How does the increase in intracellular calcium cause contraction?
Posted July 22, 2021
Muscles contract in response to depolarization, activation of G-protein-coupled receptors and other stimuli. Changes in intracellular Ca2+ concentration play an important role in the excitation-contraction-relaxation cycle of skeletal muscle. Calcium levels within muscle cells are controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum that’s present in the sarcoplasm. Intracellular calcium concentrations increase when the sarcoplasmic reticulum releases calcium ions. Contraction is triggered when these calcium ions bind to the protein complex troponin, uncovering the active-binding sites on actin. As soon as the actin-binding sites are exposed, the high-energy myosin head bridges the gap, resulting in the formation of a cross-bridge. On binding to the actin, P1 is released and myosin changes to a lower energy state. An acute increase of Ca2+ results in excessive muscle contraction causing rigidity leading to mitochondrial damage, disorganization of myofibrils and muscle weakness. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, decreasing intracellular calcium levels, which allows the muscle cell to relax.