Answer

Gap junctions are direct electrical and chemical connections between the cytoplasmic compartments of two cells. They are created by the "docking" of two membrane-bound pore complexes called connexons (one on each cell). Connexons are themselves hexamers of monomeric proteins called connexins. In contrast to chemical synapses, the transmission at gap junctions is practically instantaneous – the spread of current is no longer limited by the need to engage synaptic machinery – and bidirectional – stimulating either neuron will stimulate the other. This speed comes at a price: because most gap junctions simply pass current, the response of the "post-synaptic"^† cell has to be directly proportional to the behavior of the pre-synaptic cell. This means that these synapses are sign-preserving, like excitatory chemical synapses, rather than sign-inverting, like inhibitory synapses. There is a slight difference though: because they are bi-directional, gap junctional synapses are neither excitatory nor inhibitory – they are synchronizing.

The presence of a direct chemical connection between two cells is a potential liability – chemical events that would be best contained in one cell, like gene regulatory signals or viral infections, could spread to all gap-junctionally connected cells. Several factors prevent this. First, the diameter of these pores is just 1.5 nanometers, so ions and second messenger molecules can flow through them but proteins cannot. Second, most gap junctions close in response to an increase in calcium or a drop in pH. These signals tend to predict imminent cell death, and so closing gap junctions prevents the "rot" from spreading.

In terms of signal transmission, a network of gap-junctionally connected spiking neurons behaves like a single "super-neuron". This "super-neuron" has a greater downstream effect – many neurons fire at once – but also a higher current-injection threshold for firing, since current can leak out through the gap junctions, rather than across the membrane (this is most easily seen if you consider the equivalent circuit diagram for such a network).

Perhaps the most successful use of gap junctions in biology is in the heart, where electrical connections ensure that cardiac myocytes fire at the same time (synchrony), never fire out of phase (higher threshold), and successfully induce contraction (larger downstream effects). Unfortunately for us and the Egyptians, the heart is not the brain, so it is not an example of gap junctions in neuroscience.

Luckily, Kandel provides several examples in the below-referenced pages, including one from his favorite organism, the sea slug Aplysia californica. This slug produces an ink jet to ward off predators. The ink jet is controlled by three gap-junctionally connected cells, ensuring that when ink is to be released, it is released rapidly and in bulk, and that otherwise it is safely stored. A similar mechanism controls escape behavior in squid.

^† This term is being used very loosely here – since the synapses are bidirectional, one man's post-synapse is another man's pre-synapse.

Refs

• Kandel & Schwartz 5e, Cells at an Electrical Synapse Are Connected by Gap-Junction Channels, pp 180-5.