The Response of Denervated Muscle to Long-Term Stimulation (1985, Revisited here in 2014)

Gerta Vrbová, a guide and a friend for a generation of neuro-myologists - Her scientific legacies and relations with colleagues

Gerta Sidonová - Vrbová, (Trnava, Slovakia, November 28, 1926 - London, UK, October 2, 2020) has been a key neuroscientist, who for almost half a century has contributed important findings and hypotheses on the relationships between motoneurons and skeletal muscle fibers, in particular on the differentiation and extent of plasticity of the peculiar characteristics of the different types of fibers present in mammalian muscles. This issue, Ejtm 31 (1), 2021, opens with the personal obituary authored by Dirk Pette, who remember his lifelong collaboration with Gerta, describing the many molecular and metabolic events that occur by changing the pattern of activation of adult muscle fibers through neuromuscular low frequency electrical stimulation. To honor the many scientific legacies of Gerta Vrbová and her impact on a generation of researchers studying myology and managements of neuromuscular disorders I add here additional examples of Gerta's scientific heritage and of her relations with colleagues.

Muscle spindles of the rat sternomastoid muscle

The sternomastoid (SM) muscle in rodents presents a peculiar distribution of fiber types with a steep gradient from the ventral, superficial, white portion to the dorsal, deep, red region, where muscle spindles are restricted. Cross section of the medial longitudinal third of the rat SM contains around 10,000 muscle fibers with a mean diameter of 51.28±12.62 (μm +/- SD). Transverse sections stained by Succinate Dehydrogenase (SDH) reaction clearly presents two distinct regions: the dorsal deep red portion encompassing a 40% cross section area contains a high percentage of packed SDH-positive muscle fibers, and the ventral superficial region which contains mainly SDH-negative muscle fibers. Indeed, the ventral superficial region of the rat SM muscle contains mainly fast 2B muscle fibers. These acidic ATPase pH 4.3-negative and SDH-negative 2B muscle fibers are the largest of the SM muscle, while the acidic ATPase pH 4.3-positive and SDH-positive Type 1 muscle fibers are the smallest. Here we show that in thin transverse cryosections only 2 or 3 muscle spindle are observed in the central part of the dorsal deep red portion of the SM muscle. Azan Mallory stained sections allow at the same time to count the spindles and to evaluate aging fibrosis of the skeletal muscle tissue. Though restricted in the muscle red region, SM spindles are embedded in perimysium, whose changes may influence their reflex activity. Our findings confirm that any comparisons of changes in number and percentage of muscle spindles and muscle fibers of the rat SM muscle will require morphometry of the whole muscle cross-section. Muscle biopsies of SM muscle from large mammals will only provide partial data on the size of the different types of muscle fibers biased by sampling. Nonetheless, histology of muscle tissue continue to provide practical and low-cost quantitative data to follow-up translational studies in rodents and beyond.

The duty cycle in Functional Electrical Stimulation research. Part II: Duty cycle multiplicity and domain reporting

In part I of this review, we introduced the duty cycle as a fundamental parameter in controlling the effect of electrical stimulation pulse trains on muscle structural and functional properties with special emphasis on fatigue. Following on from a survey of the literature, we discuss here the relative ability of intermittent and continuous stimulation to fatigue muscle. In addition, pertinent literature is explored on a more deeper level, highlighting contentions regarding the duty cycle across studies. In response to literature inconsistencies, we propose frameworks upon which the duty cycle parameter may be specified. We present the idea of domain reporting for the duty cycle, and illustrate with practical examples. In addition we dig further into the literature and present a set of notations that have been used by different researchers to report the duty cycle. We also propose the idea of the duty cycle multiple, which together with domain reporting, will help researchers understand more precisely duty cycles of electrical stimulation. As a case study, we also show how the duty cycle has been looked at by researchers in the context of pressure sore attenuation in patients. Together with part I, it is hoped that the frameworks suggested provide a complete picture of how duty cycle has been discussed across the literature, and gives researchers a more trans-theoretical basis upon which they may report the duty cycle in their studies. This may also lead to a more precise specification of electrical stimulation protocols used in patients.

