THE CEREBRUM (CEREBRAL HEMISPHERES ) :::
It consists of two symmetrical hemispheres which are separated by a deep furrow called the falx carebri and joined by a pulley like band known as corpus callosum The right side of the body is represented on the left hemisphere and vice versa Each hemisphere has five main lobes so divided by four main fissures ::
Lobes:(1) Frontal lobe (2) Parietal lobe (3) Temporal lobe (4) Occipital lobe and (5) Limbic area
fissures : (1) Central sulcus (2) Parieto-occipital sulcus (3) callosomarginal fissure and (4) Sylvian fissure
The right and left cerebral hemispheres are identical with slight additional individual function Grey matter of the cerebrum forms the covering surface layer called the cortex it is composed of nerve cells with related fibres white matter is situated underneath the gray matter inside the cerebrum there are various scattered nuclei They are made up of gray matter and are bilaterally symmetrical various functions of Cerebral cortex are as follows ::
1.Motor Functions :(a) General motor functions-volitional (b) Special motor functions-regulation of muscle tone equilibrium and control of posture through autonomic nervous system (c) Control over lower centres and centres for speech smell and control of eye movements etc.
2.Sensory Function : It helps in analysing sensation of touch tactile discrimination temperature pain pressure vibration and stereognostic sensation it also governs special senses like taste smell vision and hearing
3.It governs conditional reflexes
4.Cerebral hemisphere is the seat for various functions of intelligence e.g. memory planning judgement etc
Functionally the cerebral cortex is like the store house of a highly sophisticated compact computer wherein the information of the body is constantly being fed stored and at times the solution to the information of other parts is passed on subconsciously and consiciously as required The cortex has well defind sites wherein specific information is stored or passed on to different parts of the body these sites have been defined as area of the cortex Each area is surrounded by its psycho-area which analyses and interprets the various impressions-psychosensory pyschoauditory etc The central sulcus divides the cortex into two important gyri precentral gyrus and postcentral gyrus the precentral gyrus is the motor cortex while the postcentral gyrus is the sensory cortex
The sensory cortex is divided into areas representing the areas of the body surface (on the opposite side) The important sense appreciating areas such as the hand and the face have a much larger representation than the less sensitive areas such as the back The front of the body is more sensitive then the back front to fight and back for flight damage to the sensory cortex makes the subject unable to indentify the exact site of stimulation alththough he is still aware of the stimulus
The motor cortex like the sensory cortex is divided into areas representing the parts of the body whose movements it controls the hand especially the thumb and the facial muscles have a larger representation than other muscle group thus the areas of cortical localisation are represented as per complexity of movements and not the magnitude of the muscles concerned Stimulation of various areas causes complete co-oridnated movements with prime movers the synergists and antagonists working cordially without our consciousness Damage causes paralysis of movements that occur as a willful act although reflex and involuntary movement can still occur
The frontal lobe mainly consists of the speech centre where thoughts are formulated into words Since the right side of the body is represented in the left hemisphere it is a dominant hemisphere and contains the motor cortex of the right hand Right hemisphere is less dominant and probably contains equivalent areas dealing with visual rather than verbal information
The temporal lobe consists of the auditory cortex which receives the impulses from both ears Speech heard by the auditory cortex is decoded and understood in this area the temporal lobe is largely a sensory association area Impulses concerned with what is seen heard felt etc are co-ordinated and understood here local injury can cause strange behaviour associated with complex visual and auditory hallucinations
The parietal cortex is concerned with the higher appreciation of sensation such as the ability to assess the weight texture and identity of an object that is the stereognostic sensation
The occipital lobe consists mainly of the visual cortex it receives light impulses from the opposite visual field Damage to this part causes blindness
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