During skeletal muscle contraction, calcium released from the sarcoplasmic reticulum binds to which molecule to initiate cross-bridge cycling?

• A Myosin heavy chain
• B Actin directly
• C Troponin C on the thin filament ✓
• D Titin

Explanation

At rest, tropomyosin blocks the myosin-binding sites on actin. When an action potential triggers Ca2+ release from the SR via ryanodine receptors, Ca2+ binds to troponin C (the Ca2+-binding subunit of the troponin complex). This shifts troponin T and moves tropomyosin laterally, exposing the myosin-binding sites on actin. Myosin heads then bind actin and undergo the power stroke, consuming ATP. Relaxation occurs when Ca2+ is re-sequestered by SERCA pumps.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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