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.
 Heart muscle contraction is activated by a synchronized systolic Ca release from sarcoplasmic reticulum (SR) via Ca sparks.