How are skeletal muscles stimulated to contract?

• A. By a hormone signal
• B. By a nerve (motor neuron)
• C. By stretching
• D. By a chemical signal from blood

Answer: (B)

Skeletal muscles contract when a motor neuron sends a signal to the muscle fiber it controls. The nerve impulse arrives at the neuromuscular junction and triggers the release of acetylcholine into the synaptic cleft. Acetylcholine binds receptors on the motor end plate, creating an action potential that travels along the sarcolemma and into the T-tubules. This depolarization opens channels in the sarcoplasmic reticulum, releasing calcium into the cytosol. Calcium binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin, allowing cross-bridge cycling with myosin heads powered by ATP. The contraction continues as long as calcium remains elevated and acetylcholine signaling persists; it ends when acetylcholine is degraded by acetylcholinesterase. While hormones in the blood or stretch can modulate muscle function or trigger reflexive contractions, the direct, immediate trigger for skeletal muscle contraction is the motor neuron impulse delivering acetylcholine at the neuromuscular junction.