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Date posted: November 27, 2011

Dr Bindu K   BHMS,MD(Hom)
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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