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At least 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Plastid sedimentation kinetics in roots of wild-type and starch-deficient mutants of Arabidopsis

Sedimentation and movement of plastids in columella cells of the root cap were measured in seedlings of wild-type, a reduced starch mutant, and a starchless mutant of Arabidopsis. To assay for sedimentation, we used both linear measurements and the change of angle from the cell center as indices in vertical and reoriented plants with the aid of computer-assisted image analysis. Seedlings were fixed at short periods after reorientation, and plastid sedimentation correlated with starch content in the three strains of Arabidopsis. Amyloplasts of wild-type seedlings showed the greatest sedimentation, whereas plastids of the starchless mutant showed no significant sedimentation in the vertically grown and reoriented seedlings. Because previous research has shown that a full complement of starch is needed for full gravitropic sensitivity, this study correlates increased sensitivity with plastid sedimentation. However, although plastid sedimentation contributed to gravisensitivity, it was not required, because the gravitropic starchless mutant had plastids that did not sediment. This is the first study, to our knowledge, to measure plastid sedimentation in Arabidopsis roots after reorientation of seedlings. Taken together, the results of this study are consistent with the classic plastid-based and protoplast-based models of graviperception and suggest that multiple systems of perception exist in plant cells.

NASA Discipline Plant Biology↗
NASA NTRS1998

Relationship between selected orientation rest frame, circular vection and space motion sickness

Space motion sickness (SMS) and spatial orientation and motion perception disturbances occur in 70-80% of astronauts. People select "rest frames" to create the subjective sense of spatial orientation. In microgravity, the astronaut's rest frame may be based on visual scene polarity cues and on the internal head and body z axis (vertical body axis). The data reported here address the following question: Can an astronaut's orientation rest frame be related and described by other variables including circular vection response latencies and space motion sickness? The astronaut's microgravity spatial orientation rest frames were determined from inflight and postflight verbal reports. Circular vection responses were elicited by rotating a virtual room continuously at 35 degrees/s in pitch, roll and yaw with respect to the astronaut. Latency to the onset of vection was recorded from the time the crew member opened their eyes to the onset of vection. The astronauts who used visual cues exhibited significantly shorter vection latencies than those who used internal z axis cues. A negative binomial regression model was used to represent the observed total SMS symptom scores for each subject for each flight day. Orientation reference type had a significant effect, resulting in an estimated three-fold increase in the expected motion sickness score on flight day 1 for astronauts who used visual cues. The results demonstrate meaningful classification of astronauts' rest frames and their relationships to sensitivity to circular vection and SMS. Thus, it may be possible to use vection latencies to predict SMS severity and duration.

short duration↗
NASA NTRS1998

Isolation and characterization of Arabidopsis mutants defective in the induction of ethylene biosynthesis by cytokinin

Cytokinins elevate ethylene biosynthesis in etiolated Arabidopsis seedlings via a post-transcriptional modification of one isoform of the key biosynthetic enzyme ACC synthase. In order to begin to dissect the signaling events leading from cytokinin perception to this modification, we have isolated a series of mutants that lack the ethylene-mediated triple response in the presence of cytokinin due to their failure to increase ethylene biosynthesis. Analysis of genetic complementation and mapping revealed that these Cin mutants (cytokinin-insensitive) represent four distinct complementation groups, one of which, cin4, is allelic to the constitutive photomorphogenic mutant fus9/cop10. The Cin mutants have subtle effects on the morphology of adult plants. We further characterized the Cin mutants by analyzing ethylene biosynthesis in response to various other inducers and in adult tissues, as well as by assaying additional cytokinin responses. The cin3 mutant did not disrupt ethylene biosynthesis under any other conditions, nor did it disrupt any other cytokinin responses. Only cin2 disrupted ethylene biosynthesis in multiple circumstances. cin1 and cin2 made less anthocyanin in response to cytokinin. cin1 also displayed reduced shoot initiation in tissue culture in response to cytokinin, suggesting that it affects a cytokinin signaling element.

