Peripheral Nervous System

PERIPHERAL NERVOUS SYSTEM PERIPHERAL NERVOUS SYSTEM •

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It includes nerves arising from brain and spinal cord innervating different parts of the body respectively called as, cranial and spinal nerves. Comprises of all nerves which connect the central nervous system with receptors and effectors. Nerves connecting the receptor organs to the CNS are called sensory or afferent nerves; they contain sensory fibres only. Nerves connecting the effector organs (muscles and glands) to CNS are motor or efferent nerves; they contain motor fibres only. A third type of nerve, the mixed nerves have both sensory and motor nerve fibres. Nerves connected to brain are called cranial nerves while those connected to spinal cord are spinal nerves. On the basis of presence and absence of myelin sheath around the axons the nerve fibres are myelinated or nonmylinated, respectively. Each nerve is composed of many nerve fibers enclosed within a connective tissue sheath. A nerve fiber is a long axon or dendrite of a neuron, which could be either sensory or motor or both.

CRANIAL NERVES • • • •

These are 12 pairs in humans. The name, origin and nature of these nerves are as follows: Trigeminal and abducens nerves emerge through Gasserian ganglion located within the cranium. Motor component of vagus nerve controls sound production;

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Trigeminal nerve (V) is the largest. Vagus (X) nerve regulates the functions of organs in thoracic and abdominal cavities.

SPINAL NERVES • • •

These nerves originate from the spinal cord and innervate the nearby parts of the body. The numbers generally relate to the number of vertebrae All spinal nerves are essentially of mixed nature and are generally medullated named as per their site of emergence these are classified in mammals as (i) Cervical nerves (neck part) = 8 pairs (ii) Thoracic nerves (thorax part) = 12 pairs (iii) Lumber nerves (abdominal part) = 5 pairs (iv) Sacral nerves (waist or hip part) = 5 pairs (v) Coccygeal nerves (tail region) = 1 pair

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From the spinal cord arises two roots, a dorsal (sensory or afferent) root and a ventral (motor or efferent) root. The dorsal root consists of somatic sensory and visceral sensory fibres; the cell bodies of these fibres are situated outside the spinal cord in a dorsal root ganglion. Both dorsal and ventral roots unite together outside the spinal cord and immediately divide into following three branches: (i) Ramus dorsalis It consists of somatic sensory fibres and innervates the skin, muscles of the dorsal body wall. (ii) Ramus ventralis It is the thickest branch and contains somatic motor and sensory fibres innervating skeletal muscle, bones etc., of rest body parts. (iii) Ramus communicans This branches with visceral component joins the sympathetic ganglion of autonomic nervous system. In mammals it has two branches, gray ramus communicans and white ramus communicans.

REFLEX ACTION •

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It is spontaneous, involuntary and immediate response generated at the non-conscious level stimulated through specific receptors. The sensory impulse is directly and automatically converted into a motor effect through nearest part of CNS operating in a manner of reflex arc. E.g., touching the lower limb of a decapitated frog with acid swab causes withdrawal of limb, center of this reflex is the spinal cord hence if it is damaged the reflex disappears e.g. Knee jerk reflex, sudden blinking of eye. Reflex or the pathway of impulse from receptor to effecter as follows; Stimulus → receptor → sensory neuron → interneuron motor neuron → effecter. The direct contact between sensory and motor neuron is made within the CNS.

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TYPES OF RESPONSE (i) Unconditioned reflexes are the inborn (or innate) reflexes independent of previous learning or experience, generated against related stimulus. • Such responses are fixed for a particular stimulus. Examples: Taste of milk causes salivation in a newborn though it has not tasted it before; salivation in dogs by sight or smell of meat; constriction of pupil in newly born baby if illuminated with bright light. (ii) Conditioned reflexes are based upon previous learning or experience; hence, also called as learned or acquired reflexes, the response is unrelated to the type of stimulus. It is found in higher animals. • Salivation in dog caused by the sound of bell (I.P. Pavlov, 1934). • The response of an individual in a society depends upon situation as a whole, on the conditioning in the society.

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