Abstract
As female astronauts participate in space flight more and more frequently, there is a demand for research on how the female body adapts to the microgravity environment. In particular, there is very little research on how the neuromuscular system reacts to gravitational unloading in women. We aimed to estimate changes in motor evoked potentials (MEPs) in the lower leg muscles in women after 3-day exposure to Dry Immersion (DI), which is one of the most widely used ground models of microgravity. Six healthy female volunteers (mean age 30.17 ± 5.5 years) with a natural menstrual cycle participated in this experiment. MEPs were recorded from the gastrocnemius and soleus muscles twice before DI, on the day of DI completion, and 3 days after DI, during the recovery period. To evoke motor responses, transcranial and trans-spinal magnetic stimulation was applied. We showed that changes in MEP characteristics after DI exposure were different depending on the stimulation site, but were similar for both muscles. For trans-spinal stimulation, MEP thresholds decreased compared to baseline values, and amplitudes, on the contrary, increased, resembling the phenomenon of hypogravitational hyperreflexia. This finding is in line with data observed in other experiments on both male and female participants. MEPs to transcranial stimulation had an opposing dynamic, which may have resulted from the small group size and large inter-subject variability, or from hormonal fluctuations during the menstrual cycle. Central motor conduction time remained unchanged, suggesting that pyramidal tract conductibility was not affected by DI exposure. More research is needed to explore the underlying mechanisms.
Highlights
Nowadays women participate in space flights (SF) alongside men, and the differences between the sexes should be considered when training crewmembers for space missions
We evaluated central motor conduction time (CMCT), which is calculated with the following formula: CMCT = cortical motor evoked potentials (MEPs) latency − spinal MEP latency
Cortical MEP thresholds were higher right after Dry Immersion (DI) and in the soleus muscle they even showed a tendency to increase during the recovery period
Summary
Nowadays women participate in space flights (SF) alongside men, and the differences between the sexes should be considered when training crewmembers for space missions. One of the important lines of research in space physiology is studying how weightlessness affects motor system function. The complex of changes occurring in human motor function under the conditions of real or simulated microgravity is called hypogravitational motor syndrome (Kozlovskaya et al, 1988), and it is defined by a deficit in vestibular, proprioception, and support afferent activity (Pechenkova et al, 2019), and substantial alterations in the functional (e.g., atony, a decline in speed-force qualities) and structural (e.g., atrophy and a phenotype deterioration) characteristics of skeletal muscles (Kozlovskaya et al, 1988; Kozlovskaya and Kirenskaya, 2004; Koppelmans et al, 2017; Amirova et al, 2020). In experiments with animal models it was shown that a hindlimb suspension in rats results in nerve fiber demyelinization, which in turn may play a role in the development of hypogravitational motor syndrome (Islamov et al, 2013)
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