Final answer:
Acetylcholine receptor-channels allow diffusion of both Na+ and K+ ions when open, playing a key role in generating an action potential and muscle contraction.
Step-by-step explanation:
Acetylcholine receptor-channels allow the diffusion of both sodium (Na+) and potassium (K+) ions when open. These ligand-gated channels are specific types of ion channels that open in response to the binding of a chemical signal, in this case, acetylcholine. Upon activation, these ion channels allow the rapid diffusion of Na+ ions into the cell, contributing to an action potential which leads to cellular responses such as muscle contraction. Simultaneously, K+ ions may also diffuse through the channel but in the opposite direction as part of restoring the membrane potential after an action potential. The precise activity of these channels is crucial for proper electrical signal transmission and is therefore vital in nerve and muscle function.