Electrical synapse

An electrical synapse is a mechanical and electrically conductive link between two abutting neuron cells that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. At gap junctions, such cells approach within about 3.5 nm of each other (Kandel et al. 2000), a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapses (Hormuzdi et al. 2004). In organisms, electrical synapse-based systems co-exist with chemical-based, but are limited to systems that require the fastest possible response.

Structure


Each gap junction (aka nexus junction) contains numerous gap junction channels which cross the membranes of both cells (Gibson et al., 2004). With a lumen diameter of about 1.2 to 2.0 nm (Bennet and Zukin, 2004; Hormuzdi et al., 2004), the pore of a gap junction channel is wide enough to allow ions and even medium sized molecules like signaling molecules to flow from one cell to the next (Kandel et al., 2000, p. 178-180; Hormuzdi et al., 2004), thereby connecting the two cells' cytoplasm. Thus when the voltage of one cell changes, ions may move through from one cell to the next, carrying positive charge with them and depolarizing the postsynaptic cell.

Gap junction channels are composed of two hemi-channels called connexons in vertebrates, one contributed by each cell at the synapse (Kandel et al., 2000, p. 178; Bennet and Zukin, 2004; Hormuzdi et al., 2004). Connexons are formed by six 7.5 nm long, four-pass membrane-spanning protein subunits called connexins, which may be identical or slightly different from one another (Bennet and Zukin, 2004).

Effects
Without the need for receptors to recognize chemical messengers, signaling at electrical synapses is more rapid than that which occurs across chemical synapses, the predominant kind of junctions between neurons. The synaptic delay for a chemical synapse is typically about 2 ms, while the synaptic delay for an electrical synapse may be about 0.2 ms. However, the difference in speed between chemical and electrical synapses is not as important in mammals as it is in cold-blooded animals (Bennet and Zukin, 2004).

The relative speed of electrical synapses also allows for many neurons to fire synchronously (Kandel et al., 2000, p. 180; Bennet and Zukin, 2004; Gibson et al., 2004). Because of the speed of transmission, electrical synapses are found in escape mechanisms and other processes that require quick responses, such as the response to danger of the sea hare Aplysia, which quickly releases large quantities of ink to obscure enemies' vision (Kandel et al., 2000).

Normally current carried by ions could travel in either direction through this type of synapse (Hormuzdi et al., 2004). However, sometimes the junctions are rectifying synapses (Hormuzdi et al., 2004), containing voltage-dependent gates that open in response to a depolarization and prevent current from traveling in one of the two directions (Kandel et al., 2000, p. 180). Some channels may also close in response to increased calcium (Ca++) or hydrogen (H+) ion concentration so as not to spread damage from one cell to another (Kandel et al., 2000, p. 180).

There is also evidence for "plasticity" at some of these synapses—that is, that the electrical connection they establish can strengthen or weaken as a result of activity.

Electrical synapses are abundant in the retina and cerebral cortex of vertebrates.

History
The model of a reticular network of directly interconnected cells was one of the early hypotheses for the organization of the nervous system at the beginning of the 20th century. This reticular hypothesis was considered to conflict directly with the now predominant neuron doctrine, a model in which isolated, individual neurons signal to each other chemically across synaptic gaps. These two models came into sharp contrast at the award ceremony for the 1906 Nobel Prize in Physiology or Medicine, in which the award went jointly to Camillo Golgi, a reticularist and hugely famous cell biologist, and Santiago Ramón y Cajal, the champion of the neuron doctrine and the father of modern neuroscience. Golgi delivered his Nobel lecture first, in part detailing evidence for a reticular model of the nervous system. Ramón y Cajal then took the podium and refuted Golgi's conclusions in his lecture. Modern understanding of the coexistence of chemical and electrical synapses, however, suggests that both models are physiologically significant; it could be said that the Nobel committee acted with great foresight in awarding the Prize jointly.

There was substantial debate on whether the transmission of information between neurons was chemical or electrical in the first decades of the twentieth century, but chemical synaptic transmission was seen as the only answer after Otto Loewi's demonstration of chemical communication between neurons and heart muscle. Thus, the discovery of electrical communication was surprising. Electrical synpases were first demonstrated between escape-related giant neurons in crayfish in the late 1950s, and later found in vertebrates.