Question

Which one of the following correctly describes the sequence of events leading to muscle contraction following acetylcholine release at the neuromuscular junction?

• A. Increase of Ca$^{++}$ in sarcoplasm, unmasking of active sites for myosin, ‘Z' lines pulled inwards
• B. Increase of Ca$^{++}$ in sarcoplasm, masking of active sites for myosin, ‘Z' lines pulled inwards
• C. Increase of Ca$^{++}$ in sarcoplasm, unmasking of active sites for myosin, ‘Z' lines pulled outwards
• D. Decrease of Ca$^{++}$ in sarcoplasm, masking of active sites for myosin, ‘Z' lines pulled inwards

Hint:

To contract a muscle, calcium must go UP (to bind troponin), active sites must be UNMASKED (so myosin can grab actin), and the sarcomere must shorten, which means Z-lines are pulled INWARDS.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question tests our understanding of the sliding filament theory and the excitation-contraction coupling process in skeletal muscle contraction.

Step 2: Key Formula or Approach:
Trace the biochemical and physical events that occur immediately after the neurotransmitter acetylcholine (ACh) is released into the synaptic cleft at the neuromuscular junction.

Step 3: Detailed Explanation:
Let's break down the sequence of events step by step:

• 1. Action Potential & Calcium Release:
- Acetylcholine binds to receptors on the sarcolemma, generating an action potential.
- This electrical signal propagates down the transverse tubules (T-tubules) and triggers the release of calcium ions ($\text{Ca}^{2+}$) from the sarcoplasmic reticulum into the sarcoplasm.
- This leads to a rapid increase of $\text{Ca^{2+}$ in the sarcoplasm}.

• 2. Unmasking of Myosin Binding Sites:
- The released $\text{Ca}^{2+}$ binds to the subunit troponin C on the actin (thin) filaments.
- This binding induces a conformational change in the troponin-tropomyosin complex, shifting tropomyosin away from the active sites on the actin strand.
- This process represents the unmasking of active sites for myosin.

• 3. Cross-bridge Cycling and Sarcomere Shortening:
- Once the active sites are exposed, energized myosin heads bind to actin to form cross-bridges.
- Myosin performs the "power stroke," pulling the thin actin filaments towards the center of the sarcomere (A-band).
- Because actin filaments are anchored to the Z-lines, this pulling action pulls the Z-lines inwards, shortening the sarcomere and resulting in muscle contraction.

Step 4: Final Answer:
The correct sequence is: Increase of $\text{Ca}^{2+}$ in sarcoplasm $\rightarrow$ unmasking of active sites for myosin $\rightarrow$ Z-lines pulled inwards. This matches option (A).