E. S. Tomilovskaya

Long-term dry immersion: review and prospects

Dry immersion, which is a ground-based model of prolonged conditions of microgravity, is widely used in Russia but is less well known elsewhere. Dry immersion involves immersing the subject in thermoneutral water covered with an elastic waterproof fabric. As a result, the immersed subject, who is freely suspended in the water mass, remains dry. For a relatively short duration, the model can faithfully reproduce most physiological effects of actual microgravity, including centralization of body fluids, support unloading, and hypokinesia. Unlike bed rest, dry immersion provides a unique opportunity to study the physiological effects of the lack of a supporting structure for the body (a phenomenon we call ‘supportlessness’). In this review, we attempt to provide a detailed description of dry immersion. The main sections of the paper discuss the changes induced by long-term dry immersion in the neuromuscular and sensorimotor systems, fluid–electrolyte regulation, the cardiovascular system, metabolism, blood and immunity, respiration, and thermoregulation. The long-term effects of dry immersion are compared with those of bed rest and actual space flight. The actual and potential uses of dry immersion are discussed in the context of fundamental studies and applications for medical support during space flight and terrestrial health care.

Cortical reorganization in an astronaut's brain after long-duration spaceflight

To date, hampered physiological function after exposure to microgravity has been primarily attributed to deprived peripheral neuro-sensory systems. For the first time, this study elucidates alterations in human brain function after long-duration spaceflight. More specifically, we found significant differences in resting-state functional connectivity between motor cortex and cerebellum, as well as changes within the default mode network. In addition, the cosmonaut showed changes in the supplementary motor areas during a motor imagery task. These results highlight the underlying neural basis for the observed physiological deconditioning due to spaceflight and are relevant for future interplanetary missions and vestibular patients.To date, hampered physiological function after exposure to microgravity has been primarily attributed to deprived peripheral neuro-sensory systems. For the first time, this study elucidates alterations in human brain function after long-duration spaceflight. More specifically, we found significant differences in resting-state functional connectivity between motor cortex and cerebellum, as well as changes within the default mode network. In addition, the cosmonaut showed changes in the supplementary motor areas during a motor imagery task. These results highlight the underlying neural basis for the observed physiological deconditioning due to spaceflight and are relevant for future interplanetary missions and vestibular patients.

The effect of spaceflight and microgravity on the human brain

Microgravity, confinement, isolation, and immobilization are just some of the features astronauts have to cope with during space missions. Consequently, long-duration space travel can have detrimental effects on human physiology. Although research has focused on the cardiovascular and musculoskeletal system in particular, the exact impact of spaceflight on the human central nervous system remains to be determined. Previous studies have reported psychological problems, cephalic fluid shifts, neurovestibular problems, and cognitive alterations, but there is paucity in the knowledge of the underlying neural substrates. Previous space analogue studies and preliminary spaceflight studies have shown an involvement of the cerebellum, cortical sensorimotor, and somatosensory areas and the vestibular pathways. Extending this knowledge is crucial, especially in view of long-duration interplanetary missions (e.g., Mars missions) and space tourism. In addition, the acquired insight could be relevant for vestibular patients, patients with neurodegenerative disorders, as well as the elderly population, coping with multisensory deficit syndromes, immobilization, and inactivity.

Spaceflight-induced neuroplasticity in humans as measured by MRI: what do we know so far?

Space travel poses an enormous challenge on the human body; microgravity, ionizing radiation, absence of circadian rhythm, confinement and isolation are just some of the features associated with it. Obviously, all of the latter can have an impact on human physiology and even induce detrimental changes. Some organ systems have been studied thoroughly under space conditions, however, not much is known on the functional and morphological effects of spaceflight on the human central nervous system. Previous studies have already shown that central nervous system changes occur during and after spaceflight in the form of neurovestibular problems, alterations in cognitive function and sensory perception, cephalic fluid shifts and psychological disturbances. However, little is known about the underlying neural substrates. In this review, we discuss the current limited knowledge on neuroplastic changes in the human central nervous system associated with spaceflight (actual or simulated) as measured by magnetic resonance imaging-based techniques. Furthermore, we discuss these findings as well as their future perspectives, since this can encourage future research into this delicate and intriguing aspect of spaceflight. Currently, the literature suffers from heterogeneous experimental set-ups and therefore, the lack of comparability of findings among studies. However, the cerebellum, cortical sensorimotor and somatosensory areas and vestibular-related pathways seem to be involved across different studies, suggesting that these brain regions are most affected by (simulated) spaceflight. Extending this knowledge is crucial, especially with the eye on long-duration interplanetary missions (e.g. Mars) and space tourism.

