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PubMed1998

Effect of gravity on vestibular neural development.

The timing, molecular basis, and morphophysiological and behavioral consequences of the interaction between external environment and the internal genetic pool that shapes the nervous system over a lifetime remain important questions in basic neuroscientific research. Space station offers the opportunity to study this interaction over several life cycles in a variety of organisms. This short review considers past work in altered gravity, particularly on the vestibular system, as the basis for proposing future research on space station, and discusses the equipment necessary to achieve goals. It is stressed that, in keeping with the international investment being made in this research endeavor, both the questions asked and the technologies to be developed should be bold. Advantage must be taken of this unique research environment to expand the frontiers of neuroscience.

Aging↗

PubMed2001

[A forward leaning posture affects more the amplitudes of the centre of pressure displacements than those of the centre of gravity].

OBJECTIVE: An ordinary and practical method to assess the postural control mechanisms is to measure, through a force platform, the point of application of the resultant reaction forces. The recorded signal, the centre of pressure CP, is used to compute the motions of the vertical projection of the centre of gravity (CG(v)) and those of the difference CP-CG(v). The former is considered in this postural task as the controlled variable whereas the latter is thought to express the ankle resultant stiffness. Interestingly, the CG(v) motions can be extracted from the CP trajectory through a simple mathematical model. MATERIALS AND METHODS: The relationship between CP and the elementary motions CP-CG(v) and CG(v) has been tested in the present study by comparing the postural effects induced by a leaning forward position to a normal one consisting to stand still. RESULTS: A large increase is noticed for the amplitudes of the CP-CG(v) motions for both medio-lateral ML and antero-posterior AP directions. On the other hand, a similar effect appears for the CG(v) motions but only on the ML direction. Nevertheless, the frequency analysis indicates that the constancy of the CG(v) motions on the AP direction is accompanied by a change in the frequency distribution, as assessed by the MPF parameters. DISCUSSION-CONCLUSION: These features highlight the necessity, to interpret rigorously the postural performance, to take into account in separate ways each of the elementary CP-CG(v) and CG(v) motions which, combined, constitute the CP displacements.

Adult↗

PubMedDate not supplied

On the physics of unstable infiltration, seepage, and gravity drainage in partially saturated tuffs.

To improve understanding of the physics of dynamic instabilities in unsaturated flow processes within the Paintbrush nonwelded unit (PTn) and the middle nonlithophysal portion of the Topopah Spring welded tuff unit (TSw) of Yucca Mountain, we analyzed data from a series of infiltration tests carried out at two sites (Alcove 4 and Alcove 6) in the Exploratory Studies Facility (ESF), using analytical and empirical functions. The analysis of infiltration rates measured at both sites showed three temporal scales of infiltration rate: (1) a macro-scale trend of overall decreasing flow, (2) a meso-scale trend of fast and slow motion exhibiting three-stage variations of the flow rate (decreasing, increasing, and [again] decreasing flow rate, as observed in soils in the presence of entrapped air), and (3) micro-scale (high frequency) fluctuations. Infiltration tests in the nonwelded unit at Alcove 4 indicate that this unit may effectively dampen episodic fast infiltration events; however, well-known Kostyakov, Horton, and Philip equations do not satisfactorily describe the observed trends of the infiltration rate. Instead, a Weibull distribution model can most accurately describe experimentally determined time trends of the infiltration rate. Infiltration tests in highly permeable, fractured, welded tuff at Alcove 6 indicate that the infiltration rate exhibits pulsation, which may have been caused by multiple threshold effects and water-air redistribution between fractures and matrix. The empirical relationships between the extrinsic seepage from fractures, matrix imbibition, and gravity drainage versus the infiltration rate, as well as scaling and self-similarity for the leading edge of the water front are the hallmark of the nonlinear dynamic processes in water flow under episodic infiltration through fractured tuff. Based on the analysis of experimental data, we propose a conceptual model of a dynamic fracture flow and fracture-matrix interaction in fractured tuff, incorporating the time-dependent processes of water redistribution in the fracture-matrix system.

Filtration↗

PubMed1998

The 3-D motion of the centre of gravity of the human body during level walking. I. Normal subjects at low and intermediate walking speeds.

