Abstract
The aim of this study was to analyze the mechanisms of the development of hypogravitational hyperreflexia in the motoneuron pool of gravity-dependent muscles such as the gastrocnemius and soleus muscles of the leg under the conditions of five-day “dry” immersion in healthy volunteers using the method of transcranial and trans-spinal magnetic stimulation. The essence of the method lies in the stimulation of the areas of interest (motor areas of the cerebral cortex and lumbosacral thickening) with an electromagnetic stimulus. The study involved 10 subjects at the age of 29.9 ± 6.9 years, with no history of movement disorders or neurological diseases. The excitability of the motor neuron pool in both muscles was judged by the values of the thresholds and amplitudes of the motor response caused by transcranial and trans-spinal magnetic stimulations. A general pattern manifested in a significant decrease in thresholds and an increase in the amplitudes of motor responses caused by trans-spinal magnetic stimulation in both muscles gas been discovered. Specifically, the threshold of spinal evoked motor responses in both muscles decreased by 20%, and the amplitude increased by 150% after the end of immersion. The data obtained during the experiment confirm the spinal nature of the origin of hypogravitational hyperreflexia.
Highlights
This fact was confirmed by the results of post-flight examinations of the members of the Salyut-6 mission and model experiments, such as dry immersion (DI) and head-down tilt bed rest (HDBR) [18]
After determining the threshold of the EMR, the stimulation strength was gradually increased in steps of 5– 10% until the evoked motor responses of the maximum amplitude or up to 100% of the maximum output of the magnetic field were obtained
The thresholds of the EMRs for trans-spinal stimulation of m. soleus on the day of the end of the DI were significantly lower than the background values, remaining reduced on the third day of the recovery period (Fig. 3c)
Summary
We can conclude that a decrease in amplitude is possible but not necessary for a state of hypogravitational hyperreflexia, while a decrease in thresholds can be considered a characteristic feature of this phenomenon. This fact was confirmed by the results of post-flight examinations of the members of the Salyut-6 mission and model experiments, such as dry immersion (DI) and head-down tilt bed rest (HDBR) [18]. In “acute” experiments on cats, it was found that with the help of epidural stimulation of the segments of the lumbar thickening it is possible to induce locomotor activity in intact and to varying degrees devoid of supraspinal influences (decerebrated and spinal) cats. The purpose of the present study was to estimate the contribution of the spinal and supraspinal structures to the development of hyperreflexia caused by support unloading under DI conditions for five days
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