A study of functional denervation in fast and slow muscles of dystrophic mice of various ages

Abstract

It has been claimed that some fibers in dystrophic mouse muscle are denervated and, furthermore, that there is no increase in motor unit size which would be expected to occur if reinnervation took place. If these observations are correct, it ought to be possible to demonstrate a mechanical correlate of the denervation phenomenon. The functional innervation ratio (ratio of maximum twitch tension in response to nerve stimulation × 100 to maximum twitch tension in response to direct muscle stimulation) has been determined for the extensor digitorum longus and soleus muscles of normal ( C57B1 6J ) and dystrophic ( dy 2J dy 2J ) mice of various ages. It is shown that although a fall in the functional innervation ratio is seen in both muscles, it occurs some time after the symptoms of the disease are apparent. This suggests that denervation may not be the primary event in murine muscular dystrophy. The fall in functional innervation ratio is apparently not due to an enhanced susceptibility of the dystrophic neuromuscular junction to anoxia, because dystrophic muscles showed the same response to a raised bath temperature as did normal muscles.