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Fig. 116 shows an action potential registered in a skeletal muscle fibre by means of an intracellular electrode. In this case the difference in potential across the membrane at rest was 85 millivolts (i.e. the charge on the inner side was 85 millivolts). Under the influence of an isolated stimulus (the moment of its application is indicated by the arrow) the difference in potential on the membrane began to fall sharply, and was reduced to zero, after which it reappeared but with the opposite sign; the inner side of the membrane became electrically positive in relation to its outer one. When the

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PIC. 116. Action potential of a skeletal muscle fibre registered by meaus of an intracellular microelectrode

a — depolarization phase; b — repo-larlzatlon phase; e — negative afterpotential. The moment of stimulation is Indicated by the arrow

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reversal of potential had reached 30 millivolts, a restorative process began by which the membrane potential reverted to its initial level.

Ascending and descending phases are distinguished on the curve of the action potential. Since the initial polarization of the membrane disappears in the ascending phase it is called the depolarization phase (Fig. 116a); correspondingly the descending phase, during which the membrane polarization reverts to the resting level, is known as the repolarization phase (Fig. 116b).

The length of the action potential in nerve and skeletal muscle fibres varies between 0.1 and 5.0 milliseconds, during which repolarization phase is always longer than the depolarization. Cooling of the fibre by 10°C makes the action potential approximately three times longer, especially its descending phase.

AFTER-POTENTIALS

The action potential is accompanied, as a rule, with so-called after-potentials, which were first registered by Vorontsov (1926) and were later studied in detail by Erlanger and Gasser and others.

After-potentials may be negative or positive. The amplitude of either does not exceed several millivolts, while their duration varies between a few milliseconds and a hundred or more milliseconds. After-potentials are associated with the restorative processes that slowly develop in nerve and muscle fibres after excitation.

fic. 117. The action potential of a squid giant axon recorded bv means of an intracellular micro-electrode (after Hodgkin and Huxley). A positive after-potential (a) is distinctly seen. The time Intervals in 500 oscillations per second are shown at the bottom

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the beginning of the negative after-potential (Fig. 116c). The membrane remains partly depolarized for a certain time, and complete restoration to the initial potential of 85 millivolts occurs after approximately 15 milliseconds. Negative after-potential is often called afterdepolarization of the membrane.

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).