Which of the following correctly lists the steps of muscle relaxation in order, starting from the cessation of a nerve impulse?

• A. Calcium ions are released from the sarcoplasmic reticulum → Acetylcholinesterase breaks down acetylcholine → Troponin and tropomyosin block actin binding sites → Muscle fiber relaxes
• B. Acetylcholinesterase breaks down acetylcholine → Calcium ions are pumped back into the sarcoplasmic reticulum → Troponin and tropomyosin block actin binding sites → Muscle fiber relaxes
• C. Troponin and tropomyosin block actin binding sites → Acetylcholinesterase breaks down acetylcholine → Calcium ions are pumped back into the sarcoplasmic reticulum → Muscle fiber relaxes
• D. Acetylcholine is released into the synaptic cleft → Calcium ions are pumped back into the sarcoplasmic reticulum → Troponin and tropomyosin block actin binding sites → Muscle fiber relaxes

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the process of muscle relaxation, which begins with the cessation of a nerve impulse. This stops the release of acetylcholine into the synaptic cleft.
2. Step 2: Acetylcholinesterase, an enzyme, breaks down acetylcholine in the synaptic cleft, preventing further stimulation of the muscle fiber.
3. Step 3: Calcium ions are pumped back into the sarcoplasmic reticulum via active transport, reducing their concentration in the cytosol.
4. Step 4: Troponin and tropomyosin return to their resting positions, blocking the binding sites on actin filaments and preventing cross-bridge formation with myosin.
5. Step 5: With no cross-bridge cycling occurring, the muscle fiber relaxes and returns to its resting state.