Muscles contract through a sliding filament mechanism where actin and myosin filaments interact. Energy from ATP hydrolysis causes the myosin head to undergo conformational changes, attaching and detaching from actin and sliding the filaments past each other. This shortens the sarcomere. Tropomyosin normally blocks the actin binding sites, but calcium released during muscle stimulation displaces tropomyosin, allowing the cross-bridge cycle and contraction. When calcium is reabsorbed, tropomyosin resets and the muscle relaxes.

1. Muscles

10. The Cross Bridge Cycle

12. 2. The ATP molecule is then hydrolysed while the myosin head is unattached. The ADP & Pi formed remain bound to the myosin head.

18. Neuromuscular junction: Note Ach = Acetylcholine

19. Sarcoplasmic Reticulum