What Causes Rigor Mortis?

After death, several changes occur in the body. First, it goes through a primary flaccidity (all muscles in the body becomes weak). Then, the corpse enters a stage of muscle stiffness, which is called rigor mortis. After this stage again, the corpse undergoes a stage secondary flaccidity due to autolysis (self-destruction of tissues by the body enzymes present in the cells).¹

Rigor mortis is an important phenomenon that occurs in the corpse which gives crucial information regarding the time of death. Rigor mortis approximately start 2-6 hours after death, it full establishes in the body by 12 hours after death. Persists for about another 12 hours and then gradually disappear within another 12 hours. Altogether after about 36 hours after death rigor mortis will not be present in the corpse.²

Rigor mortis is stiffening of the body after death due to muscle contraction. This occurs because of the failure of enzymatic processes and the buildup of waste in the body.

When a person dies respiration stops, there is no more oxygen and ATP (adenosine triphosphate) is not produced. Depletion of ATP leads to formation of stable bonds between actin and myosin, which prevents relaxation of muscle fibers. Therefore, muscles remain in a contracted state. Another reason for muscle stiffness is increased concentration of lactic acid. As lactic acid builds up in muscle tissue, it causes the muscle fibers to shorten and stiffen. All of the above reasons cause rigor mortis.³

Here, we will be talking about the skeletal muscle contraction in the body. Skeletal muscles are also called striated muscles because when looked from a microscope it’s seen as streaks of fiber. Skeletal muscles mainly have two parts: thick filament called the myosin and thin filament called actin. There are other molecules as well, but these two are the main molecules involved in muscle contractions. This muscle contraction is called the sliding filament theory.

A signal from the brain travels through the nerves and reach the motor end plate (which is the junction between a nerve and a muscle). The impulse stimulates each muscle sarcomere. The head of the myosin is stimulated and it pulls the actin filament to the center of the sarcomere. This happens simultaneously in all sarcomeres and this create a muscle contraction. As you see in the diagram the sarcomeres become shorter.

Actin and myosin have a cycling event, myosin reach for actin binds with it release it and binds again in a new cycle. This cycling process needs energy which is taken from the adenosine triphosphate (ATP) molecule. If ATP is not present myosin will stay attached to actin continuously. With the energy from ATP actin and myosin separate and actin returns back, the length of the sarcomere returns and muscle goes into a relaxed stage.⁴

Rigor mortis is stiffening of the body after death due to muscle contraction. It is an important phenomenon that occur in the corpse which give crucial information regarding the time of death. Rigor mortis approximately start 2-6 hours after death, it full establishes in the body by 12 hours and gradually disappear after 24 hours. This occurs because of the failure of enzymatic processes and the buildup of waste in the body. After death there is no oxygen and no production of ATP in the body. Depletion of ATP leads to formation of stable bonds between actin and myosin, which prevents relaxation of muscle fibers. Another reason for muscle stiffness is increased concentration of lactic acid. As lactic acid builds up in muscle tissue, it causes the muscle fibers to shorten and stiffen. All of the above reasons cause rigor mortis.

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