BIMODAL GATING OF THE NA+ CHANNEL

Inactivation of voltage-gated ion channels, whether they are selective for Na+, K+ or Ca2+, probably never involves their total closure, and some.flow of ion current persists if large enough test pulses are app lied. Incomplete inactivation was first reported for the Na+ channels of the squid giant axon, but has since been observed in other types of peripheral nerve and, more recently, in muscle fibres and the neurons of mammalian brain. The phenomenon is therefore widespread and has im portant implications for the functioning of voltage-gated channels in a variety of situations. It is best described in terms of a gating mec hanism that switches the channel from an initial mode in which it has a high probability of opening to one in which the probability is great ly lowered, but not reduced to zero.