What is the role of the motor neuron and the neuromuscular junction in initiating muscle contraction?

• A. An alpha motor neuron releases acetylcholine at the NMJ, causing a depolarizing end-plate potential that triggers the muscle action potential and contraction.
• B. Motor neurons directly depolarize muscle fibers without neurotransmitters.
• C. NMJ inhibits contraction by blocking calcium entry.
• D. Acetylcholine at NMJ hyperpolarizes muscle membrane.

Answer: (A)

The signal to start a muscle contraction comes from the motor neuron releasing acetylcholine at the neuromuscular junction. This acetylcholine binds to nicotinic receptors on the motor end plate, opening channels that depolarize the muscle membrane and create a local end-plate potential. If this depolarization reaches threshold, it triggers an action potential in the muscle fiber, which then leads to calcium release inside the muscle and the cycling of actin-mmyosin cross-bridges that cause contraction. The other options aren’t correct because the muscle fiber’s initial depolarization and subsequent contraction rely on a chemical signal from the neuron, acetylcholine at the NMJ is excitatory rather than inhibitory, and it does not hyperpolarize the membrane or block calcium entry.