Final answer:
An action potential in a motor neuron triggers the release of the neurotransmitter acetylcholine, which binds to receptors on the muscle fiber causing depolarization, leading to an action potential that results in muscle contraction.
Step-by-step explanation:
When a motor neuron is activated by an action potential, the following sequence of events occurs to stimulate a muscle contraction. Initially, the action potential travels along the axon of the motor neuron to the neuromuscular junction (NMJ). Here, the axon terminal releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft.
The ACh molecules then diffuse across the synaptic cleft and bind to ACh receptors on the sarcolemma at the motor end plate. This binding causes the receptors, which are also ion channels, to open and allow positively charged ions, particularly Na+ ions, into the muscle fiber, leading to depolarization. This step briefly changes the muscle fiber's membrane potential to become less negative.
Subsequently, this depolarization triggers an action potential within the muscle fiber itself that travels down the T tubules, leading to the release of calcium ions (Ca2+) from the sarcoplasmic reticulum. The influx of calcium ions causes the muscle fiber to contract. Thus, the initiation of an action potential in a motor neuron ultimately results in muscle contraction. The entire process transpires in a fraction of a second.