Stimulus induced repetitive muscle potentials in the gracile axonal dystrophy (GAD) mouse

Gracile axonal dystrophy (GAD) is an inherited neurodegenerative disease in the mouse with progressive sensory ataxia and motor paresis. Electromyographic examination was conducted in 25 unanaesthetized GAD mice and 24 controls of the same strain during 6, 9 and 12 wk of postnatal life. Among 9 and 12 wk old mice, about 75% showed resting spontaneous activities--either fibrillation or fasciculation or both. On stimulation of the tibial nerve at ankle, with single pulse, the EMG showed repetitive muscle potentials of large amplitude, following "M" response. The frequency and duration of the train of these stimulus--induced repetitive muscle potentials (SIRMP) were almost constant in a given animal. The SIRMP failed to reappear in response to the second stimulus within 5 s when twin pulses were applied or within 5 min when tetanic stimuli were applied, indicating their fatigability. It is concluded that the SIRMP originate from immature, supplementary motor endplates that develop at ultraterminal nerve sproutings induced by denervation and reinnervation and possibly are due to hyperexcitable trigger points in the peripheral nerve endings. The EMG abnormalities in the GAD mouse are very much similar to those observed in human syndromes with hyperexcitable peripheral nerves that result in sustained muscle activity like neurotonia and hence the GAD mouse is a good model of such motor abnormalities in man.