Which extrinsic muscle is primarily affected first during rigor mortis?

Rigor mortis is a postmortem change characterized by the stiffening of muscles due to biochemical processes that occur after death. The order in which muscles experience rigor mortis typically correlates with their size and the metabolic activity that occurs in them.

The eyelids, particularly the small muscles within them, tend to be among the first to exhibit rigor mortis. This is because these smaller muscles become more rigid sooner due to their unique composition and relative lack of blood supply at the time of death. Additionally, the eyelids are activated by complex neural mechanisms, and once neuronal activity ceases, the small muscle fibers can contract and stiffen due to the depletion of ATP (adenosine triphosphate), which is crucial for muscle relaxation.

In contrast, the orbicularis oris, deltoid, and flexor digitorum muscles are larger muscles that typically take longer to undergo this process. The eyelids' rapid onset of rigor mortis makes them the first group of extrinsic muscles to be affected in this phenomenon, leading to the conclusion that they are primarily impacted first during rigor mortis.