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At least 163 records · Page 9Linked to original sources
PubMed1995

Vestibular plasticity following orbital spaceflight: recovery from postflight postural instability.

Results of previous studies suggested that the vestibular mediated postural instability observed in astronauts upon return to earth from orbital spaceflight may be exacerbated by an increased weighting of visual inputs for spatial orientation and control of movement. This study was performed to better understand the roles of visual and somatosensory contributions to recovery of normal sensori-motor postural control in returning astronauts. Preflight and postflight, 23 astronaut volunteers were presented randomly with three trials of six sensory organization test (SOT) conditions in the EquiTest system test battery. Sagittal plane center-of-gravity (COG) excursions computed from ground reaction forces were significantly higher on landing day than preflight for those test conditions presenting sway-referenced visual and/or somatosensory orientation cues. The ratio of summed peak-to-peak COG sway amplitudes on the two sway-referenced vision tests (SOTs 3 + 6) compared to the two eyes closed tests (SOTs 2 + 5) was increased on landing day, indicating an increased reliance on visual orientation cues for postural control. The ratio of peak-to-peak COG excursions on sway-referenced surfaces (SOTs 4, 5 & 6) to an earth fixed support surfaces (SOTs 1, 2 & 3) increased even more after landing suggesting primary reliance on somatosensory orientation cues for recovery of postflight postural stability. Readaptation to sway-referenced support surfaces took longer than readaptation to sway-referenced vision. The increased reliance on visual and somatosensory inputs disappeared in all astronauts 4-8 days following return to earth.

Astronauts↗
PubMedDate not supplied

Evaluation of transit-time and electromagnetic flow measurement in a chronically instrumented nonhuman primate model.

The Physiology Research Branch at Brooks AFB conducts both human and nonhuman primate experiments to determine the effects of microgravity and hypergravity on the cardiovascular system and to identify the particular mechanisms that invoke these responses. Primary investigative efforts in our nonhuman primate model require the determination of total peripheral resistance, systemic arterial compliance, and pressure-volume loop characteristics. These calculations require beat-to-beat measurement of aortic flow. This study evaluated accuracy, linearity, biocompatability, and anatomical features of commercially available electromagnetic (EMF) and transit-time flow measurement techniques. Five rhesus monkeys were instrumented with either EMF (3 subjects) or transit-time (2 subjects) flow sensors encircling the proximal ascending aorta. Cardiac outputs computed from these transducers taken over ranges of 0.5 to 2.0 L/min were compared to values obtained using thermodilution. In vivo experiments demonstrated that the EMF probe produced an average error of 15% (r = .896) and 8.6% average linearity per reading, and the transit-time flow probe produced an average error of 6% (r = .955) and 5.3% average linearity per reading. Postoperative performance and biocompatability of the probes were maintained throughout the study. The transit-time sensors provided the advantages of greater accuracy, smaller size, and lighter weight than the EMF probes. In conclusion, the characteristic features and performance of the transit-time sensors were superior to those of the EMF sensors in this study.

Animals↗
PubMed1995

Inertial acceleration as a measure of linear vection: an alternative to magnitude estimation.

The present study focused on the development of a procedure to assess perceived self-motion induced by visual surround motion--vection. Using an apparatus that permitted independent control of visual and inertial stimuli, prone observers were translated along their head x-axis (fore/aft). The observers' task was to report the direction of self-motion during passive forward and backward translations of their bodies coupled with exposure to various visual surround conditions. The proportion of "forward" responses was used to calculate each observer's point of subjective equality (PSE) for each surround condition. The results showed that the moving visual stimulus produced a significant shift in the PSE when data from the moving surround condition were compared with the stationary surround and no-vision condition. Further, the results indicated that vection increased monotonically with surround velocities between 4 and 40 degrees/sec. It was concluded that linear vection can be measured in terms of changes in the amplitude of whole-body inertial acceleration required to elicit equivalent numbers of "forward" and "backward" self-motion reports.

Acceleration↗
PubMed1996

Role of physical countermaneuvers in the management of orthostatic hypotension: efficacy and biofeedback augmentation.

