Transmission of nerve impulses

At excited stage

If a stimulus is applied at one end it will give rise to an excitation. This leads to variation of a membrane potential accompanied by a change in the permeability of the membrane. This sudden reversal of the resting potential is called the action potential and is due to the sudden depolarization of the nerve membrane. The excited potion of the fiber becomes electrically negative in relation to the adjacent area at rest. The inside of the membrane becomes positively charged. The increase in permeability to Na+ is only momentary. It lasts only for a fraction of 1 ms after which resting potential is restored.

At the end of the region of depolarization the next part of the axon is excited due to a small local current formed as a result of the action potential. Thus depolarization of the nest segment of the axoplasm takes place. There is therefore a series of local currents propagated along the axon.

At recovery stage

In the meantime the depolarized area begins to recover because the Na+ begins to diffuse out due to the activity of the Na+/K+ pump. Thus the impulse passes along the axon in the form of a wave. The whole lasts only for about 1-2 ms. Once the resting potential is restored another impulse can be transmitted.

The wave of excitation causes two phases in the action potential and is said to be biphasic. During the period between repolarisation and depolarization nerve impulses cannot pass. This is known as the absolute refractory period. This is followed by the relative refractory period lasting for about 4 to 8 ms during which time the nerve reverts to its original excitability.

The lowest strength of stimulus required to produce an action potential is known as the threshold of stimulus. Threshold is a critical level, that is, the point at which a stimulus becomes receptable or is at significant intensity to produce an effect. Stimulations below the threshold level fail to elicit an action potential. Stimulations whose intensity is above the threshold level do not alter the magnitude of the action potential. It remains the same, that is, the wave of excitation travels at a maximum speed. This characteristic of the nerve impulse is known as the all or non law. This all or none law character of the action potential allow their transmission for long distance without falling out.

The movement of ions during the generation of a nerve impulse

. Na+ ion gated channels are closed during the resting potential. At the time of excitation the Na+ gated channels open allowing Na+ to enter at that point causing depolarization of a nerve cell. This causes the potential of the neurone to rise from -70 to -30 mV. This change in resting potential opens more Na+ gated channels allowing excess Na+ to rush into the nerve cell.

2. The excess Na+ in the cytoplasm makes the potential difference to reach up to +40 mV. The Na+ gated channels open for only about 5.5 ms before shutting down.

3. These gated channels close and become inactive. This allows the K+ gated channels to open and K+ moves outside. This outward flow of K+ leads to the rapid fall of potential towards the resting potential level.

4. During the refractive period (0.5 to 2 ms) the membrane is unable to react to addition stimulus. This has two phases:

a) Absolute refractive period

The charge is briefly reduced below the resting potential of -70mV due to excess loss of K+

b) Relative refractive period

During this short period the inactivated Na+ gated channels return to their original closed but active state and the K+ gated channels close, preparing the system to work again.

The biological significance of the refractive period is to limit the number of action potential that can be generated per second.