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FUNCTIONAL AREAS OF BRAIN
Dr. Bindu. K BHMS,MD(Hom)
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Functions of brain can be studied by dividing it into different lobes(frontal
lobe, parietal lobe, temporal lobe and occipital lobe.) or by dividing it into
different areas based on histological structural differences(ie-Brodmann’s
areas—47 areas)
BROADMANN’S CLASSIFICATION OF FUNCTIONAL AREAS
It was a mere historical accident that Broadmann in his original study of
the monkey brain started to cut the serial sections in the horizontal plane. He
named the first cell mass that he encountered as area one(1). The subsequent
groups of cells that he visualized were called in ascending numerical order. In
the finalized map that is seen today the numbrers occur in a jumbled manner
because of the fact that reading is done in an anterior-posterior direction.
There are nearly 5 areas described by Broadmann. The workers who came to the
field later used rather artificial methods to subdivide Broadmann’s areas into
smaller and smaller subdivisions so that they ended up by describing a mosaic
pattern consisting of quiet a large number of islets with a confusing array of
numerical labels. There has been a conscious attempts at simpliflying and
rationalizing the Broadmann map by various workers,like Vogt,Von Bonin and
Bailey.
Classification of Von Bonnin and Bailey
The area FA of Von Bonnin and Bailey corresponds to area 4 of Broadmann. It is
the agranular cortex in the depth in the central fissure and extending
anteriorly to the frontal surface of the pre-central gyrus. The gray mater is
quiet thick in this area and the anterior border is determined by the presence
of the giant pyramid cells of Betz in the 5th layer. FB roghly corresponds to
Brodmann’s area 6, the is still thick and agranular but lacks Betz cells. FC is
analogous to area 8 and is a transitional band with an illdeveloped granular
layer. The reminder of the frontal lobe designated as FD corresponding to areas
9 to 12 is quiet uniform in structure. PB lies almost entirely buried in the
depth of the central fissure and consists of dust-like cells(Konio cortex) of
sensory cortex. PC occupying the free face of the post-central gyrus looses the
excessive granulation and the six layers are more or less uniform size and
character. According to Von Bonin and Bailey,there is no distinct area 2, and
thus PB and PC will correspond to areas 3 and 1 of Brodmann. The areas OC,OB and
OA in the occipital lobe correspond to 17,18 and 19 of Brodmann. While OC is a
sharply defined area, OB merges in sensibly with OA. The temporal lobe which
does not contain motor cells is divided into TA,TE and TC.
Methods of localization of cerebral cortex
A rough and ready method is to divide them into anatomical, physiological,
pathological and clinical methods. The anatomical method may further be
subdivided into macroscopical and microscopical observations and the other
methods can be split up further.
The gyri and sulci and the fissures and lobes which are outstanding features on
the surfaces of the cerebral cortex provide very convenient landmarks for
labeling the cerebral cortex. Thus the pre central gyrus has been conferred with
the role of centre of voluntary motor activity and the occipital lobe with the
interpretation of visual sensation. The simplicity of this method limits its
usefulness as an exact scientific tool in the study of cortical function. The
microscopic method, on the other hand, provides the most elegant means of
outlining cortical activity and has been extensively used in this area of
investigation. Areas with large pyramidal cells (agranular cortex) would control
motor function, while konio-cortical areas (granular cortex) would subserve the
function of sensory appreciation. A group of cells may have more than one
function, and all functions need not be represented by differing cell
patterns.similarly, the fibre tracts gives an indication of the associative
function the connected areas.
MAJOR FISSURES AND SULCI OF THE BRAIN
There are six main fissures and sulci:-
the longitudinal cerebral fissure
the transverse cerebral fissure
lateral sulcus (fissure)
the central sulcus
the parieto occipital sulcus, and
the calcarine sulcus.
THE LONGITUDINAL CEREBRAL FISSURE (SAGITTAL FISSURE)
It partially seperates the cerebral hemispheres. In situ, this contains the falx
cerebri. The longitudinal cerebral fissure seperates the cerebral hemispheres in
the frontal and occipital regions, but between these parts of the brain, the
fissure extends only as far as the corpus callosum.
THE TRANSVERSE CEREBRAL FISSURE
It seperates the cerebral hemisphres superiorly from the cerebellum,
mid-brain and diencephalon inferiorly. The tentorium cerebelli lies in the
posterior part of this fissure.
