Action potential 1. Action potentials arriving at the presynaptic terminal cause voltage- gated Ca2+ channels to open. Ca2+ 1 2 3 Presynaptic terminal Synaptic vesicle 2. Ca2+ diffuse into the cell and cause synaptic vesicles to release acetylcholine, a neurotransmitter molecule. Voltage-gated Ca2+ channel Synaptic cleft 3. Acetylcholine diffuses from the presynaptic terminal across the synaptic cleft. Acetylcholine Postsynaptic membrane Na+ Acetylcholine bound to receptor site opens ligand-gated Na+ channel 4. Acetylcholine molecules combine with their receptor sites and cause ligand-gated Na+ channels to open. Na+ diffuse into the cell and causes depolarization. If depolarization reaches threshold, an action potential is produced in the postsynaptic cell. 4 4

Action potential Ca2+ Presynaptic terminal Voltage-gated Ca2+ channel Action potentials arriving at the presynaptic terminal cause voltage- gated Ca2+ channels to open.

Ca2+ Synaptic vesicle Acetylcholine Ca2+ diffuse into the cell and cause synaptic vesicles to release acetylcholine, a neurotransmitter molecule.

Ca2+ Presynaptic terminal Synaptic cleft Acetylcholine Acetylcholine diffuses from the presynaptic terminal across the synaptic cleft.

Presynaptic terminal Synaptic cleft Acetylcholine Postsynaptic membrane

Na+ Acetylcholine bound to receptor site opens ligand-gated Na+ channel Acetylcholine molecules combine with their receptor sites and cause ligand-gated Na+ channels to open.

Na+ Na+ diffuse into the cell and causes depolarization. If depolarization reaches threshold, an action potential is produced in the postsynaptic cell.