Non-NASA Center↗
NASA NTRS1998

Nonphotic entrainment of the human circadian pacemaker

In organisms as diverse as single-celled algae and humans, light is the primary stimulus mediating entrainment of the circadian biological clock. Reports that some totally blind individuals appear entrained to the 24-h day have suggested that nonphotic stimuli may also be effective circadian synchronizers in humans, although the nonphotic stimuli are probably comparatively weak synchronizers, because the circadian rhythms of many totally blind individuals "free run" even when they maintain a 24-h activity-rest schedule. To investigate entrainment by nonphotic synchronizers, we studied the endogenous circadian melatonin and core body temperature rhythms of 15 totally blind subjects who lacked conscious light perception and exhibited no suppression of plasma melatonin in response to ocular bright-light exposure. Nine of these fifteen blind individuals were able to maintain synchronization to the 24-h day, albeit often at an atypical phase angle of entrainment. Nonphotic stimuli also synchronized the endogenous circadian rhythms of a totally blind individual to a non-24-h schedule while living in constant near darkness. We conclude that nonphotic stimuli can entrain the human circadian pacemaker in some individuals lacking ocular circadian photoreception.

NASA Discipline Regulatory Physiology↗
NASA NTRS1997

Light-regulated root gravitropism: a role for, and characterization of, a calcium/calmodulin-dependent protein kinase homolog

Roots of many species grow downward (orthogravitropism) only when illuminated. Previous work suggests that this is a calcium-regulated response and that both calmodulin and calcium/calmodulin-dependent kinases participate in transducing gravity and light stimuli. A genomic sequence has been obtained for a calcium/calmodulin-dependent kinase homolog (MCK1) expressed in root caps, the site of perception for both light and gravity. This homolog consists of 7265 base pairs and contains 11 exons and 10 introns. Since MCK1 is expressed constitutively in both light and dark, it is unlikely that the light directly affects MCK1 expression, though the activity of the protein may be affected by light. In cultivars showing light-regulated gravitropism, we hypothesize that MCK1, or a homolog, functions in establishing the auxin asymmetry necessary for orthogravitropism.

NASA Discipline Plant Biology↗
NASA NTRS1997

A novel culture morphology resulting from applied mechanical strain

To demonstrate that cells both perceive and respond to external force, a strain/relaxation regimen was applied to normal human fetal and aged dermal fibroblasts cultured as monolayers on flexible membranes. The precisely controlled protocol of stretch (20% elongation of the culture membrane) at 6.67 cycles/min caused a progressive change in the monolayers, such that the original randomly distributed pattern of cells became a symmetric, radial distribution as the cell bodies aligned parallel to the applied force. High cell density interfered with the success of re-alignment in the fetal cell cultures observed, which may reflect a preference in this cell strain for cell-cell over cell-matrix contacts. The chronologically aged cells observed did not demonstrate this feature, aligning efficiently at all seeding densities examined. The role of microfilaments in force perception and transmission was investigated through the addition of cytochalasin D in graded doses. Both intercellular interactions and cytoskeletal integrity mediate the morphological response to mechanical strain.

NASA Discipline Cell Biology↗
NASA NTRS1997

Spaceflight exposure effects on transcription, activity, and localization of alcohol dehydrogenase in the roots of Arabidopsis thaliana

Although considerable research and speculation have been directed toward understanding a plant's perception of gravity and the resulting gravitropic responses, little is known about the role of gravity-dependent physical processes in normal physiological function. These studies were conducted to determine whether the roots of plants exposed to spaceflight conditions may be experiencing hypoxia. Arabidopsis thaliana (L.) Heynh. plants were grown in agar medium during 6 or 11 d of spaceflight exposure on shuttle missions STS-54 (CHROMEX-03) and STS-68 (CHROMEX-05), respectively. The analysis included measurement of agar redox potential and root alcohol dehydrogenase (ADH) activity, localization, and expression. ADH activity increased by 89% as a result of spaceflight exposure for both CHROMEX-03 and -05 experiments, and ADH RNase protection assays revealed a 136% increase in ADH mRNA. The increase in ADH activity associated with the spaceflight roots was realized by a 28% decrease in oxygen availability in a ground-based study; however, no reduction in redox potential was observed in measurements of the spaceflight bulk agar. Spaceflight exposure appears to effect a hypoxic response in the roots of agar-grown plants that may be caused by changes in gravity-mediated fluid and/or gas behavior.

