End-plates, transmission and contractile characteristics of muscles without spindles in the hereditary sensory neuropathy of the Sprawling mouse.

In the mutant mouse Sprawling, a deficiency of sensory ganglion cells is associated with a failure of the development of spindles and tendon organs particularly affecting muscles of the hindleg. Electrophysiological and morphological investigations were made on nerve, muscle and the neuromuscular junction of soleus and extensor digitorum longus (EDL). It was found that the absence of sensory innervation had no effect on the development of muscle bulk, on the fibre diameters or on histochemical profiles. The elimination of polyneuronal innervation proceeded at the normal rate and was complete by 3 wks of age. The strength of contractile responses and the number of motor units were normal in both muscles. End-plate areas were measured and the size distribution found to be normal in soleus. In EDL, however, there was a preponderance of small end-plates and a relative deficiency of large ones. This was associated with an abnormally low frequency of miniature end-plate potentials and of the mean quantal content of transmission. There was also a reduction in the complexity of the postsynaptic specialization at end-plates in the EDL. Localized axonal swellings packed with neurofilaments were observed in preterminal motor nerve fibres, which suggested an abnormality of axonal transport. There was no evidence of denervation of muscle fibres. The contractile characteristics of soleus and EDL were still distinguishable as slow and fast-twitch, respectively, despite the abnormalities found, and it seems likely that impulse traffic in the lower motoneuron is only marginally affected, if at all. An abnormality of axonal transport may be responsible for the inability of motoneurons to maintain large end-plates in fast-twitch muscles.