Question

1. what has to be released by the sarcolemma and sarcoplasmic reticulum in order to allow for the myosin head to attach to actin?

• phosphorus
• sodium
• calcium
• atp

Explanation:

When a muscle fiber is stimulated, the sarcoplasmic reticulum releases calcium ions (\(Ca^{2+}\)). These \(Ca^{2+}\) bind to troponin on the actin filaments. This binding causes a conformational change in troponin - tropomyosin complex, which exposes the myosin - binding sites on actin. Then, the myosin head can attach to actin. Phosphorus is part of many biological molecules like ATP but is not directly released in this context for myosin - actin binding. Sodium is involved in action potential generation in muscle cells (like in the sarcolemma depolarization) but not for the direct myosin - actin attachment. ATP is used for the power stroke of the myosin head (after the initial binding) and for detaching the myosin head from actin, not for the initial attachment.

Answer:

Calcium