OBJECTIVE: To measure the mechanical energy changes of the centre of gravity (CG) of the body in the forward, lateral and vertical direction during normal level walking at intermediate and low speeds. DESIGN: Eight healthy adults performed successive walks at speeds ranging from 0.25 to 1.75 m s(-1) over a dedicated force platform system. BACKGROUND: In previous studies, it was shown that the motion of the CG during gait can be altered more than the motion of individual segments. However, more detailed normative data are needed for clinical analysis. METHODS: The positive work done during the step to accelerate the body CG in the forward direction, W(f), to lift it, W(v), to accelerate it in the lateral direction, W(I), and the actual work done by the muscles to maintain its motion with respect to the ground ('external' work), W(ext), were measured. This allowed the calculation of the pendulum-like transfer between gravitational potential energy and kinetic energy of the CG, (percentage recovery, R). At the optimal speed of about 1.3 m s(-1), this transfer allows saving of as much as 65% of the muscular work which would have been otherwise needed to keep the body in motion with respect to the ground. The distance covered by the CG at each step either forward (step length, S(I)), or vertically (vertical displacement, S(v)) was also recorded. RESULTS: W(I) was, as a median, only 1.6-5.9% of W(ext). This ratio was higher, the lower the speed. At each step, W(ext) is needed to sustain two distinct increments of the total mechanical energy of the CG, E(tot). The increment a takes place during the double stance phase; the increment b takes place during the single stance phase. Both of these increments increased with speed. Over the speed range analyzed, the power spent to to sustain the a increment was 2.8-3.9 times higher than the power spent to sustain the b increment.

Journal Article↗

PubMed1998

The 3-D motion of the centre of gravity of the human body during level walking. II. Lower limb amputees.

OBJECTIVE: To analyse the motion of the centre of gravity (CG) of the body during gait in unilateral lower limb amputees with good kinematic patterns. DESIGN: Three transtibial (below-knee, BK) and four transfemoral (above-knee, AK) amputees were required to perform successive walks over a 2.4 m long force plate, at freely chosen cadence and speed. BACKGROUND: In previous studies it has been shown that in unilateral lower limb amputee gait, the motion of the CG can be more asymmetric than might be suspected from kinematic analysis. METHODS: The mechanical energy changes of the CG due to its motion in the vertical, forward and lateral direction were measured. Gait speed ranged 0.75-1.32 m s(-1) in the different subjects. This allowed calculation of (a) the positive work done by muscles to maintain the motion of the CG with respect to the ground ('external' work, W(ext)) and (b) the amount of the pendulum-like, energy-saving transfer between gravitational potential energy and kinetic energy of the CG during each step (percent recovery, R). Step length and vertical displacement of the CG were also measured. RESULTS: The recorded variables were kept within the normal limits, calculated in a previous work, when an average was made of the steps performed on the prosthetic (P) and on the normal (N) limb. Asymmetries were found, however, between the P and the N step. In BK amputees, the P step R was 5% greater and W(ext) was 21% lower than in the N step; in AK amputees, in the P step R was 54% greater and W(ext) was 66% lower than in the N step. Asymmetries were also found in the relative magnitude of the external work provided by each lower limb during the single stance as compared with the double stance: a marked deficit of work occurred at the P to N transition.

Journal Article↗

PubMed1997

In-shoe force measurements from locomotion in simulated zero gravity during parabolic flight.

INTRODUCTION:: No effective countermeasure for space-induced bone loss has yet been identified. It has been hypothesized that an effective exercise regimen would elicit loads on the lower extremity which resemble those encountered on Earth. Although a treadmill has been used on shuttle flights, the loads to which the lower extremity was exposed have not yet been quantified. It is believed that these loads are much less than the loads experienced in 1G. The purpose of this study was to determine the magnitude of lower extremity loading during tethered treadmill exercise in a 0G environment. METHODS:: Data were collected on five subjects (avg. ht. 177.3+/-10.1 cm, avg. mass 78.3+/-18.0 kg) onboard the KC-135, a NASA airplane used to simulate periods of zero gravity through parabolic flight. Subjects ambulated at 4 speeds: a walk (1.56m/sec), fast walk (2.0m/sec) slow jog (2.75m/sec), and jog (3.35m/sec) on the NASA treadmill operated in either a passive or motorized mode. Each subject wore a harness connected to the Subject Load Device (SLD) to tether them to the treadmill. The tension in the SLD was subjectively adjusted for comfort by each subject. Force data were collected at 60 Hz using Pedar insoles. The number of parabolas per subject was variable due to motion sickness and hardware problems. RESULTS:: Analysis of the insole data showed that the average SLD load was only 35.2% BW, although the values ranged from 20.1% to 56.6%. Maximum ground reaction force values increased with increasing speed and were not affected by treadmill mode. The impulse was higher during walking with the treadmill in the passive mode than in the active mode, but this difference diminished with increasing speed. Subjects tended to run on their forefeet, as shown from the extremely small heel impulse values. At higher speeds, heel contact was absent, while forefoot impulse became more pronounced. DISCUSSION:: All force values were lower than those reported from 1G studies, where typical peak ground reaction forces are 1.2xBW and 2.5xBW for walking and running, respectively. At every speed, the ratio of the rearfoot to forefoot impulse was much lower than reported from 1G studies, and this ratio decreased with increasing speed. CONCLUSIONS:: If the exposure to forces similar to those in 1G is a requirement for countermeasures against space-induced osteoporosis, the loads in the SLD must be greatly increased and should be directly measured before exercise.

