In the process of excitation-contraction coupling in skeletal muscle, which step is indicated by 'a' if it refers to the release of calcium ions from the sarcoplasmic reticulum?

• A. Myosin heads bind to actin forming cross-bridges.
• B. The action potential travels along the sarcolemma and down the T-tubules.
• C. Acetylcholine is released from the motor neuron into the synaptic cleft.
• D. Calcium ions are released from the sarcoplasmic reticulum into the cytosol.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of excitation-contraction coupling in skeletal muscle. This process links the electrical signal (action potential) to the mechanical contraction of the muscle fibers.
2. Step 2: Recognize the role of calcium ions in this process. Calcium ions are stored in the sarcoplasmic reticulum and are released into the cytosol when triggered by an action potential.
3. Step 3: Identify the sequence of events in excitation-contraction coupling: (1) Acetylcholine is released from the motor neuron into the synaptic cleft, (2) The action potential travels along the sarcolemma and down the T-tubules, (3) Calcium ions are released from the sarcoplasmic reticulum into the cytosol, and (4) Myosin heads bind to actin forming cross-bridges.
4. Step 4: Focus on the specific step indicated by 'a' in the problem. The release of calcium ions from the sarcoplasmic reticulum into the cytosol is the key event that allows the muscle contraction to proceed.
5. Step 5: Relate this step to its physiological importance. Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the actin binding sites, enabling myosin heads to form cross-bridges with actin and initiate contraction.