STIMULATION (THE "COCK" STAGE)
Screens 3 to 6 show an animation of the sliding filament model of muscular contraction.

The muscle is relaxed and few Ca²⁺ ions are present in the cytoplasm of the muscle cell. Cytoplasmic Ca²⁺ ions have been actively taken up and stored by the sarcoplasmic reticulum, a modified endoplasmic reticulum unique to muscle cells. Also, there is a molecule of ATP bound to the myosin head.

The troponin complex on the actin filament directly across from the myosin head is positioned such that the threadlike tropomyosin protein is blocking the active site located underneath it. Ca²⁺ ions are needed to unblock the active site.

Contraction begins when the action potential traveling in the T tubules reaches the sarcoplasmic reticulum. The action potential induces the opening of calcium channels in the sarcoplasmic reticulum membrane, which results in the release of stored Ca²⁺ ions into the cytoplasm.