THE LATERAL SULCUS
It begins inferiorly on the inferior surface of the cerebral hemispheres as
a deep furrow and extends posteriorly, seperating the frontal and temporal
lobes.posteriorly, the lateral sulcus seperates partly the parietal and frontal
lobes.
THE CENTRAL SULCUS
It is a prominent groove running infero-anteriorly from about the middle of
the superior margin of the cerebral hemispheres, stopping just short of the
lateral sulcus.
The central sulcus is an important landmark of the cerebral cortex because the
motor cortex (pre-central gyrus) lies anterior to it and the general sensory
cortex (post-central gyrus) lies posterior to it. The superior end of the
central sulcus is located about 1 ¼ cm posterior to the mid-point of a line
joining the inion and the nasion and its inferior end is about 5 cm superior to
the external acoustic meatus.
THE PARIETO-OCCIPITAL SULCUS
It seperates the parietal and occipital lobes of the brain on the medial
aspect of the brain. It extends from the calcarine sulcus to the superior border
and continues for a short distance on the supero-lateral surface.
THE CALCARINE SULCUS
It is on the medial surface of the brain. It commences near the occipital
pole and runs anteriorly, taking a curved course and joining the parieto-occipital
sulcus at an acute angle.
THE MAIN LOBES OF THE CEREBRAL HEMISPHERES.
There are four main lobes of the cerebral hemispheres.
THE FRONTAL LOBES
They are the largest of all the lobes of the brain. They form the anterior parts
of the cerebral hemispheres. The frontal lobes lie anterior to the central sulci
and superior to the lateral sulci. Their lateral and superior surfaces extend
posterior to the coronal sulcus.the basal surfaces of the frontal lobes rest on
the orbital parts of the frontal bone in the anterior cranial fossa. The orbital
gyri form impressions in the orbital plates of the frontal bone. The olfactory
bulbs rest on the cribriform plates.
THE PARIETAL LOBES
The parietal lobes are related to the internal aspects of the posterior and
superior parts of the parietal bones. Each parietal lobe is bounded anteriorly
by the superior part of the line joining the parieto-occipital sulcus and the
pre-occipital notch. The inferior boundary of each lobe is indicated by an
imaginary line extending from the posterior ramus of the lateral sulcus to the
inferior end of the posterior boundary.
THE TEMPORAL LOBES
The temporal lobes lie inferior to the lateral sulci. Their convex anterior
ends, called, temporal poles, fit into the anterior and lateral parts of the
middle cranial fossa. Their posterior parts lie against the middle one-third of
the inferior part of the parietal bone.
THE OCCIPITAL LOBES
These are small and are located posterior to the parieto-occipital sulci. They
rest on the tentorium cerebelli, superior to the posterior cranial fossa. They
contain the visual cortex.
FUNCTIONS OF CERTAIN SPECIFIC CORTICAL AREAS
Post-central gyrus – primary sensory areas
Pre-central gyrus – voluntary motor area (primary motor area)
These primary sensory and motor areas have highly specific functions, while
other areas of the cortex perform more general functions that we call
association or cerebration.
SPECIFIC FUNCTIONS OF THE PRIMARY SENSORY AREAS
The primary sensory areas all have certain functions in common. For
instance, somatic sensory areas, visual sensory areas and auditory sensory areas
all have spatial localizations of signals from the peripheral receptors.
The functional ability of the parietal cortex is recognising the following types
of functions.
1. tactile discrimination
2. tactile localisation
3. thermal perception of minute changes
4. weight discrimination (stereognosis)
5. vibratory sensation
6. taste sensation
All the sensory tracts after successive relays at different levels end in
different parts of the cerebral cortex. The somaesthetic sensations are relayed
to the post-central cortex.(areas 3,1,2) of the parietal lobe. The body is
represented in a reverse order in areas 3,1,2. The extent of representations
varies depending upon the type of function. The sensation from the tongue or
face has wider representation than that from the limbs.
Besides the above functions of the parietal cortex, it is observed that the
post-central gyrus is also involved in the modulation of afferent input.
Apart from areas 3,1 and 2 of the parietal cortex, additional sensory cortices
are also found in the motor cortex and in the ectosylvian gyrus.
SECONDARY SENSORY AREA
The secondary sensory area or sensory association areas extend 1 to 5 cms in
one or more directions from the primary sensory areas. Each time a primary
sensory area recieves a sensory signal, secondary signals spread, after a delay
of a few milli-seconds into the respective association areas as well. Part of
this spread occurs directly from the primary area through subcortical fiber
tracts, but a major part also occurs in the thalamus, beginning in the sensory
relay nuclei, passing next to corresponding thalamic association areas, and then
travelling to the association cortex.
