The Hodgkin–Huxley analysis of nervous conduction, which we examined in the chapter 5, showed that the sodium and potassium conductances of the axon membrane are switched on and off (or ‘gated’) by changes in membrane potential. Hence the channels through which the sodium and potassium ions flow are themselves gated by changes in membrane potential. In this chapter we examine some of the properties of these voltage-gated channels.

Voltage-gated sodium channels

Voltage clamp studies of the Hodgkin–Huxley type can provide information about the overall actions of large numbers of channels, but they cannot tell us how the individual channels behave. Two methods have been developed for this: fluctuation analysis and the patch clamp technique. Fluctuation analysis was developed first, but the patch clamp method allows more direct observation if the individual channel currents are large enough.

Patch clamping

The patch clamp technique has the very great advantage that the current flow through individual ion channels can be measured. An outline of the method is given in chapter 2. For voltage-gated channels, the stimulus for channel opening is a clamped depolarization applied to the patch of membrane containing the channel.

The first single-channel records of sodium channel currents were obtained by Sigworth & Neher (1980) by patch clamping myoballs, which are spherical cells prepared by tissue culture of embryonic rat muscle in the presence of colchicine. They used pipettes containing tetraethylammonium (TEA) to block any potassium channels present and α-bungarotoxin to block acetylcholine channels (p. 114).