Excitatory Synapses Operating by Electrical Transmission

Abstract

As provisionally defined in the Introduction, a synapse is a structure anatomically differentiated and functionally specialized for transmission between excitable cells. The present chapter includes a wide variety of electrically transmitting synapses, beginning with the simplest and least differentiated type, the septal synapse. In all cases the transmission is effected by the depolarizing action of an electric current that is generated either by impulses or by synaptic potentials in the presynaptic component. In the most differentiated type, the giant motor synapses of the crayfish, the synaptic membrane is a very efficient electrical rectifier, so one-way transmission is ensured just as effectively as with a chemically transmitting synapse. Nevertheless, even in this synapse the functional arrangements are much simpler than for chemically transmitting synapses, where there are of necessity in the transmitting element the complex phenomena of manufacture, mobilization and release of the transmitter substances, and in the recipient element there are the specialized receptor sites with the associated ionic permeability mechanisms (Chapters IV, V and VI). Because of this greater simplicity it seems likely that the electrical synapses represent the most primitive mode of communication between excitable cells, as for example in coelenterates. There are fascinating fields for investigation on the comparative physiology of synaptic transmission in these primitive organisms.