Final answer:
Calcium required for skeletal muscle contraction is released from the sarcoplasmic reticulum (SR) following the depolarization of the sarcolemma and subsequent signaling down the T-tubules. This results in the binding of calcium to troponin, the exposure of active sites on actin, and the formation of cross-bridges between actin and myosin, leading to muscle contraction.
Step-by-step explanation:
From which structure(s) is the calcium released to initiate skeletal muscle contraction?
The calcium required to initiate skeletal muscle contraction is released from the sarcoplasmic reticulum (SR), a specialized form of smooth endoplasmic reticulum found in muscle cells. When a muscle fiber receives a neural signal, an action potential travels along the sarcolemma and down the T-tubules—extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. The action potential triggers the opening of calcium channels in the SR, leading to the release of calcium ions (Ca2+) into the cytosol.
Once in the cytosol, calcium binds to the protein troponin, causing a conformational change that moves the tropomyosin protein away from the active sites on actin filaments. This exposure of the active sites allows the myosin heads to bind to them, forming cross-bridges. As ATP is hydrolyzed, the myosin heads pull on the actin filaments, causing muscle contraction. This process is crucial in the mechanism of muscle contraction, known as the excitation-contraction coupling.