Twitch of a single motor unit is called:

Fasciculations are jerky, visible contractions of groups of muscle fibers that occur as a result of pathologic discharge of spinal motor neurons. Fibrillations are fine, irregular contractions of individual muscle fibers. They may occur due to denervation hypersensitivity resulting from LMN lesion. Usually, the contractions are not visible grossly. Myoclonic jerk or clonus: Regular, rhythmic contractions of a muscle (entire muscle) subjected to sudden, maintained stretch. It is due to increased g-efferent discharge.