Sodium and potassium channels in regenerating and developing mammalian myelinated nerves.

A study has been made of how the normal complementary distribution of sodium and potassium channels in mammalian myelinated nerve fibres (all the sodium channels being in the node with all the potassium channels in the internode) is altered in regenerating and in developing rabbit sciatic nerves. In regenerating nerve fibres, where a marked increase in the number of nodes per unit length occurs, there is a corresponding increase in the sodium channel content (determined from the maximum saturable binding of labelled saxitoxin), consistent with the idea that the number of sodium channels per node remains roughly constant. The use of 4-aminopyridine, which by blocking potassium channels prolongs the action potential, has shown that both in regenerating nerve fibres and in developing nerve fibres potassium currents contribute to the mammalian action potential. In both cases, with the passage of time, the sensitivity to 4-aminopyridine progressively decreases.