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PubMed2004

Importance of direction of vibration on the onset of Soret-driven convection under gravity or weightlessness.

This paper considers the influence of the direction of vibration on the stability threshold of two-dimensional Soret-driven convection. The configuration is an infinite layer filled with a binary mixture, which can be heated from below or from above. The limiting case of high-frequency and small-amplitude vibration is considered for which the time-averaged formulation has been adopted. The linear stability analysis of the quasi-mechanical equilibrium shows that the problem depends on five non-dimensional parameters. These include the thermal Rayleigh number (Ra(T)), the vibrational parameter (R), the Prandtl number (Pr), the Lewis number (Le), the separation ratio (S) and the orientation of vibration with respect to the horizontal heated plate (alpha). For different sets of parameters, the bifurcation diagrams are plotted Ra(c) = f(S) and k(c) = g(S), which are the critical thermal Rayleigh and the critical wave numbers, respectively. Our results indicate that, relative to the classical case of static gravity, vibration may affect all regions in Ra(c)-S stability diagram. In the case of mono-cellular convection, by using a regular perturbation method, a closed-form relation for the critical Rayleigh number is found. Several physical situations in the presence or in the absence of gravity (micro-gravity) are discussed.

Journal Article↗
PubMed1990

Vector-free gravity disrupts synapse formation in cell culture.

Terrestrial organisms evolved under and are subjected to the constancy of gravity. The organisms having adapted to this environmental factor, it is possible that embryonic development may be modified by exposure to altered gravity. To test the effects of gravity on embryonic development, we monitored the formation of nerve-associated acetylcholine receptor patches (NARPs) as an index of synaptogenesis. Embryonic spinal neuron and myotomal myocyte cocultures were placed in a horizontally rotating clinostat. From the cell's perspective, this results in the cancellation of the gravitational vector because of continuous averaging, thus mimicking the reduced gravitational force encountered in space. NARPs from cultures in which nerve-muscle contact was established before the onset of rotation were unaffected. In contrast, cultures in which nerve contact took place during rotation showed a marked inhibition of NARPs. Moreover, in the myocytes which did exhibit NARPs, the area of the patch was significantly reduced compared with control sister cultures. Several paradigms were used to ascertain that these findings did not result simply from loss of contact between neurites and myocytes, accelerated diffusion of a putative aggregating factor secreted by neurites, or from turbulence in the medium. Our data suggest that the process of synapse formation is sensitive to the gravitational vector. Embryonic development of the nervous system, in space, may therefore be markedly different from that normally occurring on earth.

Animals↗
PubMed1995

Influence of gravity and body position on normal oropharyngeal swallowing.

The influence of gravity, if any, on pharyngeal bolus transport is unknown. The aim of the present study was to evaluate the effect of gravity and body position on the radiological and manometric events during the normal oropharyngeal swallow. In eight healthy male volunteers, we performed simultaneous videoradiography and manometry, using a manometry catheter incorporating a perfused sleeve measuring upper esophageal sphincter pressure and three solid-state transducers recording pharyngeal pressures. Swallows of 5 and 10 ml of barium were performed in three different body positions: upright, horizontal, and 30 degrees head down (inverted). Hypopharyngeal intrabolus pressure increased significantly in the horizontal and inverted positions compared with upright, P = 0.0001. This resulted in increased maximal sphincter diameters during bolus flow (P = 0.0001) and shorter duration of sphincter opening (P = 0.03). As a result, transsphincteric flow increased in the horizontal and inverted positions, P = 0.04. Total swallowing duration, oral and pharyngeal transit time, pharyngeal peristaltic amplitude and duration, the length of the bolus in the pharynx, and excursions of the hyoid and larynx were unaffected by body position. We conclude that intrabolus pressure is an important determinant of upper esophageal sphincter opening in the healthy oropharynx and that gravity does not influence pharyngeal bolus transport. The healthy upper esophageal sphincter has residual opening capacity that can be demonstrated by altering body posture.

Adult↗
PubMed1996

Cutaneous microvascular flow in the foot during simulated variable gravities.

