Molecular Mechanisms of Muscle Tone Impairment under Conditions of Real and Simulated Space Flight.

Kozlovskaya et al. [1] and Grigoriev et al. [2] showed that enormous loss of muscle stiffness (atonia) develops in humans under true (space flight) and simulated microgravity conditions as early as after the first days of exposure. This phenomenon is attributed to the inactivation of slow motor units and called reflectory atonia. However, a lot of evidence indicating that even isolated muscle or a single fiber possesses substantial stiffness was published at the end of the 20th century. This intrinsic stiffness is determined by the active component, i.e. the ability to form actin-myosin cross-bridges during muscle stretch and contraction, as well as by cytoskeletal and extracellular matrix proteins, capable of resisting muscle stretch. The main facts on intrinsic muscle stiffness under conditions of gravitational unloading are considered in this review. The data obtained in studies of humans under dry immersion and rodent hindlimb suspension is analyzed. The results and hypotheses regarding reduced probability of cross-bridge formation in an atrophying muscle due to increased interfilament spacing are described. The evidence of cytoskeletal protein (titin, nebulin, etc.) degradation during gravitational unloading is also discussed. The possible mechanisms underlying structural changes in skeletal muscle collagen and its role in reducing intrinsic muscle stiffness are presented. The molecular mechanisms of changes in intrinsic stiffness during space flight and simulated microgravity are reviewed.

Early Deсline in Rat Soleus Passive Tension with Hindlimb Unloading: Inactivation of Cross-bridges or Activation of Calpains?

The study was aimed at testing the hypotheses about the role of cross-bridges and calpains in reduction of rat soleus passive tension under conditions of hindlimb unloading. For this purpose, we used an inhibitor of µ-calpain PD 150606 as well as a blocker of actomyosin interaction (blebbistatin). It was found for the first time that a decrease in passive tension of rat soleus after 3-day hindlimb unloading is associated with the activity of µ-calpain and does not depend on the processes of cross-bridges formation.

Eukaryotic elongation factor 2 kinase activation in M. soleus under 14-day hindlimb unloading of rats.

Functional unloading of m. soleus of male Wistar rats was found to cause a reduction in protein synthesis. The level of phosphorylation of the translation elongation factor 2 (eEF2) and the eEF2 kinase (eEF2k) activity in m. soleus after 14 days of unloading were assessed. Rats were divided into the control group (C) and the group with hindlimb unloading for 14 days (HU14). The level of eEF2 phosphorylation in group HU14 was 80%, whereas in the control is was 40%. The indices of eEF2k expression and protein content in group HU14 increased compared to group C.

[EFFECT OF SHORT-TERM DRY IMMERSION ON PROTEOLYTIC SIGNALING IN HUMAN SOLEUS MUSCLE].

The signaling processes initiating proteolytic events in m. soleus of humans during short-term exposure in the non-weight bearing conditions were analyzed. Dry immersion (DI) was used to induce weight deprivation over 3 days. Western blotting was used to define the IRS-1 content, total and phosphorylated neuronal NO-synthase (nNOS), AMP-activated protein kinase (AMPK) that control the anabolic and catabolic pathways, and concentrations of cytoskeletal protein desmin and Ca²âº-activated protease calpin. Already on day-3 of DI calpain-dependent proteolysis manifests itself by reductions in both the total content and level of nNOS phosphorilation. Moreover, AMPK phosphorilation was decreased drastically.

[Changes in the Functional Connectivity of Motor Zones in the Use of Multimodal Exoskeleton Complex "Regent" in the Neurorehabilitation of Post-Stroke Patients].

The article discusses the effect of a course of treatment with the use of multimodal complex exoskeleton (MCE) "Regent" on the reorganization of cortical locomotor zones in 14 patients with post-stroke hemiparesis, mainly atthe chronic stage of the disease. Before the course of treatment, we identified specific areas of activation in the primary sensorimotor and supplementary motor areas and the inferior parietal lobules in both affected and healthy hemispheres by means of functional MRI (fMRI) with the use of special passive sensorimotor paradigms. After the course of treatment with MCE, we observed an improvement of temporal characteristics of walking; it was accompanied by a decrease in the activation zones of inferior parietal lobules, especially in the healthy hemisphere, and by a significant increase in the activation zone of primary sensorimotor and supplementary motor areas. The analysis of the functional connectivity of studied zones before and after the course of treatment with MCE showed significant changes in intra- and interhemispheric interactions.

[Evaluation of changes in the cortical gait control in post-stroke patients induced by the use of the "Regent" soft exoskeleton complex (SEC) by navigated transcranial magnetic stimulation].

