MUSCLE PHYSIOLOGY

Detailed explanation-1: -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.

Detailed explanation-2: -Calcium triggers contraction in striated muscle. (A) Actomyosin in striated muscle. (1) Striated muscle in the relaxed state has tropomyosin covering myosin-binding sites on actin. (2) Calcium binds to troponin C, which induces a conformational change in the troponin complex.

Detailed explanation-3: -Troponin plays a central role in the calcium-regulation of muscle contraction: Troponin is the sole calcium-binding component of thin filaments (actin-tropomyosin-troponin complex) of striated muscles.

Detailed explanation-4: -(a) In the absence of Ca2+, tropomyosin prevents the binding of myosin heads to actin. (b) When the [Ca2+] rises, Ca2+ binds to a subunit of troponin, which causes the tropomyosin to shift slightly into the groove of the actin filament. The shift in position of tropomyosin allows the myosin heads to bind to actin.