myosin cross bridge attach to the actin filament when sarcoplasmic reticulum is stimulated to release which of the following?
(a) Calcium ions
(b) actycholine
(c) troponin
(d) adenosine triphosphate (ATP)
Step in contraction (sliding filament theory):
(1) The sarcoplasmic reticulum (SR) releases calcium ions (Ca²⁺).
(2) Calcium binds to troponin, which shifts tropomyosin, exposing binding sites on actin.
(3) Then myosin cross-bridges attach to actin.
(4) ATP is needed later for detachment and power stroke, but the key trigger is calcium release.
π Correct Answer: (a) Calcium ions ✅
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