15.1.9—Cholinergic synapse structure and function

A cholinergic synapse has a presynaptic membrane, a synaptic cleft and a postsynaptic membrane. The cleft separates the cells, so an electrical impulse cannot cross it directly.

1. An action potential reaches the presynaptic membrane and opens voltage-gated Ca²⁺ channels.
2. Ca²⁺ enters the presynaptic neurone and causes vesicles to fuse with the membrane, releasing acetylcholine (ACh).
3. ACh diffuses across the cleft and temporarily binds to receptors on the postsynaptic membrane.
4. The receptors open Na⁺ channels; Na⁺ entry depolarises the postsynaptic membrane and can initiate a new impulse.
5. Acetylcholinesterase breaks down ACh, preventing continued stimulation; choline can be recycled to make more ACh.

Transmission is directional because release occurs at the presynaptic membrane and receptors are on the postsynaptic membrane. The chemical step also introduces a short delay compared with direct electrical continuity.

ACh is a temporary messenger, not a permanent bridge between neurones. This card ends when the next cell is depolarised; the neuromuscular junction and muscle contraction are covered separately.