The mechanisms underlying the locomotion recovery in poststroke patients remain unknown. Navigated transcranial magnetic stimulation (nTMS) is a new method to evaluate the functional state of the motor system. Using of the exoskeleton complex (EC) allow to correct walking pattern significantly. The aim of this study was to evaluate the capability of nTMS to assess changes in gait cortical control using EC in poststroke patients. 14 patients suffered subcortical stroke, mean age was 53.0 years [49, 62], mean duration of a stroke of 14.2 [7.0; 23.0] months were included. All patients trained with EC for 10 times and also received standardized physical therapy. All patients underwent nTMS, as well as clinical assessment using a Fugl-Meyer Scale lower extremity section and 10 m walking test before and after trains. A significant decrease of time to walk 10 meter was observed, while Fugl-Meyer Score remained unchanged. Patients showed the significant reduction of the average latency of motor responses from the affected hemisphere and different patterns of size and localization changes in both legs' cortical motor areas. Navigates TMS may demonstrate individual patterns of changes in cortical representation of leg muscles in post-stroke patients with damage of various motor system elements, while using exoskeleton complex. Thus, navigated TMS may be used not only for verification of neuroplasticity process, but it may also provide its detailed description.

[Mehanisms of Influences of Electrical Spinal Cord Stimulation on Autonomic Functions.]

Recently transcutateous electrical spinal cord stimulation began to be used both for experimental studies of motor functions regulation and for rehabilitation of motor functions in patients with spinal cord injury. The spinal cord is a very important center of vital functions regulation and the spinal cord stimulation directed to the activation of spinal locomotor related networks will affect visceral systems as well. This circumstance is necessary to take into account when this new method will be used for rehabilitation as well as for the studies on healthy subjects. Here the review of publications related to effects of electrical spinal cord stimulation on peripheral and cerebral circulation, on the cardiovascular, respiratory, excretory, and digestive systems of mammals is presented.

[The Effects of Optokinetic Stimulation on Visual-Manual Tracking under Support-Proprioceptive Deprivation.]

To determine the effects of additional visual stimuli (retinal optokinetic stimulation - ROKS) on characteristics of the visual-manual tracking (VMT) without support and with decreased proprioceptive afferentation there was used a horizontal "dry" immersion. Altogether 18 subjects aged from 19 to 31 years participated in the study and they were submerged into the immersion bath from 5 to 7 days. There was evaluated performance of the VMT in tasks to pursue the jerky (saccadically) and smooth (linear, pendular and circular) movement of a point visual stimulus before, during and after immersion. Eye movements were recorded using electrooculography, hand movements - by ajoystick with a biological visual feedback (one of the two visible stimuli on the screen matched the current angle of thejoystick handle). Computerized visual stimulation programs were presented to subject using a virtual reality glasses with and without additional ROKS. We analyzed time, amplitude and velocity characteristics of the visual and manual tracking (VT and MT), including efficiency ratio (e) and gain (g) as ratios of respectively amplitudes and velocities of eyes/hand movements to the stimulus movement. Without ROKS efficiency ratio and gain of both VT and MT were significantly decreased in comparison with baseline during the whole immersion and up to the R + 3 day after. The most pronounced deterioration was observed in parameters of the VT. When using a "threshold" ROKS parameters of the VT and MT weren't changed before immersion, during and after immersion - they improved (on 1-5-7 days during immersion and on R + 3 day after there was a significant improvement of the VT and MT in comparison to same tests without ROKS). The greatest impact of the ROKS was observed in parameters of the VT. Evaluations of the vestibular function (VF) were performed before and after immersion using videooculography approach. We analyzed statical torsional otolith-cervical-ocular reflex (OCOR), dynamical vestibular-cervical-ocular reactions (VCOR) and vestibular reactivity (VR), spontaneous eye movements (SpEM). On R + 1 day after immersion in 28% of subjects there was found a significant decrease of the gain of OCOR (0.12 value in comparison to 0.25 baseline) with simultaneous significant increase of parameters of VCOR/VR. There was found a correlation between parameters of VT and MT, between VF and VT and no correlation between VF and MT. It was found that removal of the support and minimization of the proprioceptive afferentation has a greater impact-upon accuracy of the VT then accuracy of the MT. When using ROKS correlations between the studied parameters were not only preserved, but also intensified. The results obtained indicate the development of sensory deprivation (and afferent deficit) under the immersion exposure and the possibility of its correction with additional ROCKS.

[Comparative Analisys of Efficacy of Countermeasure Provided by Different Modes of Locomotor Training in Space Flight.]

