic
cortex are involved in emotions: feeling and expression of emotions, emotional
memories, and recognition of the signs of emotions in other people.
The cerebral cortex surrounds the cerebral hemispheres. In humans the cortex is
greatly convoluted with sulci (small grooves) fissures (large grooves) and gyri (bulges
between the sulci and fissures). The cerebral cortex consists mostly of glia and cell
bodies, dendrites, and interconnecting axons of neurons. Cell bodies predominate and
the cerebral cortex has a grayish brown appearance – thus the name “gray matter”.
Different regions of the cerebral cortex perform different functions. The cerebral
cortex can be divided into four lobes based on the natural structural fissures and gyri.
Two of the deepest fissures are the central fissure and the lateral fissure. These are the
frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.
However, the functioning of the cerebral cortex is better understood by looking at the
various areas of the brain.
4
The primary visual cortex receives visual information. The primary auditory cortex
receives auditory information. The primary somatosensory (somesthetic) cortex
receives information from different regions of the body including information
concerning taste. Most sensory information from the body is sent to the contralateral
hemisphere (information from the right side of the body to the left hemisphere and
information from the left side of the body to the right hemisphere). The only
exceptions are smell and taste. The primary motor cortex is directly involved in the
control of movement. Again the control is contralateral. The different association
areas which occupy most of the surface of the cerebral cortex accomplish the functions
carried out between sensation and action – the perceiving, learning, remembering,
planning, deciding. The central fissure divides the brain into the rostral and caudal
regions. The rostral region is involved in movement related activities such as planning
and executing behaviours. The caudal region is involved in perceiving and learning.
The sensory association area analyses sensory information received from the primary
somatosensory cortex and perception takes place – memories are also stored there.
Damage to the somatosensory association area cause deficits related to
somatosensation and the environment in general such as difficulty in perception,
trouble drawing maps, or following maps, etc. Damage to the visual association area
will not cause blindness but may cause deficits in recognition. Damage to the auditory
5
association area may have difficulty in perceiving speech or even producing
meaningful speech.
Damage to regions of the association cortex at the junction of the three posterior lobes
(where somatosensory, visual, and auditory functions overlap) may have difficulty
reading or writing.
The frontal association area is involved in planning and execution of movements. The
motor association cortex (the premotor cortex) directly controls behaviour. The rest
of the frontal lobe known as the prefrontal cortex is involved in planning and strategy.
Although the cerebral hemispheres cooperate with each other, they do not perform
identical functions. Some functions are lateralised – located primarily on one side of
the brain. The corpus callosum – a large band of axons that connects corresponding
association areas of the left and right hemisphere – plays a role in unifying the
experiences of the left and right hemispheres thus ensuring that our perception and
experience of the world is a unified whole. In individuals who have had their corpus
callosum seem to have two brains working in the head – independently of each other.
Such individuals are termed as split-brain individuals.
6
The knowledge that each hemisphere carries out a different function comes from the
work of Roger Sperry who carried out research on split-brain individuals and was the
Nobel prize in medicine (1981) for his research in this field.
In a typical test situation, the subject is seated
in front of a screen that hides his hands from
view. His gaze is fixed at a spot on the center of
the screen. The word “nut” is flashed for onetenth of a second on the left side of the screen
such that the information is available only to
the left visual field.
This information is
transmitted to the right hemisphere. Sperry
noticed that the subject can easily pick up a nut
from a pile of unseen objects with his left hand.
But he cannot name the object he has seen.
In another situation, the word “hat-band” is
flashed in such a way that the word “hat” gets
communicated to the right hemisphere and the
word “band” to the left hemisphere. When
asked what word was seen, the subject says
“band”. When asked to clarify what type of
band, the subject makes various guesses such s
rubber-band, rock nd roll band, band of
robbers, etc.
Tests with other word
combinations such as “key case” and “suitcase”
have yielded similar results.
If a split-brain subject is blindfolded and a
familiar object is placed in the left hand, he can
demonstrate its use with appropriate gestures.
But if asked what is going on while he is
gesturing (because he is blindfolded and can’t
see his gestures), he cannot express his gestures
in language. This is the case as long as the left
hemisphere receives no sensory input. But if
the subject’s right hand accidently touches the
object or if the object makes a characteristic
sound (like the jingle of a key chain), the left
hemisphere immediately recognizes the object
and communicates it through language.
7
From this, Sperry concluded that the left hemisphere is the “talking” hemisphere.
However, the right hemisphere does have some linguistic capabilities. It recognizes
the meaning of simple, concrete objects and can write a little. In one situation, a splitbrain subject is shown a list of common objects (like cup, knife, book, glass) for long
enough for it to be registered in both hemispheres. Then the list is removed and one
of the words from the list is flashed for 1/10th of a second to the right hemisphere. If
the subject is asked to write what he saw, his left hand will write the word correctly.
But when asked what he has written, because the left hemisphere has no input, he will
simple make guesses. Thus, the right hemisphere can only comprehend very simple
language like simple nouns and respond to it by picking it up or gesturing
appropriately. If presented with a simple command like wink or nod or smile – it
cannot respond – so it cannot comprehend even the simplest form of abstract
language.
In general, the left hemisphere participates in the analysis of information – the
extraction of the elements that make up the whole of the experience. Thus the left
hemisphere is good at recognizing serial events and controlling sequences of
behaviour. Thus activities such as talking, understanding the speech of others,
reading and writing are lateralized to the left hemisphere in most people. The left
hemisphere governs our ability to express ourselves in language. It can perform
complicated logical activities and is skilled in mathematical computations.
