Reinnervation often causes a change in myofibers, the target fiber, characterized by absence of oxidative enzyme activity in the center, surrounded by a rim of more intense than normal activity. Target fibers may be confused with central cores, which occur in a congenital myopathy-central core myopathy.
In the EMG, large motor units appear as polyphasic or giant motor unit potentials. Denervated muscle is overexcitable. Spontaneous discharges from individual myofibers are picked up by the EMG as fibrillations. Spontaneous firing of an entire motor unit causes cotraction of a small group of myofibers that appears as a ripple on the surface of the muscle (fasciculation). Fasciculations of the tongue are an important sign of motor neuron disease. Since denervation does not cause myonecrosis, there is no elevation of CK.
Denervation atrophy is caused by peripheral neuropathies and motor neuron diseases. The most common motor neuron disease in adults is amyotrophic lateral sclerosis. In children, it is the autosomal recessive spinal muscular atrophy and its variants . Lower motor neuron damage may also be caused by enteroviruses which include the poliomyelitis virus and some arthropod borne viruses, especially West Nile Virus.