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Cerebrum (cerebral cortex)
The cerebrum, consists of two identical convoluted
hemispheres, each is formed of 5 lobes. It contains
grey matter in its cortex and in deeper cerebral nuclei.
Most of what are considered to be higher functions of
the brain are formed by the cerebrum, such as
voluntary initiation of movements, final sensory
perception, conscious thoughts, language, personality
traits, and factors we associate with the mind or
intellect.
- The cerebrum is the only structure of the
telencephalon.
- It is the largest portion of the brain (accounting
for 80% of its weight). And it is the brain region
primarily responsible for mental functions.
- There are right and left cerebral hemispheres,
which are connected by a large fiber tract, called
corpus callosum.
The cerebral cortex:
The cerebrum consists of an outer cerebral cortex,
composed of 2-4 mm of gray matter (consists
primarily of densely packaged cell bodies and their
dendrites as well as glial cells) and underling white
matter (formed of bundles or tracts of mylinated nerve
fibers (Axons), its white appearance is due to lipid
composition of the myelin.
The cerebral cortex is characterized by numerous
folds and grooves called convolutions.
The elevated folds are called gyri, and the depressed
grooves are the sulci.
Each cerebral hemisphere is subdivided by deep sulci
or fissures into five lobes, these are
The frontal, parietal, temporal, occipital lobes, which
are visible from the surface and the deep insula.
Frontal lobe:
Is the anterior portion of each cerebral hemisphere. A
deep fissure, called the central sulcus, separates the
frontal central lobe from the parietal lobe.
The precentral gyrus, involved in motor control, is
located in the frontal lobe just in front of the central
sulcus. The neuron cell bodies located her are called
upper motor neurons, because of their role in muscle
regulation.
The post central gyrus, located just behind the central
sulcus in the parietal lobe is the primary area of the
cortex responsible for the perception of somatesthetic
sensation “means body feelings” (sensations arising
from cutaneous, muscle, tendon, and joint receptors.
Stimulation of specific areas of the precentral gyrus→
specific movements.
Stimulation of specific areas of the post central
gyrus→ produces sensations in specific parts of the
body.
Representation of the body in the cerebral cortex:
Typical maps of the sensory and motor cortex shows
that:
- The body is represented in an upside down picture
with the superior regions of the cortex devoted to
the toes and the inferior regions devoted to the
head.
- The area of representation does not correspond to
the size of the body parts being served, but on the
number of receptors in it. The body part with the
highest density of receptors; are represented by
the largest area of the sensory cortex, and the
body regions with the greatest motor innervations
are represented by the largest areas of the motor
cortex.
- The hand and face, which have a high density of
sensory receptors and motor innervation, are
served by larger areas of the precentral and
postcentral gyri than is the rest of the body.
- There is crossed representation of the body i-e the
left cerebral cortex controls the functions of the
right side of the body and the right cortex controls
the left body side.
The frontal lobe: Is responsible for three main
functions
1- Voluntary motor control.
2- Speaking ability.
3- Elaboration of thoughts.
* The area of the frontal lobe immediately in front
of the central sulcus and adjacent to the
somatosensory cortex is called the primary motor
cortex, it controls voluntary movements produced by
skeletal muscles.
The parietal lobe: is primarily responsible for
receiving and processing sensory input such as
touch, pressure, heat, cold, and pain from the
surface of the body. The parietal lobe also perceives
awareness of the body position, a process called
proprioception.
The temporal lobe:
*Contains auditory centers that receive sensory
fibers from the cochlea of each ear.
* Involved in interpretation and association of
auditory and visual informations.
The occipital lobe:
Is the primary area responsible for vision and for
coordination of eye movements.
The insula:
*Is implicated in memory encoding.
*Integration of sensory information (pain) with
visceral responses. In particular, insula seems to be
involved in coordinating the cardiovascular
responses to stress.
The cerebral cortex is organized into layers and
functional columns:
 The cerebral cortex is organized into six welldefined layers based on varying distribution of cell
bodies and locally associated fibers of several
distinctive cell types.
 Theses layers are organized into functional
vertical columns that extend perpendicularly from
the surface down through the depth of the cortex
to the underling white matter.
 The neurons within a given column are believed to
function as a team with each cell being involved in
different aspect of the same specific activity.
Other regions of the nervous system besides the
primary motor cortex are important in motor control.
The motor cortex is not the only region of the brain
involved with motor control.
First: - lower brain regions and the spinal cord control
involuntary skeletal muscle activity, such as the
maintenance of posture.
Second: - Although the motor cortex can activate
motor neurons to bring about muscle contraction, the
motor cortex itself does not initiate voluntary
movements. The motor cortex is activated by wide
spread pattern of neuronal discharge that gives the
readiness potential, which occurs 750 msec before
specific electrical activity is detected in the motor
cortex.
The high motor areas of the brain believed to be
involved in this voluntary decision- making period
include the supplementary motor are, the premotor area,
and the posterior parietal cortex. These four regions
develop a motor program for the specific voluntary
task and then initiate the appropriate pattern in the
primary motor cortex to bring about the sequenced
contraction of appropriate muscles to accomplish
desired complex movement.
The supplementary motor area
Lies on the medial (inner) surface of each hemisphere
anterior to (in front of) the primary motor cortex. It
appears to play a preparatory role in programming
complex sequence of movements. e.g opening or
closing the hand. Lesion in this area will not result in
paralysis, but they do interfere with performance of
more complex, useful integrated movements.
Premotor area
Located on the lateral surface of each hemisphere in
front of the primary motor cortex, is believed to be
important in orienting the body and arms towards a
specific target. The premotor cortex is guided by
sensory input from the posterior parietal cortex, a
region that lies posterior to (in back of )the primary
somatosensory cortex.