Journal Article↗

PubMed2001

Leaf senescence under various gravity conditions: relevance to the dynamics of plant hormones.

Effects of simulated microgravity and hypergravity on the senescence of oat leaf segments excised from the primary leaves of 8-d-old green seedlings were studied using a 3-dimensional (D) clinostat as a simulator of weightlessness and a centrifuge, respectively. During the incubation with water under 1-g conditions at 25 degrees C in the dark, the loss of chlorophyll of the segments was found dramatically immediately after leaf excision, and leaf color completely turned to yellow after 3-d to 4-d incubation. In this case kinetin (10 micromolar) was effective in retarding senescence. The application of simulated microgravity conditions on a 3-D clinostat enhanced chlorophyll loss in the presence or absence of kinetin. The loss of chlorophyll was also enhanced by hypergravity conditions (ca. 8 to 16 g), but the effect was smaller than that of simulated microgravity conditions on the clinostat. Jasmonates (JAs) and abscisic acid (ABA) promoted senescence under simulated microgravity conditions on the clinostat as well as under 1-g conditions. After 2-d incubation with water or 5-d incubation with kinetin, the endogenous levels of JAs and ABA of the segments kept under simulated microgravity conditions on the clinostat remained higher than those kept under 1-g conditions. These findings suggest that physiological processes of leaf senescence and the dynamics of endogenous plant hormone levels are substantially affected by gravity.

Abscisic Acid↗

PubMed2001

Gravity-dependent reactions of the moss Pohlia nutans protonemata.

In darkness, protonemata of Pohlia nutans (Hedw.) grew negatively gravitropically (upwards). However, not all filaments became gravitropic immediately after transfer to darkness. Some of them (~20%) for several days grew in different directions with respect to gravity. The apical cells of those protonemata predominantly contained multiple chloroplasts. The intensity of chlorophyll fluorescence rapidly decreased in the apical cells of such protonemata while starch content increased in comparison with upright growing protonemata. Light, especially in the red and blue part of the spectrum, inhibited protonemal gravitropism. Red light induced stronger inhibitory effects than blue light. Red light of 1.0 to 1.5 micromoles m-2 s-1 intensity induced bud differentiation in apical cells on almost all side branches of main protonemal filaments. Bright fluorescence of F-actin bundles in the tip of apical protonematal cells and a delicately fluorescing network enclosing plastids basal to the tip in a sedimentation zone were visualized. Bright fluorescence of actin as local patches and fine prominent axially oriented bundles was observed in cells of gametophore buds.

Actin Cytoskeleton↗

PubMed2003

The teleost fish medaka (Oryzias latipes) as genetic model to study gravity dependent bone homeostasis in vivo.

Long-term space flight and microgravity result in bone loss that can be explained by reduced activity of bone-forming cells (osteoblasts) and/or an increase in activity of bone resorbing cells (osteoclasts). Osteoprotegerin (OPG) has been shown to regulate the balance between osteoblast and osteoclast cell numbers and is involved in maintaining constant bone mass under normal gravitational conditions. The small bony fish medaka (Oryzias latipes) has attracted increasing attention as a genetic model system to study normal embryonic developmental and pathological processes. To analyze the molecular mechanisms of bone formation in this small vertebrate, we have isolated two opg genes, opgl and opg2, from medaka. Our phylogenetic analysis reveals that both genes originated from a common ancestor by fish-specific gene duplication and represent the orthologs of the mammalian opg gene. Both opg genes are differentially expressed during embryonic and larval development, in adult tissues and in cultured primary osteoblast-like cells. Furthermore, we have characterized the opg2 promoter region and identified consensus binding sites for the transcription factor core-binding-factor-1A (CBFA1). In mammals, CBFA1 has been shown to be a regulator of opg expression and to be essential for several steps during osteoblast differentiation. Here we show that sequence and expression domains of opg, cbfal and a member of the dlx gene family are highly conserved between medaka and higher vertebrates. This suggests that not only single genes but entire genetic networks for bone formation are conserved between teleosts and mammals. These findings will open medaka fish as a genetic model to monitor bone formation under different gravity conditions in a living whole animal allowing the identification of novel factors involved in bone homeostasis.

