What happens to the endplate potentials on the muscle fibre when the firing frequency of the motoneuron decreases?

Question

A multiple choice question I came across which confused me a little. The options were:

A)The amplitude decreases

B)The duration decreases

C)The frequency decreases

I know that the force of contraction of a muscle fiber decreases when the frequency of the motor neuron decreases. Thus, I would assume that the frequency of the local end-plate potential would also decrease, in order for this to make sense. Following this logic, I answered C, but I am unsure if my reasoning is correct, this answer seems a bit too straightforward.

I considered the answer could be A, but I felt that it wouldn't make sense for the amplitude of the end-plate potential to have any effect on the force of contraction (because the action potential illicited by the end-plate potential is an "all or nothing" event).

I'm a bit stuck on this question and unsure if my logic makes sense, so I was wondering if anyone could help me out.

Answer 1

The answer is A. End-plate potentials (EPPs) are not action potentials.

Action potentials are "all or none" potentials because they occur only for sufficiently large stimuli (White, 2002).

Increasing or decreasing the number of motor neurons active at any one time changes the amount of force produced by a muscle (Purves, et al. 2001). How?

EPPs are generated at the neuromuscular junctions, and are generated by a conductance increase in the end-plate membrane (Van der Kloot & Cohen, 1984).

Feher (2012) points out that:

The end-plate potential is a graded potential (it is not all-or-none) that propagates electrotonically to the neighboring patch of muscle fiber membrane where it initiates an action potential on the muscle much like it does on unmyelinated nerves.

With higher current, you can activate more patches of muscle fibre through more action potentials, creating a stronger contraction.

A reduction in amplitude will result in failure to reach threshold in a proportion of muscle fibres, which results in muscle weakness (Vincent et al. 2007).

Therefore, the answer in your paper is A – The amplitude in the EPP decreases with a reduction in motoneuron firing frequency.

References

Feher, J. (2012). 3.6 - The Neuromuscular Junction and Excitation-Contraction Coupling. In Quantitative Human Physiology: An Introduction, Academic Press, pp259-269. https://doi.org/10.1016/B978-0-12-382163-8.00029-3

Purves D, Augustine GJ, Fitzpatrick D, et al. (2001). The Regulation of Muscle Force. In. Neuroscience. 2nd edition. Sinauer Associates. Available from: https://www.ncbi.nlm.nih.gov/books/NBK11021/

Van der Kloot, W., & Cohen, I. S. (1984). End-plate potentials in a model muscle fiber. Corrections for the effects of membrane potential on currents and on channel lifetimes. Biophysical journal, 45(5), 905–911. https://doi.org/10.1016/S0006-3495(84)84237-X

Vincent, A., Buckley, C. & Burke, G. (2007). Chapter 90 - Neuromuscular Junction Disorders. In Neurology and Clinical Neuroscience, Mosby, pp1223-1234. https://doi.org/10.1016/B978-0-323-03354-1.50094-8

White, J. A. (2002). Action Potential. In Encyclopedia of the Human Brain, Academic Press, pp1-12 https://doi.org/10.1016/B0-12-227210-2/00004-2