Research articleEffect of partial denervation on motor units in the ageing rat medial gastronemius
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Neuroprotective effects of exercise on the aging human neuromuscular system
2021, Experimental GerontologyCitation Excerpt :The age-related loss of the MN appears to be preceded by decrements in metabolic capacity, as well as structural changes, including myelin sheath irregularities, myelin ovoids, macrophages overloaded with myelin debris, and Wallerian degeneration (Larsson and Ansved, 1995). Some researchers have used the term “sick MNs” when describing this state (Einsiedel and Luff, 1992; McComas et al., 1971), as it is associated with significant loss of MN function across a range of domains, including reduced action potential conduction velocity, impaired axoplasmic transport (Brunetti et al., 1987; Kanda and Hashizume, 1989; McMartin and O'Connor, 1979; Pestronk et al., 1980), and, ultimately, reduced capacity for regeneration (Einsiedel and Luff, 1992). However, it should be noted that the “sick MN” terminology has fallen out of favour as it is important not to conflate aging with disease.
The emerging role of the sympathetic nervous system in skeletal muscle motor innervation and sarcopenia
2021, Ageing Research ReviewsCitation Excerpt :Despite its relevance to the onset and outcome of sarcopenia, the etiology of age-related skeletal muscle denervation remains unknown (Delbono, 2011a). Several groups have reported skeletal muscle denervation and reinnervation and motor unit remodeling or loss in aging rodents or humans (Doherty et al., 1993; Einsiedel and Luff, 1992; Hashizume et al., 1988; Johnson et al., 1995; Kanda and Hashizume, 1989, 1992; Larsson and Ansved, 1995; Larsson et al., 1991, 2019; Larsson and Edstrom, 1986; Zhang et al., 1996). Motor-unit remodeling leads to changes in fiber-type composition (Pette and Staron, 2001; Schiaffino and Reggiani, 1994, 1996).
Motor unit number estimation (MUNE): Where are we now?
2018, Clinical NeurophysiologyCitation Excerpt :The differences in the reported studies seem to be related to the methodologies employed, ranging from 30% to 50% between age 30 and 70 years. The increase in MU size accompanying this age-related decline in MUNE is less than found in neurogenic disorders, perhaps reflecting a reduced capacity for compensatory reinnervation with aging (Einsiedel and Luff, 1992; Shefner, 2001). Exceptionally gifted, aged athletes show relative preservation in the number of MUs in their tested muscles and, in these subjects, these MUs were more stable than in an age-matched non-athletic population (Power et al., 2016).
Age-associated alterations of the neuromuscular junction
2011, Experimental GerontologyCitation Excerpt :A study in soleus of 4- and 24-month-old rats found a significant age-related degeneration of endplates leading to a loss of muscle fibers and decrease in the size of motor units (Gutmann and Hanzlikova, 1966). A loss of muscle fibers and an increase in innervation ratio leading to a 30–40% loss in motor units and preferential loss of fast motor units were demonstrated in old rats as well as in humans (Edstrom and Larsson, 1987; Einsiedel and Luff, 1992a,b). As imaging techniques have advanced over the last few decades, our understanding of neuromuscular synapse formation and molecular players that regulate its function have been greatly enhanced.
Muscle diseases and aging
2007, Handbook of Clinical NeurologyCitation Excerpt :Studies in animals as well as in humans have also shown that there is reduction in motor unit numbers with aging (Brooks and Faulkner, 1994; Kadhiresan et al., 1996; Liu et al., 1996; Larsson et al., 2001; McNeil et al., 2005,). This decrease involves all components of the motor unit including myelinated motor axons as well as a reduction in the size and numbers of spinal motor neurons (Hashizume et al., 1988; Einsiedel and Luff, 1992; Doherty et al., 1993a, 1993b; Liu et al., 1996; Zhang et al., 1996; McNeil et al., 2005). There is good evidence from some studies that the type of motor neuron and consequent muscle fiber loss is preferential and involves mainly the fast firing larger motor neurons and type II muscle fibers (Hashizume et al., 1988; Doherty et al., 1993a; Kadhiresan et al., 1996).