Integration of sensory, spinal, and volitional descending inputs in regulation of human locomotion

We reported previously that both transcutaneous electrical spinal cord stimulation and direct pressure stimulation of the plantar surfaces of the feet can elicit rhythmic involuntary step-like movements in noninjured subjects with their legs in a gravity-neutral apparatus. The present experiments investigated the convergence of spinal and plantar pressure stimulation and voluntary effort in the activation of locomotor movements in uninjured subjects under full body weight support in a vertical position. For all conditions, leg movements were analyzed using electromyographic (EMG) recordings and optical motion capture of joint kinematics. Spinal cord stimulation elicited rhythmic hip and knee flexion movements accompanied by EMG bursting activity in the hamstrings of 6/6 subjects. Similarly, plantar stimulation induced bursting EMG activity in the ankle flexor and extensor muscles in 5/6 subjects. Moreover, the combination of spinal and plantar stimulation exhibited a synergistic effect in all six subjects, eliciting greater motor responses than either modality alone. While the motor responses to spinal vs. plantar stimulation seems to activate distinct but overlapping spinal neural networks, when engaged simultaneously, the stepping responses were functionally complementary. As observed during induced (involuntary) stepping, the most significant modulation of voluntary stepping occurred in response to the combination of spinal and plantar stimulation. In light of the known automaticity and plasticity of spinal networks in absence of supraspinal input, these findings support the hypothesis that spinal and plantar stimulation may be effective tools for enhancing the recovery of motor control in individuals with neurological injuries and disorders.

Effects of gravitational unloading on back muscles tone

The study involved 12 healthy volunteers who were exposed to 6-h or 3-day dry immersion (DI). The back muscle tone was recorded by resonance vibrography using parameters of transverse stiffness of the muscles under study. The measurements in 3-day DI were performed twice before DI, daily in the course of DI, and twice after its completion; in the short-term (6-h) DI, the testing was carried out twice before DI, 1 and 4 h after the beginning of DI, and during the first hour after DI completion. It has been shown that the absence of support is followed by a sharp decrease in the back extensor muscles tone, which has the maximal values during the first hours and days of DI. The possible role of back muscle atony in the development of well-known phenomena of the spine length increase and back pain appearance observed at the beginning of space flights and in the first days after landing, as well as under the conditions of simulated microgravity (DI and head-down bed rest), is discussed.

Effects of mechanical stimulation of sole support zones on the H-reflex characteristics under conditions of support unloading

The effects of mechanical stimulation of the sole support zones on the state of the m. soleus moto-neuron pool in humans have been studied under 7-day support unloading conditions, which were simulated using a dry immersion (DI) model. The excitability of the m. soleus motoneuron pool was estimated by the H-reflex amplitude normalized by the maximal M-wave amplitude before, during, and after immersion exposure. The results were compared for two groups of volunteers, i.e. a control group, which was subjected only to immersion exposure, and a test group, in which mechanical stimulation in locomotion regimens was applied daily during DI. The relative H-reflex amplitude increased during immersion in the control group, whereas such alterations were not detected in the test group.

Effect of support deprivation on the order of motor unit recruitment

Motor unit (MU) activity in the leg extensors has been tested by maintenance of a small isometric effort under the conditions of support deprivation enabled by dry immersion with simultaneous mechanic stimulation of the foot support zones. The analysis of MU interspike interval (ISI) histograms in the heads of two leg extensors (m. soleus and m. gastrocnemius lat.) revealed that the order of MU recruitment is extremely dependent on the support input activity. In immersion, the order of MU recruitment was rear-ranged during the isometric effort maintenance test, which revealed weaker involvement of small tonic MUs. Large MUs with longer ISIs (up to 260 ms) and high variability replaced small tonic MUs with relatively short ISIs (100 ms) and low variability. Daily support stimulation under the conditions of immersion was favorable for maintaining the normal pattern of MU recruitment.

Brain Tissue–Volume Changes in Cosmonauts

Changes in Brain Volume in Cosmonauts Ten cosmonauts, who spent an average of 189 days in space, had changes in brain volumes — mainly decreased cortical volume and increased CSF subarachnoid and v...

Functional activity of the liver under the conditions of immersion and effects of countermeasures

Ultrasound investigations (USI) of the liver, organs and vessels of the gastroduodenal area, as well as blood biochemistry, were performed in two groups of male volunteers on the 4th day of their stay in the conditions of “dry” immersion with and without the application of countermeasures, including the support load imitator (SLI) or high-frequency electromyostimulation. Using 13С-methacetin breath test (13C-MBT), two other groups were investigated for the effect of immersion on the detoxification activity and metabolic capacity of the liver and the efficacy of SLI. The performed USIs have identified deceleration in the hepatic venous flow and the signs of plethora in the abdominal venous system. Elevated blood levels were detected in pepsinogen, pancreatic amylase, bilirubin total, due to its unconjugated fraction, insulin, and C-peptide. The 13C-MBT has shown a slowdown in the rate of 13C-methacetin inactivation and a reduction in the hepatic metabolic capacity. The application of countermeasures during the immersion has not affected the ultrasound patterns of the hemodynamic rearrangement in both the liver and the abdomen. High frequency electromyostimulation during the immersion has neutralized the changes in all biochemical indicators except C-peptide, while the application of SLI has led to the restoration of only pepsinogen and amylase to the initial values. In addition, the use of SLI during the immersion counteracted the reduction in the 13C-methacetin inactivation rate and did not substantially affect the reduction in the metabolic capacity of the liver.