The contraction of skeletal muscles primarily depends on which substance?

The contraction of skeletal muscles primarily depends on calcium because it plays a crucial role in the process of muscle contraction. When a muscle is stimulated by a nerve impulse, there is a release of calcium ions from the sarcoplasmic reticulum, a structure that stores calcium within the muscle cells.

These calcium ions bind to troponin, a regulatory protein that is part of the thin filaments in muscle fibers. When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from myosin-binding sites on actin filaments. This exposure allows the myosin heads to attach, and the process of muscle contraction begins as the myosin heads pull on the actin filaments, leading to muscle shortening.

Therefore, calcium is essential for initiating and regulating the contraction of skeletal muscles, making it the correct answer in this context.