STS-68 Shuttle Project↗
NASA NTRS1997

Mechanical signals in plant development: a new method for single cell studies

Cell division, which is critical to plant development and morphology, requires the orchestration of hundreds of intracellular processes. In the end, however, cells must make critical decisions, based on a discrete set of mechanical signals such as stress, strain, and shear, to divide in such a way that they will survive the mechanical loads generated by turgor pressure and cell enlargement within the growing tissues. Here we report on a method whereby tobacco protoplasts swirled into a 1.5% agarose entrapment medium will survive and divide. The application of a controlled mechanical load to agarose blocks containing protoplasts orients the primary division plane of the embedded cells. Photoelastic analysis of the agarose entrapment medium can identify the lines of principal stress within the agarose, confirming the hypothesis that cells divide either parallel or perpendicular to the principal stress tensors. The coincidence between the orientation of the new division wall and the orientation of the principal stress tensors suggests that the perception of mechanical stress is a characteristic of individual plant cells. The ability of a cell to determine a shear-free orientation for a new partition wall may be related to the applied load through the deformation of the matrix material. In an isotropic matrix a uniaxial load will produce a rotationally symmetric strain field, which will define a shear-free plane. Where high stress intensities combine with the loading geometry to produce multiaxial loads there will be no axis of rotational symmetry and hence no shear free plane. This suggests that two mechanisms may be orienting the division plane, one a mechanism that works in rotationally symmetrical fields, yielding divisions perpendicular to the compressive tensor, parallel to the long axis of the cell, and one in asymmetric fields, yielding divisions parallel to the short axis of the cell and the compressive tensor.

NASA Discipline Number 40-50↗
NASA NTRS1996

The barberplaid illusion: plaid motion is biased by elongated apertures

The perceived direction of motion of plaids windowed by elongated spatial Gaussians is biased toward the window's long axis. The bias increases as the relative angle between the plaid motion and the long axis of the window increases, peaks at a relative angle of approximately 45 deg, and then decreases. The bias increases as the window is made narrower (at fixed height) and decreases as the component spatial frequency increases (at fixed aperture size). We examine several models of human motion processing (cross-correlation, motion-energy, intersection-of-constraints, and vector-sum), and show that none of these standard models can predict our data. We conclude that spatial integration of motion signals plays a crucial role in plaid motion perception and that current models must be explicitly expanded to include such spatial interactions.

NASA Center ARC↗
NASA NTRS1994

Space flight and neurovestibular adaptation

Space flight represents a form of sensory stimulus rearrangement requiring modification of established terrestrial response patterns through central reinterpretation. Evidence of sensory reinterpretation is manifested as postflight modifications of eye/head coordination, locomotor patterns, postural control strategies, and illusory perceptions of self or surround motion in conjunction with head movements. Under normal preflight conditions, the head is stabilized during locomotion, but immediately postflight reduced head stability, coupled with inappropriate eye/head coordination, results in modifications of gait. Postflight postural control exhibits increased dependence on vision which compensates for inappropriate interpretation of otolith and proprioceptive inputs. Eye movements compensatory for perceived self motion, rather than actual head movements have been observed postflight. Overall, the in-flight adaptive modification of head stabilization strategies, changes in head/eye coordination, illusionary motion, and postural control are maladaptive for a return to the terrestrial environment.

