Introduction

In Chapter 6 we described how electrical potentials may be recorded across membranes. In the absence of any externally applied currents, or spontaneous activity, these potentials take the form of steady-state membrane potentials. In excitable cells, and as a result of transient changes in the membrane properties, we can also record transient changes of the membrane potential. Some of those changes result in reversal of the polarity of the membrane potential. In this chapter we will examine the basis of this electrical activity which is the action potential. The action potential is a transient change in transmembrane electrical potential.

Although action potentials can be generated in different ways (and they may have a variety of different waveforms) the underlying electrical event is always a change in membrane conductances. The purpose of this chapter is to analyse this conductance change, so that we are able to understand action potential generation in any excitable cell.

Propagated and local action potentials

Action potentials can be recorded either as propagating electrical waves, or as electrical events that take place at a specific point in a cell membrane (local action potential). In the inset of Figure 7.1 an action potential propagates from left to right after electrical stimulation. Records obtained from the shaded area of the inset are displayed in the rest of the figure.