The Important Condition Affecting the Passage of a Nerve Impulse – Explained!

1. Repeated stimulation does not lessen the excitability and conductivity of a nerve fibre or nerve but if the interval between two stimuli is shorter than the relative refractory period, the second and subsequent stimuli produce progressively smaller impulses than the first.

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2. Temperature also plays an important role in the passage of a nerve impulse. Below a certain temperature the propagation of the excitatory process in the nerve does not take place.

3. pH value also regulates the conduction and propagation of nerve impulses. Actually pH value does not affect the nerve impulses but alters the excitability and after potentials.

Passes of nerve impulses through the nerve fiber:

The nerve fibre or axon, as you have already studied in the beginning of the chapter, is like a cylinder, the interior of which is filled with axoplasm (i.e., the cytoplasm of the nerve cell) and the exterior of which is covered with a thin membrane the axon membrane.

The axon membrane is made up of lipo-proteins and is about 100A in thickness. It has some minute pores or channels of 7 to 10A in diameter through which water and other substances corresponding to the pores, can pass in and out of the axon memb­rane, this states that axon membrane is semi permeable in nature.

The axon membrane exhibits a bioelectrical potential or membrane potential which according to Bernstein’s membrane-ion theory modi­fied by Hodgkin and Huxleys (1952), is caused by unequal concen­trations of various ions in and outside the cell due to the semiperme­able nature of axon membrane.

Membrane permeability to different ions varies-and changes regularly depending upon the physiological state of the nerve fibres.

The membrane potential is normally about 85 mV. Due to semipermeable nature of the axon membrane and con­centration gradient certain ions are transported from the axoplasm (i.e., the protoplasm of the axon) into the interstitial fluid (i.e., the fluid surrounding the axon) and certain other ions from the interstitial fluid into the axoplasm.

On account of this, the concentration of ions in the inner axoplasm differs from that of outer interstitial fluid.

In the resting state of the nerve, i.e., when the nerve is not being sti­mulated, the sodium ions are being actively transported from inside to the outside; with the result the concentration of sodium ions inside the nerve fibre (i.e., the axoplasm) is made almost negligible and that of potassium ions is more.

Normally the concentration of sodium ions in the interstitial fluid is about 100 times more than that in the inner axoplasm.

This is simply due to the movements of sodium ions from the axoplasm into the interstitial fluid.

The outward transport of sodium ions is known as sodium-pump. Steady existence of sodium pump requires a certain amount of energy which is supplied by the ATP.

Due to this sodium pump the axon membrane towards its inner side becomes electronegative, while towards outside electropositive. Actually in this state the nerve fibre is said to be in the normal pola­rized state.

When the axon is excited by an adequate stimulus of any sort (mechanical, chemical or electrical), the axon membrane immediately becomes depolarized and the sodium pump stops.

Now the permea­bility of the axon membrane to the sodium ions is suddenly increased (about ten times to that of potassium). This results into the diffusion of sodium ions from the exterior into the axoplasm.

The flow of sodium ions from the exterior, i.e., interstitial fluid into the axoplasm ineretore, begins to exceed the out flow of potassium ions considerably.

The inward movement of sodium ions is so heavy that the inside of the axon membrane not only gets depolarized, but the inside or the interior of the axon membrane becomes positive to that of outside.

This is just the opposite of the resting state of axon-membrane and it is called reverse potential or reverse polarization.

This change in specific permeability and subsequent depolarization starts from the point where the stimulus was received and spreads in both direction along the nerve fibre in the form of depolarized wave which constitu­tes the nerve impulse.

If the stimulus is too weak then this change stops after transmitting a short distance and normal polarization will be restored, we would then say that the strength of the stimulus was lower than the threshold.

When the stimulus is strong depolarization spreads throughout the length of the axon or nerve fibre.

As soon as the wave of depolarization has travelled the entire length of the fibre, no more sodium ions can enter from outside because, now the axon membrane becomes totally impermeable to sodium ions due to simultaneous increase in potassium permeability.

A large quantity of potassium from the interstitial fluid diffuses through the membrane into the axoplasm of the axon and sodium ions begin to diffuse outside.

Because of this the reverse potential disappears and the normal resting membrane potential returns. This process is known as repolarization.

This repolarization starts exactly on the same spot where the depolarization had started and then continues to advance from there in both directions.

Now once again sodium pump starts functioning. The entire process of depolarization and repolarization is completed within a fraction of a second and the fibre is ready for a fresh impulse.