Final answer:
When acetylcholine binds to its receptors, an influx of sodium ions (Na+) occurs, resulting in depolarization and the initiation of an action potential which eventually leads to muscle contraction.
Step-by-step explanation:
Upon the binding of acetylcholine (ACh), a neurotransmitter, to receptors in the motor end-plate of a cell, there is a rapid influx of sodium (Na+) ions into the muscle cell. This influx of sodium triggers a depolarization event, referred to as an end-plate potential, which spreads along the sarcolemma to create an action potential that leads to muscle contraction. In the process of neurotransmission, calcium ions (Ca2+) inside the nerve cell prompt the release of ACh into the synaptic cleft, where it can bind with the ACh receptors on the motor end plate, opening sodium channels and allowing Na+ ions to rush into the cell.