Sport has many beneficial effects: activation of blood circulation, strengthening of muscles and bones, consumption of lipids and carbohydrates… But there is one effect that scientists had not, until now, clearly documented: when muscles are subjected to training, they themselves stimulate the growth of motor neurons, cells that control muscles to allow voluntary movements. Previous studies had indeed highlighted a potential link of this nature between muscle activity and nerve growth, but not of this importance.
In 2023, researchers at the prestigious Massachusetts Institute of Technology (MIT) were able to restore mobility to mice that had suffered a major muscle injury. To do this, they implanted muscle tissue on the injury before stimulating it. Ultimately, the graft allowed the mice to regain their motor functions. How?
When the researchers analyzed the graft, they found that exercise had caused it to produce biochemical signals known to promote the growth of nerves and blood vessels. It was interesting because we always think that nerves control muscles, but we don't think that muscles communicate with nerves, note in a communicates researcher Ritu Raman. So we started thinking that muscle stimulation was promoting nerve growth. People said that maybe it was, but there are hundreds of other types of cells in an animal, and it's very difficult to prove that the nerve is growing more because of the muscle, rather than the immune system or some other factor playing a role.“.
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Four times faster growth
A new study, published on November 10, 2024 in the journal Advanced Healthcare Materials, allowed to verify In vitro The role of muscle in nerve growth. In this study, the MIT researchers meticulously isolated muscle cells from all influences to avoid bias. They created, in the laboratory, a tiny muscle tissue with mouse cells, genetically modified to contract in response to light. They could thus allow it to exercise as if it were in an animal's body.
They then collected the solution in which the tissue was bathed in order to use the substances it had secreted. And they placed this solution in a container containing mouse motor neurons. The effect was rapid: the nerve cells quickly began to grow, four times faster than those that had not received the solution. These neurons " grow much further and faster, and the effect is quite immediate" comments Ritu Raman.
They are, as the researchers suspected, exposed to myokines. These proteins, biochemical factors, are secreted by the repeated contraction of a muscle. Produced by muscle cells, they are released into the circulatory system and can modulate cellular activity in the body.
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Mechanical and biochemical growth
But how exactly do neurons respond to these cellular signals? To understand this, scientists studied changes in gene expression in these nerve cells. We found that many of the genes upregulated in exercise-stimulated neurons were not only related to neuron growth, but also to neuron maturation, their ability to communicate with muscles and other nerves, and axon maturity, explains the researcher. Exercise appears to impact not only the growth of neurons, but also their maturity and proper functioning.“.
If muscles have a biochemical effect on neurons, via myokines, they also have a mechanical impact. Indeed, motor neurons move at the same time as the muscles to which they are linked when they contract and relax. Can these movements also promote the growth of nerve cells? By exercising only a set of neurons, without muscle and for 30 minutes a day, the researchers noted growth as high as during the "bath" with myokines.
MIT scientists found that motor neuron growth increased significantly over 5 days in response to biochemical (left) and mechanical (right) cues related to exercise. The green ball represents a group of neurons growing outward into long tails, or axons. Credit: Angel Bu
“ This is a good sign because it tells us that the biochemical and physical effects of exercise are equally important.", notes Ritu Raman. However, despite a similar morphology between mechanically and biochemically stimulated neurons, the mechanical action seemed to have a weaker effect than the other on gene expression. A difference, perhaps due to the protocol used, which should be better understood in future studies.
Although still preliminary, these results offer hope for better care for people with neurodegenerative diseases. By targeting their muscles, researchers hope to one day partially restore their mobility.
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