Question

Which of the following temporal sequences is correct for excitation-contraction coupling in skeletal muscle?

A. Increased intracellular [Ca²⁺]; action potential in the muscle membrane; crossbridge formation
B. Action potential in the muscle membrane; depolarization of the T tubules; release of Ca²⁺ from the sarcoplasmic reticulum (SR)
C. Action potential in the muscle membrane; splitting of adenosine triphosphate (ATP); binding of Ca²⁺ to troponin C
D. Release of Ca²⁺ from the SR; depolarization of the T tubules; binding of Ca²⁺ to troponin C

Answer 1

The correct sequence for excitation-contraction coupling is the action potential in the muscle membrane, followed by depolarization of the T tubules, and culminating in the release of calcium ions from the sarcoplasmic reticulum.

Step-by-step explanation:

The correct temporal sequence for excitation-contraction coupling in skeletal muscle is given by option B, which states: Action potential in the muscle membrane; depolarization of the T tubules; release of Ca²⁺ from the sarcoplasmic reticulum (SR). To break it down further:

1. Acetylcholine is released from the axon terminal and binds to receptors on the sarcolemma.
2. An action potential is generated and propagates along the sarcolemma and travels down the T tubule.
3. The change in voltage due to the action potential triggers the release of Ca²⁺ from the sarcoplasmic reticulum.
4. Released Ca²⁺ binds to troponin, leading to conformational changes that facilitate crossbridge formation and muscle contraction.

This sequence of events ensures that the muscle contraction is precisely controlled and that the power of the contraction is proportionate to the signal delivered by the nervous system.