What are the two main parts of skeletal muscle fibers?

At the microscopic level, the muscle fibers that attach to your bones have two main parts: thick filaments and thin filaments. The thick filaments are made up of the protein molecule myosin and thin filaments are made up of the protein molecule actin.

The fibers include some other proteins as well, but actin and myosin are the main players in rigor mortis.

When you move, let's say to scratch your head or lift a weight, a nerve impulse triggers a biochemical reaction that makes myosin stick to actin. When these two molecules lock, they pull the muscle's thick and thin filament toward one another. When thousands of filaments pull together simultaneously and repeatedly, you have a muscle contraction.

Once the myosin and actin molecules stick together, they remain that way until another molecule, adenosine triphosphate (ATP) grabs hold of the myosin, forcing it to let go.

Your body uses the oxygen in the air that you breathe to make ATP. Of course, when you die your oxygen supply ends, and without ATP, the thick and thin filaments stay stuck together - resulting in rigor mortis.

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