Which ion is crucial for triggering contraction in both skeletal and smooth muscle?

Calcium ions (Ca²⁺) play a pivotal role in the contraction mechanism of both skeletal and smooth muscle types. In skeletal muscle, when a nerve impulse triggers an action potential, calcium is released from the sarcoplasmic reticulum into the cytoplasm. This increase in intracellular calcium concentration allows for the binding of calcium to troponin, which leads to a conformational change that moves tropomyosin away from the binding sites on actin filaments. This exposes the binding sites for myosin, allowing cross-bridge formation and subsequent muscle contraction.

In smooth muscle, the contraction mechanism is somewhat different but still heavily relies on calcium. Calcium enters the smooth muscle cells through voltage-gated calcium channels or is released from the sarcoplasmic reticulum. The rise in calcium levels activates calmodulin, which then activates myosin light-chain kinase (MLCK). Activated MLCK phosphorylates the myosin light chains, allowing myosin to interact with actin, leading to contraction.

Thus, in both skeletal and smooth muscles, calcium serves as the key signaling molecule that triggers contraction by facilitating the interaction between actin and myosin filaments.