Effects of long-term space flights on the organization of the horizontal gaze fixation reaction

The results of the Russian-Austrian space experiment Monimir, which was a part of the international space program Austromir, are presented. The characteristics of the horizontal gaze fixation reaction (hGFR) to the visual targets were studied during long-term space flights. Seven crewmembers of the space station Mir participated in our experiment. The subjects were tested four times before the flight, five times during the flight, and three to four times after landing. During the flight and after accomplishing, the characteristics of gaze fixation reaction changed regularly: the reaction time and coefficient of the gain of vestibular-ocular reflex increased; the velocities of eye-head movements increased and decreased. These changes were indicative of a disturbed control of the vestibular-ocular reflex under microgravity conditions because of variability of the vestibular input activity. The cosmonauts that had flight and non-flight professional specializations differed in strategies of their adaptation to the microgravity conditions. In the former, exposure to microgravity was accompanied by gaze hypermetry and inhibition of head movements; conversely, in the latter, the velocity of head movements increased, whereas that of saccades decreased.     

Vestibular function and space motion sickness

The vestibular system plays an important role in intersensory interactions and gravitation is a natural stimulus for its receptors. Weightlessness alters the input signals of the otoliths and their effect on the pattern and dynamics of changes in the vestibular function (VF), which is accompanied by development of space adaptation syndrome (SAS) and space motion sickness (SMS). These changes occur both during the spaceflight (SF) and after returning to Earth, but the mechanisms of their development are still poorly understood and require special studies. In total, 47 Russian cosmonauts (crewmembers of long-term International Space Station (ISS) missions) have participated in the studies into VF before and after SF and nine of them, in onboard studies during SF (129–215 days) as a part of the Virtual space experiment (stage 1). Electro- and video-oculography are used to record spontaneous eye movements (SpEM), static vestibular–ocular responses during head tilts to the right or left shoulder (static otolith–cervical–ocular reflex, OCOR), and dynamic vestibular-ocular response during the head rotation around the longitudinal axis of the body. The examination is accompanied by personal and questionnaire survey on subjective responses and complaints of cosmonauts about SAS and SMS. Significant changes in SpEM (drifts of eyes, spontaneous and gaze-evoked nystagmus, and arbitrary saccades) and a decrease in OCOR (statistically significant decrease in the amplitude of ocular counter-rolling in response to head tilts up to its absence or inversion, an atypical OCOR) are observed during SF. An atypical OCOR is observed at the beginning of adaptation to weightlessness in seven of the nine cosmonauts (the first one to two weeks of SF) and repeatedly throughout the flight in all cosmonauts regardless of whether it is their first flight or not. Atypical vestibular responses after SF, similar to the responses during SF, are observed in several cosmonauts by day 9 after flight. It has been shown that atypical OCOR variants are more frequently observed in the subjects lacking any previous space experience, as well as a more pronounced decrease in this response with a concurrent increase in the response of the semicircular canals. It is also demonstrated that repeated SFs lead to a considerable shortening in the after-flight readaptation to terrestrial conditions and a considerable decrease in the degree of vestibular disorders. In the initial period of SF, the changes in VF are correlated with the complaints and manifestations of SAS and SMS; however, the complaints and the corresponding symptoms are unobservable during the further flight despite significant changes in the VF state. The patterns of the VF disorders associated with the impact of weightlessness and observed during and after SF are very similar, allowing these disorders to be regarded as SAS and SMS of different severities (intensities).     

Changes in the gaze fixation reaction in rhesus monkeys during the initial stage of water immersion

The gaze fixation reaction was studied in three rhesus monkeys before and during thermoneutral (34.5 degrees C) water immersion to the mid-chest level. The angular vestibulo-ocular reflex gain increased and the head angular velocity decreased significantly in all monkeys in 5 h after the start of immersion. Additionally, one animal was immersed to the neck level. Two hours in the condition of more pronounced support deprivation decreased significantly angular velocity of the head but not increased the angular vestibulo-ocular reflex gain. Therefore, support deprivation act upon the head movement control first.     

