Varied morphology of spontaneous single muscle fiber discharges.

Fibrillation potentials and positive sharp waves have rather stereotypical morphologies generally conforming to a triphasic initially positive and biphasic initially positive waveform, respectively. Careful attention during the needle electromyographic examination of muscle tissue with unstable resting membrane potentials, however, frequently reveals spontaneous potentials with appearances conforming to not only those described above but also waveforms with morphologies that can best be characterized as a blend of both a fibrillation potential and positive sharp wave. These "hybrid" or composite waveforms are hypothesized to arise from an electrical summation of two single muscle fibers repetitively discharging in a time-locked manner. Five individuals with lower motor neuron disorders displaying spontaneous discharges at rest during the needle electromyographic were examined. The spontaneous discharges were categorized according to their morphology. Stereotypical positive and negative onset fibrillation potentials and positive sharp waves from these patients were digitized. Computer simulations summating the digitized waveforms were successful in reproducing all of the observed "hybrid" waveforms documented in the patients. The supposition of two single muscle fibers discharging within a temporal domain conducive to electrical summation is an extension of the concept that electrical summation of muscle fibers comprising a motor unit summate to generate an electrical representation of that motor unit.