14.12: Synapses électriques
Electrical synapses found in all nervous systems play important and unique roles. In these synapses, the presynaptic and postsynaptic membranes are very close together (3.5 nm) and are actually physically connected by channel proteins forming gap junctions.
Gap junctions allow the current to pass directly from one cell to the next. In contrast, in the chemical synapse, the neurotransmitters carry the information through the synaptic cleft from one neuron to the next. They consist of two hexameric connexin hemichannels or connexons contributed by each of the adjacent cells. These hemichannels make contact between the two cell membranes by forming a continuous bridge between the cytoplasm of the two connecting cells. The opening of the connexon pore is more like the shutter of a camera where the connexins in the hemichannel rotate slightly with respect to one another for ion passage. In addition to the ions, other molecules, such as ATP, can also diffuse through the large gap junction pores.
Signaling in electrical synapses is virtually instantaneous. Some electrical synapses are bidirectional too. Electrical synapses are not easily blocked and are important for synchronizing the electrical activity of a group of neurons. For example, electrical synapses in the thalamus are thought to regulate slow-wave sleep, and disruption of these synapses can cause seizures. In the intestinal smooth muscle cells, electrical synapses provide electrical rhythmicity contributing to peristaltic intestinal activity vital for normal functioning of the gastrointestinal tract.
