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Positive after-potential is expressed in an intensified normal polarization, or hyperpolarization, of the membrane, and is well seen in unmyelinated nerve fibres (p. 69). In an unmyelinated giant axon of squid, for example, the descending phase of the action potential is transformed directly into a positive after-potential whose amplitude reaches approximately 15 millivolts; only then does the membrane potential revert to the initial resting level (Fig. 117).

In myelinated nerve fibres after-changes of potential are more complex in character; a negative after-potential is often replaced by a positive after-potential, then new electronegative phase sometimes develops, and only then is the resting potential completely restored.

fig. 118. Summation of afterpotentials in the phrenic nerve of a cat during brief stimulation with rhythmic impulses. Thje ascending part of the action potentials is not visible. Recordings were begun with negative after-potentials (a) passing into positive potentials (b). The upper curve shows the response to an isolated stimulus. With a growing frequency of stimuli the positive after-potential is sharply increased (after Gasser)

During a rhythmic stimulation of a nerve after-potentials become cumulative, so that their amplitude and duration increase (Fig. 118).

THE ION THEORY OF THE ORIGIN OF ACTION POTENTIALS

The reason for the appearance of an action potential in nerve and muscle fibres is a change in the ion permeability of the membrane.

As it has already been pointed out, membrane permeability to potassium in a state of rest exceeds that to sodium. As a consequence, the flow of positively charged potassium ions from the protoplasm to the surrounding fluid exceeds the contrary flow of sodium cations from the outside into the cell, so that at rest the outer side of the membrane is electrically positive to the inner one.

Under the effect of a stimulus on the cell, membrane permeability to sodium ions increases markedly to a point where it is approximately ten times that to potassium ions. The flow of positive sodium ions from the surrounding fluid into the protoplasm therefore begins to exceed the outflow of potassium ions considerably, which reverses the sign of the membrane charge, its outer surface becoming electrically positive to its inner surface. The change is registered as •a rise of the action potential curve (depolarization phase).

The increase in the permeability to sodium ions continues in nerve fibres for only a very short time, and is followed by the appearance of restorative processes in the cell that lead to a new decrease in membrane permeability to sodium ions and an increase in that* to potassium ions.

The process leading to a fall in the sodium permeability of the membrane is called inactivation by Hodgkin. As a result of inactivation, the flow of positive sodium ions into the protoplasm is sharply reduced, w’hile a simultaneous increase in potassium permeability intensifies the flow of positive potassium ions out of the protoplasm into the surrounding medium. The two processes result in repolarization of the membrane; its outer surface again acquires a positive charge, and the inner surface a negative one. The change is registered as a descending part of the action-potential curve (repolarization phase).