What is the primary mechanism responsible for rigor mortis?

Rigor mortis is primarily caused by the depletion of ATP in muscle cells following death. When an organism dies, the processes that generate ATP, such as aerobic respiration, cease due to a lack of oxygen and nutrient supply. This depletion of ATP leads to an inability for muscle fibers to undergo relaxation, as ATP is necessary for detaching myosin heads from actin filaments during muscle contraction. Without sufficient ATP, the myosin heads remain attached to the actin, resulting in a sustained state of contraction or rigidity, characteristic of rigor mortis.

In this state, calcium ions may also play a role in initiating muscle contraction. After death, calcium ions leak into the cytosol due to compromised membrane integrity, which further contributes to muscle contraction. However, the underlying cause of the rigidity associated with rigor mortis is principally linked to ATP depletion, which prevents muscle relaxation.