What is a synaptic gap?

This is the junction between two neurons. At the one end you have the pre-synaptic membrane - the terminal end of the previous neuron, the pre-synaptic neuron - and on the other side you have the post-synaptic membrane, part of the post-synaptic neuron. Action potentials typically cross the cleft by the use of neurotransmitters. Examples include ACh (Acetyl Choline), often used in the synaptic clefts of parasympathetic neurons.