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Name: William M. R.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 2001

What exactly is rigor mortis? What causes it (well, obviously death) but biochemically speaking, what biological changes go on at the molecular or physiological level to make the body stiff? How long does it take for onset, and how long does it last, and why does it finally go away? Thank you for your time in answering this question.

Let me see if I remember this from my physiology days..... At the tissue level of muscles, the functional unit is called a sarcomere. This is made of 2 types of filaments-actin and myosin. Myosin filaments are thick and have little "heads" along their length that look kind of like the heads on a golf club. The actin filaments are thinner, but made of 2 strings kind of wound around each other like 2 strings of pearls. Along the length of the actin filaments there are molecules called troponin. During a muscle contraction, thousands of sarcomeres made up of actin and myosin are activated by calcium which pours into the tissue. The calcium binds to the troponin on the actin and causes a site on the actin to be exposed which allows the heads on the myosin to attach there. The energy for this is provided by ATP. When that happens the myosin and actin are pulled toward each other shortening the sarcomere. Because muscle is made of millions of sarcomeres all shortening the overall result is that the whole muscle shortens. Once the contraction is finished, the calcium lets go, and the myosin lets go and the muscle returns to its original length. So-your question was about rigor mortis!! After death, calcium is still present in the tissue so contraction can still occur. But the myosin cannot let go of the actin filaments because ATP is no longer being synthesized. So the muscles are stuck in contraction. IT lasts for about 24 hours, but then dissipates after another 12 hours as muscles begin to deteriorate.


This is a very interesting question but hard to answer briefly, so read on. The key players are actin, myosin, and ATP. Actin and myosin are proteins in your muscles; the best way to describe them is by using an analogy. Imagine stretching a rope between two cars that are maybe 100 feet apart. The rope is actin in our analogy. Now you and a bunch of your friends gather at the center of the rope, half of you facing one car, and half the other. You all start to pull on the rope, with a hand over hand motion, and sure enough, the cars move towards you. You and each one of your friends are acting very much like individual myosin molecules all working together in your muscles. In our analogy, the movement of the cars towards each other is like a muscle contraction. In muscles, myosin "walks" along actin with a grasp, pull, release action. Each cycle requires that a myosin molecule bind and break down one ATP molecule for energy.

Now to understand rigor mortis, follow the ATP (and especially the breakdown of ATP) during the above cycle:

1. Myosin binds a molecule of ATP (the myosin is not holding the actin tightly yet)

2. The myosin catalyzes the breakdown of the ATP to ADP and Pi (inorganic phosphate), releasing energy that is temporarily stored in the shape of the myosin molecule. The Pi is released from myosin and floats away. This step is immediately followed by step 3.

3. The myosin (with ADP bound) now grips the actin tightly, and then converts the stored energy from step 2 into motion by pulling along the actin "rope".

4. A fresh ATP replaces the ADP on the myosin. The myosin does not let go UNTIL the fresh ATP replaces the ADP.

Now for rigor mortis. When an organism dies, lots of myosin will have ATP bound, ready for a stimulus to start a muscle contraction. This would be like step 1 above. Note that myosin is not gripping actin tightly in step 1. With time, ATP will spontaneously degrade to ADP and Pi; as this happens in a dead person's muscles, we find ourselves in the same situation as in step 2. This starts the chain of events leading to Step 3, even in a dead person, Thus, we have a muscle contraction in a dead person. These random muscle contractions lead to the odd movements of facial and limb muscles in the dead.

But there's more. Note step 4. The myosin stays stuck to the actin UNTIL it is freed by the attachment of a fresh ATP. In the dead, there is no source of ATP, so the myosin STAYS stuck to the actin. Hence, the stiffness (rigor mortis) of death. And finally, the muscle proteins will eventually start to degrade (decompose). As they do, they will release their grip, and the stiffness will go away.

If you followed this, then you should understand why meat that is butchered and immediately frozen or eaten is usually tougher than meat that is butchered, and then "aged" in a cooler for a period of time.

Paul Mahoney, Ph.D.

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