The association areas of the cerebral cortex are
involved in many higher functions.
The motor, sensory, and language areas account to
about half of the total cerebral cortex. The remaining
is called the association areas. Are involved in higher
functions, there are three association areas:
 The prefrontal association area.
 The parietal-temporal – occipital association
area.
 The limbic association area.
The prefrontal association area
Is the front portion of the frontal lobe just anterior to
the premotor cortex.
Functions:
1- Planning for voluntary activity.
2- Weighing consequences of future actions and
choosing between different options for various
social or physical situations,
3- Personality traits.
Lesion in this area results in changes in personality
and social behavior.
The parietal-temporal-occipital association cortex
Is found at the interface of the three lobes for which it
is named.
In this strategic location, it pools and integrates
somatic, auditory, and visual sensations projected
from these three lobes for complex perceptual
processing.
Functions:
It enables us to “get the complete picture “of the
relationship of various parts of our bodies with the
external world. e.g it integrates visual information
with propioceptive input to enable you to place what
you are seeing in proper prespective, such as realizing
that a bottle is in an upright position in spite of the
angle from which you view it.
This region is also involved in the language pathway
connecting Wernicke’s area to the visual and auditory
cortices.
The limbic association area
Is located mostly on the bottom and adjoining inner
portion of each temporal lobe.
Functions:
This area is concerned primarily with motivation and
emotion and is extensively involved in memory.
Indeed, all association areas appear to be involved in
sophisticated mental events such as memory, thinking,
decision making, creativity, and self –consciousness.
None of these higher brain functions are controlled by
specific cortical region. All are believed to depend on
complex interrelated pathways involved several
different regions. The cortical association areas are all
interconnected by bundles of fibers within the cerebral
white matter. Collectively, the association areas
integrat, diverse information for purposeful action.
The cerebral hemispheres have some degree of
specialization
The cortical areas described thus far appear to be
equally distributed in both the right and left
hemispheres, except for the language areas, which are
found only on one side, usually the left.
The left side is also more commonly the dominant
hemisphere for fine motor control. Thus most people
are right handed because the left side of the brain
controls the right side of the body.
The left side also excels in the performance of logical,
analytical, sequential, and verbal tasks, such as math,
language forms, and philosophy.
The right side excels in non language skills, especially
spatial perception and artistic and musical endeavors.
The two hemispheres share information and
complement each other. In some individuals the skills
associated with one hemisphere appear to be more
developed. Left hemisphere dominance tends to be
associated with “thinkers”, whereas the right
hemispheric skills dominate in “creators”
Language ability has several discrete components
controlled by different regions of the cortex:
The areas of the brain responsible for language ability
are found in the left hemisphere in the majority of the
population. The primary areas of cortical
specialization for language are the Broca‫̉̉׳‬
which is responsible for speaking ability, is located in
left frontal lobe in close association with the motor
areas of the cortex that control the muscle necessary
for articulation. Wernick‫׳‬s area, located in the left
cortex at the juncture of parietal, temporal, and
occipital lobes is concerned with language
comprehension. It plays a critical role in
understanding both spoken and written messages.
Visualizing the Brain
The following techniques are now used
1- x-ray computerized tomography(CT).
2- Positron immersion tomography (PET).
3- Magnetic resonant image (MRI).
CT:
Involves complex computer manipulation of data
obtained from x-ray absorption by tissues of different
densities. Using this technique soft tissues as the brain
can be observed at different depths.
PET:
Radio- isotopes that emit positrons are injected into
the blood stream. Positrons are like electrons but
carry positive charges, the collision of a positron and
electron results in emission of gamma rays, which can
be detected and used to pinpoint brain cells that are
most active.
Scientists used this to study brain metabolism, drug
distribution in the brain, and changes in blood flow as
a result of brain activity.
MRI:
This technique is based on the concept that protons
(H+) respond to magnetic field. The magnetic field is
used to align the protons, which emits a detectable
radio-wave signal when appropriately stimulated.
With this technique excellent images could be obtained
without subjecting the person to any known danger.
EEG (electroencephalogram):
The synaptic potentials produced at the cell bodies and
dendrites of the cerebral cortex create electrical
currents that can be measured by electrodes placed on
the scalp. A record of these electrical currents is called
electroencephalogram or EEG.
EEG is used clinically to diagnose epilepsy and other
abnormal states, distinguish various stages of sleep.
Absence of an EEG can be used to signify brain death.
There are normally four types of EEG patterns.
*Alpha waves:
-Recorded from the parietal and occipital regions.
- In awake and relaxed person but with eyes closed.
- These waves are rhythmic oscillations of 10-12
cycles/ second.
- In child below age of 8 years the frequency is lower
about 4-7 cycles/ second.
*Beta waves:
- Are stronger from the frontal lobe.
- Are produced by visual stimuli and mental activity.
- Because they respond to stimuli from receptors and
are super imposed on continuous activity they are
called evoked activity
- Frequency of beta waves is 13-25 cycles/ second.
*Theta waves:
- Are emitted from the temporal and occipital lobes.
- Frequency 5-8 cycles/ second.
- Common in newborn infants.
- Its presence in adults generally indicates sever
emotional stress and can be a warning of brain
breakdown.
*Delta waves:
-Emitted in a general pattern from the cerebral cortex.
-Have a low frequency 1-5 cycles/ second.
- Common during sleep and in an awake infant.
During sleep we have 2 types of EEG patterns
corresponding to the 2 phases of sleep:
*Rapid eye movement sleep, when dreams occur. The
EEG is similar to wakefulness
* Non-Rapid eye movement sleep, resting sleep, the
EEG display large slow delta waves (high amplitude
and slow frequency) superimposed on it is sleep
spindles.