MEDICAL PHYSIOLOGY

Detailed explanation-1: -(1) Calcium binds to troponin C, causing the conformational shift in tropomyosin that reveals myosin-binding sites on actin. (2) ATP then binds to myosin. (3) ATP is then hydrolyzed. (4) A cross-bridge forms and myosin binds to a new position on actin.

Detailed explanation-2: -Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to begin cross-bridge cycling. Then the sarcomere shortens and the muscle contracts. In the absence of calcium, this binding does not occur, so the presence of free calcium is an important regulator of muscle contraction.

Detailed explanation-3: -The contraction of skeletal muscles is an energy-requiring process. In order to perform the mechanical work of contraction, actin and myosin utilize the chemical energy of the molecule adenosine triphosphate (ATP).

Detailed explanation-4: -Adenosine triphosphate (ATP) is the sole fuel for muscle contraction. During near maximal intense exercise the muscle store of ATP will be depleted in < 1s, therefore, to maintain normal contractile function ATP must be continually resynthesized.

Detailed explanation-5: -During muscle contraction, Ca2+ is important for triggering the conformational changes in the thin filaments, and ATP is important for providing the contractile forces.