En passant synapses are where one neuron joins another at a place besides the axon terminal. They have the same function and effect, merely in a different place.
En passant and neural synapses are physiologically the same in that they do the same thing. Both receive action potentials and have gated ion channels and release neurotransmitters across the synapse to the next neuron.
However, en passant synapses are located on the stem of the axon or the body of the neuron rather than at the axon terminal.
‘En passant’ is French for ‘in passing’, because the synapses are used only as the electrical signal passes by to the axon terminal.
Synapses can be of three sorts: axosomatic, where the axon joins to a soma or body of another neuron; axodendritic, where an axon leads off from a dendrite, the small branches leading off from the soma; or axoaxonic, where one axon joins to another along its length.