The general function of the sensory association area is to provide a higher
level of interpretation of the sensory experiences.
Destruction of the sensory association areas greatly reduces the capability of
brain to analyze different characteristics of sensory experiences. For instance,
damage in the temporal lobe below and behind the primary auditory areas in the
dominant sphere of the brain often causes a person to lose his ability to
understand words or other auditory experiences even though he hears them.
Like wise, destruction of the visual association area in Broadmann’s area 18 &
19 of the occipital lobe or the presence of a brain tumour or other lesions in
these areas, does not cause blindness or prevent normal activation of the
primary visual cortex but greatly reduce the person’s ability to interpret what
is seen. Such a person often loses the ability to recognize the meanings of
words, a condition that is called “ word blindness” or dyslexia.
Destruction of the somatic
sensory association area in the parietal cortex posterior to primary somatic
area 1 causes the person to lose spatial perception for location of the
different parts of the body. In the case of the hand has been “lost”, the skills
of the hand are greatly reduced. Thus, this area of the cortex seems to be
necessary for interpretation of somatic sensory experiences.
INTERPRETATIVE FUNCTION OF THE POSTERIOR SUPERIOR TEMPORAL LOBE – the
general interpretative area (WERNICKE’S AREA)
The somatic, visual and auditory association areas, which can actually be called
interpretative area, all meet one another in the posterior part of the angular
gyrus where the temporal, parietal and occipital lobes all come together. This
area of confluence of the different sensory interpretative areas is especially
highly developed in the dominant side of the brain – the left side in the
right-handed persons. And, it plays the greatest single role of any part of the
cerebral cortex in the higher levels of brain function that we call cerebration.
Therefore, this region has frequently been called by different names suggestive
of the area having almost global importance. The knowing area, the tertiary
association area and so forth. The temporal portion of the general
interpretative area is called Wernicke’s area in honor of the neurologist who
first described its special significance in intellectual processes.
Following severe damage in the
general interpretative area, a person might hear perfectly well and even
recognize different words but still might be unable to arrang these words into a
coherent thought. Like wise, the person may be able to read words from the
printed page but be unable to recognize the thought that is conveyed. In
addition, the person has similar difficulties in understanding the higher levels
of meaning of somatic sensory experiences, even though there is no loss of
sensation itself.
Dominant hemisphere: The general interpretative functions of Wernick’s
area and of the angular gyrus and also the functions of the speech and motor
control areas are usually much more highly developed in one cerebral hemisphere
than in the other. This is called the dominant hemisphere. In at least 9 or 10
persons the left hemisphere is the dominant one. At birth, Wernicke’s area of
the brain is often as much as 50 percent larger in the left hemisphere than in
the right. Therefore, it is easy to understand why the left side of the brain
might become dominant over the right side. However, it for some reason the
dominant Wernicke’s area is removed in early childhood, the opposite side of the
brain can develop full dominant characteristics.
Wernicke’s area in the non-dominant hemisphere: when the Wernick’s area
in the dominant hemisphere is destroyed, the person normally loses almost all
intellectual functions associated with language or symbolism, such as ability to
read , ability to perform mathematical operations, and even the ability to think
through logical problems. How ever, other types of interpretative capabilities,
some of which undoubtedly utilize the temporal lobe and angular gyrus regions of
the opposite hemisphere are retained. Psychological studies in patients with
damage to their non-dominant hemispheres have suggest that this hemispheres may
be especially important for understanding and interpreting music, non-verbal
visual experiences, spatial relationships between the person and the
surroundings, and probably also interpreting many somatic experiences related to
use of the limbs and hands. Thus, even though we speak of the dominant
hemisphere, this dominance is primarily for language or symbolism- related
intellectual functions, the opposite hemisphere can actually be dominant for
some other types of intelligence.
An area for recognition of faces.
An interesting type of brain abnormality called prosophenosia is the inability
to recognize faces. This occurs in persons who have extensive damage on the
medial undersides of both occipital lobes and along the medioventral surfaces of
the temporal lobes. Loss of these face recognition areas, strangely enough,
results in very little other abnormality of brain function.
The occipital portion of this area is contiguous with the primary visual cortex
and the temporal portion is closely associated with the limbic system that has
to do with emotions, brain activation and control of one’s behavioral response
to the environment.
