Question

testing a series of pharmaceutical agents for their effects on

the motor system. In the initial state of the system, electrically

stimulating the nerve cell will generate a muscle contraction in

proportion to the intensity of the stimulation.

When tested by stimulating the nerve, the muscle again contracts with normal force and duration.

Agent X is then added to the preparation and this time, electrically stimulating the

nerve fails to elicit any muscle contraction. Direct stimulation of the muscle, elicits normal contraction.

Q)Assuming Agent X alters ion conductances.

Nicotine, an acetylcholine agonist, is added to the preparation. Would

the response of the muscle in the presence of both nicotine and Agent

X be different or the same as the response in the presence of nicotine

alone? Justify your answer.

however, elicits normal contraction.

Answer 1

There two key things to remember:

1. Agent X seems to inhibit neurons' capability of transmitting action potentials by interrupting one type of element (ion channels) of the cycle (enter of Ca2+ > enter of Na+ > getaway of K+) from the cell.
2. The response of the muscle in presence of nicotine is a prolonged contraction, since the nicotinic acetylcholine receptors (nAChR) have more affinity for this ligand than for acetylcholine, these receptors are the 1st step in the action potential cycle (they're involved in the entering of Ca2+ to cell through nAChR since these are ligand-gated ion channels).

Therefore, the response of the muscle in presence of Agent X and Nicotine is: different than in "nicotine only" assay, but, there are 2 types of different:

1. Assuming Agent X affects negatively the entering of Ca2+ ions to cell: the muscle would only contract when stimulated electrically.
2. Assuming Agent X affects negatively the entering/exiting of Na+/K+ ions to cell: the muscle wouldn't contract since there won't be occurring action potentials than can stimulate the motor neurons.