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Human Body
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Thirty-one pairs of spinal nerves emerge from the spinal cord at regular intervals. They connect the whole body below the neck, i.e. the trunk and the limbs, to the brain through the spinal cord. The head and the neck are directly connected to the brain by twelve pairs of cranial nerves. Each pair is composed of (i) sensory pathways which carry the message of sensation to the brain, and (ii) motor pathways which carry the orders of movement from the brain to the muscles in the trunk and the limbs.
The other function of the spinal cord is to provide ‘reflex centres' for immediate response to certain incoming stimuli. Actions performed automaticlly in response to a stimulus without conscious decision or thought are called Teflex actions. Many such actions are handled by the spinal cord without involvement of the brain. 2. Peripheral Nervous System
The peripheral nerves penetrate every part of the body to provide the links between the brain and the outside world. They branch out from the brain and spinal cord, and run as slender threads through the head, trunk and limbs. Depending on whether they emerge from the brain or spinal cord, they are classified as "cranial nerves” and "spinal nerves". A functional distinction is between sensory nerves which carry messages to C.N.S., and motor nerves which carry impulses away from the C.N.S. There also are some mixed nerves.
The cranial nerves are a heterogeneous group with little in common other than their origin in the brain. The cranial nerves are arranged symmetrically and are usually described as 12 pairs.? Actually, however, the olfactory nerves are not really a "pair", but comprising about 15 or 20 on each side. Some of them are, bowever, exclusively sensory, others exclusively motor and still others mixed. The cranial nerves supply mainly the head and the neck. The extensively 1. Sensory nerves are also called 'afferent', while the motor nerves aro
called 'efferent'. 2. The twelve cranial nerves are : I. olfactory, II. optic, III. oculomotor,
IV. trochlear, V. trigeminal, VI. abducens, VII. facial, VIII, acoustic, · IX. glossopharyngeal, X. vagus, XI, spinal accessory and XII. hypo
glossal nerves.
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