Our objective was to understand how weight bearing with varying gravitational fields affects blood perfusion in the sole of the foot. Human subjects underwent whole body tilting at four angles: upright [1 gravitational vector from head to foot (Gz)], 22 degrees (0.38 Gz), 10 degrees (0.17 Gz), and supine (0 Gz), simulating the gravitational fields of Earth, Mars, Moon, and microgravity, respectively. Cutaneous capillary blood flow was monitored on the plantar surface of the heel by laser Doppler flowmetry while weight-bearing load was measured. At each tilt angle, subjects increased weight bearing on one foot in graded load increments of 1 kg beginning with zero. The weight bearing at which null flow first occurred was determined as the closing load. Subsequently, the weight bearing was reduced in reverse steps until blood flow returned (opening load). Mean closing loads for simulated Earth gravity, Mars gravity, Moon gravity, and microgravity were 9.1, 4.6, 4.4, and 3.6 kg, respectively. Mean opening loads were 7.9, 4.1, 3.5, and 3.1 kg, respectively. Mean arterial pressures in the foot (MAP(foot)) calculated for each simulated gravitational field were 192, 127, 106, and 87 mmHg, respectively. Closing load and opening load were significantly correlated with MAP(foot) (r =0.70, 0.72, respectively) and were significantly different (P < 0.001) from each other. The data suggest that decreased local arterial pressure in the foot lowers tolerance to external compression. Consequently, the human foot sole may be more prone to cutaneous ischemia during load bearing in microgravity than on Earth.

Adolescent↗
PubMed1987

Gravity-independent inequality in pulmonary blood flow in humans.

Single-photon emission computerized tomography of the lung with 99mTc-labeled human albumin macroaggregates (99mTc-MAA) was used in six healthy subjects to study the three-dimensional distribution of pulmonary blood flow. 99mTc-MAA was injected while the subjects were resting in the supine position and holding their lung volume at normal end expiration. Tomography was performed on each subject from 120 projections of radioactivity in the lungs acquired with a rotating gamma camera. To minimize lung motion artifacts, the subjects were asked to hold their breath at end expiration during the 10-s duration of data acquisition in each projectional angle. Perfusion images of lung slices (11 mm thick) were reconstructed, and the radioactivity within each slice was expressed per unit lung volume of 3.7 X 3.7 X 11 mm. Perfusion images of a midcoronal slice from each subject manifested a concentric pattern of radioactivity that decreased significantly from the center to the periphery, suggesting that blood flow rate per unit lung volume was up to 10 times larger near the central region. This gradient in activity between the center and the periphery of the coronary slices was gravity independent as the subjects were supine. Images of sagittal slices from the middle of the right lung also manifested a similar pattern of concentric gradient in activity, with the vertical distribution (gravity related) almost comparable with the horizontal distribution (gravity independent). These results indicate that pulmonary blood flow in resting supine humans is spatially stratified with a marked central-to-peripheral gradient in all directions. It appears that zone 4 (reduced blood flow) is not a phenomenon limited to the dependent region of the lung as commonly thought but rather is a manifestation of this spatial distribution whereby blood flow is lowest in all peripheral regions of the lung.

Adult↗
PubMed1991

Relative contribution of gravity to pulmonary perfusion heterogeneity.

We designed a series of experiments and analyses to quantify the contribution of gravity to pulmonary perfusion heterogeneity. Regional pulmonary perfusion was measured in five anesthetized and ventilated dogs in both supine and prone positions by use of radiolabeled microspheres injected during apnea at functional residual capacity. Measurements of flow were repeated in each position, and the sequence of positions was prospectively designed to nullify any effect of order. The lungs of each animal were excised, perfused with saline until clear, dried at an inflation pressure of 25 cmH2O, and cut into 1.9-cm3 pieces. Each piece was weighed and the radioactivity determined in a scintillation counter. Measurement errors were minimized by excluding lung pieces that had greater than 25% airway and weighed less than 10 mg or greater than 60 mg. Weight-normalized flows in each position and repetition were determined for each lung piece. An analysis of variance model was used to identify the percentage of variation in regional flow that was due to position (supine vs. prone), to random error and time (measurement and repetition), and to structure, where structure was defined as the component of flow that remained constant across position and replication. The contributions of position, error/time, and structure to the total variability of flow across the five dogs were 7.8 +/- 0.6, 8.4 +/- 8.3, and 83.8 +/- 8.4%, (SD), respectively. Because the contribution of position represents the additive effect of gravity between two opposite positions, the contribution of gravity to perfusion heterogeneity in one position may be as little as 4%.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Supine exercise during lower body negative pressure effectively simulates upright exercise in normal gravity.

