During a twitch in a skeletal muscle fiber, which of the following occurs?

• A. A single, brief contraction and relaxation cycle is produced in response to one action potential.
• B. Multiple action potentials are required to generate a single twitch.
• C. The muscle fiber remains in a sustained contraction until ATP is depleted.
• D. Calcium ions are permanently removed from the sarcoplasm.

Answer: (A)

Verified step by step guidance

1. Understand the concept of a muscle twitch: A muscle twitch is the response of a skeletal muscle fiber to a single action potential. It consists of three phases: the latent period, contraction phase, and relaxation phase.
2. Recognize the role of action potentials: A single action potential triggers the release of calcium ions from the sarcoplasmic reticulum, initiating the contraction process. This results in a single, brief contraction and relaxation cycle.
3. Clarify why multiple action potentials are not required: A single twitch is produced by one action potential. Multiple action potentials would lead to summation or tetanus, not a single twitch.
4. Explain the role of ATP: ATP is required for muscle contraction and relaxation, but during a twitch, the muscle fiber does not remain in sustained contraction until ATP is depleted. Sustained contraction occurs in tetanus, not a twitch.
5. Discuss calcium ion dynamics: Calcium ions are temporarily released into the sarcoplasm during a twitch and are then actively pumped back into the sarcoplasmic reticulum. They are not permanently removed from the sarcoplasm.