What causes rigor mortis? Rigor mortis is caused by the muscles in the body being unable to contract.
Rigor mortis is Latin. Rigor means “stiff” and mortis means “death”. It happens because of the way that we extend and contract our muscles. If you want to reach out and pick up a cup of coffee, the movement starts with your brain sending the instructions through your nerves. This signal is electrical and is called an action potential. It crosses the nerve cell and reaches a muscle cell. The neurotransmitter acetylcholine binds to receptors on the outside of the muscle cell and starts a chemical reaction. To contract your muscle and reach for the coffee cup, the acetylcholine opens membrane channels in the muscle fibers. Sodium ions flood in, which tells the muscle fiber to release the calcium ions it has stored. The calcium ions spread through the muscle fiber and make the chains of proteins that make up the muscle cells change. Muscle fibers are made up of chains of a protein called actin that make thin filaments and chains of a protein called myosin that make thick filaments. The calcium makes the protein between these two fibers contract, and the actin and myosin form bridges. These bridges constantly connect and disconnect, making the fibers slide past each other and the muscle contract. This takes energy and the muscle cells get energy from adenosine triphosphate (ATP). Cells have some ATP stored, but they can get more from the glucose that is in the blood. The cells need oxygen to convert ATP into usable energy.
When you have finished lifting the coffee cup and want to relax your muscles, the process reverses. Your brain stops sending the neurotransmitter and the sodium ions stop flowing in. The calcium moves back into storage in the muscle fibers and the proteins uncontract. The bridges between the actin and the myosin break and the chains of proteins slide back past each other to their original position. It takes more ATP to get the calcium to move out of the muscle fibers and for this process to happen.
So, what causes rigor mortis? When you die, the body no longer transports oxygen, which means that there is no way to convert ATP to energy. This means that there is no way to remove the calcium ions that are in the muscles. The actin and the myosin bridges stay intact and the muscles remain rigid. Rigor mortis starts approximately two hours after death, but that depends on a host of conditions, such as temperature, age, amount of body fat, and others. Rigor mortis starts in the small muscles, such as those in the face, and works its way to the larger muscles. This is because the bigger muscles have a larger store of ATP that they can work through even after death. Once that has been completely depleted, the muscles go stiff. Rigor mortis disappears between about 15 hours and 36 hours after death because the proteins in the muscles start to break down and they can’t keep the muscles contracted any longer. This also depends on temperature, age, and a lot of other factors. Forensic pathologists can use the state of rigor mortis to make an approximate guess at the time of death. The position the body is in can also tell if the person died in a different place, as well.
There are other things that can be used to tell the time of death as well. Pallor mortis is the first thing that happens after death and is increased paleness in the body because the blood is no longer circulating. It usually happens 15 minutes after death. Algor mortis is how much the body has cooled after death. The environment obviously plays a big part in this, but once our bodies can no longer produce heat, all things being equal, we cool at a standard rate. Livor mortis is where the blood collects in different parts of the body. Once the heart is no longer pumping, gravity pulls on the blood vessels and blood pools in the lowest places. This happens within half an hour of death and it can be used to tell what position the body was left in after death. The blood discolors those areas and after a certain amount of time the discoloration becomes permanent. And this is what I learned today.
Sources
Before you click on these, be warned that some of them have pictures of rigor mortis that you may not want to see.
https://www.sciencedirect.com/topics/medicine-and-dentistry/rigor-mortis
https://en.wikipedia.org/wiki/Rigor_mortis
https://www.visiblebody.com/learn/muscular/muscle-contractions
https://www.thoughtco.com/what-causes-rigor-mortis-601995
https://www.ncbi.nlm.nih.gov/books/NBK549867/