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The sciatic nerve was crushed in 5-6-day-old rats and the time course of recovery and changes in physiological and morphological properties of reinnervated fast and slow muscles was compared. The maximal tetanic tension developed by the reinnervated muscles was recorded at different times from about 18 days of age, when functional recovery was first seen, until 2 months. The maximal indirectly elicited tetanic tension of the reinnervated slow soleus muscle gradually increased from 55% of normal at 18 days to 75% of normal at 2 months. In contrast, the tension of the reinnervated fast muscle extensor digitorum longus (e.d.l.) fell sharply from 70% of normal at 18 days to 40% at 21 days and remained at that level till the end of the study. The total number of muscle fibres in control, reinnervated and denervated e.d.l. muscles was counted. At 18 days the number of fibres in the reinnervated e.d.l. was similar to normal but by 1 month it had fallen to one-third. This decrease did not take place in permanently denervated muscles until at least 35 days. Loss of fibres in the reinnervated soleus was small. During the early stages of reinnervation the contraction and relaxation of the fast muscles was very prolonged. By 1 month the time taken to reach peak twitch tension had decreased to normal values but the relaxation was still slower and remained so for several months. The study of fatigue resistance showed that at 18 days the reinnervated fast muscles were as fatigable as normal muscles from animals of the same age. The fatigability of normal muscles increased with age to adult levels, but the reinnervated muscles became more fatigue resistant and remained so. Our findings suggest that fast muscles become selectively impaired after nerve injury at 6 days because they lose a large number of fibres after reinnervation.

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