How do astronauts' muscles and nerves respond to reduced gravity?

Skeletal muscles are an important part of your musculoskeletal system. They serve a variety of functions. Among the many functions performed by skeletal muscles, one of them is maintaining our posture.

On Earth, the musculoskeletal system must support the body’s weight, and the bones and postural muscles are permanently burdened by gravity. But what happens to these muscles when they have no gravity to resist?

This question is a topic of interest to many scientists. Recently, a team of scientists from Japan set out to find the answer. They study the response of neuromuscular properties to gravitational unloading and share research-based insights into how astronauts can avoid neuromuscular problems during extended spaceflight.

The group explored how the morphological, functional and metabolic properties of the neuromuscular system adapt to reduced anti-gravity activity. Using human and rodent simulation models, they first investigated how afferent and efferent motoneuron activity controls neuromuscular properties. According to their review, afferent neural activity (which includes signals sent from skeletal muscles to the central nervous system during muscle activity) is critical for controlling brain activity and muscle properties.

Muscle atrophy eventually arises from inhibition of anti-gravity muscle activity as it causes remodeling of the sarcomere, the structural unit of muscle, reducing their number. In addition, the amplitude of the electromyogram soleus and adductor longus is reduced, which is a sign of anti-gravity muscle activity. This suggests that nerves, as well as muscles, are affected by a low-gravity environment.

Gravity unloading causes decreased motor control, mechanical changes, and impaired coordination of antagonistic muscles. Despite their constant exercise aboard the ISS, crew members who were in space reportedly had difficulty walking.

To combat the effects of lesser gravity on the neuromuscular system and protect their physical health, astronauts on the International Space Station must use treadmills, bicycle ergometers, and resistance training equipment. This exercise-based prevention approach does not always work to stop certain unwanted neuromuscular changes.

Additional challenges may arise when astronauts are exposed to a microgravity environment for six months or more. This review, therefore, could have significant implications in space research, with particular emphasis on astronaut health.

The scientists noted, “Changes in muscle properties due to gravitational unloading may be related to decreased neural activity and mechanical stress dependent on contraction and/or stretching. Stimulating the soleus muscle adequately seems to reduce the likelihood of its atrophy. So astronauts must walk or run slowly while exercising with a rear foot-strike landing.”

“Regular passive stretching of the soleus also appears to be effective. Thus, information from unique perspectives, such as those discussed in this review, may play an important role in developing appropriate countermeasures against neuromuscular problems for future long-term human space exploration missions. ISS astronauts will be grateful to the research team for sharing this meaningful insight. In the meantime, let’s wish the researchers success for their next mission!”

Journal Reference:

1. Takashi Ohira, Fuminori Kawano, et al. Neuromuscular properties responses to disassembly and potential precautions during space exploration missions. Neuroscience & Biobehavioral Reviews. DOI: 10.1016/j.neubiorev.2022.104617

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