A review of the effect of exercise training on neuromuscular junction in throughout life: A logical analysis of animal experimental studies

Neuromuscular junction is a synaptic junction in which motor neurons are connected to muscle fibers. Neuromuscular junction is a complex structure that changes under the influence of various stages of development as well as environmental changes such as pathological conditions and genetic and non-genetic diseases and physiological conditions such as physical activity and exercise training. During about a first third of life, the growth of the nervous system naturally promotes an increase in strength and muscle mass. However, with age, the function of the nervous system also slowly decreases. Neuromuscular junction is one of the structures that are damaged due to aging processes. Neuromuscular junction are important for understanding the basic concepts, diagnosis and treatment of atrophy and reduction of muscle mass. An active lifestyle is particularly important for understanding the function of neuromuscular junctions in changes in muscle mass and their application in clinical settings. Exercise training and physical activity as a metabolic and neuromotor stress can improve the overall function of the neuromuscular junction by improving neuromuscular signaling, and improving muscle function, a factor known to aid growth. Exercise training leads to impressive changes in the neuromuscular junction of young and old animals. However, large studies aimed at investigating these effects have introduced methodological variables that show effects on the results under study. Therefore, the aim of this study was to systematically investigate the effects of exercise training on neuromuscular junction and related diseases in the lives of young, adult and old animals.

In this systematic review article, the results of articles presented on the effect of exercise on neuromuscular junction in Google Scholar, Web of science, PubMed databases, which included five thousand article titles, were reviewed and analyzed. In this study, two independent evaluators, WKN and EFG, reviewed all titles during the analysis. Then, the abstracts of the articles were evaluated using PRISMA. Of the articles reviewed, 109 articles were used for the initial analysis of chicks. The selection criteria were such that the abstract of the article should have sufficient data on the parameters of the neuromuscular junction components, the animals studied as well as the therapies, and the measures used in these studies. Among the articles, 26 full-text articles were analyzed and among these 26 articles, 11 articles were used for systematic review. The results of the study showed that the breeds of animals used in the selected articles were rodents including SD (Sprague Dawley) rats, Fisher rats, Vistar rats and C57BL / 6NNia rats. In terms of age and gender, all articles provided information on the effects of exercise on the neuromuscular components of both young and adult animals. Among these articles, four articles examined the effects of exercise on the parameters of neuromuscular connection and in all studies, male animals were used. Most of the exercises used in the research were endurance exercises. Only two articles were done on strength training. The duration of the training intervention in the research varied from 6 weeks to 15 weeks. The number of training sessions was 5 times a week. None of the articles examined seasonal exercise to prescribe exercise intensity.

Regarding the effect of training on the presynaptic part, the results showed that endurance training caused hypertrophy of the presynaptic part, including area, length, total area and complexity of neuromuscular connection. Endurance training had an effect on the morphological adaptation of the neuromuscular junction in muscles such as the soleus muscle and to a lesser extent in the long toe extensor muscles of the gluteus maximus as well as the soleus muscle. Most of the results obtained from the systematic study showed that in some of the analyzed parameters such as mean and total branch length as well as branch complexity, there are high levels of heterogeneity. The results also showed that endurance training did not have a significant effect on the postsynaptic sections of slow-acting neuromuscular fibers. However, many changes were observed in fast-twitch fibers. In addition, the analysis shows the general effects of an increase in the overall environment and end-plate dispersion as well as a decrease in the drug spot environment before and after synaptic mating of fast-twitch myofibers. The whole postsynaptic section showed similar changes to the process of presynaptic changes. Although thethey were more dependent on the type of exercise provided. In endurance training, for example, it reduced the spotty environment of slow-fiber neuromuscular junction in the soleus muscles, while not showing morphological compatibility with strength training. Regarding the effect of aging on neuromuscular connection, the results showed that the aging process is naturally associated with various adaptations in neuromuscular connection. Exercise also increases longevity and also strengthens the structure of the neuromuscular junction in old age. Studies in young and old animals during exercise have shown that adaptations that occur in the structure of the neuromuscular junction may vary with age. Exercise can be positively involved in the up regulation of genes and the protein expression of several molecules and growth factors. Physical activity and exercise naturally increase the expression of growth factor such as neurotrophic factors which can effecting on quality of function an Structure of neuromuscular junction.

In general, the research findings showed that the neuromuscular junctions, including presynaptic and postsynaptic, as well as its morphological structure are affected by different developmental stages and undergo changes during development from embryonic to old age. Also, pathological conditions such as the type of disease or injury can cause changes in the structural components of the neuromuscular junction, which ultimately lead to changes in the quality of motor function. Also, exercise can play an effective role in improving the structural and functional quality of the neuromuscular junction. Therefore, regular exercise can be a preventive method in old age and its negative effects on muscle mass and motor function, which is created by improving neuromuscular connections. Due to the difference in results between aerobic and resistance training methods, more research is needed by applying the main variables of training, including training intensity, training volume, etc. for better conclusions in this regard.