Mechanism of Conduction of Action Potential

MECHANISM OF CONDUCTION OF ACTION POTENTIAL

Depolarization occurs first at the site of stimulation in the nerve fiber. It causes depolarization of the neighboring areas. Like this, depolarization travels throughout the nerve fiber. Depolarization is followed by repolarization.

CONDUCTION THROUGH MYELINATED NERVE FIBER – SALTATORY CONDUCTION

Saltatory conduction is the form of conduction of nerve impulse in which, the impulse jumps from one node to another. Conduction of impulse through a myelinated nerve fiber is about 50 times faster than through a nonmyelinated fiber. It is because the action potential jumps from one node to another node of Ranvier instead of travelling through the entire nerve fiber.

Mechanism of Saltatory Conduction

Myelin sheath is not permeable to ions. So, the entry of sodium from extracellular fluid into nerve fiber occurs only in the node of Ranvier, where the myelin sheath is absent. It causes depolarization in the node and not in the internode. Thus, depolarization occurs at successive nodes. So, the action potential jumps from one node to another. Hence, it is called saltatory conduction (saltare = jumping).

REFRACTORY PERIOD

Refractory period is the period at which the nerve does not give any response to a stimulus.

TYPES OF REFRACTORY PERIOD

Refractory period is of two types:

1. Absolute Refractory Period

Absolute refractory period is the period during which the nerve does not show any response at all, whatever may be the strength of stimulus.

2. Relative Refractory Period

It is the period, during which the nerve fiber shows response, if the strength of stimulus is increased to

maximum. Absolute refractory period corresponds to the period from the time when firing level is reached till the time when one third of repolarization is completed. Relative refractory period extends through rest of the repolarization period.

SUMMATION

When one subliminal stimulus is applied, it does not produce any response in the nerve fiber because, the subliminal stimulus is very weak. However, if two or more subliminal stimuli are applied within a short interval of about 0.5 millisecond, the response is produced. It is because the subliminal stimuli are summed up together to become strong enough to produce the response.

This phenomenon is known as summation.

ADAPTATION

While stimulating a nerve fiber continuously, the excitability of the nerve fiber is greater in the beginning. Later the response decreases slowly and finally the nerve fiber does not show any response at all. This phenomenon is known as adaptation or accommodation.

Cause for Adaptation

When a nerve fiber is stimulated continuously, depolarization occurs continuously. Continuous depolarization inactivates the sodium pump and increases the efflux of potassium ions.

INFATIGABILITY

Nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time. The reason is that nerve fiber can conduct only one action potential at a time. At that time, it is completely refractory and does not conduct another action potential.

ALL-OR-NONE LAW

All-or-none law states that when a nerve is stimulated by a stimulus it gives maximum response or does not give response at all. Refer Chapter 90 for more details on all-or-none law