Classification of nerve fibres

This slideshow classify the nerve fibers of human ervous system

1. CLASSIFICATION OF NERVE FIBRES FATIMA WAHID MANGRIO

2. BASIS OF CLASSIFICATION Nerve fibers are classified by six different methods. The basis of classification differs in each method . Classification of nerve fibers 1. Depending upon structure 2. Depending upon distribution 3. Depending upon origin 4. Depending upon function 5. Depending upon secretion of neurotransmitter 6. Depending upon diameter and conduction of impulse ( Erlanger Gasser classification )

3. DEPENDING UPON STRUCTURE Myelinated Nerve Fibers Myelinated nerve fibers are the nerve fibers that are covered by myelin sheath . ii. Non-myelinated Nerve Fibers Non myelinated nerve fibers are the nerve fibers which are not covered by myelin sheath.

4. DEPENDING UPON DISTRIBUTION Nerve fibers are classified into two types, on the basis of distribution : i . Somatic Nerve Fibers Somatic nerve fibers supply the skeletal muscles of the body. ii. Visceral or Autonomic Nerve Fibers Autonomic nerve fibers supply the various internal organs of the body.

5. DEPENDING UPON ORIGIN On the basis of origin, nerve fibers are divided into two types: i . Cranial Nerve Fibers Nerve fibers arising from brain are called cranial nerve fibers. ii. Spinal Nerve Fibers Nerve fibers arising from spinal cord are called spinal nerve fibers.

6. DEPENDING UPON FUNCTION Functionally, nerve fibers are classified into two types : i . Sensory Nerve Fibers Sensory nerve fibers carry sensory impulses from different parts of the body to the central nervous system. These nerve fibers are also known as afferent nerve fibers . ii. Motor Nerve Fibers Motor nerve fibers carry motor impulses from central nervous system to different parts of the body. These nerve fibers are also called efferent nerve fibers.

7. DEPENDING UPON SECRETION OF NEUROTRANSMITTER Depending upon the neurotransmitter substance secreted , nerve fibers are divided into two types: i . Adrenergic Nerve Fibers Adrenergic nerve fibers secrete noradrenaline . An adrenergic nerve fiber  is a  neuron for which the   neurotransmitter is either  adrenaline (epinephrine ), noradrenaline or dopamine . Adrenergic nerve terminals are found in the   sympathetic nervous system , This system increases heart rate, slows digestion, dilates pupils.

8. ii. Cholinergic Nerve Fibers Cholinergic nerve fibers secrete acetylcholine . the primary transmitter of nerve impulses within the parasympathetic nervous system—i.e., that part of the autonomic nervous system that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows the heart rate.

9. DEPENDING UPON DIAMETER AND CONDUCTION OF IMPULSE Erlanger and Gasser classified the nerve fibers into three major types, on the basis of amount of myelination, their diameters, and their propagation. i . Type A nerve fibers ii. Type B nerve fibers iii. Type C nerve fibers .

10. Among these fibers, type A nerve fibers are the thickest fibers and type C nerve fibers are the thinnest fibers. Type C fibers are also known as Type IV fibers. Except type C fibers, all the nerve fibers are myelinated . Type A nerve fibers are divided into four types: a. Type A alpha or Type I nerve fibers b. Type A beta or Type II nerve fibers c. Type A gamma nerve fibers d. Type A delta or Type III nerve fibers.

11. A FIBERS A fibers are the largest diameter axons (5–20 m) and are myelinated. The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are A fibers , as are the axons of motor neurons that conduct impulses to skeletal muscles . A fibers have a brief absolute refractory period and conduct nerve impulses (action potentials) at speeds of 12 to 130 m/sec (27–290 mi/ hr ).

12. B FIBERS B fibers are axons with diameter of (2–3 m). Like A fibers, B fibers are myelinated and exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/ hr ). B fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.

13. C FIBERS C fibers are the smallest diameter axons (0.5–1.5 m) and all are unmyelinated. Nerve impulse propagation along a C fiber ranges from 0.5 to 2 m/sec (1–4 mi/ hr ). C fibers exhibit the longest periods. These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin, and pain impulses from the viscera.

14. Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are C fibers . Examples of motor functions of B and C fibers are constricting and dilating the pupils, increasing and decreasing the heart rate , and contracting and relaxing the urinary bladder . The C fibers constitute more than one half of the sensory fibers in most peripheral nerves, as well as all the postganglionic autonomic fibers.