The human brain

The human brain

The appearance of Dulci and gyro increases the surface area for the neurons many times, without increasing the size of the brain. There are specific areas on the brain for specific functions. Thalamus integrates sensory, motor and visceral activities. Hypothalamus controls various visceral and vasomotor activities. It maintains a biological clock for our body. Parkinson ism occurs due to lesion of corpus stratum. There is reduction in speed or bradykinesia,i.e. slow movement or akinesia, i.e. no movement with muscle rigidity. There is free flow of information in the central nervous system, between two hemispheres through the commissariat fibers;
between various parts of one hemisphere through the association fibers and between upper and lower parts through the projection fibers. Internal capsule contains lots of fibers packed in its ''limbs''. It is supplied by the ''end'' of the human being concerned, if not treated properly.The cerebrum is made of two cerebral hemispheres which are incompletely separated from each other by the median plane by the corpus callous. Each hemisphere contains a cavity, called the lateral ventricle.

Dissection

Keep the cerebrum so that the longitudinal fissure faces superiorly. Identify the convex strong band of white matter, the corpus callosum binding parts of the medial surfaces of the two cerebral hemisphers. Define splenium as the thick rounded part of corpus callosum. Divide the corpus callosum in the median plane starting from the splenium towards the trunk, genu and rostrum. Inferior to the trunk of corpus callosum extend the incision into the tela choroidea of the lateral and third ventricles, and the interthalamic adhesion connecting the medial surfaces of two thalami.Identify the thin septum pellucidum connecting the inferior surfaces of corpus callosum to a curved band of white matter-anterior column of the fornix. Look for the anterior commissure just at the anterior end of the anterior column of fornix. Turn the brain upside down and identify optic chiasma. Divide the optic chiasma, anterior communicating artery, infundibulum and a thin groove between the adjacent mamillary bodies, posterior cerebral artery close to its origin. Carry the line of division around the midbrain to join the two ends of the median cut. Separate the right and left cerebral hemispheres. In the two hemisphere, identify the three surfaces, four borders, three poles. Identify the central sulcus, posterior ramus of lateral sulcus, parieto-occipital sulcus and preoccipital notch and demarcate the four lobes of the superolateral surface of each cerebral hemisphere. Strip the meninges from the surfaces. Identify the vessels on the surfaces of hemisphere surface, medial surface and inferior of hemisphere. Make thin slice through the part of the calcarine sulcus, posterior to its junction with the parieto-occipital sulcus. Identify the stria running through it. On cutting series of thin slices try to trace the extent of visual stria. 

Motor areas 

Primary motor area : It is located in the precentral gyrus, including the anterior wall of central sulcus,and in the anterior part of paracentral lobule on the medial surface of cerebral hemispheres. This correspond to area 4 of brodmann. Electrical stimulation of primary motor area elicits contraction of muscles that are mainly on the opposite side of body. Although cortical control of musculature is mainly contralateral, there is significant ipsilateral control of most of the muscles of the head and axial muscles of the body. The contralateral half of the body is represented as upside down, except the face. The pharyngeal region, tongue are represented in
the most ventral and lower part of percentral gyrus, followed  by the face, hand, arm, trunk and thigh. The remainder of leg, foot and perineum is on the medial surface of hemisphere in the paracentral lobule. Another significant feature in this area is that the size of the cortical area for a particular part of the body is determined by the functional importance of the part and its need for sensitivity and and intricacy of the movements of that region. The area for the face, especially the larynx and lips, is therefore disproportionately large and a large area is assigned to the hand particularly the thumb and index finger. Movements of joints are represented rather than individual muscles.

Premotor area :This area coincides with the brodmann's area 6 and is situated anterior to motor area in the superolateral and medial surfaces of the hemisphere. The premotor area contributes to motor function by its direct contribution to the pyramidal and other descending motor pathways and by its influence on the primary motor cortex.In general,the primary motor area is the cortex in which execution of movements originates and relatively simple movements are maintained. In contrast, the premotor area programmes skilled motor activity and thus directs the primary motor area in its execution. The premotor and primary motor areas are together
referred to as the primary somato-motor area. Both these areas give origin to corticospinal and cerebellum after relay in ventral intermediate nucleus of thalamus.

