What is the role of calcium in muscle contraction?
A.
Calcium binds to tropomyosin to expose the myosin-binding sites on actin.
B.
Calcium is released from the sarcoplasmic reticulum to initiate the sliding of actin and myosin filaments.
C.
Calcium activates the motor neurons to stimulate muscle contraction.
D.
Calcium is required for the relaxation of muscles after contraction.
The Correct Answer and Explanation is:
The correct answer is B. Calcium is released from the sarcoplasmic reticulum to initiate the sliding of actin and myosin filaments.
Explanation:
Muscle contraction is a complex process involving the interaction of actin and myosin filaments within muscle cells. Calcium plays a critical role in this process, specifically in the initiation of contraction.
- Calcium Release and Initiation of Contraction:
- Sarcoplasmic Reticulum (SR): The sarcoplasmic reticulum is a specialized organelle in muscle cells that stores calcium ions. When a muscle fiber is stimulated by a motor neuron, an action potential travels along the muscle cell membrane (sarcolemma) and reaches the SR.
- Calcium Release: In response to this action potential, calcium channels in the SR open, releasing calcium ions into the cytoplasm of the muscle cell. This release of calcium is the initial trigger for muscle contraction.
- Interaction with Actin and Myosin:
- Troponin-Tropomyosin Complex: Within the muscle cell, actin filaments are associated with regulatory proteins troponin and tropomyosin. Troponin is sensitive to calcium ions. When calcium binds to troponin, it induces a conformational change in the troponin-tropomyosin complex, which moves tropomyosin away from the myosin-binding sites on actin filaments.
- Cross-Bridge Formation: With the binding sites exposed, myosin heads, which are part of the thick filaments, can attach to actin filaments, forming cross-bridges. This interaction is powered by ATP hydrolysis, causing the myosin heads to pivot and pull the actin filaments towards the center of the sarcomere, leading to muscle contraction.
- End of Contraction and Muscle Relaxation:
- Calcium Reuptake: After the contraction, calcium ions are actively transported back into the SR. The removal of calcium from the cytoplasm allows tropomyosin to cover the myosin-binding sites on actin, leading to muscle relaxation.
Thus, calcium’s primary role is in initiating the contraction by enabling the interaction between actin and myosin. While it also plays a part in muscle relaxation through reuptake into the SR, its most crucial function is in the initial activation of muscle contraction.