SYNAPTIC TRANSMISSION 

The key feature of electrical synapses is that the pre and postsynaptic neurones are physically coupled by structures that allow electrical continuity between cells.

Electrical synapses are characterised by the very close apposition of the pre- and postsynaptic membranes (resulting in a synaptic cleft of only 3 nm) and the presence of gap-junctions that connect the intracellular space of the two neurones.

Gap-junctions are formed by complementary hemi-channels associated with the pre- and post synaptic membrane that provide a low-resistance pathway between the two cells.

These hemi-channels are known as connexons which are made up of the protein connexin. Each connexon is formed from six connexin molecules which extend a uniform distance outside the cells. Alignment of connexons from each cell across the gap results in the formation of aqueous pores roughly 2 nm in diameter between the two cells that functionally define the gap junction.

As a consequence of these gap-junctions an action potential in the presynaptic neurones is able to traverse the synaptic cleft and depolarise the postsynaptic neurone.

In most cases the action potential is not faithfully reproduced in the postsynaptic neurones because the gap-junctions have resistance that reduces its magnitude. There are actually two different classes of electrical synapse: Rectifying synapses can only pass information from the pre- to the postsynaptic neurone. In the diagram opposite two neurones are coupled by a rectifying synapse. Their membrane potentials are recorded by the voltmeters designated 1 and 2 respectively and their membrane potentials plotted below. Using the buttons below, investigate the effect of stimulating neurone 1 and then 2 neurone on the membrane potential of the other neurone. Click on buttons to stimulate neurones     Note that the action potential in neurone 1 causes a subthreshold depolarisation of neurone 2 but not vice versa.

Bidirectional (or reciprocal) synapses are able to pass information in either direction. In the diagram opposite two neurones are coupled by a bidirectional synapse. Their membrane potentials are recorded by the voltmeters designated 3 and 4 respectively and their membrane potentials plotted below. Using the buttons below, investigate the effect of stimulating neurone 3 and then neurone 4 on the membrane potential of the other neurone. Click on buttons to stimulate neurones     Note that the action potential in neurone 3 causes a subthreshold depolarisation of neurone 4 and vice versa.

Electrical synapses were first described in the crayfish where they are involved in the fairly simple escape reflex that these crustaceans use to avoid predators. In this instance they are useful because there is no significant delay between the pre- and postsynaptic neurone and hence communication is very rapid. Although electrical synapses are fairly rare in human but they are found in some very important locations including the retina, heart and digestive tract.