Re: What exactly happens at the cell/molecular level of a muscle with a cramp?

Hi Terry,

Your question about cramps is a good one. Unfortunately, there's no one good answer. There are many ways cramps can be triggered. Normal muscle contraction is the result of a series of biophysical and biochemical steps which include the following: an electrical message is transmitted down a nerve, the nerve releases neurotransmitter onto receptors in the muscle, which causes a change in the distribution of charged ions on either side of the membrane. This change in ion distribution spreads over the muscle membrane, causing proteins in the membrane to let calcium into the muscle, which leads to biochemical interactions between actin and myosin. These interactions need energy (ATP) to go to completion.

A cramp (too much contraction) could occur if something went wrong with the nerve, or muscle electrical activity, or the biochemical interactions within the muscle cell. And in fact, cramps (or what are sometimes called 'myotonias' and basically the same thing) can occur due to any of these things. Rigor mortis is a sort of big cramp you get after dying, due to the lack of ATP. In every case, of course, the actin-myosin interactions (which you should be able to read about in about any textbook, or look here: http://bio.winona.msu s.edu/berg/ANIMTNS/muscle.htm) are still going on, as during normal muscle contraction. It's just that the contraction is harder, longer, and uncontrolled... leading often to pain and (later) fatigue.

There is a nice discussion about cramps, their causes, and some medical advice at: http://www.myodynamics.com/cramps. html

Dave Featherstone
Dept. of Biology
University of Utah