7.2 Synapses and Neural Integration

A synapse is a junction between neurons or between a neuron and a receptor or effector cell. At a chemical synapse, the presynaptic terminal and postsynaptic membrane are separated by a cleft about tens of nanometres wide. Arrival of an action potential depolarizes the terminal, opening voltage-gated Ca²⁺ channels. Ca²⁺ enters and triggers docked vesicles to fuse with the presynaptic membrane.

Neurotransmitter released by exocytosis diffuses across the cleft and binds receptors in the postsynaptic membrane. Ligand-gated channels change ion permeability, generating a graded postsynaptic potential. If depolarization at the axon hillock reaches threshold, the postsynaptic neuron produces its own action potential. The neurotransmitter does not itself become the action potential.

At a cholinergic synapse, acetylcholine binds cholinergic receptors; at many neuromuscular junctions this opens cation channels and depolarizes the muscle membrane. Acetylcholinesterase hydrolyses acetylcholine in the cleft. Choline is taken back into the terminal, used to resynthesize acetylcholine and repackaged into vesicles. Rapid clearance keeps separate presynaptic spikes from becoming one prolonged signal.

Chemical synapses are directional because vesicle release machinery is concentrated presynaptically and receptors are concentrated postsynaptically. They also introduce a short synaptic delay, but permit amplification, inhibition, plasticity and integration. An electrical synapse through gap junctions has different properties and should not be used as the model for acetylcholine transmission.