Final answer:
Through ACh receptors on postsynaptic membranes, Na+ ions flow into the cell, resulting in a depolarization that triggers an action potential and subsequent muscle contraction.
Step-by-step explanation:
When acetylcholine (ACh) is released at a synapse, such as the neuromuscular junction, it binds to ACh receptors on the postsynaptic membrane. This binding causes specific ion channels, particularly ligand-gated channels, to open and allow Na+ ions (sodium ions) to flow into the postsynaptic cell. This influx of Na+ ions leads to a depolarization of the postsynaptic membrane, effectively reducing the voltage difference between the inside and outside of the cell. This depolarization, known as an end-plate potential, spreads across the membrane, creating an action potential. This action potential propagates across the cell, causing a further wave of depolarization and leading to muscle contraction.