Review, Tutorial↗
NASA NTRS2003

Evaluation of Neutral Body Posture on Shuttle Mission STS-57 (SPACEHAB-1)

Research has shown that the space environment induces physiological changes in the human body, such as fluid shifts in the upper body and chest cavity, spinal lengthening, muscular atrophy, space motion sickness, cardiopulmonary deconditioning, and bone mass loss, as well as some changes in visual perception. These require a period of adaptation and can substantially affect both crew member performance and posture. These physiological effects, when work activities are conducted, have been known to impact the body's center of gravity, reach, flexibility, and dexterity. All these aspects of posture must be considered to safely and efficiently design space systems and hardware. NASA has documented its microgravity body posture in the Man-Systems Integration Standards (MSIS); the space community uses the MSIS posture to design workstations and tools for space application. However, the microgravity body posture should be further investigated for several reasons, including small sample size in previous studies, possible imprecision, and lack of detail. JSC undertook this study to investigate human body posture exhibited under microgravity conditions. STS-57 crew members were instructed to assume a relaxed posture that was not oriented to any work area or task. Crew members were asked to don shorts and tank tops and to be blindfolded while data were recorded. Video data were acquired once during the mission from each of the six crew members. No one crew member exhibited the typical NBP called out in the MSIS; one composite posture is not adequate. A range of postures may be more constructive for design purposes. Future evaluations should define precise posture requirements for workstation, glove box, maintenance, foot-restraint, and handhold activities.

Mount, Frances E.↗
NASA NTRS1995

Looking around: 35 years of oculomotor modeling

Eye movements have attracted an unusually large number of researchers from many disparate fields, especially over the past 35 years. The lure of this system stemmed from its apparent simplicity of description, measurement, and analysis, as well as the promise of providing a "window in the mind." Investigators in areas ranging from biological control systems and neurological diagnosis to applications in advertising and flight simulation expected eye movements to provide clear indicators of what the sensory-motor system was accomplishing and what the brain found to be of interest. The parallels between compensatory eye movements and perception of spatial orientation have been a subject for active study in visual-vestibular interaction, where substantial knowledge has accumulated through experiments largely guided by the challenge of proving or disproving model predictions. Even though oculomotor control has arguably benefited more from systems theory than any other branch of motor control, many of the original goals remain largely unfulfilled. This paper considers some of the promising potential benefits of eye movement research and compares accomplishments with anticipated results. Four topics are considered in greater detail: (i) the definition of oculomotor system input and output, (ii) optimization of the eye movement system, (iii) the relationship between compensatory eye movements and spatial orientation through the "internal model," and (iv) the significance of eye movements as measured in (outer) space.

Flight Experiment↗
NASA NTRS1995

Modeling human vestibular responses during eccentric rotation and off vertical axis rotation

A mathematical model has been developed to help explain human multi-sensory interactions. The most important constituent of the model is the hypothesis that the nervous system incorporates knowledge of sensory dynamics into an "internal model" of these dynamics. This internal model allows the nervous system to integrate the sensory information from many different sensors into a coherent estimate of self-motion. The essence of the model is unchanged from a previously published model of monkey eye movement responses; only a few variables have been adjusted to yield the prediction of human responses. During eccentric rotation, the model predicts that the axis of eye rotation shifts slightly toward alignment with gravito-inertial force. The model also predicts that the time course of the perception of tilt following the acceleration phase of eccentric rotation is much slower than that during deceleration. During off vertical axis rotation (OVAR) the model predicts a small horizontal bias along with small horizontal, vertical, and torsional oscillations. Following OVAR stimulation, when stopped right- or left-side down, a small vertical component is predicted that decays with the horizontal post-rotatory response. All of the predictions are consistent with measurements of human responses.

