Although we are focusing in this chapter on synapses that require a chemical substance to mediate synaptic transmission, the brain also contains electrical synapses (M. V. Bennett, 2000). At electrical synapses (or gap junctions), the presynaptic membrane comes even closer to the postsynaptic membrane than it does at chemical synapses; the gap at an electrical synapse (see Figure 1a) is about 10 times narrower than the gap at a chemical synapse. At electrical synapses, the facing membranes of the two cells have large channels that allow ions to flow from one neuron directly into the other (see Figure 1b). That means that electrical current associated with neural activity in one neuron can flow directly across the gap junction to affect the other neuron.
Transmission at these synapses closely resembles action potential conduction along the axon. Electrical synapses therefore work with practically no time delay, in contrast to chemical synapses, where the delay is about a millisecond. Because they’re so fast, electrical synapses are often found in neural circuits that mediate escape behaviors in invertebrates. They are also found where many fibers must be activated synchronously, as in the system for moving our eyes. Clinically, electrical synapses are suspected of contributing to the spread of seizure discharges in epilepsy (Szente et al., 2002), which we discuss at the end of this chapter.
Bennett, M. V. (2000). Electrical synapses, a personal perspective (or history). Brain Research Reviews 32: 16–28.
Szente, M., Gajda, Z., Said Ali, K., and Hermesz, E. (2002). Involvement of electrical coupling in the in vivo ictal epileptiform activity induced by 4-aminopyridine in the neocortex. Neuroscience 115: 1067–1078.