Myokines in Home-Based Functional Electrical Stimulation-Induced Recovery of Skeletal Muscle in Elderly and Permanent Denervation

Neuromuscular disorders, disuse, inadequate nutrition, metabolic diseases, cancer and aging produce muscle atrophy and this implies that there are different types of molecular triggers and signaling pathways for muscle wasting. Exercise and muscle contractions may counteract muscle atrophy by releasing a group of peptides, termed myokines, to protect the functionality and to enhance the exercise capacity of skeletal muscle. In this review, we are looking at the role of myokines in the recovery of permanent denervated and elderly skeletal muscle tissue. Since sub-clinical denervation events contribute to both atrophy and the decreased contractile speed of aged muscle, we saw a parallel to spinal cord injury and decided to look at both groups together. The muscle from lifelong active seniors has more muscle bulk and more slow fiber-type groupings than those of sedentary seniors, demonstrating that physical activity maintains slow motoneurons that reinnervate the transiently denervated muscle fibers. Furthermore, we summarized the evidence that muscle degeneration occur with irreversible Conus and Cauda Equina syndrome, a spinal cord injury in which the human leg muscles may be permanently disconnected from the peripheral nervous system. In these patients, suffering with an estreme case of muscle disuse, a complete loss of muscle fibers occurs within five to ten years after injury. Their recovered tetanic contractility, induced by home-based Functional Electrical Stimulation, can restore the muscle size and function in compliant Spinal Cord Injury patients, allowing them to perform electrical stimulation-supported stand-up training. Myokines are produced and released by muscle fibers under contraction and exert both local and systemic effects. Changes in patterns of myokine secretion, particularly of IGF-1 isoforms, occur in long-term Spinal Cord Injury persons and also in very aged people. Their modulation in Spinal Cord Injury and late aging are also key factors of home-based Functional Electrical Stimulation - mediated muscle recovery. Thus, Functional Electrical Stimulation should be prescribed in critical care units and nursing facilities, if persons are unable or reluctant to exercise. This will result in less frequent hospitalizations and a reduced burden on patients' families and public health services.

Revisiting the peculiar regional distribution of muscle fiber types in rat Sternomastoid Muscle

The sternomastoid (SM) muscle in rodents is known to have a peculiar distribution of fiber types with a steep gradient from surface to deep region. We here further characterize this peculiar regional distribution by quantitative histochemical morphometrys. In Hematoxylin-Eosin (H-E) stained transverse cryosections harvested in the medial portion of the muscle we counted around 10.000 myofibers with a mean diameter of 51.3±12.6 (μm). Cryisections of the SM stained by SDH reaction clearly show two distinct regions, toward the deep surface of the muscle a 40% area that contains packed SDH-positive myofibers, while the remaining area of the SM toward the external surface presents a more checker-board appearance. On the other hand, in the deep region of SM type 1 (slow contracting) muscle fibers, caracterized by positive acidic ATPase pH 4.35 reaction, are only the 24.5% of the fibers in the deep area of SM muscles, being restricted to the deepest region. The 75.5% of the myofibers in the deep region are of the fast contracting types (either 48.4% 2A, SDH -positive fibers or 27.1% 2B, SDH-negative fibers, respectively). As expected the 2B muscle fibers, acidic ATPase pH 4.3-negative and SDH-negative, present the largest size, while Type 1 fibers, acidic ATPase pH 4.3-positive and SDH-positive, present the smallest size in rat SM muscle. Based on present and previous observations, comparison of change in absolute number and/or percentage of the fiber types in any experimental model of muscle atrophy/hypertrophy/plasticity/pathology /recovery in the rat SM, and possibly of all mammals, will ask for morphometry of the whole muscle cross-sections, muscle sampling by bioptic approches will provide only comparable data on the size of the different types of muscle fibers.