Exercise within a lower body negative pressure (LBNP) chamber in supine posture was compared with similar exercise against Earth's gravity (without LBNP) in upright posture in nine healthy male volunteers. We measured footward force with a force plate, pressure in soleus and tibialis anterior muscles of the leg with transducer-tipped catheters, calf volume by strain gauge plethysmography, heart rate, and systolic and diastolic blood pressures during two conditions: 1) exercise in supine posture within an LBNP chamber during 100-mmHg LBNP (exercise-LBNP) and 2) exercise in upright posture against Earth's gravity without LBNP (exercise-1 G). Subjects exercised their ankle joints (dorsi- and plantarflexions) for 5 min during exercise-LBNP and for 5 min during exercise-1 G. Mean footward force produced during exercise-LBNP (743 +/- 37 N) was similar to that produced during exercise-1 G (701 +/- 24 N). Peak contraction pressure in the antigravity soleus muscle during exercise-LBNP (115 +/- 10 mmHg) was also similar to that during exercise-1 G (103 +/- 13 mmHg). Calf volume increased significantly by 3.3 +/- 0.5% during exercise-LBNP compared with baseline values. Calf volume did not increase significantly during exercise-1 G. Heart rate was significantly higher during exercise-LBNP (99 +/- 5 beats/min) than during exercise-1 G (81 +/- 3 beats/min). These results indicate that exercise in supine posture within an LBNP chamber can produce similar musculoskeletal stress in the legs and greater systemic cardiovascular stress than exercise in the upright posture against Earth's gravity.

Adult↗
PubMed1996

Pulmonary blood flow distribution in standing horses is not dominated by gravity.

Recent studies using microspheres in dogs, pigs and goats have demonstrated considerable heterogeneity of pulmonary perfusion within isogravitational planes. These studies demonstrate a minimal role of gravity in determining pulmonary blood flow distribution. To test whether a gravitational gradient would be more apparent in an animal with large vertical lung height, we measured perfusion heterogeneity in horses (vertical lung height = approximately 55 cm). Four unanesthetized Thoroughbred geldings (422-500 kg) were studied awake in the standing position with fluorescent microspheres injected into a central vein. Between 1,621 and 2,503 pieces (1.3 cm3 in volume) were obtained from the lungs of each horse with spatial coordinates, and blood flow was determined for each piece. The coefficient of variation of blood flow throughout the lungs ranged between 22 and 57% among the horses. Considerable heterogeneity was seen in each isogravitational plane. The relationship between blood flow and vertical height up the lung was characterized by the slope and correlation coefficient of a least squares regression analysis. The slopes within each horse ranged from -0.052 to +0.021 relative flow units/cm height up the lung, and the correlation coefficients varied from 0.12 to 0.75. A positive slope, indicating that flow increased with vertical distance up the lung (opposite to gravity), was observed in three of the four horses. In addition, blood flow was uniformly low in three of the four horses in the most cranial portions of the lungs. We conclude that in lungs of resting unanesthetized horses, animals with a large lung height, there is no consistent vertical gradient to pulmonary blood flow and there is a considerable degree of perfusion heterogeneity, indicating that gravity alone does not play the major role in determining blood flow distribution.

Animals↗
PubMed1998

Choroidal readaptation to gravity in rats after spaceflight and head-down tilt.

To determine when choroidal structures were restored after readaptation to Earth gravity or orthostatic position, fine structure and protein distribution were studied in rat choroid plexus dissected either 6 h [Space Life Sciences-2 (SLS-2) experiments] or 2 days [National Institutes of Health-Rodent 1 (NIH-R1) experiments] after a spaceflight, or 6 h after head-down tilt (HDT) experiments. Apical alterations were noted in choroidal cells from SLS-2 and HDT animals, confirming that weightlessness impaired choroidal structures and functions. However, the presence of small apical microvilli and kinocilia and the absence of vesicle accumulations showed that the apical organization began to be restored rapidly after landing. Very enlarged apical microvilli appeared after 2 days on Earth, suggesting increased choroidal activity. However, as distributions of ezrin and carbonic anhydrase II remained altered in both flight and suspended animals after readaptation to Earth gravity, it was concluded that choroidal structures and functions were not completely restored, even after 2 days in Earth's gravity.

