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

Abstract

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

Maintaining motor units into old age: running the final common pathway

“…The authors reported very interesting and exciting data from both rodent and human models that investigated the age-related loss of MNs and the resulting effects on skeletal muscle. We would like to take this opportunity to provide support for the findings reported by Mosole et al 1 by integrating findings we have reported in masters athletes 2-5 and to add to this valuable discussion.…”

“…We read with great interest a recent article from Mosole et al 1 that synthesized the literature and presented some new data to provide clarity on whether life-long exercise promotes reinnervation of muscle and survival of spinal [motor neurons (MNs)] and [motor units (MUs)] into old age. The authors reported very interesting and exciting data from both rodent and human models that investigated the age-related loss of MNs and the resulting effects on skeletal muscle.…”

“… 15 These electrophysiological measures of fiber grouping are consistent with histological evidence of increased MU homogeneity in advanced age. 1 , 14 , 16 , 17 Masters athletes have provided an interesting model in humans to explore healthy adult aging in a population free from many confounding factors known to accelerate the aging process such as obesity and immobility/disuse. Both masters athletes and healthy older adults seem to experience MU remodeling but the masters athletes exhibit less MU loss and more effective reinnervation and more stable neuromuscular transmission, whereas the age-matched controls presented with more extensive remodeling and less stable neuromuscular transmission.…”

Old muscle in young body: an aphorism describing the Chronic Fatigue Syndrome

“… 26 , 27 This hypothesis will also be discussed using data from the most recent literature on the specific metabolic-functional muscle markers of CFS comparing them with those derived from sarcopenic muscles. 28-30 …”

Exciting perspectives for Translational Myology in the Abstracts of the 2018Spring PaduaMuscleDays: Giovanni Salviati Memorial – Chapter IV - Abstracts of March 17, 2018

“…Based on present and previous observations changes in absolute and relative number of fiber types, in any experimental model, 246 will ask for morphometry of the whole muscle cross-section, while after muscle sampling only the size of the different types of muscle fibers could be computed. 247 …”