To Reverse Atrophy of Human Muscles in Complete SCI Lower Motor Neuron Denervation by Home-Based Functional Electrical Stimulation

After spinal cord injury (SCI), patients spend daily several hours in wheelchairs, sitting on their hamstring muscles. SCI causes muscle atrophy and wasting, which is especially severe after complete and permanent damage to lower motor neurons. A European Union (EU)-supported work demonstrates that electrical fields produced by large electrodes and purpose-developed electrical stimulators recover both quadriceps and hamstring muscles, producing a cushioning effect capable of benefitting SCI patients, even in the worst case of complete and long-term lower motor neuron denervation of leg muscles. We reported that 20 out of 25 patients completed a 2-year h-bFES program, which resulted in (1) a 35% increase in cross-sectional area of the quadriceps muscles (P < 0.001), (2) a 75% increase in mean diameter of quadriceps muscle fibers (P < 0.001), and (3) improvement of the ultrastructural organization of contractile machinery and of the Ca2+-handling system. Though not expected, after 2 years during which the 20 subjects performed 5 days per week h-bFES of the atrophic quadriceps muscles, the CT cross-sectional area of the hamstring muscles also augmented, increasing from 26.9+/-8.4 (cm²) to 30.7+/-9.8 (cm²), representing a significant (p ≤ 0.05) 15% increase. Here we show by quantitative muscle color computed tomography (QMC-CT) that h-bFES-induced tissue improvements are present also in the hamstring muscles: a once supposed drawback (lack of specificity of muscle activation by large surface electrodes) is responsible for a major positive clinical effect. Interestingly, 2 years of home-based FES by large surface electrodes reversed also the denervation-induced skin atrophy, increasing epidermis thickness. Finally, we would like to attract attention of the readers to quantitative muscle color computed tomography (QMC-CT), a sensitive quantitative imaging analysis of anatomically defined skeletal muscles introduced by our group to monitor atrophy/degeneration of skeletal muscle tissue. Worldwide acceptance of QMC-CT will provide physicians an improved tool to quantitate skeletal muscle atrophy/degeneration before and during rehabilitation strategies so that therapy for mobility-impaired persons can be better prescribed, evaluated, and altered where needed.

Mobility disorders and pain, interrelations that need new research concepts and advanced clinical commitments

This Perspective will discuss topics recently suggested by Prof. Helmut Kern, Vienna, Austria, to advance the research activities of his team, that is: Topic A, 10 years post RISE; Topic B, New research for new solutions on old research questions; Topic C, Working groups on nerve regeneration, training-parameters of seniors in different ages, muscle adaptation; and studies of connective tissue and cartilage. This Perspective summarizes some of the basic concepts and of the evidence-based tools for developing further translational research activities. Clinically relevant results will ask for continuous interests of Basic and Applied Myologists and for the support during the next five to ten years of public and private granting agencies. All together, they will end in protocols, devices and multidisciplinary managements for persons suffering with muscle denervation, neuromuscular-related or non-related pain and for the increasing population of old, older and oldest senior citizens in Europe and beyond.

Comparison of strategies and performance of functional electrical stimulation cycling in spinal cord injury pilots for competition in the first ever CYBATHLON

Functional Electrical Stimulation (FES) can elicit muscular contraction and restore motor function in paralyzed limbs. FES is a rehabilitation technique applied to various sensorimotor deficiencies and in different functional situations, e.g. grasping, walking, standing, transfer, cycling and rowing. FES can be combined with mechanical devices. FES-assisted cycling is mainly used in clinical environments for training sessions on cycle ergometers, but it has also been adapted for mobile devices, usually tricycles. In October 2016, twelve teams participated in the CYBATHLON competition in the FES-cycling discipline for persons with motor-complete spinal cord injury. It was the first event of this kind and a wide variety of strategies, techniques and designs were employed by the different teams in the competition. The approaches of the teams are detailed in this special issue. We hope that the knowledge contained herein, together with recent positive results of FES for denervated degenerating muscles, will provide a solid basis to encourage improvements in FES equipment and open new opportunities for many patients in need of safe and effective FES management. We hope to see further developments and/or the benefit of new training strategies at future FES competitions, e.g. at the Cybathlon 2020 (www.cybathlon.ethz.ch).

