Question

during excitation of a muscle fiber, which one of the following events comes immediately after the release of acetylcholine from the motor neuron?

o action potential arrives at the synaptic knob of the motor neuron
o sodium enters the muscle cell
o acetylcholine diffuses across the synaptic cleft
o acetylcholine binds to its receptors

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Explanation:

When acetylcholine is released from the motor neuron, it first has to cross the synaptic cleft to reach the muscle - fiber side. After diffusing across the synaptic cleft, it can bind to its receptors on the muscle fiber. Action potential arriving at the synaptic knob is the event that leads to acetylcholine release. Sodium entering the muscle cell occurs after acetylcholine binds to its receptors and causes depolarization.

Answer:

Acetylcholine diffuses across the synaptic cleft