LONG-TERM PERSISTENCE OF ENLARGED MOTOR UNITS IN PARTIALLY DENERVATED HINDLIMB MUSCLES OF CATS

1. It was the aim of this study to determine the effect that regenerating motor axons would have on enlarged or sprouted motor units that had been established for a relatively long time. 2. The fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus were partially denervated by unilateral section of the L7 ventral root in 12-wk-old kittens. After 200+ days single motor units were isolated, and their isometric contractile properties were determined. FDL units were also tested for their resistance to fatigue and categorized as fast-twitch-fatigable (FF), fast-twitch-fatigue-resistant (FR), and slow-twitch-fatigue-resistant (S). 3. It had been established previously that regenerating motor axons via L7 returned to the experimental muscles by ~ 100 days. Therefore from 100 to 200 days it was assumed that the sprouted units would be in competition with the regenerating axons. The extent of the original denervations was variable and was estimated from the contralateral side. In soleus most denervations ranged from 83 to 99%; in FDL, from 37 to 81%. 4. In both soleus and FDL there was no evidence that the motor units had sprouted to any less extent than found previously. Within some soleus muscles the largest motor units were developing substantially more force than was expected. However, determination of mean fiber cross-sectional area from muscles frozen, sectioned, and prepared for histochemical analysis showed that this was attributable to increased mean cross-sectional area of the type I fibers. 5. The innervation ratio of the largest soleus units was estimated from unit force, an assumed value for specific force and mean cross-sectional area of type I fibers. For 11 units this gave a mean of 3,580 ± 800 (SD) fibers. 6. The presence of polyneuronal innervation was investigated between pairs of sprouted units and none was found. It was also checked for between the ventral roots containing sprouted axons and regenerating axons. Only in soleus was there some evidence of a small amount of polyneuronal innervation between sprouted and regenerated axons. 7. All units were tested for their response to high-frequency tetanic stimulation (300 Hz). Most units in soleus, particularly the largest units, could not sustain force at high frequencies; units in FDL were unaffected. 8. It is concluded that the terminals of sprouted motor units that have had some time to establish themselves are not repressed by regenerating motor axons. This is despite the fact htat several of the units had sprouted to at least 10 times their mean normal size and force development failed with high-frequency stimulation.