Use it or Lose It: Tonic Activity of Slow Motoneurons Promotes Their Survival and Preferentially Increases Slow Fiber-Type Groupings in Muscles of Old Lifelong Recreational Sportsmen

Histochemistry, immuno-histochemistry, gel electrophoresis of single muscle fibers and electromyography of aging muscles and nerves suggest that: i) denervation contributes to muscle atrophy, ii) impaired mobility accelerates the process, and iii) lifelong running protects against loss of motor units. Recent corroborating results on the muscle effects of Functional Electrical Stimulation (FES) of aged muscles will be also mentioned, but we will in particular discuss how and why a lifelong increased physical activity sustains reinnervation of muscle fibers. By analyzing distribution and density of muscle fibers co-expressing fast and slow Myosin Heavy Chains (MHC) we are able to distinguish the transforming muscle fibers due to activity related plasticity, to those that adapt muscle fiber properties to denervation and reinnervation. In muscle biopsies from septuagenarians with a history of lifelong high-level recreational activity we recently observed in comparison to sedentary seniors: 1. decreased proportion of small-size angular myofibers (denervated muscle fibers); 2. considerable increase of fiber-type groupings of the slow type (reinnervated muscle fibers); 3. sparse presence of muscle fibers co-expressing fast and slow MHC. Immuno-histochemical characteristics fluctuate from those with scarce fiber-type modulation and groupings to almost complete transformed muscles, going through a process in which isolated fibers co-expressing fast and slow MHC fill the gaps among fiber groupings. Data suggest that lifelong high-level exercise allows the body to adapt to the consequences of the age-related denervation and that it preserves muscle structure and function by saving otherwise lost muscle fibers through recruitment to different slow motor units. This is an opposite behavior of that described in long term denervated or resting muscles. These effects of lifelong high level activity seems to act primarily on motor neurons, in particular on those always more active, i.e., on the slow motoneurons. The preferential reinnervation that follows along decades of increased activity maintains neuron and myofibers. All together the results open interesting perspectives for applications of FES and electroceuticals for rejuvenation of aged muscles to delay functional decline and loss of independence that are unavoidable burdens of advanced aging.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01679977.

Editorial: The EJTM Special "Mobility in Elderly"

Not available.

3D False Color Computed Tomography for Diagnosis and Follow-Up of Permanent Denervated Human Muscles Submitted to Home-Based Functional Electrical Stimulation

This report outlines the use of a customized false-color 3D computed tomography (CT) protocol for the imaging of the rectus femoris of spinal cord injury (SCI) patients suffering from complete and permanent denervation, as characterized by complete Conus and Cauda Equina syndrome. This muscle imaging method elicits the progression of the syndrome from initial atrophy to eventual degeneration, as well as the extent to which patients' quadriceps could be recovered during four years of home-based functional electrical stimulation (h-b FES). Patients were pre-selected from several European hospitals and functionally tested by, and enrolled in the EU Commission Shared Cost Project RISE (Contract n. QLG5-CT-2001-02191) at the Department of Physical Medicine, Wilhelminenspital, Vienna, Austria. Denervated muscles were electrically stimulated using a custom-designed stimulator, large surface electrodes, and customized progressive stimulation settings. Spiral CT images and specialized computational tools were used to isolate the rectus femoris muscle and produce 3D and 2D reconstructions of the denervated muscles. The cross sections of the muscles were determined by 2D Color CT, while muscle volumes were reconstructed by 3D Color CT. Shape, volume, and density changes were measured over the entirety of each rectus femoris muscle. Changes in tissue composition within the muscle were visualized by associating different colors to specified Hounsfield unit (HU) values for fat, (yellow: [-200; -10]), loose connective tissue or atrophic muscle, (cyan: [-9; 40]), and normal muscle, fascia and tendons included, (red: [41; 200]). The results from this analysis are presented as the average HU values within the rectus femoris muscle reconstruction, as well as the percentage of these tissues with respect to the total muscle volume. Results from this study demonstrate that h-b FES induces a compliance-dependent recovery of muscle volume and size of muscle fibers, as evidenced by the gain and loss in muscle mass. These results highlight the particular utility of this modality in the quantitative longitudinal assessment of the responses of skeletal muscle to long-term denervation and h-b FES recovery.

