Which of the following correctly lists the steps of muscle relaxation in the proper sequence?

• A. Muscle fiber relaxes, acetylcholine is released, calcium ions are pumped into the sarcoplasmic reticulum, troponin-tropomyosin complex covers binding sites.
• B. Acetylcholine is broken down, calcium ions are pumped back into the sarcoplasmic reticulum, troponin-tropomyosin complex covers binding sites, muscle fiber relaxes.
• C. Troponin-tropomyosin complex covers binding sites, acetylcholine is released, calcium ions are released from the sarcoplasmic reticulum, muscle fiber relaxes.
• D. Calcium ions are released from the sarcoplasmic reticulum, acetylcholine is broken down, muscle fiber relaxes, troponin-tropomyosin complex covers binding sites.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the process of muscle contraction and relaxation. Muscle relaxation occurs after contraction and involves the cessation of nerve signals, removal of calcium ions, and the covering of actin binding sites by the troponin-tropomyosin complex.
2. Step 2: Recall that acetylcholine (ACh) is a neurotransmitter responsible for initiating muscle contraction. During relaxation, acetylcholine is broken down by the enzyme acetylcholinesterase, stopping the signal for contraction.
3. Step 3: Calcium ions, which were released from the sarcoplasmic reticulum during contraction, are actively pumped back into the sarcoplasmic reticulum by calcium pumps. This step is crucial for stopping the interaction between actin and myosin.
4. Step 4: As calcium levels decrease in the cytoplasm, the troponin-tropomyosin complex re-covers the binding sites on actin filaments, preventing further cross-bridge formation between actin and myosin.
5. Step 5: With the binding sites covered and the absence of nerve signals, the muscle fiber relaxes, returning to its resting state.