What is Rigor Mortis?
Hi, and welcome to this video on rigor mortis! Today, we’ll be going into detail about what this process is and what causes it to happen. Let’s get started!
When a person dies, a long series of irreversible biochemical reactions and processes begin immediately, affecting all body tissues. This continuum of biochemical processes is known as decomposition, and results in several postmortem changes, some of which are visible with the naked eye, others seen only under a microscope.
Postmortem changes are typically classified based on the order of appearance and include immediate changes, early changes, and late changes.
Early postmortem changes generally occur in the first 24 hours following death and include three visible changes, known as the ‘‘classical triad of post-mortem changes’’ or just “classic triad”:
- Postmortem Hypostasis (Livor Mortis, or Lividity)
- Postmortem rigidity (Rigor Mortis)
- Postmortem cooling (Algor Mortis)
Early postmortem changes include the progression of degradative changes in cellular structure and function known as postmortem autolysis.
As cellular metabolic activity declines and eventually stops altogether, autolysis is initiated by the release of digestive enzymes from lysosomes. The release of these enzymes is the beginning of an irreversible process that ultimately results in the cell being broken down.
As the cells break down, calcium is released, which initiates muscle contraction and begins the process of rigor mortis.
The development of these changes and eventual resolution of rigor mortis is decided by several factors including:
- The age and size of the individual
- Temperature of the body
- Environmental conditions (such as humidity and ambient temperature)
- Presence of sepsis, infectious conditions, injury, or toxic exposure
- Premorbid illness or disease
- Exercise or strenuous physical activity before death.
Rigor mortis occurs uniformly in all the muscles of the body, and the process generally unfolds in three distinct phases:
- Primary flaccidity or delay period
- Rigor mortis or Rigid Stage
- Secondary flaccidity.
As blood stops circulating, the brain sends fewer and fewer signals to the rest of the body, causing the pupils to dilate, eyelids to lose their tension, the jaw to become slack, and all of the muscles in the body to relax and lose their tone. This initial phase is called ”primary flaccidity”, and it becomes fully present within an hour or two of death. This phase lasts as long as the supply of ATP remains sufficient to support the detachment of actin-myosin cross-bridges.
ATP is the main source of energy for all cellular processes and provides the energy for the contraction and relaxation of muscle fibers. Let’s go over the steps involved:
- After signaling from a motor neuron, ATP provides for the active transport of sodium and potassium ions across the sarcolemma, which envelops the fibers of the skeletal muscles. Calcium ions are then released from the sarcoplasmic reticulum and trigger the muscle contraction cycle by binding to the protein complex, troponin, which exposes the active binding site on actin.
- When signaling from the motor neuron ends, ATP provides the energy for the active transport of calcium out of the sarcoplasm back into the sarcoplasmic reticulum.
- ATP then provides the energy for the process known as myosin-actin cycling. This involves ATP preparing myosin to do two things: reach forward and bind to actin, forming actin-myosin cross-bridges, and pull the actin filaments closer, causing the sarcomere to shorten.
When signaling stops, ATP once again attaches to myosin, causing the actin-myosin cross-bridge to detach. This releases actin, which allows the muscle to relax, ready to repeat the process again in a new cycle.
As circulation stops, a series of changes takes place that begins with a rapid depletion of stored ATP and a shift from aerobic metabolism to the less efficient anaerobic metabolism. With this shift, the production of ATP is drastically reduced, causing accelerated glycolysis and lactic acid production.
While ATP continues to be formed by anaerobic glycolysis, the muscles will remain flaccid, but as time progresses, lactic acid builds up, resulting in a decrease in intracellular pH. As the intracellular acidity increases, the transmembrane potential of the cell decreases and Ca+ begins to diffuse from extracellular fluid and leak through the membrane of the sarcoplasmic reticulum, acting as the trigger for contraction. Glycolysis stops when the pH reaches 5.8 and ATP is no longer synthesized.
Once ATP is entirely consumed, irreversible cross-bridges are formed between actin and myosin filaments, and rigidity appears in the body as a whole. The development of this transient postmortem change begins the second phase.
Before the development of postmortem stiffening, the nurse should place the body in a supine position, with proper alignment of the arms and legs. Close the eyelids and mouth. If dentures are in place, they should not be removed, if not, an attempt should be made to insert them. Maintaining proper alignment, raise the head of the bed 30° and maintain this elevation throughout care and transport. To prevent blood from settling in the face, a small pillow may be placed under the head and shoulders, but care should be exercised to maintain proper head and neck alignment.
The rigor mortis phase typically begins about 2-3 hours after death, although this can vary. Rigor mortis is completely formed (fixed) about 12 hours after death and can last for up to 48 hours after onset.
All muscles are concurrently involved in the process, but rigidity is typically first observed in the small muscle groups such
- the eyelids,
- the jaw,
- the neck,
- and upper shoulders,
then, over the next 4–6 hours, gradually becomes apparent in larger muscle groups of the torso and limbs, until the body is essentially stiff.
As decomposition continues, during the last phase known as Secondary Flaccidity, the body returns to a fully flaccid state. Autolysis is generally credited to be the underlying cause of this phenomenon. Again, the time frame can vary, but typically muscle stiffness will start to recede 24 to 36 hours after reaching maximal or fixed rigor, generally in the same order as it began.
Let’s go over a few review questions before we go:
1. Which of the following statements about rigor mortis is not true?
- The development of rigor mortis is governed by several factors, including size and temperature of the body as well as environmental conditions.
- Muscle rigidity typically begins about 2-3 hours after death, is completely formed (or fixed) about 12 hours after death, and lasts up to 48 hours after onset.
- Immediately after muscle stiffness begins to form, the body should be placed in a supine position, with proper alignment of the arms and legs, mouth and eyes closed, and the head elevated on a small pillow.
- Muscle rigidity is typically first observed in the small muscle groups of the jaw, the neck and upper shoulders, gradually becoming apparent in larger muscle groups of the torso and limbs, until the entire body is stiff.
2. Which statement best describes the development of rigor mortis?
- After death, once ATP is entirely consumed, irreversible actin-myosin cross-bridges are formed, preventing muscle relaxation.
- With cessation of circulation after death, the body shifts to anaerobic metabolism, causing build-up of lactic acid which causes intense muscle stiffness.
- With the shift to aerobic metabolism, the production of ATP is drastically reduced, preventing cross-bridge formation, resulting in irreversible muscle rigidity.
- After death, signaling from a motor neuron stops and ATP remains attached to myosin, preventing cross-bridge detachment and muscle relaxation.
3. ATP is the main source of energy for all cellular processes. Which statement best describes its role in the development of rigor mortis?
- As ATP continues to be formed by anaerobic glycolysis, the muscles will remain rigid and will be unable to relax.
- If ATP molecules are not available to attach to myosin allowing the cross-bridge to detach, myosin would remain bound to actin indefinitely preventing muscle relaxation, resulting in rigor mortis.
- When ATP binds to troponin, it prevents cross-bridges from forming between actin and myosin filaments, resulting in irreversible muscle rigidity.
- ATP provides the energy for the actin-myosin cross-bridge to detach, which prevents the muscle from relaxing, resulting in rigor mortis.
That’s all for this review! Thanks for watching, and happy studying!