Fibrillation also occurs with individual skeletal muscle fibers. This happens when muscle ﬁbers lose contact with their innervating axon producing a spontaneous action potential, "fibrillation potential" that results in the muscle fiber's contraction. These contractions are not visible under the skin and are detectable through needle electromyography (EMG) and ultrasound. Fibrillations do not occur in healthy individuals. They are a major symptom in acute and severe peripheral nerve disorders, in myopathies in which muscle ﬁbers are split or inﬂammed, and lower motor neuron lesions.
They contrast with fasciculations that are visible spontaneous contractions involving small groups of muscle fibers. Also, fasciculation does not necessarily denote pathology, as does fibrillation, although it can be seen in lower motor neuron lesions as well.