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THE BRAIN AND NERVOUS SYSTEM

The Nervous system is the governing agency of the body. It controls all muscular movements, whether voluntary or involuntary. It is responsible for all conscious, subconscious and unconscious thoughts and regulates many vital processes such as circulation, digestion and elimination. Constantly sensitive to changes on inside and outside of the body, the nervous system detects and different all kinds of stimuli. Depending on the nature of the stimuls, this system may react immediately, may delay response or may never react. For example, the pupil of the eye immediately narrows in a bright light. The nervous system is made up of two main divisions 1. The central nervous system composed of the brain and spinal cord and 2. the peripheral nervous system. The latter consists of twelve pairs of nerves arising from the brain., 31 pairs of nerves coming from the spinal cord and the nerves of the automatic nervous system which supply the internal organs and blood vessels.

The Brain 

The brain is the control station for nerve impulses. It is composed chiefly of nerve cells with their fibers interwoven in a complex relay system. In the average adult, it weight approximately 45 ounces. At birth it weight only eleven to thirteen ounces, but it increases in weight until about the 20th year. After this there is steady loss of weight for the reminder of the persons life. The brain is divided into four parts 1. the medulla oblongata, continuous above with the mid brain and below with the spinal cord 2. The mid brain, the part between the cerebellum and cerebellum 3. The cerebrum and 4. The cerebral hemispheres. The medulla oblongata is about three fourth of an inch to one inch in length. Externally it looks like an expand part of the spinal cord. Internally however, its structure is quite complex and consists of nerve tracts passing into the brain. From some of the nuclei come fibers that eventually emerge to form the 8th, 9th, 10th and 12th cranial nerves, cells centers in this area are also concerned with swallowing, vomiting, breathing, speech, digestion, metabolism, and the beating of the heart. In the medula oblongata, the large bundles of fibers, which originated in the two halves of cerebrum and which transmit the impulses of voluntary movements, cross to the opposite side. Thus movement in the right arm for example is controlled by the centres in the left half of the cerebrum. Lying above the medula oblongata and continuous with it is the pons. It is made up of massive bundles of fibers that start in the cerebrum and sweep backward to the cerebrum. This connection makes possible many skilled acts that require coordination of sight, hearing muscular movement and various other sensations. The playing of a musical instrument is an example of such as act. The pons contains a space called the fourth ventricle. In the floor of this ventricle is the nucleus of the vi cranial nerve. The pones is concerned with contraction of the pupil of the eye, blinking, and with some vocal reactions, including laughter.

The cerebrum the second largest part of the brain, is back of the pons. It lies in the back of the skull. It is made up of many narrow, leaf like folds arranged into two large masses, and of a middle portion. Rich in cells, it has many complex connections with the brain above it and with the spinal cord below. The chief functions of the cerebellum is to co-ordinate more or less complex movements into special acts. This may be movement in different parts of the same limb, combined action of the limbs or combined action of the head and limbs and body. For example picking up a pencil, writing with it, and laying it down again requires smooth interaction of many muscle groups. The cerebellum correlates the action of the various groups. To do this range , direction, rate and force of movement must be synchronised and maintained with the movements of the eye. Disease in the cerebellum does not cause paralysis. It does produce disturbance of muscular co-ordination tremors, staggering gait and excessive relaxation of the muscles result from disease in this part of the brain. The mid brain is a small area between the pons and the cerebrum. It is an important relay station for the sensory impulses. It also governs some muscle activity of a reflex nature. Many of the involuntary acts of the eye, such as narrowing of the pupil in bright light, originate here. The 3rd 4th and 5th craniel nerves originate from cell collections in this area of the brain. Just above the mid brain is an important group of nuclei. Here the two large nerves from the eyes meet and part of their fibers cross to the opposite sides. Other cells of this region are concerned with such vital functions as regulation of body temperature, metabolism, and heart rate, sexual development, sleep and body use of fat and water are all influenced by this region in the brain. The Thalamus, which is found next to this group of cells, contains another group of nuclei which integrate sensations of many sorts. Also it is the sight of crude form of consciousness and plays a role in the production of emotion. When this part of the brain is disease spontaneous laughter or crying may occur. The crude emotional response that arise are further elaborated and controlled by the cerebral cortex.

Above these nuclei are two cerebral hemispheres which represent 70 percent of the entirenervous system. This is the area of the nervous system in which all the sensory experiences are mixed and blended. Specific sensory impulses thus become associated with many others and expand the experience and consciousness. The individual capacity for many and varied activities memory emotion, and ideas is dependent on the action of this part of the nervous system. The surfaces of the hemispheres are marked by large, rounded folds and deep grooves. Party on the aixs of the main grooves and party or imaginary lines, the cerebrum is divided into fine lobes, frontal parietal, insula temporal and occipital. Each lobe has special functions but these functions are only partially understood. The occipital lobes at the back of the skull are the site where visual impressions are made. Colour size, form, movement and distance are evaluated in this portion of the brain, leading to the identification of a particular object. Also the difference between similar objects are discerned. For example, two object high in the air can be recognised as a bird and an airplane on the basic of past experience. Injury to this area may cause blindness. The temporal lobes receive the fibers concerned with hearing, speech balance and smell. Diseases related to these lobes cause loss of smell, or they may be responsible for imaginary smells.

