Synapses and neurotransmitters

1. An action potential arrives at the presynaptic knob and opens voltage-gated Ca²⁺ channels. Ca²⁺ enters by facilitated diffusion.
2. Ca²⁺ makes vesicles containing ACh move to and fuse with the presynaptic membrane. ACh is released by exocytosis.
3. ACh diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
4. The receptors are linked to Na⁺ channels, which open. Na⁺ enters the postsynaptic neurone, so the membrane is depolarised.
5. If depolarisation reaches threshold, an action potential is set off in the postsynaptic neurone.
6. The enzyme acetylcholinesterase in the cleft hydrolyses ACh to choline and ethanoate (acetate), so the response stops. Choline is taken back into the presynaptic knob and used, with ATP from the mitochondria, to resynthesise ACh.

Synapses ensure one-way transmission because only the presynaptic knob has vesicles and only the postsynaptic membrane has receptors. A single impulse may not release enough ACh to reach threshold in the next neurone, so a synapse controls which signals are passed on.

Acetylcholine is one of many neurotransmitters; others include noradrenaline, dopamine and serotonin. Each binds only to receptors with a complementary shape. At an excitatory synapse the transmitter opens Na⁺ channels and depolarises the postsynaptic membrane. At an inhibitory synapse the transmitter (for example GABA) opens channels that make the postsynaptic membrane more negative (hyperpolarised), so an action potential is less likely. Whether the postsynaptic neurone fires depends on the combined effect of all the transmitter released onto it, from several synapses at once (spatial summation) or from impulses arriving in quick succession at one synapse (temporal summation).