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Inactivation of sodium permeability leads to suppression of depolarization and triggers off repolarization during excitation.

The increase in potassium permeability during depolarization occurs with a longer latent period than that in sodium permeability; potassium permeability grows parallel with the inactivation of the latter, thus promoting repolarization of the membrane (p. 17).

THE CRITICAL LEVEL OF DEPOLARIZATION

Registration of changes in the membrane potential of nerve fibre at the cathode has shown that an action potential arises at the moment that depolarization reaches a critical level, which depends not on the character of the stimulus applied, on the position of the electrodes, etc., but on the properties of the membrane itself, by which it is determined exclusively.

Fig. 127 shows tracings of the changes in membrane potential of a nerve fibre under the influence of prolonged (A) and short (B and C) stimuli of variable strength. As may be seen the action potential arose in all cases at the moment the resting potential dropped from 80 millivolts to the critical level of 60 millivolts. The sole difference was in the rate at which depolarization took place.

With a weak current depolarization of the membrane developed slowly, and for the action potential to appear the stimulus had therefore to be of longer duration. With a rise in current strength the rate of depolarization grew and the minimum time of stimulus action correspondingly shortened. From that it follows that the minimum stimulation time (p. 20) is the period required and sufficient for depolarization of the membrane at the cathode to reach the critical level.

fig. 127. Change in the membrane potential at the cathode of a stimulating current. Broken line indicates the critical level of depolarization. Bottom — stimuli arousing responses Л, B, and C

The faster the rate of depolarization at a given current the shorter the utilization time, and vice versa. The rate, however, depends, first, on the membrane time constant RC (p. 25) and second, on the rate of increase in sodium permeability.

In other words, the utilization time of stimulation is determined both by the passive electrical properties of the membrane — its capacitance and resistance — and by the active properties of the mechanism of change in its ion permeability.

LOCAL RESPONSE

An action potential capable of spreading along a nerve or muscle fibre is not the only form of response to stimulation. A local, unspreading response can be evoked, in addition, in any excitable formation, by application of stimuli of near-threshold strength.