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Paraphrased from the Wikipedia article "Synapse":

There are two fundamentally different kinds of synapses:

  1. Chemical synapses, which convert electrical activity in the presynaptic neuron into the release of neurotransmitters via the activation of voltage-gated calcium channels. The released neurotransmitters, which vary in type by cell, bind to receptors of the postsynaptic cell, giving rise to complex behaviors.

  2. Electrical synapses, in which presynaptic and postsynaptic are connected by "gap junctions" or synaptic clefts capable of passing electrical current, resulting in rapid signal transduction.

Are there certain structures and locations in the nervous system that are mostly or exclusively chemical synapses, and certain ones that are mostly or exclusively electrical? Or, are they mixed throughout the nervous system?

For example, one might suppose that the neurons which transmit signals from brain to limbs might be electrical, due to the apparent advantage of fast transduction times in this scenario. On the other hand, it might be advantageous for the brain to be capable of more the dynamic, albeit slower, signal transmissions made possible by chemical synapses. Are these intuitions accurate?

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Synapses are almost all chemical, the electrical synapses are more of a "special case".

The speed of transmission across limbs, etc, has very little to do with the speed of the actual synapse. The signal itself is conducted electrically inside a given cell (this is an "action potential"), aided in many cases by insulating myelination. The synapse is just the endpoint.

Some places where electrical synapses/gap junctions are present are:

  • Between cardiomyocytes, to synchronize heart contraction
  • Between certain networks of inhibitory interneurons in the CNS
  • In some (but not nearly all) reflex circuits
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