Adaptation, Physiological↗
PubMed1998

Gravity effects on upper airway area and lung volumes during parabolic flight.

We measured upper airway caliber and lung volumes in six normal subjects in the sitting and supine positions during 20-s periods in normogravity, hypergravity [1.8 + head-to-foot acceleration (Gz)], and microgravity ( approximately 0 Gz) induced by parabolic flights. Airway caliber and lung volumes were inferred by the acoustic reflection method and inductance plethysmography, respectively. In subjects in the sitting position, an increase in gravity from 0 to 1. 8 +Gz was associated with increases in the calibers of the retrobasitongue and palatopharyngeal regions (+20 and +30%, respectively) and with a concomitant 0.5-liter increase in end-expiratory lung volume (functional residual capacity, FRC). In subjects in the supine position, no changes in the areas of these regions were observed, despite significant decreases in FRC from microgravity to normogravity (-0.6 liter) and from microgravity to hypergravity (-0.5 liter). Laryngeal narrowing also occurred in both positions (about -15%) when gravity increased from 0 to 1.8 +Gz. We concluded that variation in lung volume is insufficient to explain all upper airway caliber variation but that direct gravity effects on tissues surrounding the upper airway should be taken into account.

Adult↗
PubMed1999

Gravity is an important but secondary determinant of regional pulmonary blood flow in upright primates.

Original studies leading to the gravitational model of pulmonary blood flow and contemporary studies showing gravity-independent perfusion differ in the recent use of laboratory animals instead of humans. We explored the distribution of pulmonary blood flow in baboons because their anatomy, serial distribution of vascular resistances, and hemodynamic responses to hypoxia are similar to those of humans. Four baboons were anesthetized with ketamine, intubated, and mechanically ventilated. Different colors of fluorescent microspheres were given intravenously while the animals were in the supine, prone, upright (repeated), and head-down (repeated) postures. The animals were killed, and their lungs were excised, dried, and diced into approximately 2-cm3 pieces with the spatial coordinates recorded for each piece. Regional blood flow was determined for each posture from the fluorescent signals of each piece. Perfusion heterogeneity was greatest in the upright posture and least when prone. Using multiple-stepwise regression, we estimate that 7, 5, and 25% of perfusion heterogeneity is due to gravity in the supine, prone, and upright postures, respectively. Although important, gravity is not the predominant determinant of pulmonary perfusion heterogeneity in upright primates. Because of anatomic similarities, the same may be true for humans.

Algorithms↗
PubMed2000

Selected contribution: redistribution of pulmonary perfusion during weightlessness and increased gravity.

To compare the relative contributions of gravity and vascular structure to the distribution of pulmonary blood flow, we flew with pigs on the National Aeronautics and Space Administration KC-135 aircraft. A series of parabolas created alternating weightlessness and 1.8-G conditions. Fluorescent microspheres of varying colors were injected into the pulmonary circulation to mark regional blood flow during different postural and gravitational conditions. The lungs were subsequently removed, air dried, and sectioned into approximately 2 cm(3) pieces. Flow to each piece was determined for the different conditions. Perfusion heterogeneity did not change significantly during weightlessness compared with normal and increased gravitational forces. Regional blood flow to each lung piece changed little despite alterations in posture and gravitational forces. With the use of multiple stepwise linear regression, the contributions of gravity and vascular structure to regional perfusion were separated. We conclude that both gravity and the geometry of the pulmonary vascular tree influence regional pulmonary blood flow. However, the structure of the vascular tree is the primary determinant of regional perfusion in these animals.

Animals↗
PubMed2005

Anticipating the effects of gravity when intercepting moving objects: differentiating up and down based on nonvisual cues.

