Biology 103
2002 Second Paper
On Serendip

Soon after the time of death, a body becomes rigid. This stiffening is the result of a biochemical process called Rigor Mortis, Latin for "stiffness of death". (1) This condition is common to all deceased humans, but is only a temporary state. The slang term "stiff", used to refer to a dead person, originates from rigor mortis. Within hours of the time of death, every muscle in the body contracts and remains contracted for a period of time. Before I am able to explain the biological cause of this condition, I must first describe the structure of muscles and the process of muscle contraction.

Muscles have many different levels, beginning with the individual muscle fibers. Muscle fibers are a combination of many cells but have structures similar to that of an individual cell. The organelles found in a normal cell are also found in muscle fiber, but are given different names: the plasma membrane is sarcolemma, endoplasmic reticulum is sarcoplasmic reticulum, mitochondria are sarcosomes, and the cytoplasm is called sarcoplasm. Muscle fibers are composed of tissue units called sarcomeres. Sarcomeres are connected horizontally to extend the length of the muscle fiber. The components of an individual sarcomere are a thick and thin filament, myosin and actin, respectively. The ends of a sarcomere are what are called Z lines. Rows of actin extend from these Z lines but do not meet in the middle. Spaced in between the rows of actin, and not connected to either Z line are the myosin filaments. Strands of actin take on a double helix shape and are surrounded by a long protein strand called tropomyosin spotted with various small protein complexes called troponin. Underneath the tropomyosin are myosin active sites, where myosin is able to bind to the actin. Along a strand of myosin are "heads" that protrude towards the actin. Actin and myosin are the central actors in muscle contraction. (2)

Muscle contraction begins in the brain with a nerve impulse sent down the spinal cord to a motor neuron. The action potential started in the brain is passed on to the muscle fibers through an axon where it is carried into a neuromuscular junction. (2) The neuromuscular junction, also referred to as the myoneural junction, releases acetylcholine when the action potential reaches the junction. When the acetylcholine comes into contact with receptors on the surface of the muscle fiber, a number of transmembrane channels open to allow sodium ions to enter. (3) This influx of sodium ions creates an action potential within the fiber which triggers a release of calcium ions from the sarcoplasmic reticulum.

Calcium ions filter throughout the sarcomeres and bind with the troponin complexes, causing a shift in the tropomyosin structure and exposing the myosin binding sites on the actin. A "power stroke" follows, wherein the myosin heads drop the ADP and Pi, which hold the heads in a cocked back position, and move laterally thereby moving the actin filament at the same time. Finally, ATP binds to the myosin heads, thereby detaching from the actin. Upon release from the actin, the ATP breaks down into ADP and Pi, giving energy to return the myosin to its cocked position, thereby renewing the cycle. (4)

The relaxation of a muscle depends upon the termination of the action potential beginning at the neuromuscular junction. An enzyme within the muscle fiber destroys the acetylcholine, thereby stopping the action potential that acetylcholine produces. Therefore, calcium ions are no longer released from the sarcoplasmic reticulum. In fact, the already loose calcium ions are brought back into the sarcoplasmic reticulum. Finally, the myosin and actin are unable to bind, thereby contracting, because the myosin active sites need calcium ions to be exposed. (2)

The supply of ATP is central in the continuing process of muscle contraction. ATP originates from three sources; the phosphagen system, glycogen-lactic acid system, and aerobic respiration. In the phosphagen system, muscle cells store a compound called creatine phosphate in order to replenish the ATP supply quickly. The enzyme creatine kinase breaks the phosphate from this compound and the phosphate is added to ADP. This source of ATP can only sustain muscle contraction for 8 to 10 seconds. The glycogen-lactic acid uses the muscles' supply of glycogen. Through anaerobic metabolism, the glycogen is broken down and creates ATP and the byproduct lactic acid. This method does not require oxygen and is able to supply more ATP than the phosphagen sytem, but occurs at a slower rate. Finally, the aerobic respiration allows glucose to be broken down into carbon dioxide and water in the presence of oxygen. The glucose supplies come from the muscles, the liver, food, and fatty acid. This method creates the most ATP and for extended periods of time, but takes the most time. (5)

With all of the information on how muscles work, it is now possible to explain the process behind rigor mortis. Death terminates aerobic respiration because the circulation system has ceased. (6) Therefore, the muscles rely on the phosphagen and anaerobic metabolism methods to acquire ATP. As stated above, these sources only provide a small amount of ATP. This lack of ATP disables the myosin heads from detaching from the actin. Meanwhile, calcium ions leak from extracellular fluid and the sarcoplasmic reticulum, which is unable to recall the ions, into the muscle fiber. (6) The ions perform their task as if the body were alive, disengaging the tropomyosin and troponin from the myosin active sites. The muscle contracts when the myosin shifts, but the lack of ATP prevents it from detaching, and the muscle remains contracted. Such a process occurs in all muscles as the body becomes rigid.

Rigor mortis usually sets in within four hours, first in the face and generally smaller muscles. The body reaches maximum stiffness within twelve to forty-eight hours. However, this time may vary due to environmental conditions – cooler conditions inhibit rigor mortis. (1) Rigor mortis is only a temporary condition. During the process, the body has been accumulating lactic acid through anaerobic respiration. Lactic acid lowers the pH of the muscles, and deteriorates the contraction of the muscles. (7) The body loses its rigidity due to the decay of the muscles. In conclusion, rigor mortis is the stiffening of the body due to a lack of ATP after death. Only temporary, the condition is environmentally sensitive and is believed to occur in all grown humans.

References

1)"How does Rigor Mortis work?"

2)"Contraction and Rigor Mortis"

3)"Muscles"

4)"HowStuffWorks 'How Muscles Work'"

5)"HowStuffWorks – How Exercise Works"

6)"Chapter 10: Muscle Tissue"

7) "Conversion of Muscle to Meat"


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