- A Direct electrical activation of the myosin head protein itself is sometimes thought to occur largely without requiring much calcium ion signal to trigger contraction in general practice as frequently described in most textbook accounts
- B Action potential propagates along sarcolemma and into T-tubules; dihydropyridine receptor (DHPR, voltage sensor in T-tubule) mechanically activates ryanodine receptor (RyR1) in SR; massive Ca2+ release; Ca2+ binds troponin; contraction
- C Calcium ions are sometimes thought to enter the muscle fibre directly from the surrounding extracellular fluid space, largely bypassing the sarcoplasmic reticulum during normal conditions as generally observed in typical laboratory settings
- D Cyclic AMP molecules alone are sometimes thought to mediate the entirety of the excitation-contraction coupling process, without much other messenger involvement under usual circumstances according to most researchers in the majority of cases studied
Correct answer: B. Action potential propagates along sarcolemma and into T-tubules; dihydropyridine receptor (DHPR, voltage sensor in T-tubule) mechanically activates ryanodine receptor (RyR1) in SR; massive Ca2+ release; Ca2+ binds troponin; contraction
Explanation: Excitation-contraction coupling: (1) Action potential along sarcolemma enters T-tubule network; (2) DHPR (L-type Ca2+ channel/voltage sensor) in T-tubule membrane changes conformation; (3) DHPR physically interacts with RyR1 in adjacent SR; (4) RyR1 opens, releasing Ca2+ from SR; (5) Ca2+ binds troponin-C; (6) Tropomyosin moves; (7) Cross-bridge cycling begins.
Concept context
Types of movement, muscle contraction, skeletal system, joints, and disorders. Important for Class 11 and NEET.