Question

Select the correct statements regarding mechanism of muscle contraction.
A. It is initiated by a signal sent by CNS via sensory neuron.
B. Neurotransmitter generates action potential in the sarcolemma.
C. Increased Ca⁺⁺ level leads to the binding of calcium with troponin on action filaments.
D. Masking of active site for actin is activated.
E. Utilising the energy from ATP hydrolysis to form cross bridge.
Choose the most appropriate answer from the options given below:

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

Answer 1

The Correct Option is A

Muscle contraction is a complex process involving several steps. Let’s analyze the given statements:

A. Initiation by CNS via sensory neuron: Muscle contraction is initiated by signals from the CNS, but these signals are sent via motor neurons, not sensory neurons. Sensory neurons carry signals from the periphery to the CNS. So, statement A is incorrect.
B. Neurotransmitter and action potential: Neurotransmitters (e.g., acetylcholine) released from motor neurons bind to receptors on the muscle cell membrane (sarcolemma), generating an action potential that spreads across the sarcolemma and into the T-tubules. So, statement B is correct.
C. Calcium binding to troponin: The action potential triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum. The increased Ca²⁺ concentration in the cytoplasm leads to the binding of calcium with troponin, a protein complex on the actin filaments. This binding causes a conformational change in tropomyosin, exposing the myosin-binding sites on actin. So, statement C is correct.
D. Masking of active site for actin: The active sites on actin are initially masked by tropomyosin, not activated. The increased Ca²⁺ level and its binding to troponin causes tropomyosin to move away from the myosin-binding sites on actin, unmasking them. So, statement D is incorrect.
E. ATP hydrolysis and cross-bridge formation: ATP hydrolysis provides the energy for myosin heads to bind to the exposed active sites on actin, forming cross-bridges. The subsequent ”power stroke” of the myosin heads pulls the actin filaments towards the center of the sarcomere, resulting in muscle shortening (contraction). So, statement E is correct.