Final answer:
A nervous impulse reaching a muscle cell causes the release of acetylcholine, which binds to receptors on the muscle cell and initiates a series of events leading to muscle contraction.
Step-by-step explanation:
When a nervous impulse travels from a neuron to a muscle cell, the electrical action potential reaches the end of the neuron's axon, stimulating the release of the neurotransmitter acetylcholine into the neuromuscular junction. The acetylcholine then binds to receptors on the muscle fiber's membrane, allowing the action potential to continue into the muscle fiber. This leads to the release of calcium ions (Ca²+) from the endoplasmic reticulum within the muscle cell, which is essential for muscle contraction.
The Ca²+ ions bind to actin, causing a shift in actin filaments, and expose myosin binding sites. Myosin heads then bind to these sites to form cross-bridges, ultimately causing the muscle to contract as part of the muscle contraction cycle. This process is terminated when acetylcholine is broken down by the enzyme acetylcholinesterase, and Ca²+ returns to the endoplasmic reticulum.