Caloric stimulation during short episodes of microgravity

Summary Caloric testing was performed during parabolic flight at the NASA Reduced Gravity Facility in Houston, Texas. Six test subjects were stimulated with continuous unilateral air insufflation (25°R), in a manner similar to the experiments performed in the extended weightlessness of orbital flight during the SL1 and D1 Spacelab missions. Nystagmus response was recorded by electro-oculography and eye video image. It was the purpose of the experiments to re-examine the apparent discrepancy between the disappearance of caloric nystagmus during short episodes of weightlessness and the finding that caloric responses can be elicited during periods of extended weightlessness. The present results agree with those of earlier experiments in that a prompt reduction of caloric nystagmus occurs on transition from hypergravity (1.8 G) to weightlessness. The time constant of nystagmus decay was estimated to be approximately 2–3s, a value which cannot be explained by cupular mechanics. A central gating mechanism involving the labyrinthine canal and otolithic afferents is proposed for the observed modulation of caloric nystagmus.     

Grip forces exerted against stationary held objects during gravity changes

 In the present study, grip forces exerted against a stationary held object were recorded during parabolic flights. Such flight maneuvers induce changes of gravity with two periods of hypergravity, associated with a doubling of normal terrestrial gravity, and a 20 s period of microgravity. Accordingly, the object’s weight changed from being twice as heavy as normally experienced and weightless. Grip-force recordings demonstrated that force control was seriously disturbed only during the first experience of hyper- and microgravity, with the grip forces being exceedingly high and yielding irregular fluctuations. Thereafter, however, grip force traces were smooth, the force level was scaled to the object’s weight under normal and high-G conditions, and the grip force changed in parallel with the weight during the transitions between hyper- and microgravity. In addition, during weightlessness, when virtually no force was necessary to stabilize the object, a low force was established, which obviously represented a reasonable safety margin for preventing possible perturbations. Thus, all relevant aspects of grip-force control observed under normal gravity conditions were preserved during gravity changes induced by parabolic flights. Hence, grip-force control mechanisms were able to cope with hyper- and microgravity, either by incorporating relevant receptor signals, such as those originating from cutaneous mechanoreceptors, or by adequately including perceived gravity signals into control programs. However, the adaptation to the uncommon gravity conditions was not complete following the first experience; finer tuning of the control system to both hyper- and microgravity continued over the measurement interval, presumably with a longer observation period being necessary before a stable performance can be reached. Received: 23 April 1998 / Accepted: 20 December 1998     

Transport and degradation characteristics of methotrexate dialkyl ester prodrugs across tape-stripped hairless mouse skin

A series of methotrexate dialkyl esters were examined with respect to their permeability across tape-stripped hairless mouse skin. The dialkyl esters showed a parabolic permeability versus side chain length relationship with the dimethyl ester being the most permeable compound. These compounds were also found to undergo an increased degree of degradation with increased ester chain length during the diffusion process, while with substantially reduced degradation occuring with the branched chain diisopropyl ester. No measurable methotrexate was formed during the course of the experiment, apparently due to the chemical and enzymatic stability of the intermediate - and -γ-monoesters.     

Illusions of postural,visual, and aircraft motion elicited by deep knee bends in the increased gravitoinertial force phase of parabolic flight

Summary Illusions of self motion and aircraft motion are experienced when executing deep knee bends in the high force phases of parabolic flight. The occurrence of such illusions indicates that skeletomotor control is actively calibrated to a 1 g reference level and that departures from this level affect the execution and appreciation of voluntary movements. The origin of the illusory patterns is shown to be understandable in terms of mismatches between efferent control signals and expected patterns of associated muscle spindle activity. It is shown, too, that spindle activity is interpreted within an entire context of spatial information about ongoing and intended motion of the body and whether the body is laden.Supported by NASA contracts NAS9-15147 and T-9140E     

Evaluation of the vestibular evoked myogenic potential during parabolic flight in humans

The purpose of this study was to investigate how gravity affects the vestibular evoked myogenic potential (VEMP). Eight healthy subjects (seven men, one woman; age range 19–45 years) participated in experiments in which three different gravity levels [microgravity (MG), normal gravity (NG), and hypergravity (HG)] were imposed during a parabolic flight procedure. The VEMP was evoked in response to an intense mono-aural click while the subjects kept the sternocleidomastoid (SCM) muscle contracted bilaterally. Background electromyographic activity of the SCM during the test was corrected. The p13–n23 amplitude was significantly greater under MG than under NG or HG. There was no difference in p13 latency between the three gravity levels. Possible mechanisms related to this phenomenon are discussed.     

