Fibrillation is the rapid, irregular, and unsynchronized contraction of muscle fibers.
These contractions are not visible under the skin and are detectable through needle electromyography (EMG) and ultrasound.
[9] In other cases they are a major symptom in acute and severe peripheral nerve disorders, in myopathies in which muscle fibers are split or inflamed, and in lower motor neuron lesions.
They contrast with fasciculations that are visible spontaneous contractions involving small groups of muscle fibers.
Fasciculations can be seen in lower motor neuron lesions as well, but they also do not necessarily denote pathology.