Question

Choose the correct statements regarding muscle contraction.
A. A motor neuron carries a signal sent by the Central Nervous System (CNS) to the sarcolemma of the muscle fibre.
B. The neural signal generates an action potential which causes the release of $Ca^{++$ into sarcoplasm.
C. Increase in $Ca^{++}$ inactivates the actin for breaking cross bridges.
D. Actin binds to the myosin head to form a cross bridge.
E. Shortening of sarcomere takes place, by pulling actin filaments towards the centre of 'A' band.
Choose the correct answer from the options given below:}

• A. C and E only
• B. A, B and E only
• C. A and B only
• D. C and D only

Hint:

Logic Tip: Calcium is the universal "Go!" signal for muscle contraction. It exposes binding sites (activates), it never inactivates them. Knowing Statement C is false instantly eliminates options 1 and 4.

Answer 1

The Correct Option is B

Step 1 : Understanding the Question:
This question focuses on the biochemical and mechanical sequence of events involved in skeletal muscle contraction according to the sliding filament theory. We need to evaluate various statements regarding neural stimulation, ion release, molecular interactions between actin and myosin, and the resulting structural changes in the sarcomere to identify which are factually correct according to standard physiological models.
Step 2 : Key Formulas and Approach:
The approach involves tracing the contraction process from the nervous system trigger to the mechanical shortening:
1. Initiation: Motor unit stimulation via the CNS at the neuromuscular junction.
2. Excitation-Contraction Coupling: The spread of action potential and subsequent release of Calcium ions from the sarcoplasmic reticulum.
3. Regulatory Mechanism: The role of Calcium binding to troponin to uncover active sites.
4. Cross-bridge Cycle: ATP-dependent interaction between thick (myosin) and thin (actin) filaments.
5. Structural outcome: Movement of filaments resulting in sarcomere shortening.
Step 3 : Detailed Explanation:

Statement A: Muscle contraction is initiated by a command from the Central Nervous System. This electrical signal travels via a motor neuron to the motor end plate. Upon reaching the terminal, it causes the release of acetylcholine, which depolarizes the sarcolemma. Thus, this statement accurately describes the neural initiation phase.

Statement B: The depolarization of the sarcolemma generates an action potential that travels deep into the muscle fiber via T-tubules. This electrical disturbance triggers the sarcoplasmic reticulum to release stored Calcium ions ($Ca^{++}$) into the sarcoplasm. This is a vital step in coupling excitation to contraction. Thus, this statement is correct.

Statement C: In the presence of $Ca^{++}$, the ions bind to the troponin subunit on the actin filament. This causes a shift in tropomyosin, which unmasks the active binding sites. This activates the actin for cross-bridge formation. It does not inactivate it. Therefore, this statement is incorrect.

Statement D: During the formation of a cross-bridge, the myosin head (which has bound and hydrolyzed ATP) binds to the exposed active sites on the actin filament. While the two proteins interact, the physical "binding" action is usually attributed to the myosin head attaching to the actin track. However, Statement D is often considered less precise than the mechanical descriptions of the myosin "grabbing" the actin.

Statement E: As the myosin heads pull the actin filaments toward the M-line (center of the A-band), the Z-lines move closer together. This leads to the shortening of the sarcomere, which is the functional unit of contraction. This describes the physical basis of the sliding filament theory. Thus, this statement is correct.

Step 4 : Final Answer:
By evaluating the physiological mechanics, we find that statements A, B, and E are the only ones that accurately represent the contraction process. Therefore, the correct option is (B).