Final answer:
When calcium ions are present in the cytosol of the muscle cell, it triggers a series of events that lead to muscle contraction.
Step-by-step explanation:
When calcium ions are present in the cytosol of the muscle cell, several events occur that lead to muscle contraction. First, acetylcholine (ACh) is released from synaptic vesicles and binds to receptors on the sarcolemma. This triggers an action potential that travels along the T-tubules and causes the release of calcium ions from the sarcoplasmic reticulum (SR). The calcium ions then bind to actin, allowing for the formation of cross-bridges with myosin and initiating muscle contraction.