Cardiovascular deconditioning in microgravity: some possible countermeasures

Microgravity is an extreme environment inducing relevant adaptive changes in the human body, especially after prolonged periods of exposure. Since the early sixties, numerous studies on the effects of microgravity, during manned Space flights, have produced an increasing amount of information concerning its physiological effects, globally defined "deconditioning". Microgravity deconditioning of the cardiovascular system (CVD) is briefly reviewed. It consists of: (1) a decrease of circulating blood and interstitial fluid volumes, (2) a decrease of arterial blood diastolic pressure, (3) a decrease of ventricular stroke volume, (4) a decrease of the estimated left ventricular mass and (5) resetting of the carotid baroreceptors. The negative effects of microgravity deconditioning manifest themselves mostly upon the reentry to Earth. They consist mainly of: (1) dizziness, (2) increased heart rate and heart palpitations, (3) an inability to assume the standing position (orthostatic intolerance), (4) pre-syncopal feelings due to postural stress and (5) reduced exercise capacity. To avoid these drawbacks several countermeasures have been proposed; they will be briefly mentioned with emphasis on the "Twin Bikes System" (TBS). This consists of two coupled bicycles operated by astronauts and counter-rotating along the inner wall of a cylindrical Space module, thus generating a centrifugal force vector, mimicking gravity.     

Parasympathetic heart rate modulation during parabolic flights

During parabolic flight short periods of microgravity and hypergravity are created. These changes influence cardiovascular function differently according to posture. During the 29th parabolic flight campaign of the European Space Agency (ESA), the electrocardiogram (ECG) was recorded continuously in seven healthy volunteers in two positions (standing and supine). Five different phases were differentiated: 1 g (1 g=9.81 m/s²) before and after each parabola, 1.8 g at the ascending leg of the parabola (hypergravity), 0 g at the apex, 1.6 g at the descending leg (hypergravity). We assessed heart rate variability (HRV) by indices of temporal analysis [mean RR interval (meanRR), the standard deviation of the intervals (SDRR), and the square root of the mean squared differences of successive intervals (rMSSD) and coefficient of variation (CV)]. In the supine position no significant differences were shown between different gravity phases for all HRV indices. In the standing position the 0 g phase showed a tendency towards higher values of meanRR compared to the control and to the other phases (p=NS). SDRR, rMSSD and CV were significantly higher compared to control (p<0.05). Significantly higher values for meanRR in the supine position at 1 g and hypergravity (p<0.05) were found when compared to standing. SDRR was significantly higher at 0 g in the standing position compared to supine [95 (44) ms vs. 50 (15) ms; p<0.05] and lower in other phases. rMSSD and CV showed the same trend (p=NS). We confirm that, during parabolic flights, position matters for cardiovascular measurements. Time domain indices of HRV during different gravity phases showed: (1) higher vagal modulation of the autonomic nervous system in microgravity, when compared with normo- or hypergravity in standing subjects; and (2) no differences in supine subjects between different g phases.     

Behavioral responses to partial-gravity conditions in rats

The effects of microgravity or hypergravity on living organisms have been studied extensively; however, thus far no studies have addressed the effects of “partial-gravity”, that is, the low-gravity levels between the unit gravity (1 G) on Earth and zero gravity (0 G) in space. The purpose of the present study was to examine behavioral responses in rats under partial-gravity conditions. Rat behavior was monitored by video cameras during parabolic flights. The flight trajectory was customized in order to generate graded levels of partial gravity. Gravity-dependent behavior patterns were observed in rats. In the conditions of 0.4 G through 0.2 G, rats showed startle and crouching. Hindlimb stretching emerged at 0.15 G and was more frequently observed toward 0.01 G. Different thresholds may exist for emotional and balance/posture-related behaviors.     

Time suboptimal feedback control of systems described by linear parabolic partial differential equations

Feedback control for systems described by linear parabolic partial differential equations is considered from a time suboptimal point of view. A scalar quadratic function representing the deviation from a desired distribution is first formulated, and the rate of approach to the target is maximized by minimizing this quadratic function at each time step. An orthogonal collocation technique enables an accurate low-order lumped parameter model to be constructed. A direct search optimization can then be used to obtain the optimal quadratic function and, subsequently, the optimal feedback gain matrix for control. The resulting control applied to a diffusion process takes the system to the target rapidly and in a stable manner.