Question

Muscle contraction is triggered:
A) When high levels of oxygen and sugar are released by the sarcolemma.
B) When a surplus of ATP is released by a nerve motor unit.
C) By the release of a neurotransmitter at a synapse that directly causes actin and myosin to slide.
D) By the nerve releasing a neurotransmitter, which triggers a flow of calcium that attaches to actin filaments and exposes the myosin binding

Answer 1

Hint:
The cell membrane of the muscle fibre is called sarcolemma and sarcoplasm contains fine thread-like contractile structures called myofibrils. The signal for muscle contraction comes from the nerve by triggering an action potential in the sarcolemma.

Complete step by step solution:
Action potential from sarcolemma then propagates through the T-tubules. The t-tubule membrane contains voltage-sensitive proteins, which are activated by the incoming action potential. An increase in cytoplasmic calcium initiates the contraction of each myofibril. The increase in calcium concentration is transient because calcium is rapidly pumped back into the sarcoplasmic reticulum by an abundant, calcium ATPase, in its membrane. Typically, the cytoplasmic calcium concentration is restored to resting levels within 30 milliseconds, allowing the myofibrils to relax.

Option ‘D’ is correct

Note:
In resting muscle troponin T and troponin I interact with tropomyosin to keep tropomyosin away from the groove of thin filament i.e., actin. This prevents myosin heads within actin filaments, thereby preventing any force-generating interaction. When calcium level increases, troponin C binds to calcium, and troponin I releases its hold on actin. This allows tropomyosin to slip back to its normal position and the myosin head can walk along with actin filaments.