A myoclonic jerk is a sudden, involuntary twitch of a muscle. It may be a contraction, called positive myoclonus; or a relaxation, called negative myoclonus. The most common time for people to encounter them is while falling asleep ("sleep starts") but myoclonic jerks are also a symptom of a number of neurological disorders. Hiccups are also a kind of myoclonic jerk.
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