During the phase of excitation-contraction coupling of skeletal muscle, what occurs immediately after the action potential spreads down the T tubule?

• A. Acetylcholine is released from the axon terminus.
• B. The cross bridge forms.
• C. The power stroke takes place.
• D. The sarcoplasmic reticulum releases calcium.

Answer: (D)

Verified step by step guidance

1. Understand the process of excitation-contraction coupling in skeletal muscle, which is the sequence of events that converts an action potential in a muscle fiber into a contraction.
2. Recognize that the action potential travels along the sarcolemma and down the T tubules, which are invaginations of the sarcolemma that penetrate into the cell's interior.
3. Identify that the T tubules are closely associated with the sarcoplasmic reticulum, a specialized endoplasmic reticulum that stores calcium ions.
4. After the action potential reaches the T tubules, it triggers the opening of voltage-sensitive channels in the sarcoplasmic reticulum.
5. The opening of these channels results in the release of calcium ions from the sarcoplasmic reticulum into the cytosol, which is the immediate next step in the process.