When the end-plate potential reaches threshold, which event occurs first?

• A. Voltage-gated Na+ channels open causing depolarization
• B. Voltage-gated Ca2+ channels open
• C. Muscle relaxation occurs due to Ca2+ sequestration
• D. Tropomyosin shifts to cover myosin-binding sites

Answer: (A)

When the end-plate potential reaches threshold, the membrane at the motor end plate opens voltage-gated sodium channels. This allows Na+ to rush in, producing a rapid depolarization that triggers a muscle action potential to sweep along the sarcolemma and into the T-tubules. That action potential then leads to calcium release from the sarcoplasmic reticulum and the subsequent steps of contraction. Other events listed happen later: opening of Ca2+ channels occurs as part of the excitation-contraction coupling after the action potential, Ca2+ sequestration ends contraction as the cytosolic Ca2+ is pumped back, and tropomyosin movement to expose myosin-binding sites happens only after Ca2+ binds to troponin.