Which of the following describes step three of a muscle contraction?

• A. The myosin head binds to actin forming a cross-bridge.
• B. The sarcomere shortens as the actin filaments slide over the myosin filaments.
• C. Calcium ions are released from the sarcoplasmic reticulum.
• D. ATP binds to the myosin head, causing it to detach from actin.

Answer: (A)

Verified step by step guidance

1. Understand the sequence of events in muscle contraction, which involves several key steps: initiation, cross-bridge formation, power stroke, detachment, and reactivation.
2. Recognize that muscle contraction is initiated when calcium ions are released from the sarcoplasmic reticulum, which is an early step in the process.
3. Identify that the formation of a cross-bridge occurs when the myosin head binds to actin, which is a crucial step in the contraction cycle.
4. Acknowledge that the power stroke occurs as the sarcomere shortens, with actin filaments sliding over myosin filaments, following the cross-bridge formation.
5. Note that ATP binding to the myosin head causes it to detach from actin, allowing the cycle to repeat, which is a later step in the contraction process.