In many types of nerve and muscle fibre the membrane potential does not return immediately to the base-line at the foot of the action potential, but undergoes further slow variations known as after-potentials. The nomenclature of after-potentials dates from the period before the invention of intra-cellular recording techniques when external electrodes were used, so that an alteration of potential in the same direction as the spike itself is termed a negative after-potential, while a variation in the opposite direction corresponding to a hyperpolarization of the membrane is termed a positive after-potential (see Fig. 2.3). As may be seen in Fig. 2.4b, isolated squid axons display a characteristic positive phase which is almost completely absent in the living animals (Fig. 2.4a), while frog muscle fibres have a prolonged negative after-potential (Fig. 2.4h). In some mammalian nerves, both myelinated and non-myelinated, there is first a negative and then a positive after-potential. A related phenomenon, which is most marked in the smallest fibres, is the occurrence after a period of repetitive activity of a prolonged hyperpolarization of the membrane known as the post-tetanic hyperpolarization.
There is no doubt that after-potentials are always connected with changes in membrane permeability towards specific ions, but there is more than one way in which the membrane potential can be displaced either upwards or downwards. In the isolated squid axon, for example, the positive phase arises because the potassium conductance is still relatively high at the end of the spike, whereas the sodium conductance is inactivated and is therefore below normal. The membrane potential consequently comes close to EK for a short while, and then drops back as ¿K and¿Na resume their usual resting values. The mechanism responsible for production of the positive after-potential and the post-tetanic hyperpolarization in vertebrate nerves is quite different, for it has been shown to involve an enhanced rate of extrusion of Na+ ions by the sodium pump operating in an electrogenic mode. In other cases, a change in the relative permeability of the membrane to Cl_ and K+ ions may play a part. There is also evidence that the presence of Schwann cells partially or wholly enveloping certain types of nerve fibre has important effects on the afterpotential by slightly restricting the rate of diffusion of ions in the immediate neighbourhood of the nerve membrane.
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