Motor units in a fast-twitch muscle of normal and dystrophic mice.

Isometric contractions of motor units, isolated functionally by ventral root splitting were recorded from extensor digitorum longus muscles of normal and dystrophic mice of the strain C57BL/6J dy2J/dy2J. Motor unit tetanic tension was significantly lower and both time to peak tension and to half-relaxation of the twitch were significantly prolonged in dystrophic mice relative to age-matched controls. In control mice, motor unit tetanic tension averaged 4.98% of whole muscle tension, corresponding to twenty motor units. Two out of fifteen dystrophic mice exhibited an apparent decrease in the number of motor units, but the data from the remaining thirteen mice indicated no change in relative motor unit size, and hence in the number of motor units. The two mice in which changes were seen were the most severely affected and it is suggested that the apparent reduction in the number of units might be due to some units becoming so small as to be unmeasurable. No evidence was obtained for a population of units with normal characteristics within the dystrophic muscles. There was no clear relationship between tetanic tension and the time to peak tension or to half-relaxation in units from control mice. In dystrophic mice, however, a significant correlation was seen. This possibly reflects two simultaneous effects of the dystrophic process, a loss of tension accompanied by slowing of the twitch.