Acetylcholine receptors on skeletal muscles are described as being ionotropic receptors because

they are Na+, K+ and Ca2+ ion channels.

Explanation:

Ionotropic acetylcholine receptors are also called nicotinic acetylcholine receptors because beside acetylcholine (Ch) they respond to nicotine. These receptors are  primary receptors in muscle for motor nerve-muscle communication that controls muscle contraction.

Two molecules of ACh are required for receptor to open. Since the receptors are  linked to ion channels, the channels open. Opening of the channel allows positively charged ions to move across it: sodium enters the cell and potassium exits.

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