An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction. At gap junctions, such cells approach within about 3.5 nm of each other,[1] a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapse.[2] In many animals, electrical synapse-based systems co-exist with chemical synapses.
Compared to chemical synapses, electrical synapses conduct nerve impulses faster, but unlike chemical synapses they do not have gain (the signal in the postsynaptic neuron is the same or smaller than that of the originating neuron). Electrical synapses are often found in neural systems that require the fastest possible response, such as defensive reflexes. An important characteristic of electrical synapses is that most of the time, they are bidirectional, i.e. they allow impulse transmission in either direction.[3] However, some gap junctions do allow for communication in only one direction.
Structure
Each gap junction (aka nexus junction) contains numerous gap junction channels which cross the membranes of both cells.[4] With a lumen diameter of about 1.2 to 2.0 nm,[2][5] 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,[6][2] 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 funnels are composed of two hemi-channels called connexons in vertebrates, one contributed by each cell at the synapse.[7][2][5] 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.[5]
Effects
The simplicity of electrical synapses results in synapses that are fast, but can only produce simple behaviors compared to the more complex chemical synapses.[8]
- Without the need for receptors to recognize chemical messengers, signal transmission 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 marked in mammals as it is in cold-blooded animals.[5]
- The response is always the same sign as the source. For example, depolarization of the pre-synaptic membrane will always induce a depolarization in the post-synaptic membrane, and vice versa for hyperpolarization.
- The response in the postsynaptic neuron is generally smaller in amplitude than the source. The amount of attenuation of the signal is due to the membrane resistance of the presynaptic and postsynaptic neurons.
- Long-term changes can be seen in electrical synapses. For example, changes in electrical synapses in the retina are seen during light and dark adaptations of the retina[9].
The relative speed of electrical synapses also allows for many neurons to fire synchronously.[10][4][5] 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.[1]
Normally, current carried by ions could travel in either direction through this type of synapse.[2] However, sometimes the junctions are rectifying synapses,[2] containing voltage-dependent gates that open in response to a depolarization and prevent current from traveling in one of the two directions.[10] Some channels may also close in response to increased calcium (Ca2+) or hydrogen (H+) ion concentration so as not to spread damage from one cell to another.[10]
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[11].
Electrical synapses are abundant in the retina and cerebral cortex of vertebrates.