OBJECTIVE: To evaluate the efficacy of various physical countermaneuvers in reducing orthostatic hypotension and its associated symptoms and to assess the efficacy of biofeedback training in enhancing the effectiveness of physical countermaneuvers. MATERIAL AND METHODS: In nine study subjects with neurogenic orthostatic hypotension, four training sessions on physical countermaneuvers were performed after tilt-up, three with visual feedback on the effect of physical countermaneuvers on blood pressure and other cardiovascular variables. Blood pressure change and orthostatic symptoms during tilt-up were determined, as were the changes in total peripheral resistance, stroke index, and heart rate. RESULTS: The five female and four male patients had a mean age of 53 years and a mean duration of symptoms of 4.2 years. On an orthostatic symptom scale of 0 to 10, these patients had a mean symptom score of 7.3. The increment in systolic blood pressure was better for some maneuvers (such as leg crossing and a combination) than others (such as neck flexion and abdominal contraction). Three patterns of responses to biofeedback were found. Simple maneuvers such as squatting did not improve with training; visual feedback was needed for maneuvers such as thigh contraction, and performance declined without biofeedback; the third pattern, seen in maneuvers such as leg crossing, showed continued improvement with training, even without biofeedback. A survey at 3 to 4 months after training revealed continued use of physical maneuvers (3.8 +/- 3.1 per day), increased standing time with each episode of presyncopal symptoms (8.3 +/- 5.8 minutes), and continued global symptomatic improvement. Total peripheral resistance, but not heart rate or stroke index, showed significant regression with blood pressure improvement. CONCLUSION: Physical countermaneuvers are efficacious in reducing orthostatic hypotension, can be augmented by use of biofeedback, and may significantly improve the functional outcome. The major mechanism of improvement is an increase in total peripheral resistance, presumably by reducing the vascular capacitance.

Adult↗
PubMed1993

The regulation of rat activity following exposure to hyperdynamic fields.

The microgravity of spaceflight and the hyperdynamic fields produced via centrifugation have allowed researchers to examine the effect of altered gravitational environments on the regulation of physiological systems. In previous studies, we have discussed the importance of homeostatic and circadian mechanisms for the regulation of physiological systems such as body temperature and heart rate. Rats exposed to a chronic 2G field exhibited lower mean daily body temperature and heart rate. The homeostatic component of regulation for body temperature and heart rate adapts to a new steady state after 5-6 days. However, the circadian rhythm of body temperature and heart rate became severely depressed and did not recover for approximately 7-10 days. The measurements of body temperature and heart rate suggest an adaptation of homeostatic and circadian regulatory mechanisms following 10 days exposure to 2G. However, an important function of physiological homeostasis is to respond to environmental stressors. An important question thus becomes whether the regulation of body temperature and heart rate has sufficiently recovered to respond to an environmental challenge separate from that of the hyperdynamic field. In this study, a high frequency light/dark cycle (LD 3:3) was provided for 24 hours as an environmental challenge to assess the recovery of homeostatic and circadian regulation. Previous studies have demonstrated that high frequency light dark cycles are highly effective for testing homeostatic and circadian components of physiological regulation in monkeys and rats. For example, the nocturnal rat exhibited a homeostatic increase in body temperature during the dark periods and a decrease during the light periods. In addition, the magnitude of the body temperature response exhibits a time of day variation demonstrating the effect on circadian regulation.

Animals↗
PubMed1993

Transesophageal echocardiographic evaluation of baboons during microgravity induced by parabolic flight.

