The objective of the study was to determine the influence of simulated microgravity by exposure to dry immersion on the craniomandibular system. Twelve healthy male volunteers participated in a 3-day dry immersion study. Before and immediately after exposure we measured maximal bite force using piezoresistive sensors. The mechanical properties of the jaw and cervical muscles were evaluated before, during, and after dry immersion using MyotonPRO. Because recent studies reported the effects of jaw motor activity on the postural stability of humans, stabilometric measurements of center of pressure were performed before and after dry immersion in two mandibular positions: rest position without jaw clenching, and intercuspidal position during voluntary teeth clenching. Results revealed no significant changes of maximal bite force after dry immersion. All postural parameters were significantly altered by dry immersion. There were however no significant differences in stabilometric data according to mandibular position. Moreover the masseter tonicity increased immediately after the end of dry immersion period. Dry immersion could be used as a valid model for studying the effects of microgravity on human subjects. However, 3 days appear insufficient in duration to evaluate the effects of weightlessness on maximal bite force. Our research suggests a link between postural disturbance after dry immersion and masseter tonicity.

Parameters of cardiac activity after administration of the cholinesterase inhibitor physostigmine were analyzed in newborn rats and on day 16 of postnatal development. The type of cardiovascular response to acetylcholine excess in newborns and 16-day-old rats were similar, but they significantly differed by the magnitude, which suggests that that maturation of cholinergic structures involved in the regulation of cardiac activity is completed during the early postnatal ontogeny.

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.