CVS Practice Test

The release of calcium from the sarcoplasmic reticulum (SR) is predominantly induced by an action potential traveling through the T-tubules. When an action potential reaches the muscle fiber’s membrane, it propagates along the T-tubules, leading to depolarization of the membrane. This depolarization activates voltage-sensitive dihydropyridine receptors (DHPR) that are located in the T-tubule membrane. These receptors are mechanically linked to ryanodine receptors (RyR) on the SR membrane.

As the action potential passes through the T-tubules, it causes a conformational change in the DHPR, which in turn stimulates the RyR to open. This opening allows a significant amount of calcium to flow from the SR into the cytosol, facilitating muscle contraction. This process represents the excitation-contraction coupling mechanism in skeletal muscle, where the electrical signal is converted into a mechanical response.

In contrast, the other options do not accurately describe the primary mechanism for calcium release from the SR. The binding of ATP to myosin does not cause the release of calcium; rather, it is involved in the cross-bridge cycle after calcium has already been released. A decreased calcium concentration in the cell would not