Ionic channels in nerve membranes.

Abstract In summary a wide variety of experiments with axons provides kinetic, electrochemical, and pharmacological evidence that three types of channels through the membrane contribute to the ionic permeability changes underlying action potentials. In normal function one channel accounts for most of the movements of Na ions, another accounts for most of the movements of K ions, and the last accounts for the remaining “leakage” fluxes. The Na, K, and leakage channels seem to be independent specializations of the membrane. Theoretical, electrical, and pharmacological evidence suggest that Na channels are short narrow pores that open and close in an all-or-nothing fashion. An open Na channel in a squid giant axon may have a conductance of about 0.5 nmho. Very little is known about K and leakage channels except that K channels may be longer pores with a lower conductance than Na channels.