In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell (neural or otherwise).
There are two fundamentally different types of synapses:
- In a chemical synapse, the presynaptic neuron releases a chemical called a neurotransmitter that binds to receptors located in the postsynaptic cell, usually embedded in the plasma membrane. The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron.
- In an electrical synapse, the presynaptic and postsynaptic cell membranes are connected by special channels called gap junctions that are capable of passing electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell. The main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next
Further, there is a type of signaling called “ephatic coupling”, wherein neurons become coupled not through a direct synapse, but through proximity (sharing ions in chemical synapses, and via electric fields in electical synapses). This looks (to me) to be what changes a neural network from a linear circuit into some far more “random”.