MEDICAL PHYSIOLOGY

Detailed explanation-1: -The mobilized calcium ions leave the sarcoplasmic reticulum and move into the myofibrils where they combine with troponin on the actin filaments (Figure 5.3, upper). The inhibitory action of tropomyosin is removed and the sliding movement between the actin and myosin filaments occurs (see page 38).

Detailed explanation-2: -Terminal cisternae then go on to release calcium, which binds to troponin. This releases tropomyosin, exposing active sites of the thin filament, actin.

Detailed explanation-3: -If calcium ions are released from the terminal cisternae by the action potential, they bind to troponin changing its shape. The troponin then pulls on the tropomyosin so that the binding sites on actin are exposed.

Detailed explanation-4: -If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction.