Effects of sustained stimulation on the excitability of motoneurons innervating paralyzed and control muscles

The excitability of thenar motoneurons (reflected by F-wave persistence and amplitude) and thenar muscle force were measured during a stimulation protocol (90 s of 18-Hz supramaximal electrical stimulation of the median nerve) designed to induce muscle fatigue (force decline). Data from muscles ( n = 15) paralyzed by chronic cervical spinal cord injury were compared with those obtained from control muscles ( n = 6). The persistence of F waves in both paralyzed and control muscles increased from ∼60 to ∼76% during the first 10 s of the fatigue protocol. Persistence then declined progressively to ∼33% at 90 s. These changes in F-wave persistence suggest that similar reductions occur in the excitability of the motoneurons to paralyzed and control motor units after sustained antidromic activation. Despite this, significantly larger force declines occurred in the paralyzed muscles of spinal cord-injured subjects (∼60%) than in the muscles of control subjects (∼15%). These data suggest that the decreases in motoneuron excitability for both the spinal cord-injured and control subjects are a result of activity-dependent changes in motoneuron properties that are independent of fatigue-related processes in the muscles

Embryonic neurons transplanted into the tibial nerve reinnervate muscle and reduce atrophy but NCAM expression persists

Objective: The aim of this study was to use the glycogen depletion technique to determine whether reinnervated muscle fibers could be distinguished from denervated muscle fibers by their size or by neural cell adhesion molecule (NCAM) expression. Methods: Medial gastrocnemius muscles of five adult Fischer rats were reinnervated from embryonic neurons transplanted into the distal stump of the tibial nerve. Ten weeks later, the transplants were stimulated repeatedly to deplete reinnervated muscle fibers of glycogen. Areas of reinnervated (glycogen-depleted) muscle fibers were measured and assessed for NCAM expression. The areas of muscle fibers from reinnervated, denervated (n=5) and unoperated control muscles (n=5) were compared. Results: Mean reinnervated muscle fiber area was significantly larger than the mean for denervated fibers (mean ± SE: 40 ± 6 and 10 ± 1% of unoperated control fibers, respectively). NCAM was expressed in 55 ± 7% of reinnervated fibers (mean ± SE; range: 42-77%). The mean areas of reinnervated fibers that did or did not express NCAM were similar. NCAM was only expressed in some fibers in completely denervated muscles. Discussion: Our data show that NCAM expression does not differentiate muscle denervation or reinnervation. Quantifying the area of large fibers did distinguish reinnervated muscle fibers from denervated fibers and showed that reinnervation of muscle from neurons placed in peripheral nerve is a strategy to rescue muscle from atrophy