Alterations of characteristics of horizontal gaze fixation reaction in long-term space flights

The study was performed to explore effects of long-duration SF on the characteristics of horizontal gaze fixation reaction (hGFR). Changes in GFR observed in long-duration space flight (SF) point out to serious disturbances of the VOR system due to, apparently, altered vestibular activity. Two strategies of reaction adaptation to the microgravity conditions were discovered in cosmonauts of civic occupations and pilots.     

Effect of weightlessness and artificial gravitation on thyroid gland morphology]

The investigation of the thyroid gland was carried out in Wistar rats, SPF colony 4.5--13 h and 25 days after a 18.5 days flight on board the space biosatellite "Cosmos-936". In animals subjected to weightlessness, moderate symptoms of the thyroid hypofunction were observed, statistically significant decrease in number and volume of the nuclei in calcitonin-secreting cells (C-cells) was especially pronounced during 4.5--9 h after landing. Similar but less pronounced changes were observed in C-cells of the rats subjected to artificial conditions of space flight, besides weightlessness. The similarity of the changes in the animals of both groups made it possible to connect the increasing amount of C-cells and the morphological symptoms of their functional inhibition with the effect of weightlessness and hypokinesia. During the space flight, the animals were kept under the conditions of artificial gravitation on board the biosatellite and therefore morphological peculiarities specific for the earth conditions were preserved in C-cells and the thyroid gland. Thus, it was concluded that artificial gravitation prevented the development of the thyroid changes which appeared under the influence of weightlessness.     

Effect of real and simulated weightlessness on the characteristics of the static otolith reflex

To determine the role of the support-proprioceptive factor in functioning of the vestibular system, in particular, the role of static torsional otolith-cervical-ocular reflex (OCOR), the latter was studied in 16 subjects after a seven-day “dry” horizontal immersion and in 14 cosmonauts after a prolonged exposure to weightlessness (for 126–195 days). OCOR was studied by the videooculography method during alternately tilting the head towards the right or left shoulder by an angle of 30° in the frontal plane before the flight and before immersion, as well as on days 1, 3, and 7 after the completion of the immersion experiment and on days 1 (2), 4 (5), and 8 (9) after the spaceflight. For the first time it was demonstrated that elimination of the support and minimizing the proprioceptive afferentation may lead to the absence or inversion of the static torsional OCOR, as well as to a positional nystagmus against the background of the inverted reflex. Comparison of OCOR in cosmonauts after prolonged exposure to weightlessness and in the subjects examined after immersion revealed similarity in this reaction. However, changes in OCOR after immersion were encountered only in 60% of the subjects, whereas after the spaceflight, in 90% of the cosmonauts examined. The post-flight changes in OCOR were more pronounced and long-lasting.     

The effect of space flight conditions on the rate of multiplication, morphology of cells, DNA and protein content in the ciliate Tetrahymena pyriformis

The conditions of a space flight and, in particular, the weightlessness promote an increased density of the ciliate culture, an enhanced reproduction rate, and an elevated ratio of dividing cells. The condition of weightlessness brings about some decrease in the bulk protein content of the cells determined by cytophotometry of Naphthol-yellow stained ciliates. The quantity of DNA in macronuclei was measured following the routine Feulgen procedure (its "cold" variant). The DNA content was found to remain unchanged. Some changes in the shape and size of the cells were noticed under flight conditions: ciliates that had developed in weightlessness appeared more spherical than control ones, due presumably to a decrease in the body length and to some extension in the body width. The conditions of space flight, including the weightlessness, induce changes in the physiological status of unicellular organisms. A decrease in the gravitation force may lead to a decrease in the energy expenditures for maintenance of the cell positional homeostasis.     

