In its simplest form, myoclonus consists of a muscle twitch followed by relaxation. A hiccup is an example of this type of myoclonus. Other familiar examples of myoclonus are the jerks or "sleep starts" that some people experience while drifting off to sleep.
Do you ever have the inexplicable feeling that you’re falling just as you’re drifting off into a deep sleep?
Most people who have experienced this describe it as the sudden sensation of tripping, leaping or falling into a void. Some might just keep on sleeping, but the majority of people wake up with a rapid heartbeat feeling scared.
Perhaps you’ve heard explanations like your soul is leaving your body or you’ve been possessed by dark forces when it happens, but none of this is true. This is called a myoclonic jerk and is a normal physiological process.
When we sleep, we go through several sleep cycles, each consisting of four different stages. The sensation of falling down occurs during the first stage of sleep, called NREM. There are a few theories about why this happens. One is that when your muscles relax as you’re about to fall asleep, your brain gets confused sometimes and thinks you’re falling. As a result, your muscles react by tensing up, so you ’catch yourself’ before you fall. Other scientists believe it’s a reflex that humans developed during the evolutionary process to prevent them from falling out of the trees where they once slept.
These simple forms of myoclonus occur in normal, healthy persons and cause no difficulties. When more widespread, myoclonus may involve persistent, shock-like contractions in a group of muscles. In some cases, myoclonus begins in one region of the body and spreads to muscles in other areas. More severe cases of myoclonus can distort movement and severely limit a person's ability to eat, talk, or walk. These types of myoclonus may indicate an underlying disorder in the brain or nerves.
Read & learn:http://www.mayoclinic.org/diseases-conditions/myoclonus/home/ovc-20166171