<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> <P> 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 muscle tissue by enzymes (endogenous or bacterial) during decomposition . 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 . Decomposition of the myofilaments occurs 48 to 60 hours after the peak of rigor mortis, which occurs approximately 13 hours after death . </P> <P> At the time of death, a condition called "primary flaccidity" occurs . Following this, the muscles stiffen in rigor mortis . All muscles in the body are affected . Starting between two and six hours following death, rigor mortis begins with the eyelids, neck, and jaw . The sequence may be due to different lactic acid levels among different muscles, which is directly related to the difference in glycogen levels and different types of muscle fibers . </P> <P> Rigor mortis then spreads to the other muscles, including the internal organs, within the next four to six hours . The onset of rigor mortis is affected by the individual's age, sex, physical condition, and muscular build . Rigor mortis may not be perceivable in many infant and child corpses due to their smaller muscle mass . </P>

Why does rigor mortis start at the head
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