Answer:
Release of calcium from the terminal cisterns of sarcoplasmic reticulum triggers skeletal muscle contraction.
Explanation:
Muscular contraction is a highly regulated process that depends on free calcium concentration in the cytoplasm. Amounts of cytoplasmic calcium are regulated by sarcoplasmic reticulum that functions as a storage of the ion.
When a nerve impulse reaches the membrane of a muscle fiber, through acetylcholine release, the membrane depolarizes producing the entrance of calcium from extracellular space. The impulse is transmitted along the membrane to the sarcoplasmic reticulum, from where calcium is released. At this point, tropomyosin is obstructing binding sites for myosin on the thin filament. The calcium channel in the sarcoplasmic reticulum controls the ion release, that activates and regulates muscle contraction, by increasing its cytoplasmic levels. When calcium binds to the troponin C, the troponin T alters the tropomyosin by moving it and then unblocks the binding sites, making possible the formation of cross-bridges between actin and myosin filaments. Then myosin binds to the uncovered actin-binding sites, and while doing it ATP is transformed into ADP and inorganic phosphate.
Z-bands are then pulled toward each other, thus shortening the sarcomere and the I-band, and producing muscle fiber contraction.
