Define the term: "rigor mortis"

Rigor mortis is a postmortem physiological phenomenon in which the muscles of a dead body become stiff and rigid due to a biochemical condition that prevents the relaxation of muscle fibers. It is directly related to the structure and function of sarcomeres, especially the role of actin and myosin filaments in muscle contraction. Under normal living conditions, muscle contraction is an active process that requires ATP for both the contraction and relaxation phases. After death, the production of ATP ceases completely, leading to an irreversible binding between the actin and myosin filaments in the sarcomere, causing the muscles to remain in a contracted state. This permanent cross-bridge formation without ATP results in the stiffness that characterizes rigor mortis.

The process of rigor mortis begins approximately 2 to 6 hours after death, depending on environmental conditions like temperature, cause of death and metabolic activity at the time of death. The stiffness first appears in the smaller muscles such as those of the face and jaw, and gradually spreads to the larger muscles of the trunk and limbs. The condition reaches its maximum within 12 hours, during which the entire musculature becomes stiff and inflexible. After this peak, rigor mortis begins to resolve or disappear within 24 to 48 hours as cellular enzymes (like lysosomal enzymes) start breaking down the muscle proteins during decomposition, a process called autolysis.

From a structural perspective, rigor mortis gives a practical demonstration of how critical ATP is for normal muscle function. In the living body, ATP breaks the actin-myosin cross-bridge after contraction, allowing the muscle to relax. In the absence of ATP, the myosin head remains attached to actin, locking the muscle in a contracted position.

In forensic medicine, rigor mortis is a key tool used to estimate the time since death, known as the postmortem interval (PMI). By observing the degree and location of rigor mortis in a body, forensic pathologists can estimate how long a person has been dead, especially in the early postmortem period. Thus, rigor mortis not only has physiological relevance but also major practical significance in postmortem examinations.





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