The effect of muscle activity on motor unit size in partially denervated rat soleus muscles

It has been shown previously that after section of L5 ventral ramus at 5 days the intact axons of L4 ventral ramus retain their large neonatal peripheral field in the rat soleus muscles. Soleus muscles of 5-day-old rats were partially denervated by section of their major neural input, L5 ventral ramus, and in addition paralysed with alpha-bungarotoxin for 3-5 days. The motor unit size was examined 2 months later. The tension developed by individual motor units from muscles that were partially denervated and in addition temporarily paralysed was much less than that after partial denervation alone. This reduced tension output was not due to muscle atrophy but to a smaller number of muscle fibres supplied by individual axons. Thus, unlike after partial denervation only, motoneurons were unable to maintain their large neonatal territory when the muscle was temporarily paralysed and they were unable to reoccupy this territory after the muscles recovered from the paralysis. The possibility that arrested muscle maturation due to paralysis has a permanent effect on motor unit size is discussed.