Fasciculations (muscle twitches) are the result of some kind of irritability of the nerve fibers. Because fasciculations are benign, they have not been studied particularly deeply (but, not all involuntary muscle twitches are fasciculations--more on that later). So we do not really know even where in the nerve the irritation is picked up--it could be in the cell body, could be further out in the fibers, nobody really knows. It is also thought that the exact localization of the fasciculation is random, meaning that you will feel a twitch in your arm or leg or eyelid without having necessarily irritated a nerve anywhere near the place you experience the twitch.
Causes are also only loosely understood; there are certain behaviors that can trigger fasciculations, including too little sleep, too much exercise, a lack of magnesium, and the use of stimulants (especially caffeine), but no study has been able to concretely pin a cause on fasciculation. These presumed causes are correlations; adjusting your stress, magnesium, caffeine, sleep, and exercise level can help with fasciculations, but that\'s not evidence of causation.
