Long-Term Denervation in Humans Causes Degeneration of Both Contractile and Excitation-Contraction Coupling Apparatus, Which Is Reversible by Functional Electrical Stimulation (FES): A Role for Myofiber Regeneration?

Abstract

Over the last 30 years there has been considerable interest in the use of functional electrical stimulation (FES) to restore movement to the limbs of paralyzed patients. Spinal cord injury causes a rapid loss in both muscle mass and contractile force. The atrophy is especially severe when the injury involves lower motoneurons because many months after spinal cord injury, atrophy is complicated by fibrosis and fat substitution. In this study we describe the effects of long-term lower motoneuron denervation of human muscle and present the structural results of muscle trained using FES. By means of an antibody for embryonic myosin, we demonstrate that many regenerative events continue to spontaneously occur in human long-term denervated and degenerated muscle (DDM). In addition, using electron microscopy, we describe i) the overall structure of fibers and myofibrils in long-term denervated and degenerated muscle, including the effects of FES, and ii) the structure and localization of calcium release units, or triads; the structures reputed to activate muscle contraction during excitation-contraction coupling (ECC). Both apparatus undergo disarrangement and re-organization following long-term denervation and FES, respectively. The poor excitability of human long-term DDM fibers, which extends to the first periods of FES training, may be explained in terms of the spatial disorder of the ECC apparatus. Its disorganization and re-organization following long-term denervation and FES, respectively, may play a key role in the parallel disarrangement and re-organization of the myofibrils that characterize denervation and FES training. The present structural studies demonstrate that the protocol used during FES training is effective in reverting long-term denervation atrophy and dystrophy. The mean fiber diameter in FES biopsies is 42.2 +/- 14.8 SD (p < 0.0001 vs DDM 14.9 +/- 6.0 SD); the mean percentile of myofiber area of the biopsy is 94.3 +/- 5.7 SD (p < 0.0001 vs DDM 25.7 +/- 23.7 SD); the mean percentile fat area is 2.1 +/- 2.4 SD (p < 0.001 vs DDM 12.8 +/- 12.1 SD); and the mean percentile connective tissue area is 3.6 +/- 4.6 SD (p < 0.001 vs DDM 61.6 +/- 20.1 SD). In DDM biopsies more than 50% of myofibers have diameter smaller than 10 microm, while the FES-trained subjects have more that 50% of myofibers larger than 30 microm. The recovery of muscle mass seems to be the result of both a size increase of the surviving fibers and the regeneration of new myofibers.

Stable muscle atrophy in long-term paraplegics with complete upper motor neuron lesion from 3- to 20-year SCI

“…A variable percentage of fibers were instead severely atrophic (4-23%, Table 5, column C) and resemble those found in patients that lost their peripheral nerve endings. 7 Some peculiar features allowed us to clearly distinguish large striated fibers in our patients from fibers of controls. 25 The myofibrillar apparatus, while quite well preserved, is in general less ordered than in normal muscle (Figure 5c, black arrows).…”

“…Our observations showed that: (i) human skeletal muscles survive longer (in years) than generally accepted on the basis of experiments in rodents; [4][5][6] (ii) recovery to clinically significant muscle size and function occurs in mid-term (3-5 years) lower motor neuron SCI subjects; 7,8 but (iii) starting FES within the first year post-lesion results in a better recovery of muscle function than when started many years after SCI. 9 To date, however, there is little knowledge about the longterm progression of muscle atrophy in upper motor neuron lesion (UML) patients .…”

“…This was both for the safety and to measure the actual force produced by the thigh muscles during training and functional sessions. 20 Analyses of muscle biopsies Through a small skin incision (6 mm in diameter), needle muscle biopsies were taken from the right and left vastus lateralis muscles at a single time point for each patient, as described by Kern et al 7 The resulting specimens were then prepared either for light and/or EM.…”

Motoneuron Death after Human Spinal Cord Injury

“…For example, various muscles innervated from the lesion epicenter have been unresponsive to electrical stimulation of their peripheral nerves, consistent with complete muscle denervation. [14][15][16][17][18] Other studies have shown signs of partial denervation (low motor unit counts, positive sharp waves, small amplitude compound muscle action potentials, muscle weakness that exceeds that expected from disuse atrophy, atrophied and angular muscle fibers), including indications of intramuscular motor axon sprouting, an important compensatory mechanism for recovery of muscle innervation after death of some motoneurons in a motor pool (large polyphasic motor unit potentials, stronger than usual motor unit forces, increased motor unit fiber density, increased jitter in re-innervated muscle fibers). 12,[19][20][21][22][23] In this study, we address an important and often overlooked aspect of SCI-the effect of injury on the lesioned spinal cord segment and denervation of skeletal muscle as a contributor to muscular weakness.…”

Myogenesis in human denervated muscle biopsies

“…7 Muscle biopsies from patients with spinal cord injury lasting from 9 months to 19 years also contained immature fibers, which were interpreted as regenerative in nature. 8,11,14,15 These fibers had a central nucleus and a thin cytoplasmic rim containing a few myofilaments, and expressed the embryonic isoform of the myosin heavy chain molecule (MHCemb). They represented about 1% of the total myofiber population in these muscle biopsies.…”