Which step of excitation-contraction coupling occurs immediately after the calcium channels open?

The step that occurs immediately after the calcium channels open is the movement of calcium from the cisternae of the sarcoplasmic reticulum into the sarcoplasm. This release of calcium ions is crucial for the excitation-contraction coupling process in muscle cells. When the calcium channels open, calcium ions flow into the cytoplasmic space, increasing their concentration in the sarcoplasm, which is essential for triggering further events in muscle contraction.

Once the calcium is released into the sarcoplasm, it binds to troponin, a regulatory protein associated with actin filaments. This binding causes a conformational change that exposes the myosin-binding sites on actin, enabling the subsequent steps in muscle contraction to occur, such as the formation of cross bridges between actin and myosin. Thus, the opening of the calcium channels sets off a chain reaction that is pivotal for muscle contraction, starting with the immediate release of calcium into the cytoplasm.