Question

Short answer question

List the properties of the nerve fibres.

Answer 1

i. Excitability/Irritability: Nerve fibres have polarized membranes, thus they have the ability to perceive stimulus and enter into a state of activity.

ii. Conductivity: It is the ability of the nerve to transmit impulses along the whole length of the axon.

iii. Stimulus: It is any detectable, physical, chemical, electrical change in the external or internal environment which brings about excitation in a nerve/muscle/organ/organism. A stimulus must have a minimum intensity called threshold stimulus, in order to be effective. The subliminal (weak) stimulus will have no effect while the supraliminal (strong) stimulus will produce the same degree of impulse as the threshold stimulus.

iv. Summation effect: A single subliminal stimulus will have no effect but when many such weak stimuli are given again and again they may produce an impulse due to summation of effects.

v. All or none law: The nerve will either conduct the impulse along its entire length or will not conduct the impulse at all. This occurs in the case of a subliminal or weak stimulus.

vi. Refractory period: It is the time interval (about a millisecond) during which a nerve fails to respond to a second stimulus even if it is strong.

vii. Synaptic delay: The impulse takes about 0.3 to 0.5 milliseconds to cross a synapse. It is required for the release of neurotransmitters from the axon terminal and excitation in the dendron of the next neuron.

viii. Synaptic fatigue: The transmission of nerve impulses across the synapse stops temporarily due to the depletion of the neurotransmitter.

ix. Velocity: The rate of transmission of impulse is higher in long and thick nerves. It is higher in homeotherms than in poikilotherms. The velocity of transmission is higher in voluntary fibres (100 - 120 m/s in man) as compared to autonomic or involuntary nerves (10-20 m/s). In medullated nerve fibre, the velocity of transmission is higher as an impulse has to jump from one node of Ranvier to the next.