Intercepting an object requires a precise estimate of its time of arrival at the interception point (time to contact or "TTC"). It has been proposed that knowledge about gravitational acceleration can be combined with first-order, visual-field information to provide a better estimate of TTC when catching falling objects. In this experiment, we investigated the relative role of visual and nonvisual information on motor-response timing in an interceptive task. Subjects were immersed in a stereoscopic virtual environment and asked to intercept with a virtual racket a ball falling from above or rising from below. The ball moved with different initial velocities and could accelerate, decelerate, or move at a constant speed. Depending on the direction of motion, the acceleration or deceleration of the ball could therefore be congruent or not with the acceleration that would be expected due to the force of gravity acting on the ball. Although the best success rate was observed for balls moving at a constant velocity, we systematically found a cross-effect of ball direction and acceleration on success rate and response timing. Racket motion was triggered on average 25 ms earlier when the ball fell from above than when it rose from below, whatever the ball's true acceleration. As visual-flow information was the same in both cases, this shift indicates an influence of the ball's direction relative to gravity on response timing, consistent with the anticipation of the effects of gravity on the flight of the ball.

Attention↗
PubMed1991

Spatial orientation of the vestibular system: dependence of optokinetic after-nystagmus on gravity.

1. Monkeys received optokinetic stimulation at 60 degrees/s about their yaw (animal vertical) and pitch (animal horizontal) axes, as well as about other head-centered axes in the coronal plane. The animals were upright or tilted in right-side-down positions with regard to gravity. The stimuli induced horizontal, vertical, and oblique optokinetic nystagmus (OKN). OKN was followed by optokinetic after-nystagmus (OKAN), which was recorded in darkness. 2. When monkeys were tilted, stimulation that generated horizontal or yaw axis eye velocity during OKN induced a vertical or pitch component of slow phase velocity during OKAN. This has been designated as "cross-coupling" of OKAN. Eigenvalues and eigenvectors associated with the system generating OKAN were found as a function of tilt. They were determined by use of the Levenberg-Marquardt algorithm to minimize the mean square error between the output of a model of OKAN and the data. 3. The eigenvector associated with yaw OKAN (yaw axis eigenvector) was maintained close to the spatial vertical regardless of the angle of tilt. The eigenvector associated with pitch OKAN (pitch axis eigenvector) was always aligned with the body axis. The data indicate that velocity storage can be modeled by a piecewise linear system, the structure of which is dependent on gravity and the yaw axis eigenvector, which tends to align with gravity. 4. Yaw axis eigenvectors were also determined by giving optokinetic stimulation about head-centered axes in the coronal plane with the animal in various angles of tilt. A technique using a spectral analysis of residuals was developed to estimate whether yaw and pitch OKAN slow phase velocities decayed concurrently at the same relative rate and over the same time course. The eigenvectors determined by this method were in agreement with those obtained by analyzing OKAN elicited by yaw OKN. 5. During yaw OKN with the animal in tilted positions, the mean vector of the ensuing nystagmus was closer to the body axis than to the spatial vertical. This suggests that there is suppression of the cross-coupled pitch component during OKN. The direction of the stimulus may be utilized to suppress components of velocity storage not coincident with the direction of stimulus motion. 6. There were similarities between the monkey eigenvectors and human perception of the spatial vertical, and the mean of eigenvectors for upward and downward eye velocities overlay human 1-g perceptual data.(ABSTRACT TRUNCATED AT 400 WORDS)

Acceleration↗
PubMed2005

Self-gravity driven instabilities at accelerated interfaces.

Nonlinear hydrodynamic flows are ubiquitous in the interstellar medium (ISM). Such flows play an important role in shaping atomic and molecular clouds and determining the initial conditions for star formation. One mechanism by which nonlinear flows arise is the onset and growth of interfacial instabilities. Any interface of discontinuous density is subject to a host of instabilities, including Rayleigh-Taylor, Kelvin-Helmholtz, and Richtmyer-Meshkov. As part of an ongoing study of structure formation in the ISM, Hunter, Whitaker, and Lovelace discovered an additional density interface instability. This instability is driven by self-gravity and termed the self-gravity interfacial instability (SGI). The SGI causes any displacement of the interface to grow on roughly a free-fall time scale, even when the perturbation wavelength is much less than the Jeans length. Numerical simulations have confirmed the expectations of linear theory, including the near scale invariance of the growth rate. Here, we build upon previous work by considering an initial condition in which the acceleration due to self-gravity is non-zero at the interface.

