Journal of Neurophysiology, Vol 64, Issue 4 1261-1269, Copyright © 1990 by APS
Long-term persistence of enlarged motor units in partially denervated hindlimb muscles of cats
A. R. Luff and K. Torkko
Department of Physiology, Monash University, Clayton, Victoria, Australia.
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 approximately 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.(ABSTRACT TRUNCATED AT 250 WORDS)