According to the results of the experiment performed on the ISS with participation of 15 Russian cosmonauts was fulfilled the comparative analysis of two models of preventive measures used by the Russian members of the long term space flight: the intense interval training in the aerobic-anaerobic zone power (recommended model) and the continuous low-intensity exercise in the aerobic zone energy power of the muscle activity. The interval trainings in the conditions of the space flight have provided the maintenance of the level of physical performance close to the preflight level; characterized by the maximum running speed, physiological value of work and lactate level after a standard load. The putative mechanisms of counteraction to adaptive remodeling of the propulsion system in zero gravity and enhanced understanding of the laws governing the interaction of the human body with the gravitational field of the Earth are described in the paper. The research results presented in the article show the high preventive efficacy of interval training compared with regular aerobic trainings, what is very important now in the time of searching of the means and methods of prevention of hypogravitation alterations during the interplanetary missions.

Human muscle signaling responses to 3-day head-out dry immersion.

To date little is known about catabolic NO-dependent signaling systems in human skeletal muscle during early stages of gravitational unloading. The goal of the study was to analyze signaling pathways that determine the initial development of proteolytic events in human soleus muscle during short-term gravitational unloading (simulated microgravity). Gravitational unloading was simulated by 3-day head-out dry immersion. Before and after the immersion the samples of soleus muscle were taken under local anesthesia, using biopsy technique. The content of desmin, IRS-1, phospho-AMPK, total and phospho-nNOS in soleus of 6 healthy men was determined using Western-blotting before and after the dry-immersion. Three days of the dry immersion resulted in a significant decrease in desmin, phospho-nNOS and phospho-AMPK as compared to the pre-immersion values. The results of the study suggest that proteolytic processes in human soleus at the early stage of gravitational unloading are associated with inactivation of nNOS. Reduction in AMPK phosphorylation could serve as a trigger event for the development of primary atrophic changes in skeletal muscle.

[Effects of mechanical stimulation of the soles' support zones on H-reflex characteristics under support unloading condition].

The aim of the work was to study the effects of mechanical stimulation of the soles' support zones on state of m. soleus motoneurone pool in man under 7-days support unloading conditions, which was provided by "Dry Immersion" model. Before, during and after immersion exposure the excitability of m. soleus motoneurone pool was estimated by H-reflex amplitude normalized by the maximal amplitude of M-wave. The data registered in two groups of volunteers: "control" in which only immersion exposure was used and "experimental" in which stimulation of support zones of sole was carried out during Dry Immersion were compared. During immersion relative amplitude of H-reflex increased in the control group. These alterations were not revealed in the experimental group with daily application of the support stimulation in natural locomotion regimens during immersion.

[Effect of support deprivation on the sequence of motor units recruiting].

Motor units (MUs) activity in the knee extensors were tested by sustention of a small isometric effort in the conditions of foot support deprivation by dry immersion with simultaneous mechanic stimulation of the foot support zones. Analysis of MUs inter-pulse intervals (IPI) histograms in the heads of two leg extensors (m. soleus and m. gastrocnemius lat.) demonstrated that the MUs recruiting order is much dependent on the support input activity. In immersion, rearrangement of the sequence of MUs recruiting during the isometric effort sustention test pointed to a lower involvement of small tonic MUs; large MUs with IPIs as long as 260 ms and high variability took the place of small tonic MUs with relatively short IPIs (100 ms) and weak variability. Daily support stimulation in the timeframe of immersion was favorable to maintaining the normal pattern of MUs recruitment.

[Russian system of countermeasures: the present and future].

The paper considers the fundamental principles of the Russian countermeasures system validated successfully in multiple long-term missions to the Russian space stations. The system adopted on the ISS Russian segment is detailed. In particular, it is pinpointed that physical training of cosmonauts was adjusted to the demands of the stages of ISS assembly and utilization. The locomotion training performed with departures from the flight documentation (ODFs) is described. The concluding section of the paper lays out the picture of vistas for the system of countermeasures in future exploration missions.

Effects of long-duration space flights on characteristics of the vertical gaze fixation reaction.

The aim of the study was to examine effects of long-duration exposure to weightlessness on characteristics of the vertical gaze fixation reaction (GFR). The subjects were to perform the target acquisition task on visual stimuli that appeared at a distance of 16 deg. up- and down from the primary position in a random order. Experiments were performed before launch, during flight and after landing. Before flight time of gaze fixation reaction did not exceed 650 ms. During space flight (SF) it extended up to 900-1000 ms and more. The velocities of head movement in space decreased, but the velocities of eye counterrotation decreased to a lesser degree. This difference resulted in sharp increase of vertical vestibular ocular reflex (VOR) gain (up to 4.3 values in one of the cosmonauts) during the 1st month of flight; further it decreased reaching the values of 0.5-0.7 on the 5th month of SF. After landing vertical VOR gain increased greatly again. These results in the vertical axis are in agreement with the data of Kozlovskaya et al., which showed in experiments with monkeys that horizontal VOR gain increased together with redundant inadequate responses of vestibular nucleus on vestibular stimulation and that in the course of adaptation to these conditions central nervous system inhibited vestibular input from the motor control system.