A second type of synapse that occurs between neurons is the electrical synapse. Instead of being separated by a 20-50 nm synaptic cleft, the presynaptic and postsynaptic neurons are joined by a gap junction that typically has a cleft of only 2-4 nm.
Embedded in the presynaptic and postsynaptic membranes are large open channels called connexons. Each connexon channel is composed of six protein subunits referred to as connexins. Opposing connexons span the cleft between membranes and form a continuous, water-filled channel that directly links the presynaptic and postsynaptic neurons. When an action potential arrives at the synapse, ions diffuse in both directions through the large pores formed by the connexons. Ionic current (= action potential) is quickly and directly transferred from one neuron to the other, with very little delay. Although transmission is rapid at electrical synapses, it is not easily modified. In contrast, the release and use of neurotransmitters at chemical synapses is much more adjustable, which increases neural integration. Because of their limitations, electrical synapses are much less common than chemical synapses. They are found in areas of the brain where neuronal activity needs to be synchronized, thus making it possible for groups of neurons to generate action potentials simultaneously.