NASA Discipline Neuroscience↗
NASA NTRS1994

Orientation of human optokinetic nystagmus to gravity: a model-based approach

Optokinetic nystagmus (OKN) was induced by having subjects watch a moving display in a binocular, head-fixed apparatus. The display was composed of 3.3 degrees stripes moving at 35 degrees/s for 45 s. It subtended 88 degrees horizontally by 72 degrees vertically of the central visual field and could be oriented to rotate about axes that were upright or tilted 45 degrees or 90 degrees. The head was held upright or was tilted 45 degrees left or right on the body during stimulation. Head-horizontal (yaw axis) and head-vertical (pitch axis) components of OKN were recorded with electro-oculography (EOG). Slow phase velocity vectors were determined and compared with the axis of stimulation and the spatial vertical (gravity axis). With the head upright, the axis of eye rotation during yaw axis OKN was coincident with the stimulus axis and the spatial vertical. With the head tilted, a significant vertical component of eye velocity appeared during yaw axis stimulation. As a result the axis of eye rotation shifted from the stimulus axis toward the spatial vertical. Vertical components developed within 1-2 s of stimulus onset and persisted until the end of stimulation. In the six subjects there was a mean shift of the axis of eye rotation during yaw axis stimulation of approximately 18 degrees with the head tilted 45 degrees on the body. Oblique optokinetic stimulation with the head upright was associated with a mean shift of the axis of eye rotation toward the spatial vertical of 9.2 degrees. When the head was tilted and the same oblique stimulation was given, the axis of eye rotation rotated to the other side of the spatial vertical by 5.4 degrees. This counterrotation of the axis of eye rotation is similar to the "Muller (E) effect," in which the perception of the upright is counterrotated to the opposite side of the spatial vertical when subjects are tilted in darkness. The data were simulated by a model of OKN with a "direct" and "indirect" pathway. It was assumed that the direct visual pathway is oriented in a body, not a spatial frame of reference. Despite the short optokinetic after-nystagmus time constants, strong horizontal to vertical cross-coupling could be produced if the horizontal and vertical time constants were in proper ratio and there were no suppression of nystagmus in directions orthogonal to the stimulus direction. The model demonstrates that the spatial orientation of OKN can be achieved by restructuring the system matrix of velocity storage. We conclude that an important function of velocity storage is to orient slow-phase velocity toward the spatial vertical during movement in a terrestrial environment.

Non-NASA Center↗
NASA NTRS1993

Gravitoinertial force level influences arm movement control

1. The ability to move the forearm between remembered elbow joint angles immediately after rapid increases or decreases of the background gravitoinertial force (G) level was measured. The movements had been well-practiced in a normal 1G environment before the measurements in high-(1.8G) and low-force (0G) environments. The forearm and upper arm were always unsupported to maximize the influence of altered G-loading and to minimize extraneous cues about arm position. 2. Horizontal and vertical movement planes were studied to measure the effects of varying the G load in the movement plane within a given G background. Rapid and slow movements were studied to assess the role of proprioceptive feedback. 3. G level did not affect the amplitude of rapid movements, indicating that subjects were able to plan and to generate appropriate motor commands for the new G loading of the arm. The amplitude of slow movements was affected by G level, indicating that proprioceptive feedback is influenced by G level. 4. The effects of G level were similar for horizontal and vertical movements, indicating that proprioceptive information from supporting structures, such as the shoulder joint and muscles, had a role in allowing generation of the appropriate motor commands. 5. The incidence and size of dynamic overshoots were greater in 0G and for rapid movements. This G-related change in damping suggests a decrease in muscle spindle activity in 0G. A decrease in muscle spindle activity in 0G and an increase in 1.8G are consistent with the results of our prior studies on the tonic vibration reflex, locomotion, and perception of head movement trajectory in varying force backgrounds.