Prefrontal areas
The prefrontal areas is the most anterior part of the cerebral cortex and is a
highly developed portion of the cerebral outgrowth. It consists of areas 9 to
12,13 & 14,23 and 24,32 and 44 to 47 and was formerly called the “silent areas”
of the brain. This area has a control of some types of behaviour, especially
choice of behavioural options for each social or physical situations. One of the
outstanding characteristics of a person who has lost the prefrontal areas is the
ease with which he can be distracted from a sequence of thoughts. The person
without prefrontal areas ordinarily acts precipitously in response to incoming
sensory signals, such as reacting angrily to slightest provocations. Also he is
likely to lose many or most of his morals; he has little embarrassment in
relation to his excretory, sexual and social activities; and he is prone to
quickly changing moods of sweetness, hate, joy, sadness, exhilaration and rage.
In short he is a highly distractible person with lack of ability to pursue long
and complicated thoughts.
PHYSIOLOGICAL ANATOMY OF THE MOTOR AREAS OF THE CORTEX AND THEIR
PATHWAYS TO CORD
The motor cortex lies anterior to the central sulcus, is characterized by large
pyramid shaped cells. This area is usually referred to simply as area IV which
is the number of the area in the Broadmann classification of the cortical areas.
It is frequently also called the primary motor cortex or pyramidal area. The
anterior portion of the motor cortex is usually referred to as area VI because
it is mainly composed of area VI in Broadmann’s classification. It is also
frequently referred to as either the motor association area or the pre-motor
area.
The pyramidal area and the pyramidal tract (corticospinal tract)
This area in each hemisphere contains approximately 34,000 giant Betz cells
or giant pyramidal cells, for which reason it is called the pyramidal area. The
pyramidal tract passes downwards through the brainstem, then it decussates
mainly to the opposite side to form the pyramids of the medulla. By far the
majority of the pyramidal fibres then descend in the lateral corticospinal
tracts of the cord and terminate principally on interneurons at the bases of the
dorsal horns of the grey matter. A few fibres, however, do not cross to the
opposite side but pass ipsilaterally down the cord in the ventral corticospinal
tracts and then mainly decussates to the opposite side farther down the cord.
Collaterals from the pyramidal tracts in the brain.
Even before the pyramidal tract leaves the brain, many collaterals are given
off as follows:
1. the axons from the giant Betz cells send short collaterals back to the cortex
itself. It is believed that these collaterals mainly inhibit adjacent regions of
the cortex. When the Betz cells discharges, thereby, “sharpening” the boundaries
of the excitatory signals.
2. A large body of collateral fibres pass into the striate body and putamen.
From here additional fibres extend through several neurons down the brain stem
and then into the spinal cord through the extra pyramidal tracts.
3. A moderate number of collaterals pass from the pyramidal tract into the red
nuclei. From these additional pathways pass down the cord through the
rubrospinal tract.
4. A moderate number of collaterals also deviate from the pyramidal tract into
the reticular substance of the brain stem; from here signals go to the cord via
reticulospinal tracts and others go to the cerebellum via reticulocerebellar
tracts.
5. A tremendous number of collaterals synapse in the pontile nuclei, which give
rise to the pontocerebellar fibres. Thus, whenever signals are transmitted from
the motor cortex through the pyramidal tract, simultaneous signals are
transmitted into the cerebellar hemispheres.
6. Many collaterals also terminate in the inferior olivary nuclei and from here
secondary olivocerebellar fibres transmit signals to most areas of the
cerebellum.
Thus, the basal ganglia, brain stem and the cerebellum all receive strong
signals from the pyramidal tract every time a signal is also transmitted down
the spinal cord to cause a motor activity.
Importance of the area pyramidalis:
The giant pyramidal cells of the area pyramidalis are very excitable. A
short train of stimuli almost always elicits discrete movements of muscles on
the opposite side of the body. In almost no other region of the cortex can one
be certain of achieving a motor response with such weak stimuli.
Extra pyramidal tracts:
The extra pyramidal tracts are collectively, all the tract besides the
pyramidal tract itself that transmit motor signals from the cortex to the spinal
cord.
Primary motor cortex (area IV)
The different muscle groups of the body are not represented equally in the
motor cortex. In general, the degree of representation is proportional to the
discreteness of movement required of the respective part of the body. Thus the
thumb and fingers have large representations, as is true also of the lips,
tongue and the vocal cords. The topography of the cortical motor areas shows
that the body is represented in an inverted pattern. The parts of the body
represented in this cortex are as follows from above downwards. Toes, ankle,
knee, hip, abdomen, thorax, arm, hand, fingers 5, 4, 3, 2 and thumb, face and
tongue. Recent work has shown that the arm area intervenes between the face and
the trunk areas. While there is generalized invasion of the entire body, the
parts of the face are in the normal erect orientation.