Experiments with cultures of mammalian cells aboard the biosatellite "Cosmos-782"

A considerable contribution to the investigation on biological importance of weightlessness was made by the experiments with animals in the artificial Earth satelites (AES) of "Cosmos" type. Cell cultures can serve as an ideal model to get a direct cell response to the effect of external factors. For the experiment in the AES "Cosmos-782", two thoroughly examined cell strains (L and 237) were chosen, which differed in a number of parameters (for example, duration of their mitotic cycles). Density of cell seeding and temperature of their cultivation in the laboratory experiment were calculated in such a way that the whole cycle of the culture development should take place under the conditions of weightlessness: the beginning of lag-phase--before launching and the stationary phase--after landing. The weightlessness was not shown to result in any genetical shifts revealed at chromosomal level. When cultivated after the flight, the cells do not change their mitotic cycle parameters, mitotic course and structural organization. The data obtained in the experiments with AES "Cosmos-368" and "Cosmos-782" (increase of mitotic index, some forms of mitotic pathology during the first terms of cultivation after the flight and enlargement of cellular nuclei) demonstrate the changes in the cell population which have formed under the conditions of weightlessness. Similar changes are observed while the cells propagate in the laboratory conditions. Indirect data on an earlier cell culture aging during the flight do not exclued the possibility that under weightlessness the rate of cell propagation could differ from that under gravitation.     

Gravitation and the Hemispheric Specialization of the Brain in Bilateral Symmetry Determination

We studied the influence of weightlessness on bilateral symmetry detection during prolonged space flight. Supposing that weightlessness may affect visual information processing by the right and left hemispheres in different ways, we studied this phenomenon with regard for the part of the visual field where to a stimulus was presented (the sight fixation center or the left/right half of this field). We used two types of stimuli, i.e., closed figures (polygons) and distributed figures formed by dots. There was a distinct difference between the central and noncentral presentation of stimuli under terrestrial conditions. When a stimulus was presented noncentrally (on the left or right), a manifest dominance of the horizontal axis was observed. However, there was no substantial difference while stimulating the left and right parts of the visual field. This contradicts the hypothesis on hemispheric specialization of the brain in symmetry detection. When stimuli were presented eccentrically, weightlessness did not notably influence information processing. When they were presented centrally, the predominance of the vertical axis in closed figures tended to weaken under the impact of weightlessness. However, this predominance strengthened when multicomponent figures were presented in space. The different influences of weightlessness on perceiving symmetry of stimuli of different types shows that it may be detected at various levels with different degrees of using nonvisual sensory information.     

Velocity of head movements and sensory-motor adaptation during and after short spaceflight

To investigate to time course of sensory-motor adaptation to microgravity, we tested spatially-directed voluntary head movements before, during and after short spaceflight. We also tested the re-adaptation of postural responses to sensory stimulation after space flight. The cosmonaut performed in microgravity six cycles of voluntary head rotation in pitch, roll and yaw directions. During the first days of weightlessness the angular velocity of head movements increased. Over the next days of microgravity the velocity of head movements gradually decreased. On landing day a significant decrease of head rotation velocity was observed compared to the head movement velocity before spaceflight. Re-adaptation to Earth condition measured by body sway on soft support showed similar time course, but re-adaptation measured by postural responses to vestibular galvanic stimulation was prolonged. These results showed that the angular velocity of aimed head movements of cosmonauts is a good indicator of sensory-motor adaptation in altered gravity conditions.     

Structural and metabolic characteristics of human soleus fibers after long duration spaceflight

The fiber size decline, alterations in fiber metabolic potential and increase of connective tissue component were shown in human m. vastus lateralis after short and long-duration space flights and in m.soleus and m.vastus lateralis after 120 day head down tilt bed rest. It is known from rat and monkey studies that the exposure to weightlessness leads to the most pronounced changes in postural muscles, e.g. m.soleus. It was shown that 17 day space flight induced significant decrease of fiber cross-sectional area and slow-to-fast fiber type transformation in human soleus. But in the cited work the fiber population under study was limited like in most single fiber technique analyses. The present study was purposed to investigate the structural and metabolic properties of soleus muscle in Russian cosmonauts exposed to 129-day space flight on board of the International Space Station.     