NASA Program Space Physiology and Countermeasures↗
NASA NTRS1992

Spatial orientation of the vestibular system

1. A simplified three-dimensional state space model of visual vestibular interaction was formulated. Matrix and dynamical system operators representing coupling from the semicircular canals and the visual system to the velocity storage integrator were incorporated into the model. 2. It was postulated that the system matrix for a tilted position was a composition of two linear transformations of the system matrix for the upright position. One transformation modifies the eigenvalues of the system matrix while another rotates the pitch and roll eigenvectors with the head, while maintaining the yaw axis eigenvector approximately spatially invariant. Using this representation, the response characteristics of the pitch, roll, and yaw eye velocity were obtained in terms of the eigenvalues and associated eigenvectors. 3. Using OKAN data obtained from monkeys and comparing to the model predictions, the eigenvalues and eigenvectors of the system matrix were identified as a function of tilt to the side or of tilt to the prone positions, using a modification of the Marquardt algorithm. The yaw eigenvector for right-side-down tilt and for downward pitch cross-coupling was approximately 30 degrees from the spatial vertical. For the prone position, the eigenvector was computed to be approximately 20 degrees relative to the spatial vertical. For both side-down and prone positions, oblique OKN induced along eigenvector directions generated OKAN which decayed to zero along a straight line with approximately a single time constant. This was verified by a spectral analysis of the residual sequence about the straight line fit to the decaying data. The residual sequence was associated with a narrow autocorrelation function and a wide power spectrum. 4. Parameters found using the Marquardt algorithm were incorporated into the model. Diagonal matrices in a head coordinate frame were introduced to represent the direct pathway and the coupling of the visual system to the integrator. Model simulations predicted the behavior of yaw and pitch OKN and OKAN when the animal was upright, as well as the cross-coupling in the tilted position. The trajectories in velocity space were also accurately simulated. 5. There were similarities between the monkey eigenvectors and human perception of the spatial vertical. For side-down tilts and downward eye velocity cross-coupling, there was only an Aubert (A) effect. For upward eye velocity cross-coupling there were both Muller (E) and Aubert (A) effects. The mean of the eigenvectors for upward and downward eye velocities overlay human 1 x g perceptual data.(ABSTRACT TRUNCATED AT 400 WORDS).

NASA Discipline Neuroscience↗
NASA NTRS1992

The contribution of the extracellular matrix to gravisensing in characean cells

The cell-extracellular matrix junction, which includes the cell wall and the outer surface of the plasma membrane, may be an essential region for the perception of gravity by the internodal cells of Chara corallina. Typically, when an internodal cell is oriented vertically, the downwardly directed cytoplasmic stream travels at a velocity that is 10% faster than that of the upwardly directed stream. However when the cells are treated with impermeant hydrolytic enzymes that partially digest cellulose or hemicellulose, the cells lose their ability to respond to gravity even though streaming continues. By contrast, enzymes that digest pectins have no effect on the gravity-induced polarity of cytoplasmic streaming. Furthermore, gravisensing is sensitive to protease treatment; Proteinase K, thermolysin and collagenase but not trypsin, alpha-chymotrypsin or carboxypeptidase B, inhibit gravisensing. These findings indicate that proteins in the cell-extracellular matrix junction may be required for gravisensing. Moreover, the tetrapeptide Arg-Gly-Asp-Ser (RGDS) inhibits gravisensing in a concentration-dependent manner, indicating that the gravireceptor may be an integrin-like protein. The macromolecules necessary for gravisensing have been localized to the cell ends. As a consequence of the exoplasmic site of action of the enzymes and the tetrapeptides, we interpret the results to mean that they are acting on the gravireceptor, although we cannot eliminate the possibility that they are acting on the signal transduction chain. On the whole, our observations indicate that the cell-extracellular matrix junction is a sine qua non for graviperception in statolith-free Chara internodal cells and we suggest that the gravireceptor is located in this region.

NASA Discipline Plant Biology↗
NASA NTRS1993

Perceived self-orientation and self-motion in microgravity, after landing and during preflight adaptation training

The research described in this paper is intended to support development and evaluation of preflight adaptation training (PAT) apparatus and procedures. Successful training depends on appropriate manipulation of visual and inertial stimuli that control perception of self-motion and self-orientation. For one part of this process, astronauts are trained to report their self-motion and self-orientation experiences. Before their space mission, they are exposed to the altered sensory environments produced by the PAT trainers. During and after the mission, they report their motion and orientation experiences. Subsequently, they are again exposed to the PAT trainers and are asked to describe relationships between their experiences in microgravity and following entry and their experiences in the trainers.

Space Flight↗