IMAT 2015 Q36 [Anaesthetics]

Some local anaesthetics affect nervous impulse transmission by making it more difficult for voltage­-gated sodium channels to open. In a human patient treated with these anaesthetics which of the following statements would be true?

A. The threshold potential required to generate an action potential would be lowered.

B. The axon membrane would become more difficult to repolarise.

C. It would become harder to depolarise the neuron.

D. The membrane potential of the neuron at rest would decrease below –90 mV.

E. The sodium/potassium pump may need to work faster to maintain the resting potential.

The correct choice for this question is choice C.

It would be useful to keep the central concept that has already been detailed in this question: "…making it more difficult for voltage-gated sodium channels to open." Thus, this concept regarding anaesthetics will be used to rationalize the choices below.

As a reminder, what is the role of voltage gated sodium channels in terms of nerve impulse transmission? Once a stimuli depolarizes the membrane enough to reach threshold potential, these channels respond to the change of transmembrane potential by allowing more sodium ions to pass through, which depolarizes the membrane even more. This allows for the propagation of nerve impulse transmission.

With that being said, let’s rationalize the choices given:

  • Choice A: This is not the correct choice as it is not in line with the central concept that we established earlier. Anaesthetics affect these voltage-gated channels and not the threshold potential. In such a way, regardless of how often the stimulus induces threshold potential (and regardless of how low the threshold potential is), full depolarization cannot be achieved if sodium ions cannot pass through the membrane in the first place. In addition for clarity sake, a lower threshold potential would also mean it would be easier for stimuli to evoke depolarization, which is not an objective of anaesthetics.

  • Choice B: Repolarization is not relevant to this question since it is potassium channels that mediate this and not sodium channels. During repolarization, sodium channels close and potassium channels open.

  • Choice C: This is the correct choice, since it is consistent with the central concept that we established earlier. Without voltage gated sodium channels, there will be no response to the stimuli reaching threshold potential, regardless of the strength of the stimuli. Without the opening of these sodium channels in the first place, it will be much more difficult to depolarize the neuron.

  • Choice D: Decreasing the resting membrane potential will make it easier for stimuli to induce depolarization, which is not relevant to the objective of anesthetics (which is to make depolarization HARDER), making this an incorrect consideration.

  • Choice E: Again, the resting membrane potential is not relevant to this question. The effects of anaesthetics is mainly concerned about the ability of voltage gated sodium channels to respond to stimuli depolarizing the membrane enough to reach threshold potential. In this case, voltage gated sodium channels do not play a role in maintaining resting membrane potential, which makes this an incorrect choice.

Thus, only choice C is the correct choice out of all the other choices.