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Rigor mortis[a] (from Latin rigor 'stiffness' and mortis 'of death'), orpostmortem rigidity, is the fourthstage of death. It is one of the recognizablesigns of death, characterized by stiffening of the limbs of thecorpse caused by chemical changes in themuscles postmortem (mainly calcium).[1] In humans, rigor mortis can occur as soon as four hours after death.
Afterdeath, aerobic respiration in an organism ceases, depleting the source of oxygen used in the making ofadenosine triphosphate (ATP). ATP is required to cause separation of theactin-myosincross-bridges during relaxation of muscle.[2] When oxygen is no longer present, the body may continue to produce ATP via anaerobicglycolysis. When the body'sglycogen is depleted, the ATP concentration diminishes, and the body enters rigor mortis because it is unable to break those bridges.[3][4]
Calcium enters thecytosol after death. Calcium is released into the cytosol due to the deterioration of thesarcoplasmic reticulum. Also, the breakdown of thesarcolemma causes additional calcium to enter the cytosol. The calcium activates the formation of actin-myosin cross-bridging. Once calcium is introduced into the cytosol, it binds to the troponin of thin filaments, which causes the troponin-tropomyosin complex to change shape and allow the myosin heads to bind to the active sites of actin proteins. In rigor mortis,myosin heads continue binding with the active sites of actin proteins viaadenosine diphosphate (ADP), and the muscle is unable to relax until further enzyme activity degrades the complex. Normal relaxation would occur by replacing ADP with ATP, which would destabilize the myosin-actin bond and break the cross-bridge. However, as ATP is absent, there must be a breakdown of muscletissue byenzymes (endogenous or bacterial) duringdecomposition. As part of the process of decomposition, the myosin heads are degraded by the enzymes, allowing the muscle contraction to release and the body to relax.[5]
Decomposition of the myofilaments occurs between 48 and 60 hours after the peak of rigor mortis, which occurs approximately 13 hours after death.[1]
Nysten's rule, first proposed in 1811, describes the sequential onset of rigor mortis in the variousmuscle groups. The basic sequence of the solidifying body begins from the head down the body, in the order:[6]
The rule does not occur in all cases.[7] It owes its name to the FrenchpediatricianPierre-Hubert Nysten (1771–1818).
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The degree of rigor mortis may be used inforensic pathology to determine the approximate time of death. A dead body holds its position as rigor mortis sets in. If the body is moved after death, but before rigor mortis begins, forensic techniques such aslivor mortis can be applied. Rigor mortis is known astransient evidence, as the degree to which it affects a body degrades over time.
Rigor mortis is very important in themeat industry. The onset of rigor mortis and its resolution partially determines the tenderness ofmeat. If the post-slaughter meat is immediately chilled to 15 °C (59 °F), a phenomenon known as cold shortening occurs, whereby the muscle sarcomeres shrink to a third of their original length.
Cold shortening is caused by the release of storedcalcium ions from thesarcoplasmic reticulum of muscle fibers, in response to the cold stimulus. The calcium ions trigger powerful muscle contraction aided by ATP molecules. To prevent cold shortening, a process known as electrical stimulation is carried out, especially in beef carcasses, immediately afterslaughter andskinning. In this process, thecarcass is stimulated withalternating current, causing it to contract and relax, which depletes the ATP reserve from the carcass and prevents cold shortening.[8]
