Question

Rigor mortis. Why does the body stiffen after death?

Short answer

Rigor mortis occurs due to the lack of ATP causing sustained muscle contraction after death.

Step-by-step solution

Understanding Rigor Mortis

Rigor mortis is the process where the muscles of a deceased body become stiff and hard. This stiffness occurs usually within 2 to 6 hours after death and can last up to 72 hours. The body eventually returns to a relaxed state due to decomposition.

Biochemical Background

Muscle contraction and relaxation in the living body are powered by adenosine triphosphate (ATP). After death, ATP production halts as metabolic processes stop. Without ATP, muscles cannot relax, causing them to become stiff.

Role of Calcium Ions

In living organisms, calcium ions are crucial for muscle contraction. Upon death, calcium ions flood muscle cells, promoting contraction as there is no ATP available to pump the ions back out and thus relax the muscles.

End of Rigor Mortis

Rigor mortis ends as enzymes and bacteria start decomposing the muscle tissues. This breakdown of muscle fibers leads to the natural relaxation of the muscles, making the body flaccid again.

Key concepts

Muscle Contraction

Muscle contraction is a fundamental process that allows us to move, breathe, and perform countless bodily functions. This process happens when muscles receive signals from the nervous system.
These signals cause muscle fibers to interact through a mechanism involving proteins called actin and myosin. Here’s how it works:

• Nerve signals trigger the release of calcium ions into muscle cells.
• Calcium ions bind to troponin, a protein that enables myosin to bind to actin.
• This interaction allows myosin to "pull" on actin, causing the muscle to contract.

The muscles only relax when calcium ions are removed and myosin is detached from actin, which normally requires energy.

Calcium Ions

Calcium ions play a crucial role in triggering muscle contractions by facilitating the interaction between actin and myosin proteins. When a muscle is at rest, calcium ions are stored within the sarcoplasmic reticulum, a structure inside the muscle cells.
Upon receiving a contraction signal:

• Calcium ions are released into the muscle cells.
• These ions bind to troponin, which changes shape and moves tropomyosin away from the actin binding sites.
• This allows myosin heads to attach to these sites, resulting in muscle contraction.

Normally, ATP-powered pumps would remove calcium from the muscle cell to cause relaxation. But during rigor mortis, this process is disrupted, leading to prolonged contraction.

Adenosine Triphosphate (ATP)

Adenosine triphosphate (ATP) is often called the energy currency of the cell. It provides the necessary energy for a multitude of cellular processes, including muscle contraction and relaxation. In living cells:

• ATP binds to myosin heads, providing energy for them to detach from actin filaments after a contraction.
• It powers calcium pumps that remove calcium ions from the cells, allowing muscles to return to a relaxed state.

When a person dies, ATP production ceases. Without ATP, myosin heads cannot detach from actin, and calcium ions can't be pumped back into storage, both of which contribute to rigor mortis once ATP reserves are depleted.

Decomposition

Decomposition is the natural process through which a body breaks down into simpler forms of matter. It is the reason why rigor mortis is only temporary.
After rigor mortis sets in, decomposition begins, as follows:

• Enzymes and bacteria start breaking down the muscle tissues.
• These processes cause the rigid structures of actin and myosin to degrade.
• The breakdown of muscle proteins and fibers releases the locked tension, returning the body to a flaccid state.

During decomposition, muscle tissues and fibers disintegrate, eventually overcoming the initial stiffness caused by the lack of ATP and locked-in calcium. This marks the end of rigor mortis.