Neuromuscular Adaptation to Microgravity Environment.

Abstract

The pattern of muscle activity influences the morphological, metabolic, and contractile properties of skeletal muscles. For example, the metabolic capacity of muscles is affected specifically by the type of exercise training [1][2][3]. Increased activity or over-loading by removing the synergists causes a compensatory hypertrophy [4][5][6][7][8][9][10][11]. Hypertrophy is also induced by the stretching of matured muscles in vivo [12,13] and cultured myotubes and fibroblasts [14][15][16][17]. Exercise-induced metabolic adaptation of muscles is lost when exercise training is stopped [18]. Further, muscle atrophy is induced by denervation [10,19], spinal cord transection [20], joint immobilization [21][22][23], tenotomy [24,25], spaceflight [26][27][28][29], and/or hindlimb suspension [29][30][31][32]. Most of these models are generally presumed to be models of reduced neuromuscular activity. But the level of use may not be the only factor involved in the atrophic process.There is a close association in the physiological, biochemical, and morphological properties between a motoneuron and the muscle fibers it innervates [1]. Studies to support this view are those in which chronic electrical stimulation at low frequency (1-10 Hz) changes the properties of fast-twitch muscles toward those of slow-twitch muscles [33][34][35][36][37][38].Some of the characteristics of muscle fibers are also altered following cross-innervation [39][40][41][42]. Buller et al. [40] suggested that the neural influence on muscle could be due to neurotrophic effect as well as via nerve impulses. Muscular Responses to Gravitational Unloading Morphological propertiesGravitational unloading by exposure to weightlessness [26][27][28][29][43][44][45][46][47] and/or its simulation model, hindlimb suspension [29][30][31][32][48][49][50][51][52][53][54][55][56], causes rapid atrophy in muscles, such as soleus and adductor longus, that are composed predominantly of slow fibers. The cross-sectional areas (CSAs) of both slow-and fasttwitch fibers of rats were less after 14 d of spaceflight and hindlimb suspension than those in the agematched controls [29]. But the degree of atrophy was greater in slow-than fast-twitch fibers [28,29]. Therefore, muscles composed of predominantly slow fibers are more susceptible to atrophy [26,28, 47]. The weights of fast-twitch ankle dorsi-flexors, such as the tibialis anterior (TA) and extensor digitorum longus (EDL), are also generally less after unloading than those of age-matched controls [31, 57]. But these were Japanese Journal of Physiology, 50, 303-314, 2000 Key words: atrophy, fiber-type transformation, contractile properties, neural responses, microgravity.Abstract: Morphological and/or functional characteristics of skeletal muscles have a greater adaptability in response to changes in environmental stimuli. For example, an atrophy associated with a shift of fiber characteristics toward fast-twitch type is a common adaptation of antigravity muscle to a microgravity environment. Neuromuscular responses and ...

Cell Size and Oxidative Enzyme Activity of Rat Biceps Brachii and Triceps Brachii Muscles

“…However, the mechanisms for metabolic and functional changes of fibers in the muscles are not clear. Exposure to microgravity causes fiber atrophy and type shift of fibers from high-oxidative type I to low-oxidative type II in the muscles, especially in antigravity muscles, e.g., the rat soleus muscle [31]. Decreased oxidative enzyme activity of spinal motoneurons innervating high-oxidative fibers in the muscles was observed following exposure to microgravity [32,33].…”

Gene Expression Levels of Heat Shock Proteins in the Soleus and Plantaris Muscles of Rats after Hindlimb Suspension or Spaceflight

“…Exposure to microgravity induces fiber atrophy and a shift from the slow-to the fast-twitch phenotype in fi bers of the antigravity muscles in rats [23,24]. Furthermore, decreased oxidative enzyme activities have been observed in the spinal motoneurons innervating skeletal muscles, predominantly slow-twitch muscles, following exposure to microgravity [1][2][3].…”

“…The soleus muscle, with higher mRNA expression levels of HSPs, has an oxidative metabolism that is superior to that in the plantaris muscle. Since gravitational unloading also inhibits the mitochondrial enzyme activities in the soleus muscle, but not in the plantaris muscle [23], such effects on the metabolic properties could be one of the factors responsible for the different responses of mRNA expression levels of HSPs in the two muscles. Therefore the EMG activity …”

AICAR-induced activation of AMPK negatively regulates myotube hypertrophy through the HSP72-mediated pathway in C2C12skeletal muscle cells

“…Increased loading, such as resistance training and mechanical stretching, stimulates protein synthesis and reduces protein degradation, thereby inducing muscle hypertrophy (10). On the other hand, decrease of use, such as immobilization, denervation, aging, and/or various pathological conditions, attenuates protein synthesis and increases protein degradation, resulting in atrophy (12,32). Although the process of skeletal muscle adaptation to hypertrophic and atrophic stimuli has been studied, the molecular mechanism involved in this process is not fully understood yet.…”