Which description best matches nicotinic acetylcholine receptor function at the neuromuscular junction?

• A. Influx of Na+ and K+ through ligand-gated channels
• B. Activation of G-proteins leading to cAMP
• C. Opening of Cl- channels causing hyperpolarization
• D. Activation of voltage-gated calcium channels only

Answer: (A)

Nicotinic acetylcholine receptors at the neuromuscular junction are ligand-gated ion channels that open when acetylcholine binds. When they open, positively charged ions flow through—primarily sodium entering the muscle cell and potassium leaving. This cation movement creates a depolarizing current, the end-plate potential, which, if large enough, triggers an action potential in the muscle fiber and leads to contraction. This mechanism is fast and ligand-dependent, distinguishing it from pathways that involve G-protein signaling and second messengers, chloride channels that would hyperpolarize, or voltage-gated calcium channels that respond to changes in membrane voltage rather than to a chemical signal.