How does reduced gravity affect astronauts' muscle and nerve responses?  Japanese researchers answer

How does reduced gravity affect astronauts’ muscle and nerve responses? Japanese researchers answer

Newswise – Among the many functions performed by skeletal muscle, an important one is maintaining our posture. If it weren’t for these muscles, the earth’s gravitational pull could make it difficult for us to stand and walk around. The group of muscles found primarily in our limbs, back and neck that are responsible for maintaining our posture and allowing us to move against gravity are rightly called the “anti-gravity” muscles designated.

But what happens to these muscles when there’s no gravity (or a “discharge” of gravity) for them to work against? The question may sound ridiculous to some, but not to an astronaut aboard the International Space Station (ISS)! In space, where gravity is minimal, our muscles (especially antigravity muscles) are not used as much, which can lead to their atrophy and changes in their structure and properties. In fact, it is known that the human calf muscles lose volume during a flight in space.

So how can astronauts avoid these neuromuscular problems?

A research team from Japan led by Dr. Yoshinobu Ohira from Doshisha University, Japan, went in search of the answer. The team also included Dr. Takashi Ohira, who collaborates with Doshisha University and Kindai University, Japan; dr Fuminori Kawano, associated with Doshisha University and Matsumoto University, Japan; dr Katsumasa Goto from Doshisha University and Toyohashi SOZO University, Japan; and dr Hiroshi Kaji from Kindai University. They were recently able to study the responses of neuromuscular properties to gravitational unloading and share research-based insights into how astronauts can avoid neuromuscular problems during extended spaceflight. This review made available online on March 10, 2022 and published in Volume 136 of Neuroscience and biobehaviour reviews in May 2022 – was written in response to an invitation asking the authors to contribute to a special issue. This issue, titled “Space Neurosciences,” was intended to commemorate the first human landing on the moon as part of NASA’s Apollo 11 lunar mission.

The team examined how the morphological, functional and metabolic properties of the neuromuscular system respond to reduced antigravity activities. They first looked at human and rodent simulation models and also saw how afferent and efferent motor neuron activity regulates neuromuscular properties. Their review suggests that afferent neural activity (which includes the signals sent from skeletal muscle to the central nervous system during muscle activity) plays a key role in regulating muscle properties and brain activity.

Inhibition of anti-gravity muscle activities leads to remodeling of the sarcomeres (which are the structural unit of muscles), leading to a decrease in their number, which further leads to a decrease in strength development, eventually leading to muscle atrophy. A reduction in the amplitude of electromyograms in antigravity muscles, namely the soleus and adductor longus, is also seen. This suggests that exposure to low-gravity environments affects not only muscles but also nerves.

Gravitational unloading causes a deterioration in motor control, which is thought to result in impaired coordination of antagonistic muscles and altered mechanics. Difficulty walking has also been observed in crews after space flights, although they exercised regularly on the ISS. Astronauts aboard the ISS must use treadmills, stationary bikes and resistance-training machines to counteract the effects of reduced gravity on the neuromuscular system and protect their physical health. However, these exercise-based countermeasures are not always effective in preventing certain undesirable neuromuscular changes.

Additional challenges can arise when astronauts are exposed to a microgravity environment for six months or longer; for example on the way to or from the planet Mars. This review therefore has major implications for the field of space exploration, with a particular focus on astronaut wellness (recommendations for this are mentioned by the authors).

Changes in muscle properties due to the unloading of gravity may be related to a decrease in neural activity and to contraction- and/or extension-dependent mechanical loading. Adequate stimulation of the soleus muscle appears to reduce the likelihood of atrophy. So during training, astronauts should walk or run slowly and land on their back foot (using a bungee cord would also help). Periodic passive stretching of the soleus also appears to be effective. Therefore, information from a unique perspective as discussed in this review may play an important role in the development of appropriate countermeasures against neuromuscular problems for future long-term space exploration missions. ISS astronauts will thank the research team for sharing these meaningful findings. In the meantime, we wish the researchers good luck on their next mission!

About dr Yoshinobu Ohira by Doshisha University, Japan
dr Yoshinobu Ohira is currently a visiting professor at Doshisha University Research Center for Space and Medical Sciences and Research Initiatives and Development Organization. His research interests include muscle regeneration, neuromuscular adaptation, and space environment. He works primarily in the field of life science and sports science. dr Ohira has conducted original research at several renowned institutes around the world. He has over 400 publications and 3 patents to his credit. dr Ohira has also won several prestigious awards including the NASA Cosmos Biosatellite Group Achievement Award.

Funding information
This study was supported in part by the Space DREAM project of Doshisha University for YO and the Japan Society for the Promotion of Science (JSPS) KAKENHI with grant numbers JP19K07291 for YO and JP21K21000 for TO. The donors play no part in any aspect of this manuscript.

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