The central cardiovascular responses to transient microgravity are not well understood. Theoretically, entrance into microgravity results in the loss of the hydrostatic pressure head and an increase in central venous pressure (CVP) as a consequence of augmented venous return. However, controversy exists regarding the time course and magnitude of cephalad blood volume shifts and its relationship to central atrial filling pressures. On the June 1991 STS 40 shuttle mission, pre-launch echocardiograms suggested changes in cardiac dimensions occurred while the astronauts were in the supine, feet-up position. Furthermore, a CVP line in an astronaut (n=1) demonstrated an unexpected abrupt decrease in CVP during orbital insertion. In April 1991, our laboratory performed Doppler echocardiography in 6 normal human volunteers during parabolic flight. Increases in right ventricular velocities reflecting a central shift of blood volume was demonstrated in subjects examined in the sitting position. However, test subjects examined in the horizontal positions had no significant rise in Doppler velocities. In addition, Latham et al noted variable central cardiovascular responses in chronically instrumented baboons during early microgravity. Transthoracic echocardiography (TTE) is a feasible method to noninvasively examine cardiac anatomy during parabolic flight. However, transducer placement on the chest wall is very difficult to maintain during transition to microgravity. In addition, TTE requires the use of low frequency transducers (2.5 MHz) which limits resolution. Transesophageal echocardiography (TEE) is an established imaging technique which obtains echocardiographic information from the esophagus. It is a safe procedure and provides higher quality images of cardiac structures than obtained with TTE. Since there are no interposed structures between the esophagus and the heart, higher frequency transducers can be used and resolution is enhanced. With TEE, a flexible transducer tip permits contact with the esophageal mucosa, allowing for consistent imaging. This study was designed to determine whether TEE was feasible to perform during parabolic flight and to determine whether acute central volume responses occur in acute transition to zero gravity (0G) by direct visualization of the cardiac chambers.

Animals↗
PubMed1993

Circulatory filling pressures during transient microgravity induced by parabolic flight.

Theoretical concepts hold that blood in the gravity-dependent portion of the body would relocate to more cephalad compartments under microgravity conditions. The result is an increase in blood volume in the thoracic and cardiac chambers. This increase in central volume shift should result in an increase in central atrial filling pressures. However, experimental data has been somewhat contradictory and nonconclusive to date. Early investigations of peripheral venous pressure and estimates of central venous pressure (CVP) from these data did not show an increase in CVP in the microgravity condition. However, CVP recorded in human volunteers during the parabolic flight by Norsk revealed an increase in CVP during the microgravity state. On the June 1991 STS 40 shuttle mission, a payload specialist wore a fluid line that recorded CVP during the first few hours of orbital insertion. These data revealed decreased CVP. When this CVP catheter was tested during parabolic flight in four subjects, two subjects had increased CVP recordings and two other subjects had decreased CVP measurements. In April 1991, our laboratory performed parabolic flight studies in several chronic-instrumented baboon subjects. It was again noted that centrally recorded right atrial pressure varied with exposure to microgravity, some animals having an increase and others having a decrease. Thus, data presently available has demonstrated a variable response in the mechanism not clearly defined. In April 1992, we determined a test hypothesis relating the possible mechanism of these variable pressure responses to venous pressure-volume relationships.

Animals↗
PubMed1993

Body position and volume status as determinants of cardiovascular responses to transition into microgravity in parabolic flight.

The condition of microgravity during spaceflight imposes a new challenge to the cardiovascular system and to its homeostatic mechanisms. Initial fluids shifts from the dependent parts to the upper parts of the body are supposed to induce a plethora of effects which eventually lead to the well-known puffy faces and chicken legs' of astronauts. At the same time some 2-3 kgs. in fluid is lost in urine and by diminished uptake. For research into these longer-term effects of spaceflight extensive physiologic experiments are required in space. In view of the high cost and the logistic problems related to space-research much work is done in simulation experiments like bedrest or head down tilt studies. For the very initial effects of micro-G parabolic flight can be used. In parabolic flights we have addressed the question of immediate cardiovascular effects of the transition into microgravity. Since a parabola will last for not more than some 25 seconds, one may expect to observe mainly changes in the outflow of the autonomic nervous system, reflecting in blood pressure and heart rate as easily measurable parameters. Such changes can be expected to be caused by the sudden disappearance of hydrostatic effects and the shifts of fluid from pools where it is kept under the influence of gravity. Hydrostatic effects will play a role in the position of the baroreceptors with respect to the heart: in the upright position the carotid sinuses are some 25 cm above heart level, consequently they observe a lower pressure than that at the heart. When this effect disappears in micro-G a suddenly increased pressure will be observed and the baroreflex is called into action. On the venous side blood will rush to the right atrium when it is no longer pulled down in the compliant vessels of the abdomen and legs. This may be expected to lead to increased pressures on the low-pressure side of the heart. Apart from changes in filling of the left heart this may lead to autonomic nervous effects on systemic blood pressure and heart rate as well.