Animals↗

PubMed1998

Effects of gravity on early development.

The development of embryonic and larval stages of the South African Toad Xenopus laevis D, was investigated in hyper-g up to 5 g (centrifuge), in simulated 0 g (fast-rotating clinostat), in alternating low g, hyper-g (parabolic flights) and in microgravity (Spacelab missions D1, D-2). The selected developmental stages are assumed to be very sensitive to environmental stimuli. The results showed that the developmental reaction processes run normal also in environments different to 1 g and that aberrations in behavior and morphology normalize after return to 1 g. Development, differentiation, and morphology of the gravity perceiving parts of the vestibular system (macula-organs) had not been affected by exposure to different g-levels.

Adaptation, Physiological↗

PubMed1999

Exposure to altered gravity affects all stages of endochondral cartilage differentiation.

Chondrogenesis has a number of well-defined steps: (1) condensation, which involves cell aggregation, adhesion and communication; (2) activation of cartilage genes, which is accompanied by rounding up of the cells and intracellular differentiation; and (3) production and secretion of cartilage specific matrix molecules. Our studies show that each of these steps is affected by exposure to gravitational changes. Clinorotation and centrifugation affected initial aggregation and condensation. In the CELLS experiment, where cells were exposed to microgravity after some condensation occurred preflight, intracellular differentiation and matrix production were delayed relative to controls. Once cartilage has developed, in rats, further differentiation (hypertrophy, matrix production) was also affected by spaceflight and hind limb suspension. For the process of chondrogenesis to proceed as we know it, loading and other factors present at 1g are required at each step of the process. This requirement means that not only will skeletal development and bone healing, processes involving chondrogenesis, be altered by long term exposure to microgravity, but that continuous intervention will be necessary to correct any defects produced by altered gravity environments.

Animals↗

PubMed1999

Influence of gravity on the photomorphism of secondary moss protonemata.

In dark-grown plantlets of the moss, Pottia intermedia, negatively gravitropic secondary protonemata differentiate from the superficial cells of leafy shoots. When transferred to the light, distal parts of the protonemata nearest to the apical cells begin to ramify and the apical cells of the side branches as well as of the main protonemal filaments often differentiate as buds. Dark-grown protonemata were oriented horizontally and illuminated from below with white light of different intensities. Only light with an intensity of 4.5 micrometers m-2s-1 was sufficient to induce: (a) phototropism in the apical cells, (b) light-directed initiation of primordia, and (c) directed growth of side branches and bud differentiation. Apical cells illuminated with light of lower (0.03-0.37 micrometers m-2s-1 intensity grew upwards (i.e., away from the light). It was shown that this upward growth was determined by the action of gravity. Although initiation of branch primordia was only slightly affected, their growth was strongly stimulated on the upper side of the protonemata.

Bryopsida↗

PubMed2000

Fish otolith growth in 1g and 3g depends on the gravity vector.

Size and asymmetry (size difference between the left and the right side) as well as calcium (Ca) content of inner ear otoliths of larval cichlid fish Oreochromis mossambicus were determined after a long-term stay at hypergravity conditions (3g; centrifuge). Both utricular and saccular otoliths (lapilli and sagittae, respectively) were significantly smaller after hyper-g exposure as compared to parallely raised 1g-control specimens and the absolute amount of otolith-Ca was diminished. The asymmetry of sagittae was significantly increased in the experimental animals, whereas the respective asymmetry concerning lapilli was markedly decreased. In the course of another experiment larvae were raised in aquarium hatch baskets, from which one was placed directly above aeration equipment which resulted in random water circulation shifting the fish around ("shifted" specimens). The lapillar asymmetry of the "stationary" specimens showed a highly significant increase during early development when larvae were forced to lay on their sides due to their prominent yolk-sacs. In later developmental stages, when they began to swim freely, a dramatic decrease in lapillar asymmetry was apparent. Taken together with own previous findings according to which otolith growth stops after vestibular nerve transaction, the results presented here suggest that the growth and the development of bilateral asymmetry of otoliths is guided by the environmental gravity vector, obviously involving a feedback loop between the brain and the inner ear.

