MEDICAL PHYSIOLOGY

Detailed explanation-1: -The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction.

Detailed explanation-2: -Calcium triggers contraction by reaction with regulatory proteins that in the absence of calcium prevent interaction of actin and myosin.

Detailed explanation-3: -Upon binding calcium, troponin moves tropomyosin away from the myosin-binding sites on actin (bottom), effectively unblocking it.

Detailed explanation-4: -The calcium ions bind to the troponin, changing the shape of the troponin-tropomyosin complex such that the actin binding sites are uncovered. As soon as myosin binds to actin, the cocked head of myosin releases sliding the actin fiber.

Detailed explanation-5: -Calcium initiates smooth muscle contraction by binding to calmodulin and activating the enzyme myosin light chain kinase. The activated form of myosin light chain kinase phosphorylates myosin on the 20, 000-dalton light chain and contractile activity ensues.