Reflex response changes during hyper and microgravity

This paper reports the quantitative evaluation of the H-reflex exhibited by parabolic flight with exposure to micro and high-gravity. With respect to previous findings in parabolic flights and short-term space missions, the analysis focused on reflex activity in weightlessness. The aim of this study was to investigate the effect of gravity on H-reflex and motor evoked potentials (MEP) in soleus muscle (SOL) during parabolic flight.     

Swimming velocity of Paramecium under the conditions of weightlessness

During the 6 min-lasting "free-fall conditions" (4 x 10(-6) g) of the parabolic flight of a sounding rocket Paramecium aurelia cells showed an increase of 7.5 % in their mean swimming velocity. A detailed analysis revealed that the kinetic response was transient: after 3 min the velocity decreased to the speed of the former horizontal swimming at 1 g. Control experiments simulating the influence of vibration and hypergravity during launch of the rocket lead to the conclusion that the increase of the velocity during the parabolic flight was exclusively induced by the transition to 0 g. An increased velocity was also observed under the condition of simulated weightlessness on a fast-rotating clinostat microscope.     

Behavioral and functional vestibular disorders after space flight: I. Mammals

The review presents data on functional disorders in mammals caused by changes in the vestibular system after space flight. These data show that the mammalian vestibular system responds to weightlessness dissimilarly at different ontogenetic stages. During the embryonic period, orbital space flight conditions have a little effect on the developing vestibular system and even promote normal fetal development. During the early postnatal period, when optimal sensorymotor tactics arise, long-term exposure to space flight conditions leads to the development of novel, “extraterrestrial”, sensory-motor programs that may fixate in CNS for life. In adult individuals, substantial vestibular changes and disorders may occur immediately after landing depending on the weightlessness duration. An adult organism has to solve two concurrent and mutually conflicting problems: to adapt to weightlessness and not to adapt to it in order to facilitate readaptation after return. Thus, individuals have to counteract weightlessness to retain a maximum of their pre-flight health status. The means of such a counteraction have to be adjusted according to the weightlessness duration. It is noteworthy, however, that not all functional changes occurring in adult individuals under weightlessness can be adequately accounted for. Some of them can assume a chronic or even pathological character. The review raises for the first time the question of necessity to include into the scope of studies the effect of weightlessness on a senile (senescent) organism and its vestibular system. We believe that development of space gerontology as a special branch of space biology and medicine is undoubtedly of interest and may become practically important in the future in view of the ever-growing age of space explorers.     

The dendritic spines of the pyramidal neurons in layer V of the rat sensorimotor cortex following a 14-day space flight]

There was made a quantitative study of the influence of 14 days space flight ("Kosmos-2044") on dendritic spine (DS) density of the layer V pyramidal neurons of rat sensomotor cortex. There was found an increase of the number of apical DS lying in the layers III-IV in the flight group only. Number of DS on oblique dendrites was increased in the III-IV cortical layers both in the flight and tail-suspended rats. There was also an increase in the number of DS on basal dendrites in all experimental groups. Obtained data are compared with similar 7 days flight results ("Kosmos-1667") and other data of nervous tissue plasticity in weightlessness.     

Influence of repetitive Gz acceleration on structural and metabolic profile of m. vastus lateralis in monkeys exposed to 30 day bedrest

It was shown that changes in structural and metabolic indices of extensor muscles of the lower extremities were usually found in man after exposure to space flight or to bed rest. Similar changes were also observed in monkeys, space-flown on "Kosmos" biosatellites. Response to weightlessness and to restraint was found to be different in m. soleus and in m. vastus lateralis. Therefore, it is important to study structural and metabolic changes of m. vastus lateralis fibers under conditions of gravitational unloading in monkeys, who have motor apparatus similar to that of man, and are much more fruitful object of research. It is assumed that artificial gravity can serve as a countermeasure, aimed at diminishing effects of gravitational unloading. We have studied the effect of repeated gravity overloading, created by means of a centrifuge, on structural and metabolic indices of monkey m. vastus lateralis at the background of 30 day head down tilt bed rest (BR).     

