Which sequence describes the events in generation of an action potential in a skeletal muscle fiber?

Which sequence describes the events in generation of an action potential in a skeletal muscle fiber?

• A. End-plate potential reaches threshold; voltage-gated Na+ channels open; depolarization; voltage-gated K+ channels open; repolarization
• B. Ca2+ release from SR triggers depolarization
• C. Myosin binding to actin triggers action potential
• D. ACh is released in the neuromuscular junction after AP

Answer: (A)

Generation of a skeletal muscle action potential starts at the neuromuscular junction. Acetylcholine released from the motor neuron binds to nicotinic receptors on the motor end plate, creating a graded end-plate potential. If this depolarization is large enough to reach the threshold, voltage-gated Na+ channels open, causing a rapid influx of Na+ and the upstroke of the action potential (depolarization). This is quickly followed by opening of voltage-gated K+ channels to let K+ out, driving repolarization and the return to the resting membrane potential. After the action potential, excitation-contraction coupling occurs with calcium release from the sarcoplasmic reticulum triggering contraction, but the AP itself is generated in this sequence: EPP reaching threshold → Na+ channel opening → depolarization → K+ channel opening → repolarization. The other options misplace key events: Ca2+ release from the SR happens after the action potential, not to generate it; myosin binding to actin occurs during contraction, not during AP generation; and ACh release at the NMJ happens before the end-plate potential and the muscle AP, not after.

Generation of a skeletal muscle action potential starts at the neuromuscular junction. Acetylcholine released from the motor neuron binds to nicotinic receptors on the motor end plate, creating a graded end-plate potential. If this depolarization is large enough to reach the threshold, voltage-gated Na+ channels open, causing a rapid influx of Na+ and the upstroke of the action potential (depolarization). This is quickly followed by opening of voltage-gated K+ channels to let K+ out, driving repolarization and the return to the resting membrane potential. After the action potential, excitation-contraction coupling occurs with calcium release from the sarcoplasmic reticulum triggering contraction, but the AP itself is generated in this sequence: EPP reaching threshold → Na+ channel opening → depolarization → K+ channel opening → repolarization.

The other options misplace key events: Ca2+ release from the SR happens after the action potential, not to generate it; myosin binding to actin occurs during contraction, not during AP generation; and ACh release at the NMJ happens before the end-plate potential and the muscle AP, not after.