In contrast, the right hemisphere is specialized in synthesis – putting isolated
elements together to perceive the whole. Thus, drawing, reading maps, constructing
complex objects from smaller elements is located in the right hemisphere. The right
hemisphere can comprehend only very simple language – it can respond by selecting
the relevant objects when presented with simple nouns, but cannot carry out even the
8
simplest command such as wink, nod, etc – thus it has no abstract linguistic
processing.
In normal subjects, studies have confirmed the lateralization of the different
hemispheres – verbal information is identified faster and more accurately by the left
hemisphere. In contrast, the identification of faces, facial expressions, emotions, line
slopes, dot locations are processed faster and more accurately by the right hemisphere.
It is the integration of both these experiences that leads to effective cognitive
functioning in most cases: for example, when reading a story, the right hemisphere
plays a role in decoding visual information, appreciating humour and emotional
content, deriving meaning from past associations, understanding metaphor, and
related functions. At the same time, the left hemisphere understands the syntax,
translates written words in phonetic representations, derives meaning from complex
relations among concepts and syntax. Thus, there is no activity in which only one
hemisphere is involved or to which only one hemisphere makes a contribution.
Language functioning of the brain has been further studied by different researchers.
In 1861, Paul Broca examined the brain of a patient who had suffered speech loss and
found damage in an area of the left hemisphere just above the lateral fissure in the
frontal lobe. This region, known as Broca’s area is invoved in the production of
speech. Destruction of or damage to this area results in expressive aphasia (also called
Broca’s aphasia). Destruction of the equivalent region in the right hemisphere usually
does not result in speech impairment.
In 1874, Carl Wernicke reported that damage to another area also in the left
hemisphere – this time in the temporal lobe can cause receptive aphasia – an inability
to comprehend words. Speech production is intact but meaningless. This area is
called the Wernicke’s area.
Blood supply
An intricate arterial structure supplies the brain with oxygen-rich blood. At the brain
stem, two vertebral arteries, entering through the first cervical vertebrae, join to form
the basilar artery. The basilar artery along with two internal carotid arteries, entering
through holes at the base of the skull, interconnect at the Circle of Willis. From there,
the anterior and middle cerebral arteries arise; the posterior cerebral artery arises from
the basilar system.
Cranial Nerves
There are 12 pairs of cranial nerves. Some bring information from the sense organs to
the brain; some control muscles; others are connected to glands or internal organs.
9
Cranial Nerve
Major Functions
I
Olfactory
Smell
II
Optic
vision
III
Oculomotor
eyelid and eyeball movement
IV
Trochlear
innervates superior oblique turns eye downward and
laterally
V
Trigeminal
chewing face & mouth touch & pain
VI
Abducens
turns eye laterally
VII
Facial
controls most facial expressions secretion of tears & saliva
taste
VIII
Vestibulocochlear
hearing equillibrium sensation
IX
Glossopharyngeal
taste senses carotid blood pressure
X
Vagus
senses aortic blood pressure slows heart rate stimulates
digestive organs taste
XI
Spinal Accessory
controls trapezius &
swallowing movements
XII
Hypoglossal
controls tongue movements
sternocleidomastoid
controls
The nerves are often remembered by the mnemonic ...
"On Old Olympic Towering Top, A Famous Vocal German Viewed Some Hops"
10
Basic function summary of the different areas of the brain
Structure
Area
Function
Visual cortex
Visual association area
Telencephalon
Cerebral cortex
Somatosensory
association area
Primary somatosensory
cortex
Primary motor cortex
Premotor area
Prefrontal cortex
Broca’s area
Auditory association area
Primary auditory cortex
Wernicke’s area
Corpus callosum
Limbic cortex
Limbic system
hippocampus
amygdala
Basal ganglia
11



















receiving visual information
analyses visual information from the visual cortex, recognition and
visual imagery occur
analyses sensory information from the primary somatosensory cortex,
perceives it and stores the memories
receives other sensory information – skin senses related to touch, pain,
pressure, etc
control of movement
directs and controls movement
consciousness - awareness of one's self and one's environment
thought, reasoning, planning, decision making, and other executive
functions
Speech production
analyses information received in the primary auditory cortex
receives auditory information
speech comprehension
coordinates the functioning of the two hemispheres
emotions – experience, expression, perception
motivation
learning
memory
feeling and expressing emotions, emotional memory, emotional
perception
movement
Myelencephalon
Metencephalon
Mesencephalon
Diencephalon
Structure
12
Area
Thalamus
Hypothalamus
Tectum
Superior colliculi
Inferior colliculi
Reticular formation
Tegmentum
Other areas
Cerebellum
Pons
Medulla
oblongata

























Function
voluntary movement/ motor integration
perception/ Sensory/mind-body integration
general excitability of the cerebral cortex
relay of information to and from the cerebrum
temperature
appetite
ANS functions through the endocrine system
fighting, feeding, fleeing, and mating
visual reflexes
reactions to movement
audition
general alarm and preparation for incoming information
sleep, arousal, attention focus, muscle tonus, movement
fighting, mating and other species-specific behaviour
balance/ Equilibrium of the trunk
muscle tension, spinal nerve reflexes, posture and balance of the limbs
fine motor control, eye movement
breathing/respiration
reflex centers for pupillary reflexes and eye movements
sleep and arousal
relay system for the cerebellum
breathing/respiration
heart rate/ action
blood pressure, blood vessel diameter
reflex centers for vomiting, coughing, sneezing, swallowing, and
hiccuping
Colour and Name the various parts of the brain
13
Colour and Name the various lobes of the cerebral cortex
Colour and Name the various areas of the cerebral cortex
14