Regional distribution of slow‐twitch muscle fibers after reinnervation in adult rat hindlimb muscles

Abstract

In adult rats, the sciatic nerve was unilaterally sectioned and reunited above the knee. Following a survival time of 21 weeks, five muscles were removed from both lower hindlimbs after determining their intra-limb positions. In each muscle, cryostat sections from seven equidistant proximo-distal levels were stained for myofibrillar ATPase. Intramuscular positions were determined for all slow-twitch type I fibers. Within each muscle, type I fibers were heterogeneously distributed, and the direction of type I fiber accumulation was, on average, almost identical in reinnervated muscles and contralateral controls. Furthermore, as in controls, a proximo-distal decline of type I fiber density was found in reinnervated muscles. Compared to contralateral controls, reinnervated muscles consistently showed a very high number of type I fibers at close interfiber distances, indicating respecification of muscle fiber types by the ingrowing nerve fibers. The results suggest that slow-twitch motor axons preferentially grew back toward the original slow-twitch muscle regions.

Regeneration of Reinnervated Rat Soleus Muscle Is Accompanied by Fiber Transition Toward a Faster Phenotype

“…In addition, we observed variable staining intensity of the individual I/IIA hybrid fibers ( Figures 3A and 3C, and insets), which may indicate a dynamic transition of type I to type IIA fibers, similar to the stepwise transformation of slow fibers toward fast ones in inactivity and crossreinnervation studies (Pette and Vrbová 1992). Moreover, spatial grouping of the same MHC-expressing fibers was clearly detectable in all of the examined sections ( Figures 3A-3C), which is a phenomenon seen after reinnervation (Ijkema-Paassen et al 2001;Wang et al 2002).…”

Passive and active exercise improve regeneration and muscle reinnervation after peripheral nerve injury in the rat

“…These adaptations are most evident in predominantly slow‐twitch muscles, which show the highest rate of recruitment during normal activity. Reinnervation of a muscle partially restores its phenotype, indicating the importance of the neural activity in changing the muscle fiber phenotype 32, 33. Artificially maintaining muscle activity during the period of denervation/paralysis may improve neuromuscular rehabilitation.…”

Bioengineered nerve regeneration and muscle reinnervation

“…Specificity may be defined at the level of the whole muscle, regions of the muscle or individual muscle fibre phenotype. For instance, there is evidence that a set of specific guidance cues increases the probability of slow and fast motor neurons being directed to the regions of muscle containing more slow or fast muscle fibres, respectively (Wang et al. 2002).…”