We have seen in chapter 7 that the controversy as to whether synaptic transmission was electrical or chemical in nature seemed to have been resolved by the advent of intracellular recording in the early 1950s. It was a great surprise, therefore, when intracellular recordings at the end of that decade provided clear evidence that some synapses do operate by the direct flow of current from one cell to another (see Bennett, 1985). Such synapses are described as electrotonic or electrically transmitting.

Synapses operating by electrotonic transmission

An excitatory electrotonic synapse is one in which the postsynaptic cell is directly excited by the electrotonic currents accompanying an action potential in the presynaptic axon. Electron microscopy of electrotonic synapses shows regions where the intercellular space between the two cells is much narrower than usual. These regions are known as gap junctions. As we shall see later, they contain channels which provide direct connections between the pre- and postsynaptic cells, so that current can flow readily from one cell to the other.

In order for electrotonic transmission to be effective, the electrical and geometrical characteristics of the junction must be arranged in a certain way. Figure 12.1 represents a junction between two cells, each of which is electrically excitable. The local circuit currents produced by inward movement of sodium ions in the presynaptic cell at A will be completed by currents flowing out of the synaptic cleft at B and out of the postsynaptic cell at C.