Question

Excitation-contraction coupling in skeletal muscle begins with an excitatory depolarization of the muscle fiber membrane (sarcolemma). This depolarization triggers the all-or-none opening of voltage-sensitive Na⁺ channels and an action potential that travels deep into the muscle fiber via the T tubule network. At the T tubule-sarcoplasmic reticulum "triad," the depolarization of the T tubule causes a conformational change in the dihydropyridine receptor and subsequently in the ryanodine receptor on the sarcoplasmic reticulum. The latter causes the release of Ca++ into the sarcoplasm and the binding of Ca++ to troponin C (not to calmodulin) on the actin filament. What is the initial event in excitation-contraction coupling in skeletal muscle?

1) Excitatory depolarization of the muscle fiber membrane
2) Opening of voltage-sensitive Na⁺ channels
3) Action potential traveling deep into the muscle fiber
4) Conformational change in the dihydropyridine receptor

Answer 1

The initial event in excitation-contraction coupling in skeletal muscle is the excitatory depolarization of the muscle fiber membrane (sarcolemma).

Step-by-step explanation:

The initial event in excitation-contraction coupling in skeletal muscle is the excitatory depolarization of the muscle fiber membrane (sarcolemma). This depolarization triggers the opening of voltage-sensitive Na⁺ channels and an action potential that travels deep into the muscle fiber via the T tubule network.

At the T tubule-sarcoplasmic reticulum "triad," the depolarization of the T tubule causes a conformational change in the dihydropyridine receptor and subsequently in the ryanodine receptor on the sarcoplasmic reticulum. This leads to the release of Ca++ into the sarcoplasm and the binding of Ca++ to troponin C on the actin filament.