an action potential
Channel: mohamed abdelrehim
Date: 2011-09-06

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An action potential arrives at the presynaptic terminal causing voltage-gated 
calcium ion channels to open, increasing the calcium ion permeability of the 
presynaptic terminal cell membrane. 



2+
Calcium ions (Ca) enter the presynaptic terminal and cause vesicles to release 
their neurotransmitter, acetylcholine (ACh), from the synaptic vesicles into the 
presynaptic cleft.  


Diffusion of acetylcholine across the synaptic cleft and binding of acetylcholine to 
acetylcholine receptors on the postsynaptic muscle fiber membrane causes an 
increase in the permeability of ligand-gated sodium ion channels.


The movement of sodium ions (Na) into the muscle cell results in depolarization 
of the postsynaptic membrane. Once threshold has been reached, a postsynaptic 
action potential is generated and is propagated over the muscle cell membrane.

Acetylcholine is rapidly broken down to acetic acid and choline in the synaptic 
cleft by the enzyme acetylcholinesterase.


The choline is reabsorbed by the presynaptic terminal and combined with acetic 
acid to form more acetylcholine, which enters the synaptic vesicles.