Adult↗
PubMed1993

Space flight and humoral and cellular immunity of animals.

The use of animal surrogates for the study of the effects of space flight on immune responses is desirable for several reasons. First, animal models are very useful in conditions where the use of human subjects would be difficult. This is certainly true in the space flight environment, where the number of human subjects is small, the human subjects are often too occupied with space flight duties to serve as subjects, and where sampling could be difficult under adverse conditions. Second, because of the intense interest in the immune system, many animal models and reagents already exist that are readily adaptable to space flight studies. Therefore, animal models have been used for many years for space flight studies on immunity. The predominant model used has been the rat, although recently, development of a rhesus monkey model has begun. The focus of this paper will be a description of the results of ground-based and space flight animal models to determine the effects of space flight on immune responses.

Animals↗
PubMed1995

Integrin-extracellular matrix interactions in connective tissue remodeling and osteoblast differentiation.

The differentiaton of bone cells is a complex multistep process. Bone is somewhat unusual in that it is very actively and continually remodeled in the adult and that maintenance of its mass in the mature organism is exquisitely sensitive to mechanical as well as chemical signals. Bone is also unique because it consists of a very large amount of extracellular matrix (ECM) that is mineralized. The integrin family of ECM receptors has been shown to play an important role in tissue morphogenesis in several systems. Our studies on the regulation of matrix remodeling enzymes by integrins in rabbit synovial fibroblasts show that two b1 integrin fibronectin (FN) receptor complexes (alpha 5 beta 1 and alpha 4 beta 1) cooperate in detecting subtle changes in the composition of the ECM. As a result of signal transduction by these integrins, the levels of mRNA and protein for several members of the metalloproteinase family are regulated in these cells. We have also used antibody and RGD peptide perturbation studies to determine the significance of cell/ECM interactions to normal osteogenesis. We found that interactions between the cell binding domain of FN and integrins are required for both normal morphogenesis and gene expression in cultured osteoblasts that differentiate to form bone-like tissue in culture. These data lead us to propose that beta 1 integrins play an important role in osteoblast differentiation as well as in bone remodeling.

Alkaline Phosphatase↗
PubMed1994

Cardiopulmonary adaptation to weightlessness.

The lung is profoundly affected by gravity. The absence of gravity (microgravity) removes the mechanical stresses acting on the lung paranchyma itself, resulting in a reduction in the deformation of the lung due to its own weight, and consequently altering the distribution of fresh gas ventilation within the lung. There are also changes in the mechanical forces acting on the rib cage and abdomen, which alters the manner in which the lung expands. The other way in which microgravity affects the lung is through the removal of the gravitationally induced hydrostatic gradients in vascular pressures, both within the lung itself, and within the entire body. The abolition of a pressure gradient within the pulmonary circulation would be expected to result in a greater degree of uniformity of blood flow within the lung, while the removal of the hydrostatic gradient within the body should result in an increase in venous return and intra-thoracic blood volume, with attendant changes in cardiac output, stroke volume, and pulmonary diffusing capacity. During the 9 day flight of Spacelab Life Sciences-1 (SLS-1) we collected pulmonary function test data on the crew of the mission. We compared the results obtained in microgravity with those obtained on the ground in both the standing and supine positions, preflight and in the week immediately following the mission. A number of the tests in the package were aimed at studying the anticipated changes in cardiopulmonary function, and we report those in this communication.

Adaptation, Physiological↗
PubMed1994

Altered baroreflex control of forearm vascular resistance during simulated microgravity.

