Limited fiber type grouping in self-reinnervation cat tibialis anterior muscles

Time course of functional recovery during the first 3 mo after surgical transection and repair of nerves to the feline soleus and lateral gastrocnemius muscles

Locomotion outcomes after peripheral nerve injury and repair in cats have been described in the literature for the period immediately following the injury (muscle denervation period) and then again for an ensuing period of long-term recovery (at 3 mo and longer) resulting in muscle self-reinnervation. Little is known about the changes in muscle activity and walking mechanics during midrecovery, i.e., the early reinnervation period that takes place between 5 and 10 wk of recovery. Here, we investigated hindlimb mechanics and electromyogram (EMG) activity of ankle extensors in six cats during level and slope walking before and every 2 wk thereafter in a 14-wk period of recovery after the soleus (SO) and lateral gastrocnemius (LG) muscle nerves in one hindlimb were surgically transected and repaired. We found that the continued increase in SO and LG EMG magnitudes and corresponding changes in hindlimb mechanics coincided with the formation of neuromuscular synapses revealed in muscle biopsies. Throughout the recovery period, EMG magnitude of SO and LG during the stance phase and the duration of the stance-related activity were load dependent, similar to those in the intact synergistic medial gastrocnemius and plantaris. These results and the fact that EMG activity of ankle extensors and locomotor mechanics during level and upslope walking recovered 14 wk after nerve transection and repair suggest that loss of the stretch reflex in self-reinnervated muscles may be compensated by the recovered force-dependent feedback in self-reinnervated muscles, by increased central drive, and by increased gain in intermuscular motion-dependent pathways from intact ankle extensors. NEW & NOTEWORTHY This study provides new evidence that the timeline for functional recovery of gait after peripheral nerve injury and repair is consistent with the time required for neuromuscular junctions to form and muscles to reach preoperative tensions. Our findings suggest that a permanent loss of autogenic stretch reflex in self-reinnervated muscles may be compensated by recovered intermuscular force-dependent and oligosynaptic length-dependent feedback and central drive to regain adequate locomotor output capabilities during level and upslope walking.

Short- or long-term effects of adult myoblast transfer on properties of reinnervated skeletal muscles

Skeletal muscle demonstrates a force deficit after repair of injured peripheral nerves. Data from the literature indicate that myoblast transfer enhances recovery of muscle function. Thus, we tested the hypothesis that transfer of adult myoblasts improves the properties of reinnervated rabbit tibialis anterior (TA) muscles in both the short term (4 months) and long term (14 months). Two months after transection and immediate suture of the common peroneal nerve, TA muscles were made to degenerate by cardiotoxin injection and then transplanted with adult myoblasts cultured for 13 days. Under these conditions, muscles studied at 4 months were heavier, contained larger fibers, and developed a significantly higher maximal force than muscles that had only been denervated-reinnervated. In the long term, although muscles made to degenerate were heavier and developed a significantly higher maximal force than denervated-reinnervated muscles, myoblast transfer failed to improve these parameters. However, the overall characteristics of long-term operated muscles tended clearly to approach those of the controls. Taken together, these results may have significant implications in certain orthopedic contexts, particularly after immediate or delayed muscle reinnervation.

Recovery of type I fiber regionalization in gastrocnemius medialis of the rat after reinnervation along original and foreign paths, with and without muscle rotation

After reinnervation following transection of the sciatic nerve, normal patterns of regional type I fiber distribution are known to return in rat hindlimb muscles. Here we investigate how this recovery is influenced by experimental conditions. In an initial operation, the nerve of gastrocnemius medialis (GM) of adult rats was cut close to the muscle and reinserted either (i) close to the original nerve entry, or (ii) at a more medial 'foreign' site. In other groups of animals, these nerve operations were combined with a rotation of the GM muscle around its longitudinal axis, trying to ascertain whether the position of the muscle within the limb was of importance for the reinnervation processes. In a control group the muscle was rotated but innervation remained intact. After 21 weeks, the GM muscles were removed from both hindlimbs. Cross-sections were cut at seven different levels along each muscle, and 'slow' type I fibers were identified after staining for myofibrillar ATPase. The topographical positions were mapped out for all type I fibers. In all reinnervated muscles, an extensive type I fiber grouping was seen, indicating a widespread respecification of muscle fiber properties by ingrowing 'slow' axons. Normal topographical directions of type I fiber regionalization were about equally well restored in groups with the nerve inserted at the original or at the foreign site. In rotated muscles, the direction of type I fiber regionalization was significantly less rotated than the muscle as a whole. The results suggest that ingrowing 'slow' motor axons are guided toward their normal 'slow' regions by clues which are largely independent of the i.m. path of regeneration (original vs. foreign nerve entry site) but partly dependent on the position of the target muscle within the limb (rotated vs. non-rotated cases).

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

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.