What is the motor endplate and which receptor type is present there?

• A. Region where motor neurons synapse on muscle; muscarinic receptors mediate depolarization.
• B. Junction where muscle fibers abut each other; GABA receptors mediate depolarization.
• C. Specialized postsynaptic region of a muscle fiber; nicotinic acetylcholine receptors mediate depolarization.
• D. Terminal of sensory neuron; nicotinic receptors mediate hyperpolarization.

Answer: (C)

The motor endplate is the specialized postsynaptic region of a muscle fiber where the motor neuron forms a synapse. Here, acetylcholine released from the motor neuron binds to nicotinic acetylcholine receptors on the muscle membrane. These receptors are ligand-gated ion channels that open to allow Na+ (and some K+) to flow, producing a depolarizing end-plate potential that triggers a muscle action potential and contraction. This excitatory, fast receptor action distinguishes it from other receptors: muscarinic receptors are slower, metabotropic and not responsible for the rapid depolarization at the NMJ; GABA receptors are inhibitory and not part of the neuromuscular junction; and the sensory-terminal scenario does not describe the motor endplate.