Physiology behind Rigor Mortis

Rigor Mortis/ Postmortem Rigidity is the state of contracture that develops several hours after the death of the individual. In this condition the muscle contract and become rigid even in the absence of action potential.

The basic physiology of muscle contraction include (the numbering may not coincide with the image given below)

           In a resting muscle, Tropo-myosin covers the actin’s binding site for myosin.

2         When cytosolic calcium increases after the opening of Sarcoplasmic Ca++ channel following the        action potential, Free calcium binds with the Troponin –C.

3         Calcium binding to Troponin C exposes the Mysoin- Binding site in the actin.

4         The binding of actin and Mysoin occurs and  Myosin pulls the actin towards itself producing              active tension. ATP is not needed for Cross bridge linking.

5         The dissociation of actin and myosin is energy dependent. ATP is needed for this release of actin        myosin bond.

           Once the dissociation occurs, the ATP is hydrolysed to form ADP and Inorganic phosphate and the      myosin is put back to the high energy state with high affinity for actin being ready for next cycle       of cross bridging.

Steps of Skeletal Muscle contraction
Source:http://studylib.net/doc/9410005/steps-of-a-muscle-contraction

Know how  cadaveric spasm and Rigor mortis are different. 

Lets us take a special Consideration on the point no 5 (not from diagram). The release of actin from myosin needs ATP is the most important thing to describe physiology of post mortem rigidity.

Immediately after the death, the inherent tone of the muscle is lost and the initial state of flaccidity is seen known as the stage of Primary Flaccidity. In this stage, the somatic death has occurred but not the cellular death. So, the cell responds to electrochemical stimuli.

Once death occurs at the cellular level, the membrane permeability to all ions are disrupted and large amount of calcium is released from the sarcoplasmic reticulum causing the binding of actin and myosin which brings about the contraction. This is the stage of Rigor Mortis and the cells no longer respond to electro-chemical stimuli.

The separation of actin from myosin need ATP which is depleted once the cell dies. So they cannot separate and hence the muscle remains contracted.

After several hours of death, the autolytic enzyme released from the lysosomes gradually degrade all the proteins including actin and myosin causing the loss of stiifness after 15-20 hours. And hence the stage of Secondary Flacidity develops.

Know how  cadaveric spasm and Rigor mortis are different. 

Difference between Rigor Mortis and Cadaveric Spasm

Though rigor mortis and cadaveric spasm both occur both following death but they are totally different things. Cadaveric spasm is a localized contraction of muscle that was occurring during the time of death and perisisted then after unlike Rigor mortis which developed through out the body including visceral muscle. There is a period of flaccidity before rigor mortis  develops. The following points helps us differentiate between the cadaveric spam and rigor mortis.

Know the physiology behind rigor mortis 

Character	Rigor Mortis	Cadaveric Spasm
Time Of Onset	Within 1-2 hours of death	At the time of Death
Occurrence	Occurs in all death except burn	Predisposed by sudden violent death
Muscle	Both Voluntary and involuntary Muscle Heart is the first to be involved Small muscle of digits develop it last	Local Voluntary Muscle eg. Hand with knife in suicide
Muscle Stiffening	Moderate force required to break it	Great Force required to break
Primary flaccidity	Occurs	Does not occur
Mechanism of Production	Known	Still Obscure
Response of Electrical Stimuli	Absent	Present
Medico Legal Importance	Gives idea about Time since death	Gives idea about Mode of Death