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Fibrillation is the rapid, irregular, and unsynchronized contraction of muscle fibers. An important occurrence is with regard to the heart.
Fibrillation may sometimes be used after heart surgery to stop the heart from beating while any minor leaks are stitched up.
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 can occur in healthy individuals. If the fibrillation have irregular potentials then they don't have pathological significance. In other cases 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.