Answer
Ater an excitation and a muscle contracts, Ca++ ions are released in the sarcoplasm. Ca ++ ions bind to the TnC units of troponin and troponin binds to tropomyosin forming a troponin-tropomyosin complex. Tropomyosin also binds to the active site of actin. This means that myosin cannot bind to actin unless the muscle is stimulated. Therefore, the muscle remains relaxed.
Work Step by Step
When an Action potential (AP) is triggered a wave of depolarization spreads from the motor end plate in all directions. When this wave reaches the T-tubule it travels down into the interior of the cell.. An AP opens up calcium channels in the cell ; these calcium channels are linked to calcium channels in the cisternae of the sarcoplasmic reticulum(SR) . When the calcium channels o the SR open, Ca++ ions diffuse out of the SR, down their concentration gradient into the cytosol. The Ca++ then binds to the troponin in the thin filament. This reaction causes the troponin-myosin complex to change shape; this exposes the active site on the actin filament making it again available for binding to myosin heads.
