Initial Decay (Fresh)
Autolysis (self-digestion) is the first decompositional process postmortem. As a result of immediate oxygen deprivation in the body’s cells- Carbon dioxide (CO2) levels in the blood increase, pH decreases, and bodily wastes accumulate which poison the cells. Cellular enzymes from the lysosomes, begin to dissolve the cells from inside-out. Thousands of these biological catalysts are at work lowering the activation energy of this chemical process, and helping destroy the cells. One of the primary forms of these autolytic enzymes are lipases. They help degrade/catalyze the hydrolysis of lipids. There is also an abundance of Proteases (hydrolyzes proteins) and amylases (catalyzes the hydrolysis of starch) within the body. Eventually the cells will rupture, releasing nutrient-rich fluids.
This process progresses more rapidly in tissues with a high enzymatic-content, such as the liver, and high moisture content like in the brain. Nonetheless, it will eventually affect the entire cadaver. Although the change is not apparent at first, fluid-filled blisters will appear on the skin overtime, and large sheets of skin will slough off the body (skin slippage).
The body becomes a pale white 15-25 min after death. This process only occurs in cadavers of those who are anemic (lack blood cells) and/or have less melanin in their skin (not apparent in dark-skinned individuals).
Algor Mortis refers to the cooling of the body (37°C) until it attains ambient temperature (temperature of surrounding environment).
Hypostasis or Livor Mortis is the blueish-purple discoloration of the skin due to the gravitational accumulation of the blood in the lowermost (dependent) blood vessels. Since the heart can no longer circulate the blood around the body, it settles at the lowermost point in the cadaver, depending on its position at death. Livor Mortis is most evident 2 hours after death, although it occurs as soon as 15 min.
Rigor Mortis is when the body stiffens (2-6 hours after death). It begins with the upper body (i.e eyelids, neck and jaw) moving downwards (quadriceps, calves) and following with the internal organs (i.e brain, heart). It may not be perceivable in many infant and child cadavers due to small muscle mass.
The chemical process causing stiffening occurs in the body’s muscle cells. Muscle fibers/cells are composed of thousands of tiny threads called myofibrils-each with mitochondria, multiple nuclei and a cellular membrane called a sarcolemma. These myofibrils are divided lengthwise into segments called sarcomeres, containing two strands of proteins (myofilaments) known as actin and myosin. The sliding Filament model, first proposed in 1954 by Andrew Huxley illustrates how muscles contract/relax in a body.
During contraction the two ends of the sarcomere (Z-lines) come closer together. Meanwhile, the thin filaments (double-actin strand complex) slide past the thick filaments (thicker, lumpy looking myosin strands) so actin and myosin overlap. Without this overlap of actin and myosin, the muscle remains at a relaxed state. However, actin is blocked by a ‘wall’ built with multiple proteins, tropomyosin and a complex of three polypeptides-troponin C (bonds to calcium), troponin I (inhibitory) and troponin T (bonds to tropomyosin)
The transport and storage system of the cell is known as the sarcoplasmic reticulum. Its walls are covered with multiple calcium channels (ATP is used to collect Calcium ions) which are linked to voltage sensitive proteins. When the brain sends an action-potential along the motor neuron, it synapses with a muscle cell. Its receptors are ligand-gated sodium channels- so when the motor neuron releases the neurotransmitter (acetylcholine) into the synapse this transmembrane ion-channel protein opens to allow the sodium ions to pass through as a graded potential. Depending on how strong the electric potential is, it can cause nearby voltage-gated sodium channels to open. That Action potential runs along the sarcolemma, which has lots of tubes that lye deep inside called T-tubules. When it passes through this branch of tubes, it triggers the voltage-sensitive proteins linked to the calcium channels on the sarcoplasmic reticulum. All the calcium ions are released from the channel and rush into the rest of the muscle cell. It immediately binds with the troponin C. And according to the first behavioral law of proteins- proteins tend to change shape when other molecular compounds bind to them. Thus, as the troponin reshapes, the whole “wall’ of proteins pulls away from the sites on the actin strands, as well. However, the only way the myosin heads can bind to the actin is by hydrolyzing the ATP (Adenosine Triphosphate) into ADP (Adenosine diphosphate) and the leftover phosphate – it stretches and finally binds with the actin. The myosin eventually releases all the stored energy-changing shape, it pulls on the actin strand and the muscle contracts. To relax, ATP binds to the myosin-turning the chemical energy to Kinetic energy, in order to pull it apart from the actin.
After death, the sarcoplasmic reticulum deteriorates, and releases the calcium to the cytosol. Thus, activating the formation of the actin-myosin cross-bridges (contraction). However,the actin-myosin cross-bridges are unable to separate. Due to the depletion of oxygen, ATP production declines. The cross-bridges can’t be destabilized unless ATP separates the two myofilaments. When there is no oxygen left, the body may continue to produce ATP through anaerobic glycolysis. This is essentially energy production in the absence of oxygen. The energy comes from glucose (C6H12O6) through the process of glycolysis. This molecular compound is broken down/metabolized into an alpha-keto acid called pyruvate, which temporarily converts into lactate (not to be confused with lactic acid) which in turn breaks down glucose. This cycle, accordingly, produces ATP. ATP is used during cell work via ATP hydrolysis, releasing hydrogen ions. After a long period of time, acidity increases causing the cellular cytoplasm to gel. When glycogen is depleted, the ATP concentrations diminish-causing rigor mortis. This explains why the sequence of the body’s muscles undergoing rigor mortis is so specific. The higher the levels of lactate, the more rapid rigor mortis is to appear. The muscle is unable to relax until further endogenous enzyme activity degrades the myosin heads to break the cross-bridge.