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As was indicated above, passage of an electric current through an excitable tissue primarily causes a change in membrane potential (p. 24). With a weak subthreshold current (below half threshold value) the changes of potential at the cathode are brought about by purely physical (passive) changes in the polarization of the membrane (Fig. 128A).

As current strength increases and approaches the threshold, to passive depolarization at the cathode there is added an active

Imtec

fig. 128. Local response of a nerve fibre

Л, В, C — changes in membrane potential of a nerve fibre caused by subthreshold shaks of brief duration. In curves В and C passive depolarization of the membrane is summated with active subthreshold depolarization in the form of a local response. Local responses are separated from passive changes of potential as broken lines. With a current of threshold strength (D) the local response becomes an action potential (Its peak is not shown)

subthreshold depolarization in the form of a so-called local response (Fig. 128, В and C).

The local response differs substantially in its properties from the action potential. It has no distinct threshold, and is not governed by the all-or-none law (see below), which is to be seen in the fact that the amplitude of the local response, unlike the action potential, .depends on the strength of the current applied; the stronger the stimulus the greater is the local response (see Fig. 128, В and C). During local response tissue excitability is increased, whereas it diminishes with the action potential (p. 38).

The local response, like the action potential, is brought about by an increase in the sodium permeability of the membrane, and an intensification of the inflow of those ions into the cell. With the appearance of a local response, however, the increase in sodium permeability is slight and therefore does not generate an action potential. It is not until the critical level of depolarization is reached that the local response grows into an action potential (Fig. 128D).

To sum up, the chain of phenomena developing in a nerve or muscle fibre at the cathode of a stimulating current may be expressed as follows:

passive depolarization of the membrane -> increase in sodium permeability^- increase in the flow of sodium ions across the membrane into the fibres- active depolarization of the membrane (local response and action potential).

THE ALL-OH-NONE LAW

Through study of the relationship of stimulation effects to stimulus strength an all-or-none law was established, according to which subthreshold stimulation produces no excitation, while threshold stimuli immediately produce maximum excitation unaffected by a further increase in stimulus strength.

This fact was discovered by Bowditch while studying the heart, and was later confirmed in experiments on other excitable tissues.