1. 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

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

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

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

5. Presynaptic
terminal
Synaptic cleft
Acetylcholine
Postsynaptic
membrane

6. 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.

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