Properties Of Nerve Fibers

NERVE FIBRES

MYELINATED AND UNMYELINATED NERVE FIBRES: MYELINATED NERVES 1. Multiple layers of myelin 2. Faster conduction of nerve impulse, saltatory conduction 3. E.g. All preganglionic fibers of ANS. Nerve fibers in somatic nervous system > 1 microns in diameter.

UNMYELINATED NERVES 1. No myelin covering present. 2. Slower conduction of nerve impulse. E.g. All post ganglionic fibers of ANS. Nerve fibers in somatic nervous system < 1 microns in diameter..

PROPERTIES OF NERVE FIBRES: 1. 2. 3. 4. 5.

EXCITABILITY. ALL OR NONE LAW. REFRACTORY PERIOD ACCOMODATION CONDUCTIVITY

1. EXCITABILITY: It is the property of the cells to respond to stimuli from the external or internal environment. Depends upon the: 1 Strength and duration of the stimulus. 2 Extracellular CA+ ion concentrations. • Decrease in ECF Ca + increases the excitability of nerve cells by decreasing the amount depolarization required to initiate changes that produce the action potential. • Increase in external ca+ concentration increases the stability of the membrane by decreasing its excitability.

2. ALL OR NONE LAW If the stimulus is at or above the threshold intensity, the action potential occurs with increasing amplitude and forms regardless of the strength of stimulus. But the action potential will not develop if the stimulus is lower than the threshold. 3. REFRACTORY PERIOD: The length of time a muscle remains refractory to second stimulus applied successively to a stimulus of more than threshold intensity. 4. ACCOMMODATION If a nerve is submitted to passage of constant strength of current, the site of nerve under stimulation shows decrease of excitability. Thus accommodation is rise in the threshold of the tissue during stimulation. Conductivity Conductance of nerve impulse means, spread of wave of depolarization. • Orthodromic conduction is transmission of impulses from synaptic junctions or receptors along the axons to their ends. • Antidromic conduction is conduction in opposite direction. Seen in sensory nerves supplying the blood vessels.

Types and functions of nerve fibres IMPORTANT POIINTS • Greater diameter of a nerve fiber, greater is the speed of the conduction. • The speed of conduction in myelinated fiber is 6 – 120 metre / sec • The speed of conduction in unmyelinated fiber is 1 metre / sec • Larger axons play role in proprioception, touch and pressure sensations and somatic motor functions. • Smaller axons play role in pain, and temperature sensation and autonomic functions. CLASSIFICATION

Proposed by Erlanger and Gasser

FIBRE TYPE

functions

Proprioception, somatic motor

Fibre diameter micrometer

12-20

Conduction velocity m/sec

Spi ke dura tion (mil li seco nd)

Absolute refractory period(millisecon d)

70-120

0.4- 1 0.4 – 0.5

Touch , 5-12 pressure and motor functions

30-70

A gamma

Motor to 3-6 muscle spindle

15-30

A delta

Pain, temperature ,touch

2-5

12-30

B

Preganglionic autonomic fibres

<3

3-15

1.2

1.2

0.5-2

2

2

0.7-2.3

2

2

C Dorsal root

Pain,touch, 0.4 – 1.2 temperature and impulse generated by cutaneous receptors Sympathetic Postganglionc 0.3-1.3 sympathetic nerve fibres

NOTE 1. B fibers are most susceptible to hypoxia. 2. Type A fibers are most susceptible to pressure. 3. Type C fibers are most susceptible to local anesthetic. Properties of nerves 1. Subthreshold stimuli- the stimuli by which none of axon are stimulated and no response occurs. 2. Threshold stimuli – the stimuli by which the axons with low threshold fibers are stimulated and a small potential change recorded. 3. Maximal stimulus- the stimulus that is string enough to excite all the axons in a nerve is called maximal stimulus 4. When an axon is stimulated and conducted impulse occurs, the series of potential changes and is known as action potential. 5. In mixed nerve, multiple peaks occurs ion action potential. This multi peaked action potential is called compound action potential.