Electrotonic synapses

An electrotonic synapse is one where the presynaptic cell excites the postsynaptic cell directly by means of electric current flow. Synapses of this type

Fig. 8.10. The probable arrangement of the transmembrane helices in the b2-adrenergic receptor, with adrenaline in the binding site. The molecule is drawn as seen from the outer side of the plasma membrane. From Ostrowski et al. (1992). Reprinted, with permission, from the Annual Review of Pharmacology and Toxicology, Volume 32 © 1992 by Annual Reviews www.AnnualReviews.org.

Fig. 8.10. The probable arrangement of the transmembrane helices in the b2-adrenergic receptor, with adrenaline in the binding site. The molecule is drawn as seen from the outer side of the plasma membrane. From Ostrowski et al. (1992). Reprinted, with permission, from the Annual Review of Pharmacology and Toxicology, Volume 32 © 1992 by Annual Reviews www.AnnualReviews.org.

Fig. 8.11. The structure of gap junctions, as deduced from X-ray diffraction studies on material isolated from mouse liver cells. From Makowski et al. (1984).

were first discovered in the multicellular giant fibres involved in the escape responses of crayfish and earthworms, where they serve to carry the action potential from one cell to another. They also occur between some cells in the central nervous systems of mammals and other animals, where they probably serve to promote synchrony of action in adjacent cells.

Electron microscopy of electrotonic synapses shows regions where the intercellular space between the two cells is much narrower than usual, being about 2 nm instead of 20 nm. These regions are known as 'gap junctions'. 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. Each gap junction channel is composed of a pair of hexamers, one in each of the two apposed membranes, as is shown in Fig. 8.11. The hexamers are known as connexons, and their individual subunits are proteins called con-nexins.

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