A neuromuscular junction is a type of synapse, a gap between a motor neurone and the muscle end plate known as the synaptic cleft which is approximately 50nm wide. At a neuromuscular junction an action potential passes from the presynaptic membrane to the postsynaptic membrane, also known as the junctional folds present on the muscle end plate. This is to allow signal 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 at the axon hillox) travels down the axon by the saltatory effect to reach the axon terminal. This causes the depolarisation of the presynaptic membrane which results in 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. Influx of calcium ions is an example of facilitated diffusion while release of the nerotransmitter is an example of exocitosis. At the neromuscular junction we have conversion of the electric signal into a chmical signal whereby acetylcholine acts as the signalling molecule.
- The neurotransmitter acetylcholine, then diffuses across the synaptic cleft within less than 1 ms and binds to specific receptor proteins on the postsynaptic membrance of the muscle end plate.
- The receptor protein is in fact a ligand-gated sodium channel and hence opens upon reception of signal.
- 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. Choline is then taken up to be recycled by the presynaptic membrane.
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 is greater than the threshold potential (-55mV) and hence can trigger an action potential within the junctional folds where voltage gated sodium channels exist. The action potential triggered in the membrane then lead to contraction of the muscles.  .
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