Reflex peripheral vasoconstriction induced by activation of cardiopulmonary baroreceptors in response to reduced central venous pressure (CVP) is a basic mechanism for elevating systemic vascular resistance and defending arterial blood pressure during orthostatically-induced reductions in cardiac filling and output. The sensitivity of the cardiopulmonary baroreflex response [defined as the slope of the relationship between changes in forearm vascular resistance (FVR) and CVP] and the resultant vasoconstriction are closely and inversely associated with the amount of circulating blood volume. Thus, a high-gain FVR response will be elicited by a hypovolemic state. Exposure to microgravity during spaceflight results in reduced plasma volume. It is therefore reasonable to expect that the FVR response to cardiopulmonary baroreceptor unloading would be accentuated following adaptation to microgravity. Such data could provide better insight about the physiological mechanisms underlying alterations in blood pressure control following spaceflight. We therefore exposed eleven men to 6 degrees head-down bedrest for 7 days and measured specific hemodynamic responses to low levels of the lower body negative pressure to determine if there are alterations in cardiopulmonary baroreceptor stimulus-FVR reflex response relationship during prolonged exposure to an analog of microgravity.

Adult↗
PubMed1994

Effects of 5 weeks of lower limb suspension on muscle size and strength.

Lack of weight-bearing, as occurs in space, appears to be associated with reductions in strength and mass of skeletal muscle. Very limited data, however, is at hand describing changes in skeletal muscle size and function following manned space missions. Our current knowledge therefore is mainly based on studies of space flown rats. It is obvious though that this information, only in part can be extrapolated to humans. A few bed rest studies have demonstrated that decreases in strength and muscle size are substantial. At this time, however, the magnitude or time course of such changes either in response to space flight or simulations of microgravity have not been defined. In the last few years we have employed a human model to simulate unloading of lower limb skeletal muscles that occurs in microgravity. This model was essentially adopted from the rat hindlimb suspension technique. The purpose of this study was to assess the magnitude of decreases in muscle strength and size as a result of five weeks of unilateral lower limb suspension.

Adaptation, Physiological↗
PubMed1994

Changes in hypothalamic [correction of hypothalmic] staining for c-Fos following 2G exposure in rats.

The static gravitational field of the earth has been an important selective pressure that has shaped the evolution of biological organisms. This is illustrated by the evolution of tetrapods from a water environment where gravitational force was partially negated to a terrestrial environment where gravity is of greater consequence. Terrestrial invasion resulted in a series of new structural, physiological, and behavioral features. Therefore, it is not surprising that alterations in the gravitational field can cause widespread effects in many physiological systems and behaviors. Our previous studies have demonstrated that both exposure to hyperdynamic fields and the microgravity condition of space flight have significant effects on body temperature, heartrate, activity, feeding, drinking, and circadian rhythms. However, it has not been determined whether these physiological adaptations are associated with changes in neural activity within the hypothalamic nuclei that regulate these functions. This study examined the changes in body temperature, activity, body weight and food and water intake in rats caused by exposure to a hyperdynamic field. In addition, the immediate early gene activation marker, c-Fos, was used to examine potential protein synthesis changes in the hypothalamic nuclei that regulate these functions.

Animals↗
PubMed1994

Hormonal and metabolic responses of hypophysectomized rats with head-down suspension.

The primary purpose of this investigation was to secure select anatomical and physiological measurements from hypophysectomized rats and their sham-operated control to determine how various endocrine influences could be modified by conditions of simulated microgravity. The focal point of the study was the exercise responses after head-down suspension; however, we were also interested in obtaining insights on nonexercise-related mechanisms. Since more details and information concerning this study will be published elsewhere, we will highlight those findings which warrant further research.

Animals↗
PubMed1995

Effects of hypergravity on immunologic function.

The purpose of this study was to compare the effects of hypergravity exposure (2g) with those of exposure to space flight in the Cosmos 2044 flight. To do so, rats were centrifuged continuously for 14 days. Two different experiments were carried out on tissue obtained from the centrifuged rats. In the first experiment, rat bone marrow cells were examined for their response to recombinant murine colony stimulating factor-granulocyte/monocyte (GM-CSF). In the second experiment, rat spleen and bone marrow cells were stained in with a variety of antibodies directed against cell surface antigenic markers. These cells were preserved and analyzed on a flow cytometer. The results of the studies indicated that bone marrow cells from centrifuged rats showed no significant change in response to GM-CSF as compared to bone marrow cells from control rats. Spleen cells from flown rats showed some statistically significant changes in leukocytes subset distribution, but no differences that appeared to be of biological significance. These results indicate that hypergravity did not greatly affect the same immunological parameters affected by space flight in the Cosmos 2044 mission.

Animals↗