1. Ca2+ ions attach to the troponin molecule, causing it to move.
2. As a result the tropomyosin on the actin filament shifts position, exposing a bunch of myosin binding sites.
3. Myosin heads bind with the binding sites on the actin, forming cross-bridges.
4. When these heads bind, ADP + Pi (Adenosine Diphosphate and Phosphate) is released from them.
5. The myosin changes shape, causing the heads to nod forward. This makes the attached actin move over it and the muscle contracts.
6. From the chemicals the heads released, ATP (Adenosine Triphosphate) is formed and it reattaches to them. This energy causes the heads to detach.
7. ATPase (an enzyme) on the myosin heads then hydrolyses ATP back into ADP+Pi, causing a change in the heads' shape, returning them to upright. The muscle than relaxes. THIS CYCLE REPEATS AT EVERY MUSCLE CONTRACTION.
NOTE: When ATP is absent the cross-bridges remain - you know this stiffening as RIGOR MORTIS
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