Abstract
Muscle unloading results in severe disturbance in neuromuscular function. During juvenile stages of natural development, the neuromuscular system experiences a high degree of plasticity in function and structure. This study aimed to determine whether muscle unloading imposed during juvenile development would elicit more severe disruption in neuromuscular function than when imposed on fully developed, mature neuromuscular systems. Twenty juvenile (3 months old) and 20 mature (8 months old) rats were equally divided into unloaded and control groups yielding a total of four groups (N = 10/each). Following the 2 week intervention period, soleus muscles were surgically extracted and using an ex vivo muscle stimulation and recording system, were examined for neuromuscular function. The unloading protocol was found to have elicited significant (P ≤ 0.05) declines in whole muscle wet weight in both juvenile and mature muscles, but of a similar degree (P = 0.286). Results also showed that juvenile muscles displayed significantly greater decay in peak force due to unloading than mature muscles, such a finding was also made for specific tension or force/muscle mass. When examining neuromuscular efficiency, i.e., function of the neuromuscular junction, it again was noted that juvenile systems were more negatively affected by muscle unloading than mature systems. These results indicate that juvenile neuromuscular systems are more sensitive to the effects of unloading than mature ones, and that the primary locus of this developmental related difference is likely the neuromuscular junction as indicated by age-related differences in neuromuscular transmission efficiency.
Highlights
The neuromuscular system is one of the critical integrative physiological organ systems featured in all animals, including humans (Silverthorn, 2019; American Physiological Society, 2021)
While subtotal disuse of the neuromuscular system, i.e., unloading, immobilization, bed rest, has clearly been shown to disrupt neuromuscular function in adults (Booth and Gollnick, 1983; Hackney and Ploutz-Snyder, 2012; Monti et al, 2021), little is known about how rapidly growing, maturing neuromuscular systems react to the imposition of disuse
The objective of the present investigation was to compare the deleterious effects of disuse in the form of muscle unloading on mature, fully developed vs. juvenile, rapidly developing neuromuscular systems characterized by accelerated plasticity
Summary
The neuromuscular system is one of the critical integrative physiological organ systems featured in all animals, including humans (Silverthorn, 2019; American Physiological Society, 2021). The neuromuscular junction (NMJ), which is the vital synapse joining motor neurons to the skeletal myofibers they innervate, experiences rapid rates of development during early years of adolescence (Hughes et al, 2006; Ferraro et al, 2012). This is important because disturbances in synaptic activity of the NMJ disrupts normal function of skeletal muscle and its capacity to perform the necessities, and rigors of daily living (Metter et al, 2002; Newman et al, 2006; Hairi et al, 2010). The objective of the present investigation was to compare the deleterious effects of disuse in the form of muscle unloading on mature, fully developed vs. juvenile, rapidly developing neuromuscular systems characterized by accelerated plasticity
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