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How does reduced gravity affect astronauts’ muscles and nerve responses? – News about neuroscience

Summary: Scientists reveal how astronauts can avoid neuromuscular problems that arise as a result of a longer spaceflight.

Source: Doshisha University

Among the many functions performed by the skeletal muscles, an important function is to maintain our posture. If it were not for these muscles, the gravitational force of the earth could make it difficult for us to stand and walk around. The group of muscles – most commonly found in our limbs, back and neck – that are responsible for maintaining our posture and allowing us to move against gravity are rightly called “anti-gravity muscles”.

But what happens to these muscles when there is no gravity (or a “relief” of gravitational force) for them to work against? The question may sound ridiculous to some, but not to an astronaut aboard the International Space Station (ISS)!

In outer space, where gravity is minimal, our muscles (especially the antigravity ones) are not used as much, which can result in their atrophy and changes in their structure and properties. In fact, human calf muscles are known to decrease in volume during a flight in space.

So how can astronauts avoid these neuromuscular problems?

A team of researchers from Japan led by Dr. Yoshinobu Ohira from Doshisha University, Japan set out to find the answer.

The team also included Dr. Takashi Ohira, who works with Doshisha University and Kindai University, Japan; Dr. Fuminori Kawano, affiliated with Doshisha University and Matsumoto University, Japan; Dr. Katsumasa Goto, who is with 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 gravity relief and share research-based insights on how astronauts can avoid neuromuscular problems during a longer spaceflight.

This review – which was made available online on March 10, 2022 and published in volume 136 by Neuroscience and biobehavior reviews in May 2022 – was written in response to an invitation in which the authors asked the authors to contribute to a special issue.

This issue, entitled “Space Neurosciences”, was intended to celebrate 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 simulation models for humans and rodents and also saw how afferent and efferent motoneuron activity regulated neuromuscular properties.

Their review suggests that afferent neural activity (involving 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 antigravity muscle activities results in the rebuilding of the sarcomeres (which are the structural unit of the muscles), which results in a decrease in their number, which further causes a decrease in the force development which eventually leads to muscle atrophy.

A decrease in the amplitude of the electromyograms in the antigravity muscles, namely the soleus and adductor longus, is also seen. This indicates that exposure to low gravity environments affects not only the muscles but also the nerves.

Gravity relief causes impairment of motor control, is seen as impaired coordination of antagonist muscles and altered mechanics. Walking difficulties were also observed in crews after space travel, although they regularly trained on the ISS.

Astronauts aboard the ISS must use treadmills, bicycle ergometers and strength training equipment 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 unwanted neuromuscular changes.

Additional challenges may come into play when astronauts are exposed to a microgravity environment for six months or more; for example on its way to or from the planet Mars. This review therefore has major implications in the field of space research, with particular emphasis on the well-being of astronauts (recommendations mentioned by the authors).

This shows an astronaut
Inhibition of antigravity muscle activities results in the rebuilding of the sarcomeres (which are the structural unit of the muscles), which results in a decrease in their number, which further causes a decrease in the force development which eventually leads to muscle atrophy. The image is public property

Changes in muscular properties due to gravity relief may be related to a decrease in neural activity, as well as contraction- and / or stretch-dependent mechanical stress.

Stimulating the soleus muscle adequately seems to reduce the risk of its atrophy. So, while exercising, astronauts should walk or run slowly with a rear foot-strike landing (using a bungee line would also help). Periodic passive stretching of the soleus also seems to be effective.

Thus, information from a unique perspective, discussed in this review, may play an important role in the development of appropriate countermeasures against neuromuscular problems for future long-term human space exploration missions.

See also

This shows an older man's face

ISS astronauts will thank the research team for sharing these meaningful insights. At the same time, let us wish the researchers good luck with the next assignment!

Financing: This study was supported in part by the Doshisha University Space-DREAM Project to YO and the Japan Society for the Promotion of Science (JSPS) KAKENHI, grant number JP19K07291 to YO and JP21K21000 to TO. The financiers have no role in any aspect of this manuscript.

About this neuroscientific research novelty

Author: Jun Kita
Source: Doshisha University
Contact: Jun Kita – Doshisha University
Picture: The image is public property

Original research: Closed access,
Response of neuromuscular properties to relief and potential countermeasures during space missions exploration”By Yoshinobu Ohira et al. Neuroscience and biobehavior reviews


Abstract

Response of neuromuscular properties to relief and potential countermeasures during space missions exploration

We examined the answers for the neuromuscular properties of mainly soleus and possible mechanisms. Sensory nervous activity in response to passive shortening and / or active contraction, associated with plantar flexion or dorsiflexion in the ankle, may play a significant role in the regulation of muscle properties.

Passive shortening of the muscle fibers and sarcomeres inhibits the development of tension, electromyogram (EMG) and afferent neurogram. Reconstruction of the sarcomeres, which reduces the total number of sarcomeres in a single muscle fiber, causing recovery in the length of each sarcomere, is induced in the soleus after chronic relief.

Although EMG activity and tension development in each sarcoma increase, the total tension produced by the whole muscle is still less due to the lower sarcomere number. Therefore, muscle atrophy continues to develop.

In addition, walking or slow running by landing with the rear foot with the application of greater ground reaction force, which stimulates soleus mobilization, can be an effective countermeasure. Periodic, but not chronic, passive stretching of the soleus can also be effective.

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