Sensory areas               

First somesthetic : The representation of the body in this area corresponds to that in the motor area in that contralateral half of the body is represented upside down except the face. The area of the cortex that receives sensations from a particular part of the body is not proportional to the size of that part, but rather to the intricacy of sensations received from it. Thus, the thumb, fingers, lips and tongue have a disproportionately large representation. The different sensations,i.e. cutaneous and proprioceptive are represented in different parts withinsensory area. The ventral posterior nucleus of thalamus is the main source of afferent fibres for the sensory area. This thalamic nucleus is the site of termination of all the fibres of the medial lemniscus. Most of the fibres of the spinothalamic and trogeminothalamic tracts carrying fibres for cutaneous sensibility end in anterior part of area and those for deep sensibility end in the posterior part. Somesthetic cortex is mainly in the superior parietal lobute on the precuneus on the medial surface. It coincides with areas 5 and 7 of brodmann. This receives afferents from first sensory area and has reciprocel connection with dorsal tier of nuclei of lateral mass of thalamus. Data pertaining to the general sense are integrated, permitting a comprehensive assessment of the characteristic of an object held in hand and its identification without visual aid.

Areas of special sensations
Vision : The visual area is located above and below the calcarine sulcus on the medial surface of occipital lobe. It corresponds to area 17 of brodmann. The visual area is also called the striate area because the cortex here contains the line of gennari, which is just visible to the unaided eye. The role of the second and third visual areas includes among other complex aspects of vission, the relating of present to past visual experience, with recognition of what is seen and appreciation of its significance. The three areas are linked together by association fibres. The visual areas give efferent fibres which reach frontal eye field.

Hearing :The auditory area (acoustic area) lies in the temporal lobe. Most of it is concealed as it lies in that part of superior temporal gyrus which forms inferior wall of the posterior ramus of lateral sulcus. It corresponds with areas 41 and 42 of brodmann. The medial geniculate body of the thalamus is the principal source of fibres ending in the auditory cortex with these fibres constituting the auditory radiation. The auditory radiation does not only end in the first auditory area but extends to neghbouring area as well, that is known as auditory association area or second auditory area. This area lies behind the first auditory area in superior temporal gyrus. It corresponds to area 22 of brodmann on the lateral surface of superior temporal gyrus. This region of the cortex is also known as wernicke's area and is of major importance in language functions. 

Corpus callous                
The corpus callosum is the largest commissure of the brain. It connects the two cerebral hemispheres. Since it is the neopallial commissure, it attains enormous size in man (10 cm long). The corpus callosum connects all parts of the cerebral cortex of the two sides, except the lower and anterior parts of the temporal lobes which are connected by the anterior commissure.The rostrum is directed downwards and backwards from the genu, and ends by joining the lamina terminalis, in front of the anterior commissure. It is related superiorly to the anterior horn of the lateral ventricle,and the longitudinal striae. The trunk or body is the middle part, between the genu and the splenium. It is overlapped by the gyrus cinguli and is covered by the indusium griseum and the longitu- dinal striae. The inferior surface is concave from before backwards and convex from side to side. It provides attachment to the septum pellucidum and the fornix, and forms the roof central part of the lateral ventricle. The splenium is the posterior end forming the thickest part of the occipital pole. Its lies 6 cm in front of the occipital pole. The superior surface is related to the inferior sagittal sinus and the falx cerebri. Posteriorly, it is related to the great cerebral vein, the straight sinus and the free margin on the tentorium
cerebelli.

Internal capsule
Gross anatomy : The internal capsule contains fibres going to and coming from the cerebral cortex. It can be compared to a narrow gate where the fibres are densely crowded. Small lesions of the capsule can give rise to widespread derangements of the body. When traced upwards, the fibres of the capsule diverge and are continuous with the corona radiata. When traced downwards, its fibres converge and many of them are continues with the crus cerebri of the midbrain.