A neuromuscular junction is a type of synapse, a gap between a neurone and the muscle end plate. At a neuromuscular junction an action potential passes from the presynaptic membrane to the postsynaptic membrane on the muscle end plate. This is to allow information to pass from neurone to muscle end plate which will result in the relaxation or the contraction of the muscle.
In order for this action potential to be passed on to the postsynaptic membrane several steps occur:
- An action potential (generated by a graded potential) arrives at the axon terminal. This causes the depolarisation of the presynaptic membrane which is essential to the opening of the voltage-gated calcium channels.
- Calcium ions move down their concentration gradient into the presynaptic membrane causing vesicles containing the neurotransmitter acetylcholine (the most common neurotransmitter) to fuse with the presynaptic membrane.
- The neurotransmitter acetylcholine, then diffuses across the synaptic cleft and binds to the specific receptor proteins on the postsynaptic membrance, the muscle end plate.
- When this happens, it causes ligand-gated sodium channels to open.
- Sodium ions diffuse down their concentration gradient into the postsynaptic membrane, and potassium ions diffuse out of the postsynaptic membrane. This causes the depolarisation to be passed on to the muscle end plate, and the action potential continues.
- To close the ligand-gated sodium channels and stop the depolarisation of the muscle end plate, an enzyme called acetylcholinesterase binds to acetylcholine and breaks it down. It is broken down into choline and acetate which are taken up by the presynaptic membrane to be re-used.
The EPP on the post synaptic membrane reaches -15 mV. This value is half way between the equilibrium potentials of the Sodium and Potassium ions. Importantly -15 mV is not a full depolarisation but the summation of EPP's in junctional folds trigger Action Potentials nearby which propagate into the muscle to initiate contraction.  .