<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> Rigor mortis (Latin: rigor "stiffness", mortis "of death"), the third stage of death, is one of the recognizable signs of death, caused by chemical changes in the muscles post mortem, which cause the limbs of the corpse to stiffen . In humans, rigor mortis can occur as soon as 4 hours post mortem . </P> <P> After death, respiration in an organism ceases, depleting the source of oxygen used in the making of adenosine triphosphate (ATP). ATP is required to cause separation of the actin - myosin cross-bridges during relaxation of muscle . When oxygen is no longer present, the body may continue to produce ATP via anaerobic glycolysis . When the body's glycogen is depleted, the ATP concentration diminishes, and the body enters rigor mortis because it is unable to break those bridges . </P> <P> Additionally, calcium enters the cytosol after death . Calcium is released into the cytosol due to the deterioration of the sarcoplasmic reticulum . Also, the breakdown of the sarcolemma 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 via adenosine diphosphate (ADP), and the muscle is unable to relax until further enzyme activity degrades the complex . </P>

Rigor mortis or the stiffening of the body after death occurs because
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