The word rigor mortis is made up of two words- ‘Rigor’ means ‘stiffening’ or ‘hardening’ and ‘mortis’ means ‘mortality’. It is a part of the process that takes place after death and can help to determine the time of death.
Rigor mortis is the condition after death where muscles of the body get stiff due to some chemical reaction that occurs in the myofibrils (fibres tend to get shorter). In this stage, single-cell death occurs. The condition in which rigor mortis occurs in the body indicates in what position the body was at the time of death. It can help to estimate the death time and also helps to know if the position of the body has been changed.
According to the reports, the onset of rigor mortis in India starts after one to two hours of death.
In rigor mortis, some Physico-chemical changes occur in the muscles. A voluntary muscle is composed of long fibre bundles and each fibre is made up of myofibrils that extend all over the length. Each myofibril is composed of two types of filament proteins i.e. actin filament (also known as thin filament) and myosin filament (also known as thick filament). Both these filaments are combined together to form actomyosin. Both these filaments slide past each other and result in muscle contraction.
The energy for the separation of these filaments and for the contraction of muscles is provided by ATP (Adenosine triphospahte). There are three mechanisms in the body that maintains the continuous supply of ATP for muscle contraction −
Phosphagen system.
Glycogen-lactic acid system.
Aerobic system.
When ATPase acts on the ATP, it converts the ATP into ADP and Pi. With this reaction, a huge amount of energy is also released which is responsible for muscle contraction. The phosphate which is released in this process carries another phosphorylation reaction which converts glycogen into lactic acid. Lactic acid after entering into blood reaches the liver and is converted into glycogen. The glycogen is responsible for the resynthesis of ATP.
At the time of death, muscles are relaxed because enough ATP is present to maintain the muscles in a relaxed state. After death, ATP is not resynthesized and lactates and phosphates start to accumulate in the muscles.
Dephosphorylation and deamination are responsible for changing the levels of ATP after death. After death, the calcium ions leave the sarcoplasmic reticulum in bulk and enter the sarcomere. Due to the excess release of calcium, there is a binding of the actin and myosin and contraction of muscles takes place.
In a living state, muscles are relaxed when this calcium goes back into the sarcoplasmic reticulum but this process does not occur after death because ATP is not replenished and the plasma membrane of the sarcoplasmic reticulum is disrupted. Thus the actin-myosin complex continues to remain in the contracted form and muscles become hard and rigid.
Only muscles are involved during the development of rigor. It does not involve the nervous system but paralyzed muscles tend to develop rigor mortis in a slow manner. The body can be adjusted in any posture before the onset of rigor mortis; after this, rigor will occur in that gesture only. If rigor mortis is occurring and body extremities are moved in a new gesture, then rigor will take place in a newly formed gesture but will lose proper symmetry.
There are two types of fibers in the skeleton muscles.
Red skeletal muscle (Type 1) - In Type 1 skeletal muscles, oxidative metabolism takes place due to the presence of mitochondria.
White skeletal muscle (Type2)- Type 2 muscles have a very low amount of mitochondria; thus glycolytic metabolism dominates. In both muscles, rigor occurs at different times.
The whole body experiences stiffness when the rigor is fully developed. A great degree of flexion is observed in the fingers and toes. Hips, knees, shoulders, and elbows also show some flexion.
The muscle attached to the hair follicle is called erector pilae and the occurrence of rigor in this muscle results in the development of goose flesh on the skin. Skin gets rough when rigor occurs in this muscle.
During rigor, the testis is moved into the groin and semen comes out of the seminal vesicle.
If rigor mortis occurs in pregnant women and the uterus was in labor at the time of death, the fetus is expelled out of the uterus of the mother due to contraction of the uterus caused by the rigor mortis.
Rigor mortis occurs in 4 stages −
Autolysis − This stage starts soon after death and is also known as the self-digestion stage. In this stage all the vital processes that support the life stops which include blood circulation and respiration, excretion, etc. In this stage, the body gets deficient in oxygen and cannot remove waste from the body. This results in the increased pH of the body which results in the cell death.
Bloat − In this stage, the skin starts to become a blur due to sulfur-containing compounds delivered by the membrane. The body starts to deliver a foul smell due to the putrefaction caused by the various organisms.
Active Decay − This stage marks the onset of decay of various body parts and fluid starts to come out of every tissue.
Skeletonization − It is that stage of decomposition where all the soft tissue of the body gets decomposed and the skeleton is visible.
Rigor mortis is a condition after death where muscles get stiff due to the shortening of fibers. Rigor is produced due to freezing of the body. It is caused due to inability of the body to resynthesize the ATP; this actomyosin complex continues to contract. Both voluntary and involuntary muscles are involved. It has four stages- Autolysis, bloat, active decay, and skeletonization.
Q1. Which type of reaction occurs during Rigor mortis?
Ans. Acidic reaction takes place during rigor mortis. This happens due to accumulation of phosphates and lactates.
Q2. What is the difference between rigor mortis and cadaveric spasm?
Ans. The main difference between the two is; Rigor mortis involves both voluntary and involuntary muscles whereas in cadaveric spasm only voluntary muscles are involved.
Q3. Does the body respond to electric stimuli in rigor mortis?
Ans. No, the muscles in the body do not respond to any electric stimuli.
Q4. State Nysten’s rule?
Ans. Rigor mortis involves both voluntary and involuntary muscles and all muscles are not affected by it at the same time.
Q5. What is the duration of rigor mortis in India?
Ans. Duration of rigor mortis in India is 24-38 hours in Winter and 18-36 hours in summer.
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