The neurotransmitter acetylcholine, released by a neuron at a neuromuscular junction binds to nicotinic acetylcholine receptors on the cell membrane of the skeletal muscle cell these receptors are ligand-gated ion channels that allow both na and 'to flow across the membrane when engaged with a ligand. notably, only one of the two ions crosses the membrane at appreciable rates when acetylcholine first binds to the receptor. which ion is it, and how do you know this? (use information from part a of this problem to support your answer.)