Persistent Muscle Fiber Regeneration in Long Term Denervation. Past, Present, Future

Despite the ravages of long term denervation there is structural and ultrastructural evidence for survival of muscle fibers in mammals, with some fibers surviving at least ten months in rodents and 3-6 years in humans. Further, in rodents there is evidence that muscle fibers may regenerate even after repeated damage in the absence of the nerve, and that this potential is maintained for several months after denervation. While in animal models permanently denervated muscle sooner or later loses the ability to contract, the muscles may maintain their size and ability to function if electrically stimulated soon after denervation. Whether in mammals, humans included, this is a result of persistent de novo formation of muscle fibers is an open issue we would like to explore in this review. During the past decade, we have studied muscle biopsies from the quadriceps muscle of Spinal Cord Injury (SCI) patients suffering with Conus and Cauda Equina syndrome, a condition that fully and irreversibly disconnects skeletal muscle fibers from their damaged innervating motor neurons. We have demonstrated that human denervated muscle fibers survive years of denervation and can be rescued from severe atrophy by home-based Functional Electrical Stimulation (h-bFES). Using immunohistochemistry with both non-stimulated and the h-bFES stimulated human muscle biopsies, we have observed the persistent presence of muscle fibers which are positive to labeling by an antibody which specifically recognizes the embryonic myosin heavy chain (MHCemb). Relative to the total number of fibers present, only a small percentage of these MHCemb positive fibers are detected, suggesting that they are regenerating muscle fibers and not pre-existing myofibers re-expressing embryonic isoforms. Although embryonic isoforms of acetylcholine receptors are known to be re-expressed and to spread from the end-plate to the sarcolemma of muscle fibers in early phases of muscle denervation, we suggest that the MHCemb positive muscle fibers we observe result from the activation, proliferation and fusion of satellite cells, the myogenic precursors present under the basal lamina of the muscle fibers. Using morphological features and molecular biomarkers, we show that severely atrophic muscle fibers, with a peculiar cluster reorganization of myonuclei, are present in rodent muscle seven-months after neurectomy and in human muscles 30-months after complete Conus-Cauda Equina Syndrome and that these are structurally distinct from early myotubes. Beyond reviewing evidence from rodent and human studies, we add some ultrastructural evidence of muscle fiber regeneration in long-term denervated human muscles and discuss the options to substantially increase the regenerative potential of severely denervated human muscles not having been treated with h-bFES. Some of the mandatory procedures, are ready to be translated from animal experiments to clinical studies to meet the needs of persons with long-term irreversible muscle denervation. An European Project, the trial Rise4EU (Rise for You, a personalized treatment for recovery of function of denervated muscle in long-term stable SCI) will hopefully follow.

History, Mechanisms and Clinical Value of Fibrillation Analyses in Muscle Denervation and Reinnervation by Single Fiber Electromyography and Dynamic Echomyography

This work reviews history, current clinical relevance and future of fibrillation, a functional marker of skeletal muscle denervated fibers. Fibrillations, i.e., spontaneous contraction, in denervated muscle were first described during the nineteenth century. It is known that alterations in membrane potential are responsible for the phenomenon and that they are related to changes in electrophysiological factors, cellular metabolism, cell turnover and gene expression. They are known to inhibit muscle atrophy to some degree and are used to diagnose neural injury and reinnervation that are occurring in patients. Electromyography (EMG) is useful in determining progress, prognosis and efficacy of therapeutic interventions and their eventual change. For patients with peripheral nerve injury, and thus without the option of volitional contractions, electrical muscle stimulation may be helpful in preserving the contractility and extensibility of denervated muscle tissue and in retarding/counteracting muscle atrophy. It is obvious from the paucity of recent literature that research in this area has declined over the years. This is likely a consequence of the decrease in funding available for research and the fact that the fibrillations do not appear to cause serious health issues. Nonetheless, further exploration of them as diagnostic tools in long-term denervation is merited, in particular if Single Fiber EMG (SFEMG) is combined with Dynamic Echomyography (DyEM), an Ultra Sound muscle approach we recently designed and developed to explore denervated and reinnervating muscles.