Nucleolar transformation in plants grown on clinostats

Summary Cells of carrot calli (Daucus carota L.) grown on clinostats (simulated weightlessness) exhibit increases in nucleolar number and volume. In clinostat-grown whole barley plants (Hordeum vulgare L. cv. Steptoe), nucleoli in 70% of root meristem and root cortical cells in the 1 mm root apex exhibit multiple nodulations after one day of growth. The nucleolar nodules (1.1 m mean diameter) are densely and finely fibrous, distinctly different from the nucleolus in which the content is so compact that the granular component is masked. Control nucleoli (from vertically rotated and stationary seedlings) rarely exhibit nodule-like protrusions, are not compact, and contain a well defined granular component. Proteins that are heat soluble, characteristic of many stress responses, rapidly increase in barley grown on clinostats. Barley growth on clinostats is slowly and steadily inhibited. There is no difference between vertically rotated and stationary controls for any of the parameters measured, indicating that clinostat motion per se does not affect significantly barley development. The evidence taken together suggests that barley plants germinated and grown on clinostats are stressed, the effects of which are expressed sequentially by alteration of nucleolar morphology, increased production of heat-soluble proteins, and decreased plant growth. Similar stress-related changes may be expected to occur in plants subjected to weightlessness during space flight. It is therefore of interest that nucleoli in wheat roots (Triticum aestivum L. cv. Broom) obtained from the space flight IML-1 mission show irregularity that is not observed in any of the ground controls for the flight experiment.Abbreviations Act D actinomycin D - C clinostat rotation - EM electron microscopy - LM light microscopy - R vertical rotation - rDNA ribosomal DNA - S stationary     

Bumblebee Homing: The Fine Structure of Head Turning Movements

Changes in flight direction in flying insects are largely due to roll, yaw and pitch rotations of their body. Head orientation is stabilized for most of the time by counter rotation. Here, we use high-speed video to analyse head- and body-movements of the bumblebee Bombus terrestris while approaching and departing from a food source located between three landmarks in an indoor flight-arena. The flight paths consist of almost straight flight segments that are interspersed with rapid turns. These short and fast yaw turns (“saccades”) are usually accompanied by even faster head yaw turns that change gaze direction. Since a large part of image rotation is thereby reduced to brief instants of time, this behavioural pattern facilitates depth perception from visual motion parallax during the intersaccadic intervals. The detailed analysis of the fine structure of the bees’ head turning movements shows that the time course of single head saccades is very stereotypical. We find a consistent relationship between the duration, peak velocity and amplitude of saccadic head movements, which in its main characteristics resembles the so-called "saccadic main sequence" in humans. The fact that bumblebee head saccades are highly stereotyped as in humans, may hint at a common principle, where fast and precise motor control is used to reliably reduce the time during which the retinal images moves.     

卧床前后压力感受性反射机能变化的研究

许多数据表明长期失重以后立位耐力降低可能与压力感受性反射功能的改变有关。本文比较了两组被试者15天低动力卧床前后的立位耐力。以血压调节模型为基础分析了两种不同方式卧床前后单纯立位和下身负压加立位时压力感受性反射功能的改变,并用颈部加压及下身负压对中枢调节功能改变进行了观察。结果表明严格的头低位卧床后,立位耐力下降及压力感受性反射功能改变明显大于半日平卧半日倚坐者。而压力感受性反射功能的改变,特别是中枢神经系统调节功能的紊乱,是卧床后立位耐力降低的主要原因。从这种考虑为基础,作者提出了改变失重或模拟失重状态下的血液分布,调整对压力感受器的刺激,可能是预防心血管失调的有效方法。