Do all synapses employ neurotransmitters?
Apart from the Chemical Synapse, there are Electrical Synapses.
The difference between these:
In the chemical variant a Presynaptic neuron releases chemicals (NT's) in order to generate either an EPSP or IPSP in the Postsynaptic neuron (Post-SN).
Whether this will trigger an action potential in the Post-SN is not certain, as it depends on the total of inputs from different pre-SN's a neuron receives at any time. (Summation).
In Electrical synapses however, the Electrical Pulse DOES travel directly from the Pre- to the Post-SN.
An Electrical synapse is much faster : the signal can move much faster from one cell to the next, as there is no need for the time-consuming process of NT-release.
The signal pulse WILL travel from the pre- to the post-SN: whereas in the chemical variant triggering of the NT-receptors doesn't necessarily generate an action potential due to Summation, in the electrical variety it is the electric pulse ITSELF that is transferred. This happens by means of "pushing" positively charged ions through channels in the Gap between the two neurons, thereby depolarising the Post-SN.
Because the potential itself is tranferred from one neuron to the next, the strength of the signal stays the same, or weakens: unlike the Chemical variety, the electrical transfer lacks Gain...
Also,as the Action Potential itself is transferred, the STATE of the post-SN will always be the same as that of the pre-SN: depolarisation follows depolarisation.
This is a marked difference with NT-mediated signal transfer, where an action potential ( Depolarisation ) in the pre-SN can result in an IPSP , in other words Hyper-polarisation of the post-SN...
Another difference is that, whereas chemical synapses are essentially Unidirectional (the Post-SN doesn't send a signal back), in most cases Electrical Synapses are BI-directional.
Because of these characteristics you will ususally find Electrical Synapses in simple circuits that require a VERY fast response, like defensive or escape reflexes.
Whereas in a (neuro-)chemical synapse the typical width of the synaptic cleft is 20-40 nanometer (nm), the typical width of an electrical synapse (the " Gap Junction ") is less than 4 nm.
This increased proximity enables the signal to "jump" the gap.
However, Electrical Synapses usually co-exist with their Chemical cousins on the same neurons...