For muscle contraction to occur, __ is released from the sarcoplasmic reticulum and binds with __ .
A) sodium, tropomyosin
B) calcium, troponin
C) calcium, tropomyosin
D) sodium, troponin
The correct answer and explanation is:
Correct Answer: B) calcium, troponin
Muscle contraction is a complex physiological process that begins with the stimulation of a muscle fiber by a nerve impulse. When an action potential reaches the muscle cell, it travels along the sarcolemma and dives into the cell through structures called T-tubules. This electrical signal then triggers the sarcoplasmic reticulum, a specialized organelle in muscle cells, to release calcium ions into the cytoplasm.
The presence of calcium ions is critical for contraction to occur. Calcium binds specifically to a protein called troponin, which is part of the thin filament of the muscle fiber. The thin filament is mainly composed of actin, along with two regulatory proteins—troponin and tropomyosin. In a resting muscle, tropomyosin blocks the binding sites on actin, preventing interaction with myosin, the motor protein on the thick filament.
When calcium binds to troponin, it causes a change in the shape of the troponin complex. This conformational change pulls tropomyosin away from the actin binding sites. With the binding sites exposed, myosin heads can now attach to actin, forming cross-bridges. This interaction leads to the sliding of actin filaments over myosin filaments, resulting in muscle contraction through a mechanism known as the sliding filament theory.
Once the contraction is complete or the stimulation stops, calcium ions are actively pumped back into the sarcoplasmic reticulum. As calcium levels drop in the cytoplasm, troponin returns to its original shape, allowing tropomyosin to cover the actin binding sites again. This ends the contraction process and the muscle fiber relaxes.
Therefore, calcium and troponin are essential players in the regulation of muscle contraction. Calcium initiates the process by binding to troponin, which leads to structural changes necessary for muscle fibers to contract.