Electrical Synapses and Gap Junctions

Not every synapse uses chemicals. Gap junctions directly connect the insides of neighbouring cells, allowing instant electrical coupling that synchronises neurons, heart cells and glia.

The "war of the soups and the sparks" was won by the soups — most synapses are chemical. But the sparks weren't entirely wrong. Electrical synapses exist throughout the nervous system.

Direct tunnels between cells

An electrical synapse is formed by gap junctions: clusters of channels that span the membranes of two adjacent cells, connecting their interiors. Each channel is made of two half-channels (connexons), one from each cell, built from proteins called connexins. The gap between the cells is only about 3.5 nanometres, compared with 20–40 nm at a chemical synapse. Ions and small molecules pass straight through.

Pros and cons

Electrical synapses are extremely fast — there's no delay for release and diffusion — and usually work in both directions. They are ideal for synchronising groups of cells: coupled neurons tend to fire together. But they can't easily amplify a signal or switch it from excitatory to inhibitory, and they're less adaptable than chemical synapses — though their strength can be modulated.

Where they are found

Electrical synapses were first demonstrated in the crayfish escape circuit by Edwin Furshpan and David Potter in 1959. In mammals they connect certain inhibitory interneurons in the cortex (helping produce brain rhythms), neurons in the inferior olive (which helps time movements), and cells in the retina. They are crucial outside the brain too: gap junctions spread the electrical wave through heart muscle so it contracts in a coordinated beat. Astrocytes are extensively linked by gap junctions into networks.

In disease

Mutations in connexin genes cause some forms of inherited deafness and a peripheral nerve disease (an X-linked form of Charcot–Marie–Tooth disease), and gap junctions may help spread seizure activity.