Journal Article↗
PubMed1994

GRAVITY AND LIGHT INFLUENCE THE COUNTERSHADING REFLEXES OF THE CUTTLEFISH SEPIA OFFICINALIS

Rotation (roll or pitch) of a cuttlefish away from its normal orientation produces countershading reflexes (CSRs) that consist of chromatophore expansion on the ventral body surface. When rotation is in the roll plane, the CSR has two components on each side of the body. The first (component A) consists of a unilateral expansion of chromatophores on the uppermost latero-ventral edge of the mantle, the underside of the upper fin and the uppermost side of the head; it occurs when the angle of rotation is less than 90&deg;. Further rotation (from approximately 90&deg; to approximately 180&deg;) adds the second component (component B): a unilateral expansion of the chromatophores on the upper half of the ventral surface of the mantle, funnel, head and arms. When rotation is in the pitch plane, chromatophores expand on the posterior part of the ventral mantle and fins when the head is down; when the head is up, chromatophores expand on the ventral surface of the arms, head and funnel and on the anterior part of the ventral mantle and fins. The pitch CSR is always bilateral. Destruction of the gravity or the angular acceleration receptor systems of the statocysts demonstrates that it is the gravity receptor systems that drive the CSRs. Unilateral destruction of the gravity receptor systems shows that the pitch CSR is driven bilaterally, whereas the roll CSR is driven unilaterally. Components A and B of the roll CSR are driven by input from the ipsilateral statocyst, but component A is additionally driven by light. Brain lesions provide evidence that the pathways for the CSRs run through the lateral basal lobes in the supraoesophageal part of the brain.

Journal Article↗
PubMed1984

Observations on the response of human spermatozoa to gravity, boundaries and fluid shear.

Human sperm motility response to three mechanical stimuli, gravity, fluid flow shear and rigid boundaries, was measured in a tube of 310 X 400 microns calibre. Data were gathered by cine recordings at various focussing levels d across the tube and analysed with a computerized image analysis system. The most influential stimulus was the tube wall near (more than 'at') which the swimmers tended to accumulate, leaving the fluid beyond 100 microns from the wall (d = 100) vacant of motile spermatozoa. The boundary effect was evident as soon as the spermatozoa could be viewed after loading, and accumulation, measured as frequency, as a function of d did not change with time t. This response was not significantly altered by the addition of laminar flow with a centre line velocity of about 400 microns/sec. In flow shear, spermatozoa aligned positively (in the flow direction) at the wall but negatively by about 30 microns from the wall where the velocity gradient (= shear rate) was about 3.5 sec-1. The response to gravity was relatively weak with 11 spermatozoa positive (swimming downwards) for each 9 negative. Neither the boundary effect nor the 'rheotaxic' effect were influenced by gravity as there was no statistical difference in orientation or distribution patterns between vertically and horizontally flowing suspensions. It is suggested that the boundary effect cannot be ignored in in-vitro manipulations, particularly when spermatozoa are observed or extracted. Its importance in vivo lies in the degree to which the tubes transporting motile spermatozoa seem to have mechanisms for reversing the wall accumulation tendency.

Computers↗
PubMed1969

Stimulus generalization of gravity.

In two experiments, squirrel monkeys were exposed to centrifugally generated, artificial gravity and trained to respond for food reinforcement at selected gravity (g) levels. Experiment I involved a single g value; in Exp. II, subjects were trained to discriminate among two or three g values. After training, generalization tests were administered over a 1.1-g to 2.1-g range. The major findings were as follows: (a) single-stimulus training yielded a linear relationship between percentage of responding and magnitude of gravity. (b) Two-valued discrimination training produced gradient peaks which were shifted from S(D) in a direction away from S(Delta). This effect was cancelled when S(D) was located equidistant between two S(Delta) stimuli. (c) Gradient form was independent of the S(D)-S(Delta) difference, but related to continuum location and/or intensity of discriminative stimuli.

Animals↗