What happens to myosin and actin during rigor mortis?

During rigor mortis, a condition that occurs after death, the lack of adenosine triphosphate (ATP) in muscle cells has a significant impact on the interactions between myosin and actin, the two primary proteins involved in muscle contraction. At the molecular level, ATP is essential for muscle relaxation because it is needed for the detachment of myosin heads from actin filaments.

When a muscle is at rest, myosin heads are not bound to actin, but when a muscle contracts, myosin binds to actin to form cross-bridges, and then the power stroke occurs, which pulls the actin filaments toward the center of the sarcomere. After the power stroke, ATP binds to the myosin head, causing it to detach from actin so that the cycle can repeat.

In the state of rigor mortis, the production of ATP ceases, which means that myosin heads remain bound to the actin filaments. This is what causes the muscles to stiffen and become rigid, as there is no ATP available to facilitate the release of myosin from actin. Therefore, muscle fibers remain contracted and cannot relax, leading to the characteristic hardness seen in rigor mortis.