tremor(redirected from essential tremor)
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an involuntary trembling of the entire body or parts of it. Tremors are characteristically rhythmic and stereotypic; usually they have a narrow range, most often affecting the fingers, eyelids, tongue, lower jaw, and head. In healthy subjects, tremors may be caused by muscular strain, emotional excitement, or exposure to cold. Pathological tremors may be symptoms of chronic alcoholism, thyrotoxicosis, neurotic states, acute infections, and such diseases as parkinsonism. The symptom is treated by treating the primary disease.
Eye tremor is an involuntary rapid movement of the eyeball in relation to the optic axis; its frequency is from 20 to 150 Hz, and its amplitude, from 5 to 15 seconds of arc. Visual perception itself depends to a significant degree on eye tremor, which produces constant changes in the illuminance of certain photoreceptors—that is, the photoreceptors located at the variously illuminated points of the retinal periphery. An image whose projection on the retina remains completely stable for more than a few seconds ceases to be perceived by the visual centers of the brain.