Rationale: Heart muscle contraction is  activated by a synchronized systolic Ca release from sarcoplasmic reticulum (SR) via Ca sparks.
<coherence> Rationale: Heart muscle contraction is  activated by a synchronized systolic Ca release from sarcoplasmic reticulum (SR) via Ca sparks.
Rationale: Heart muscle contraction is normally activated by a synchronized systolic Ca release from sarcoplasmic reticulum (SR) via Ca sparks.