Question

current attempt in progress
after a skeletal muscle has contracted, what most directly leads to its relaxation?
○ decreased levels of ca++ in the sarcoplasm
○ decreased levels of ca++ in the sarcoplasmic reticulum
○ decreased levels of calsequestrin
○ decreased levels of terminal cisternae

Explanation:

When a skeletal muscle contracts, calcium ions (\(Ca^{++}\)) are released from the sarcoplasmic reticulum into the sarcoplasm. These \(Ca^{++}\) ions bind to troponin, causing a conformational change that allows myosin heads to interact with actin filaments, leading to muscle contraction. For the muscle to relax, the \(Ca^{++}\) ions need to be removed from the sarcoplasm. The sarcoplasmic reticulum actively pumps \(Ca^{++}\) back into itself, reducing the \(Ca^{++}\) levels in the sarcoplasm. When \(Ca^{++}\) levels in the sarcoplasm decrease, troponin returns to its original conformation, blocking the myosin - binding sites on actin, and the muscle relaxes. Calsequestrin is a protein in the sarcoplasmic reticulum that binds \(Ca^{++}\), but a decrease in its level is not the direct cause of muscle relaxation. Terminal cisternae are part of the sarcoplasmic reticulum structure and their level decrease is not relevant to muscle relaxation. A decrease in \(Ca^{++}\) in the sarcoplasmic reticulum would mean less \(Ca^{++}\) is available to be released in the first place for contraction, not relaxation.

Answer:

decreased levels of \(Ca^{++}\) in the sarcoplasm