How Does Reducing Gravity Affect Astronaut Muscles and Nerve Response? - Neuroscience News

Summary: Researchers reveal how astronauts can avoid the neuromuscular problems that occur as a result of extended space travel.

Source: Doshisha University

Among the many functions performed by skeletal muscles, one of them is maintaining our posture. If it weren’t for these muscles, the gravitational pull of the earth might make it difficult for us to get up and walk. The group of muscles—mostly in our legs, back and neck—that are responsible for maintaining posture and allowing us to move against the force of gravity are called ‘anti-gravity’ muscles.

But what happens to these muscles when there is no gravity (or “unloading” force of gravity) to counteract them? That question may sound silly to some, but not to an astronaut on the International Space Station (ISS)!

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

So how can astronauts avoid these neuromuscular problems?

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

The team also includes Dr. Takashi Ohira, who works 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 response of neuromuscular properties to gravitational unloading, and share research-based insights into how astronauts can avoid neuromuscular problems during extended spaceflight.

This review—available online on March 10, 2022 and published in Volume 136 of Neuroscience & Biobehavioral Reviews in May 2022—written in response to an invitation asking authors to contribute to the special issue.

This issue, entitled ‘Space Neurosciences’, is intended to commemorate the first human landing on the Moon, as part of NASA’s Apollo 11 lunar mission.

The team reviewed how the morphological, functional, and metabolic properties of the neuromuscular system respond to lower anti-gravity activity. They looked at simulated models of humans and rodents first and also looked at how the activity of afferent and efferent motoneurons regulate neuromuscular properties.

Their review showed that afferent neural activity (which involves signals sent from skeletal muscles to the central nervous system during muscle activity) plays a key role in regulating muscle properties and brain activity.

Inhibiting anti-gravity muscle activity results in remodeling of the sarcomeres (which are the structural units of muscle), resulting in a decrease in their number, further leading to decreased strength development which eventually leads to muscle atrophy.

A decrease in the amplitude of the electromyogram in the anti-gravity muscles, namely the soleus and adductor longus, is also seen. This suggests that exposure to a low-gravity environment affects not only muscles, but nerves as well.

Gravity unloading causes decreased motor control, manifested by impaired antagonistic muscle coordination and altered mechanics. Difficulty walking was also observed in the crew after the space flight, even though they exercised regularly on the ISS.

Astronauts on the ISS are required to use treadmills, bicycle ergometers, and resistance training equipment to counter the reducing effects of gravity on the neuromuscular system and maintain their physical health.

However, these exercise-based precautions are not always effective in preventing certain unwanted neuromuscular changes.

Additional challenges may arise when astronauts are exposed to a microgravity environment for six months or more; for example, en route to or from the planet Mars. This review, therefore, has major implications in the field of space research, with particular emphasis on astronaut health (author’s recommendation).

Inhibiting anti-gravity muscle activity results in remodeling of the sarcomeres (which are the structural units of muscle), resulting in a decrease in their number, further leading to decreased strength development which eventually leads to muscle atrophy. Image is in public domain

Changes in muscle properties due to gravitational unloading may be related to decreased neural activity, as well as mechanical stress dependent on contraction and/or stretch.

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

Thus, information from unique perspectives, as 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

ISS astronauts will be grateful to the research team for sharing this meaningful insight. In the meantime, let’s wish the researchers good luck on their next mission!

Funding: 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) KAKEENHI, Grant number JP19K07291 to YO and JP21K21000 to TO. Funders have no role in any aspect of this manuscript.

About this neuroscience research news

Author: Jun Kita
Source: Doshisha University
Contact: Jun Kita – Doshisha University
Picture: Image is in public domain

Original Research: closed access,
“Responses of neuromuscular properties to disassembly and potential precautions during space exploration missions” by Yoshinobu Ohira et al. Neuroscience & Biobehavioral Reviews

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Abstract

Response of neuromuscular properties to disassembly and potential precautions during space exploration missions

We reviewed the neuromuscular trait responses especially the soleus and possible mechanisms. Sensory nerve activity in response to passive shortening and/or active contraction, associated with plantar-flexion or dorsi-flexion of the ankle joint, may play an important role in the regulation of muscle properties.

Passive shortening of muscle fibers and sarcomere inhibits the development of tension, electromyogram (EMG), and afferent neurogram. Sarcomere remodeling, which decreases the total number of sarcomeres in one muscle fiber causing length recovery in each sarcomere, is induced in the soleus after chronic disassembly.

Although the EMG activity and tension development in each sarcomere is increased, the total tension generated by the whole muscle is still lacking due to the lower number of sarcomeres. Because of this, muscle atrophy continues.

In addition, a slow walk or sprint with the hindfoot landing with the application of a greater ground reaction force, which stimulates the mobilization of the soleus, can be an effective preventive measure. Passive soleus stretching periodically, but not chronically, may also be effective.

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