Animals↗

PubMed1998

Dimensional symmetry breaking, information and fractal gravity in Cantorian space.

We derive the exact expectation value and standard deviation for the dimensionality of Cantorian spacetime. Connections to time symmetry breaking, information and the generalization of complex numbers are considered. Finally a link between general relativity and Cantorian spacetime is proposed which amounts to the claim that fractalization is the origin of gravity.

Fractals↗

PubMed2001

The horizontal vestibulo-ocular reflex of hypergravity rat at different gravity levels.

The horizontal vestibulo-ocular reflex (VOR) of two groups of rats was measured. One group was bred and kept under hypergravity (HG; 2.5 g) conditions, the other group lived under normal gravity (NG; 1 g). Eye position was recorded in response to horizontal rotatory stimuli. Measurements were made under NG (1 g), and during parabolic flight (0.0 and 1.8 g). For both groups, the response to a rotatory stimulus during parabolic flight is similar to the response that was observed under 1 g conditions. In general, however, the VOR of HG rats is reduced by 20-50% relative to the response of NG rats and the phase is shifted by -40 degrees. We conjecture that this amplitude reduction and phase shift are the consequence of living in a rotating system.

Animals↗

PubMed1996

Is the center of gravity controlled during upper trunk movements?

The question was addressed in this study as to whether the kinematic synergy responsible for equilibrium control during upper trunk movements may involve an actual evaluation of the weight of the body segments. Five adult subjects were asked to bend the upper trunk forward or backward to an angle of about 35 degrees, first without any load and then with a 10 kg load fixed to their shoulders. The center of gravity (CG) shift in the sagittal direction which occurred at the end of the movement was compared versus without a load. Two patterns of CG shift were identified. In the first pattern shown by three subjects in forward and three in backward trunk bending, the CG shift remained the same, while in the other pattern (two subjects for forward and two for backward trunk bending), the CG shift increased by an amount corresponding to the purely mechanical effect of the load. These results indicate that the actual weight of the segments may be evaluated on the basis of sensory inputs and may be responsible for a change in the kinematic synergy which preserves the CG control during the upper trunk movement.

Adaptation, Physiological↗

PubMed1998

Are human anticipatory postural adjustments affected by a modification of the initial position of the center of gravity?

During a lateral leg raising task, the position of the center of gravity (CG) in the horizontal plane shifts towards the supporting leg prior to the movement onset. The aim of this study was to explore whether the anticipatory postural adjustments were calibrated as a function of the initial horizontal location of the CG. Experiments were performed on 8 healthy subjects, with three initial positions of the CG (close to the supporting leg, between the two legs, close to the moving leg). Simultaneous kinematic, kinetic and electromyographic (EMG) data were recorded with the ELITE. system. The results show that the duration of the kinetic variables and EMG pattern are scaled as a function of the distance covered by the CG and constitute the means of modulating the CG shift. They suggest that the evaluation of the support conditions is necessary to calibrate the CG shift, this is done during the early phase of the postural adjustments.

Adult↗

PubMed2000

The human vertical optokinetic afternystagmus and the effect of gravity during pitch tilt.

BACKGROUND: Findings concerning human vertical optokinetic afternystagmus (VOKAN) are often not consistent and in some cases even contradictory. METHOD: VOKAN was studied in nine normal subjects using corneo-retinal potential technique and an infrared video camera detection apparatus (ISCAN). VOKAN responses were measured at three angles of pitch head (and body) tilt: 0 degrees (upright), 90 degrees (supine) and 135 degrees (45 degrees below horizontal). The optokinetic stimulus was stripe movement with a velocity of 60 degrees/s. RESULTS: Upward slow phase velocity (SP-Up) VOKAN was evoked in 6/9 subjects (67%) at 0 degrees tilt and at 90 degrees tilt, and in 4/9 (44%) at 135 degrees tilt. Downward slow phase velocity (SP-Down) VOKAN was elicited in 3/9 subjects (33%) at 0 degrees tilt; in 2/9 subjects (22%) at 90 degrees tilt and in 0/9 subjects (0%) at 135 degrees tilt. CONCLUSION: The pitch tilt, as a gravity effect, influences significantly on the appearance and the duration of VOKAN.

Adult↗