Are motoneurons involved in muscular dystrophy?

In the early seventies, a suggestion that even in muscular dystrophy a neurogenic factor may be involved, was formulated. The argument which followed this suggestion, resulted in eventual abandoning of this concept even by its author. This discussion however has never been supported by any systematic study of motoneuron activity in muscular dystrophy. We examined an activity of motoneurons supplying brachial biceps in eight controls and 26 patients affected by Duchenne muscular dystrophy by studying single motor unit (MU) potentials picked up by fine wire bipolar electrodes. In the majority of patients, MU firing rates were higher as compared to controls and increased more rapidly with increasing force level. The relationship between standard deviation of interspike intervals and their mean value, SD(x), was shifted towards the shorter intervals and lower SDs. The numerical values describing these changes were correlated with severity of the disease. There is evidence that the break-point of the function SD(x) is correlated with motoneuron properties, in particular with after-hyperpolarization duration. In muscular dystrophy, this break-point corresponds to the shorter interspike intervals. We suggest that the motoneurons in muscular dystrophy are altered either in response to the muscle degeneration, or as a result of the disease itself.