1. 1. An action potential arrives at the
presynaptic terminal causing voltage
gated Ca2+ channels to open,
increasing the Ca2+ permeability of
the presynaptic terminal.
Action
potential
Presynaptic
terminal
Ca2+
channel
ACh
Presynaptic
terminal
Acetic
acid
Ca2+
Synaptic
vesicle 1
2. Ca2+ enters the presynaptic terminal
and initiates the release of a
neurotransmitter, acetylcholine (ACh),
from synaptic vesicles in the
presynaptic terminal.
Synaptic cleft
Choline 2
3. Diffusion of ACh across the synaptic
cleft and binding of ACh to ACh
receptors on the postsynaptic muscle
fiber membrane causes an increase in
the permeability of ligand-gated Na+
channels. 7
ACh
Synaptic
cleft
Choline
Acetic
acid
Na+
ACh 3 6 5
ACh
receptor
site
Receptor
molecule
Action
potential 4
Action
potential
4. The increase in Na+ permeability
results in depolarization of the
postsynaptic membrane; once
threshold has been reached a
postsynaptic action potential results.
Acetylcholinesterase
Na+
Postsynaptic
membrane
5. Once ACh is released into the synaptic cleft it binds to the receptors for
ACh on the postsynaptic membrane and causes Na+ channels to open.
Na+
6. ACh is rapidly broken down in the synaptic cleft by etylcholinesterase to acetic acid and choline.
7. The choline is reabsorbed by the presynaptic terminal and combined
with acetic acid to form more ACh, which enters synaptic vesicles.
Acetic acid is taken up by many cell types.

2. Action
potential
Ca2+
channel
Presynaptic
terminal
Ca2+
An action potential arrives at the presynaptic terminal causing voltage
gated Ca2+ channels to open, increasing the Ca2+ permeability of
the presynaptic terminal.

3. Ca2+
channel
Presynaptic
terminal
Ca2+
ACh
Ca2+ enters the presynaptic terminal and initiates the release of a
neurotransmitter, acetylcholine (ACh), from synaptic vesicles in the
presynaptic terminal.

4. Synaptic cleft
Na+
ACh
Receptor
molecule
Na+
Diffusion of ACh across the synaptic cleft and binding of ACh to ACh
receptors on the postsynaptic muscle fiber membrane causes an increase in the permeability of ligand-gated Na+ channels.

5. Na+
Action
potential
Action
potential
Na+
The increase in Na+ permeability results in depolarization of the
postsynaptic membrane; once threshold has been reached a
postsynaptic action potential results.

6. ACh on the postsynaptic membrane and causes Na+ channels to open.
cleft
ACh
receptor
site
Postsynaptic
membrane
Na+
Once ACh is released into the synaptic cleft it binds to the receptors for
ACh on the postsynaptic membrane and causes Na+ channels to open.

7. ACh is rapidly broken down in the synaptic cleft by
Choline
Acetic
acid
ACh
receptor
site
Acetylcholinesterase
ACh is rapidly broken down in the synaptic cleft by
acetylcholinesterase to acetic acid and choline.

8. The choline is reabsorbed by the presynaptic terminal and combined
ACh
Acetic
acid
Synaptic
vesicle
Choline
ACh
Choline
The choline is reabsorbed by the presynaptic terminal and combined
with acetic acid to form more ACh, which enters synaptic vesicles.

9. Acetic acid is taken up by many cell types.
Choline
Acetic
acid
Acetic acid is taken up by many cell types.