Supplementary motor area
Areas on the medial surface rostral to the central sulcus is the supplimentary
motor area or area 2. The entire body is represented in this area also, the head
and upper extremity in the anterior part and close to the edge of the hemisphere
while the lower extremity is more posterior and nearer to the corpus callosum.
The threshold of stimulation of this area is higher than the primary area. It is
suggested by Smith et al that the supplementary motor area functions by its
relay through area 4.
Pre-motor area (area 6)
Rostral to pre-central gyrus lies an area, which when stimulated, produces
movement in the opposite arms and rotation of head and trunk to the opposite
side. This is designated area 6. These effects are seen even after the ablation
of the area 4. The control of muscular groups by area 6 is mediated through the
extra pyramidal tracts. It is generally accepted that area 6 is an integrated
center for adaptive motor activity.
Area 6 has normally an inhibitory control on some primitive reflexes. Voluntary
movements tends to become apraxic in man, ie, inability to execute purposeful
movements when area 6 is affected. If the area 6 along with area 4 on both sides
is removed, some more motor defects are seen, though posture and locomotor
activities are unaffected. Also, Hypertonicity in all the limbs. Some reflexes
like flexor reflexes are exaggarated, when areas 4 and 6 are removed together.
Suppressor areas
First described by Vogt and Vogt. They found that movements were inhibited on
stimulation of these areas. They are 2, 8,19 and 24.
Clinical signs of lesion’s of motor cortex
Lesions of motor cortex consequent to vascular accidents results in contra
lateral paralysis. Commonest cause of such a lesion is thrombosis or hemorrhage
of middle cerebral artery which supplies that portion of motor cortex. The
ensuing paralysis is called hemiplegia. This involves the face, the limbs and
the trunk muscles on one side of the body. The paralysis is typical of an upper
motor lesion. The predominant feature is spasticity of the muscles. Lesions at
the internal capsule, even though small, results in an extensive hemiplegia.
This is due to the fact that the fibers are concentrated in a compact bundle in
the internal capsule.
Hemiplegia is characterized by
loss of voluntary movement on one side. Facial muscles also show failure of
movements of movement on volition but muscles contract during emotional
expressions like laughter, anxiety or anger. Respiratory and other bilaterally
innervated muscles do not show any impairment of movement. The tone of the
muscles is excessive leading to a clasp knife rigidity. There is not much of
waisting, except a slight amount of atrophy due to prolonged disuse.
The deep reflexes are exaggerated with prolongation of relaxation. The typical
extensor plantar reflex (Barbinski’s sign) is seen. This reflex is a part of a
general withdrawal reflex and is seen in lesions of cortico spinal pathway. This
reflex can be elicited by stroking with a sharp object, the lateral aspect of
the sole of the foot, when there is dorsiflexion of the big toe and fanning of
the lateral toes. Such a reflex is seen in babies where it is attributed to the
yet unmyelinated pyramidal tract. Sometimes, an extensor pattern is observed in
deep sleep also. Patellar and ankle clonus may be present. Superficial reflexes
are also lost on the affected side.
Involvement of facial muscles
either on the same side or on the opposite side depends on the level of the
lesion. If the lesion is supranuclear (above the facial nucleus), there will be
hemiplegia with facial palsy on the opposite side of the lesion, ie, on the same
side of the paralysed limbs. If the lesion is at the level of the facial nerve
nucleus, there is hemiplegia on the opposite side with facial palsy on the same
side of the lesion. Aphasia is seen if the lesion is cortical and involves
Broca’s area (area 44)
Voluntary movement
For the execution of movement, cortex plays an important role. The motor cortex
is the upper motor key board which activates the lower motor key board (spinal
cord) for the necessary movement. In a voluntary act both pyramidal and
extrapyramidal systems are involved. The postural back ground for every
voluntary act is controlled by the extra pyramidal system.
Integrating the two hemispheres into a single mind.
Corpus callosum is not only a communicating link between the two
hemispheres, but it also co-ordinates behaviour. Though there are two
hemispheres in the brain, each is capable of neuronal processing. We act as
though we have only one mind and not two.
References
1. Gray’s Aanatomy
2. Grant’s method of Anatomy
3. Baily and Love’s – Short practice of surgery
4. Keeth. L .Moore – Clinical Anatomy
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