Gap junction · Grey Matter

A gap junction is a cluster of channels that joins the inside of two cells directly, letting ions and small molecules pass from one to the other, and between neurons it forms an electrical synapse that transmits with almost no delay.

Gap junction. A gap junction is a cluster of channels that joins the inside of two cells directly, letting ions and small molecules pass from one to the other, and between neurons it forms an electrical synapse that transmits with almost no delay.

Each channel is made of two half-channels (connexons), one from each cell, each built from six connexin proteins; they meet across a gap of only 2 to 4 nm and line up into a pore 1 to 2 nm wide. Current flows straight through, in both directions, so a voltage change in one cell appears at once, smaller, in the other. In the adult mammalian brain electrical synapses are common between inhibitory interneurons of the same type (through connexin 36), in the thalamic reticular nucleus, the inferior olive and the locus coeruleus, and astrocytes are coupled into large networks by other connexins.

They synchronise. Coupled interneurons tend to fire together, which helps them impose a common rhythm on the cells they inhibit; mice lacking connexin 36 show weaker gamma oscillations.

They do not amplify. The coupled cell receives a fraction of the signal, and because the membrane filters it, slow changes pass better than brief spikes.

In glia they spread the load. Astrocytes use their coupling to move potassium and glutamate-derived metabolites away from busy spots (spatial buffering).

An electrical synapse trades control for speed and synchrony.

It cannot change sign, amplify or be modulated much, but it makes groups of cells behave as one.

Questions: Why does the brain mostly use chemical synapses when electrical ones are faster? An electrical synapse passes current straight through gap junctions with almost no delay, in both directions, but the receiving cell gets only a fraction of the signal and its sign cannot change. A chemical synapse costs a delay (about 0.15 ms at the fastest mammalian synapses at body temperature, 0.5 to 1 ms in textbook figures) and in exchange it can amplify, since one vesicle opens hundreds of receptors. It can also turn excitation into inhibition by releasing GABA, be tuned by neuromodulators, and change its strength with experience. Electrical synapses are kept for jobs where speed and synchrony matter more than control, such as coupling interneurons into a common rhythm. How do gap junctions between interneurons help the cortex oscillate? Fast-spiking interneurons of the same type are linked by gap junctions made of connexin 36, so when some of them depolarise, current leaks into their neighbours and pulls them toward firing at the same moment. Synchronised interneurons deliver their inhibition to many pyramidal cells together, opening and closing a common window for firing, which is the core of gamma rhythms. The coupling passes slow voltage changes better than brief spikes, so it aligns the cells' rhythm more than it transmits individual spikes. Mice lacking connexin 36 still oscillate, but with weaker gamma, which shows the junctions help without being the only mechanism.