Myoclonus

Myoclonus is a sudden, brief, involuntary muscle jerk caused by a contraction or a temporary interruption of muscle activity. It can occur normally or result from neurological disease, medication, or other causes.

Positive myoclonus: A brief involuntary jerk caused by sudden muscle contraction. This is the contraction-driven form of myoclonus.
Involuntary movement: Movement that occurs without deliberate control. Myoclonus belongs to this broad class of movements.
Physiological myoclonus: Benign myoclonus that occurs in healthy people, including hiccups and sleep starts. Some jerks are normal physiological events rather than signs of disease.
Tremor: An involuntary, rhythmic oscillation of a body part caused by alternating or synchronous muscle activity. Tremor is rhythmic and sustained, unlike the typically brief, shock-like myoclonic jerk.
Negative myoclonus: A brief interruption of ongoing muscle activity that causes a sudden loss of posture or tone. This form produces a jerk-like lapse rather than an added contraction.
Muscle tone: The continuous, low-level tension present in a resting muscle. A sudden loss of tone can produce negative myoclonus.
Sleep start: A sudden muscle jerk that occurs as a person is falling asleep. This common, benign event is a familiar form of physiological myoclonus.
Chorea: A movement disorder marked by irregular, flowing, involuntary movements that shift unpredictably between body parts. Chorea consists of ongoing variable movements rather than isolated brief jerks.
Cortical myoclonus: Myoclonus generated by abnormal activity in the cerebral cortex, often involving sensory regions near the motor cortex. Cortical discharges can trigger brief jerks in specific muscles.
Motor neuron: A nerve cell that carries signals from the central nervous system to muscles. Motor-neuron output is the final neural path to a contracting muscle.

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