The parietal lobes are concerned with taste sensations and some other sensations such as the ability to judge weight, shape and textures. By the action of this area, one is able to tell what various objects are by feeling rather than by seeing them. The forntal lobes are concerned with some of the most complex abilities of the mind. Reason, emotions and judgement have their site here. In addition there are a group of large cells in the posterior region of the frontal lobs which are involved with complicated voluntary movements. For instance, the speech center is located here. It is found to predominate on the left side in right-handed individuals and vice versa. However, if the speech function is lost because of injury to only one side of the brain. It can often be re-acquired by re-education. The area responsible for these complex voluntary movements is called the motor cortex. The muscles of the body are supplied by various areas of the motor cortex. Irritation of the cells in these zones will cause spasmas of the muscles they supply. Destruction of the cells will produce complete loss of voluntary movement of the muscles. Another function of the cells of the motor cortex is to keep the muscles in balance between relaxation and contraction. If this region of the brain is seriously damaged or destroyed this inhibiting power is lost.

The frontal lobes have many connections with the thalamus as well as with other lobes of the brain. In the frontal lobes feelings or emotions are added to the other associations. The combination of feeling and knowing determines most voluntary action of the body. A baby seeing a piece of candy for the first time, may or may not react for it. But the baby who has enjoyed candy tries hard to get a piece of it, when he sees it. Thinking reasoning, judgement and imagination result as the sensory and emotional associations become more complex. The way in which these higher functions are used is a quality of personality. Disease in the frontal lobes of the cerebrum cause personality changes, errors in judgement and insight and poor emotional control. Twelve pairs of nerves arise from the brain itself. The 1st is associated with the sense of smell, the 2nd with sight, the 3rd and 4th with eye muscles of the pupil, and the 5th carries sensations from the head and mouth and cause the muscles of the jaw to move. The 6th is concerned with the movement of the eye to the side. The 7th carries impulses to all the muscles of the face. The 8th conducts impulses having to do with hearing and balance, the 9th transmits taste sensations to the posterior third of the tongue and other sensations from the throat and muscular membranes. It also aids in swallowing. The 10th is an exceedingly long nerve that extends down the neck and into the chest and abdomen. It is concerned with swallowing and talking, its action also slows the rate of the heart beat and regulates the movement of the stomach. It is a large part of the parasympathetic system in the upper part of the body, including the esophagus, stomach, intestines, liver, bronch lungs heart and blood vessels. The 11th nerve is responsible for the movement of the tongue.

The spinal cord

The nerve tracts passing to and from the brain are contained in the spinal cord, which is continuous with the lower part of the brain. It is about 18 inches long and is rounded in shape. It is large in the regions which rise to the nerves the arms and legs, since these part have many complex functions thus acquiring a large nerve supply. From the neck to the lowest part of the vertebral column, 31 pairs of nerves emerge from the spinal cord. Each nerve is attached to the cord by two roots. Because spinal cord is not as long as the vertebral column, the roots of the nerves must gradually increase in length before they car emerge from between the vertebrae. These longer nerve roots collect in amass that fills the lower end of the vertebral canal. A cross section of the spinal cord reveals a gray figure, roughly shaped like an H imposed on a white background. The nerve cells make up the grey matter, while the nerve bundles form white matter. Bundles with specific functions occupy specific areas or the spinal cord. Therefore, injury to the cord will result in certain abnormal reactions which will be evident on neurological examination.

The abnormal findings will suggest where the diseased part is located. Impulses which arise in the brain and are concerned with voluntary muscular movements are received by specific cells in the spinal cord. They relay these impulses to the nerves which control the various muscles. These cells have connections with other cells in the nervous system that act together to bring about reflex activity. Reflexes control the position of the head so that it automatically assumes the normal position with drawal on flexion reflexes pull limbs away from painful or disgreeable stimuli. Exterior reflexes straighten out the limbs and work with the flexor reflexes. Bladder and the bowel actions result from reflex actions over which there is some voluntary control. In injury to the spinal cord, the tracts allowing voluntary control may be interrupted so that the action is then reflex in origin. The cells on the front and anterior side of the spinal cord connect with cells in the cerebellum to control the direction and precision of normal muscular movement for example the swinging of the arms as one walks. Another function of these cells is the maintenance of the proper amount of constant contraction or tone of the muscles. It the muscles are too contracted, they move too slowly and rigidly. If they are too relaxed, too much stimulation is needed to make them respond. The cells along the side of the cord send out fibers that unite with others to form the sympathetic chain. These cells are concerned with the action of involuntary muscles in the intestines, arteries and other internal structures. Various glands receive fibers from these cells. Cells on the back or posterior side of the spinal cord receive the sensations of touch, pain violation, temperature, pressure and position. They them transmit these various sensations to other cells in the brain. From this it can be seen that impulses of many sorts travel down the paths in the spinal cord while others enter it and travel upward to the brain. Still others enter and travel only part of the way up and set off the spinal reflexes.

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