Question

What are the predominant determinants of the EPP onset and duration following a presynaptic action potential?

Answer 1

The End Plate Potential or EPP is a local current that develops due to the binding of acetylcholine with the acetylcholine receptors at the postsynaptic membrane.It is the forerunner of the Action potential.

The factors which determine the onset and duration of EPPs are:

1)Once the action potential reaches the neuromuscular junction, an increased permeability to Ca ions occurs in the prejunctional membrane of the nerve so thereby there is an influx of calcium ions from the ECF to the bulbous structure of the nerve..

2) Bursting of the acetylcholine containing vesicles due to the calcium ion entry.

3) Release of acetylcholine neurotransmitter into the synaptic cleft.

4) Acetylcholine esterase enzyme destroys the neurotransmitter acetylcholine released into the synaptic cleft which lysis it within seconds of its release, so this also contributes to the rate and amount of NT available for binding hence on the development and duration of the end plate potential.

5)The location and concentration of receptors for acetylcholine called end-plate receptors located on the postsynaptic membrane where an end plate potential is generated

6)Ligand-gated sodium channels causes an influx of Na ions into the postjunctional membrane. Na ion permeability of the postjunctional membrane brings the normal resting membrane potential(-90mv which is negative) to a positive value and hence closer to depolarization(more positive). The difference between the normal resting membrane potential and the more positive value is called the end plate potential. The entry is closed when Acetylcholine esterase lyses all the available acetylcholine which causes the ligand-gated Na channels to close.

7) Only a high strength EPP(generated at the motor end plate) can generate an action potential on the sarcolemma which propagates and contracts the muscle.