Question

what happens when acetylcholine stimulates its receptors in the neuromuscular junction?
the release of ca²⁺ from the sarcoplasmic reticulum decreases.
the permeability of the sarcolemma to na⁺ increases.
the positive charge on the sarcolemma decreases.
the threshold of the muscle fiber lowers.
the inhibitory effect of acetylcholinesterase is overridden.

Explanation:

When acetylcholine stimulates its receptors in the neuromuscular junction, ligand - gated ion channels (specifically for \(Na^{+}\)) open. This leads to an influx of \(Na^{+}\) ions into the muscle cell (sarcolemma). The influx of \(Na^{+}\) is a key step in initiating muscle contraction.

• For the option "The release of \(Ca^{2+}\) from the sarcoplasmic reticulum decreases": Acetylcholine binding is part of the process that actually leads to an increase in \(Ca^{2+}\) release (through a series of events like depolarization and excitation - contraction coupling), so this is incorrect.
• For the option "The positive charge on the sarcolemma decreases": When \(Na^{+}\) (a positively - charged ion) enters the cell, the positive charge on the sarcolemma (which is relatively negative at rest) becomes less negative (more positive), so this is incorrect.
• For the option "The threshold of the muscle fiber lowers": Acetylcholine binding does not directly lower the threshold of the muscle fiber. The threshold is a property related to the electrical excitability of the membrane based on ion channel characteristics and membrane potential, and acetylcholine's primary role is in ion flux, not threshold change.
• For the option "The inhibitory effect of acetylcholinesterase is overridden": Acetylcholinesterase is an enzyme that breaks down acetylcholine. Acetylcholine binding to its receptors is a separate event from overriding the effect of acetylcholinesterase. The action of acetylcholinesterase is to terminate the signal of acetylcholine, not related to the initial binding and ion flux.

Answer:

The permeability of the sarcolemma to \(Na^{+}\) increases.