The Experts below are selected from a list of 11070 Experts worldwide ranked by ideXlab platform
Don D Sheriff - One of the best experts on this subject based on the ideXlab platform.
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role of splanchnic constriction in governing the hemodynamic responses to Gravitational Stress in conscious dogs
Journal of Applied Physiology, 2011Co-Authors: Brett J Wong, Don D SheriffAbstract:Octreotide is a somatostatin analog that constricts the splanchnic circulation, thereby improving orthostatic tolerance. We tested the hypotheses that octreotide improves orthostatic tolerance by 1...
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hypotensive effect of push pull Gravitational Stress occurs after autonomic blockade
Journal of Applied Physiology, 2003Co-Authors: Don D SheriffAbstract:The “push-pull” effect denotes the reduced tolerance to +Gz (hypergravity) when +Gz Stress is preceded by exposure to hypogravity, i.e., fractional, zero, or negative Gz. Previous studies have impl...
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role of the autonomic nervous system in push pull Gravitational Stress in anesthetized rats
Journal of Applied Physiology, 2003Co-Authors: Amy L Hakeman, Don D SheriffAbstract:Tolerance to +GzStress is reduced by preceding exposure to −Gz (push-pull effect). The mechanism(s) responsible for this effect are not fully understood, although the arterial baroreceptor reflexes...
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male and female rats express similar blood pressure responses to push pull Gravitational Stress
Journal of Applied Physiology, 2002Co-Authors: Amy L Hakeman, Don D SheriffAbstract:Brief exposure to −Gz (“push”) reduces eye-level blood pressure (elbp) during subsequent exposure to +Gz(“pull”). This is called the “push-pull effect.” To evaluate the influence of gender and the ...
Tadaaki Mano - One of the best experts on this subject based on the ideXlab platform.
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Age-related influences of leg vein filling and emptying on blood volume redistribution and sympathetic reflex during lower body negative pressure in humans.
Japanese Journal of Physiology, 2002Co-Authors: Qi Fu, Yuki Niimi, Daisaku Michikami, Atsunori Kamiya, Satoshi Iwase, Tadaaki Mano, Akio SuzumuraAbstract:To test the hypothesis that leg vein filling and emptying functions could be impaired with advancing age, which would produce less blood volume redistribution toward the lower body and smaller sympathetic reflex response during mild Gravitational Stress, 9 young and 10 elderly healthy males were exposed to a lower body negative pressure (LBNP) of 15 mmHg. Venous occlusion plethysmography was used to determine the functions of the leg veins. We found that the baseline venous distensibility index (VDI) was lower (0.057 ± 0.004 vs. 0.048 ± 0.003 ml·100 ml−1·mmHg−1, young vs. elderly; p < 0.05), and half-emptying time (T1/2) was shorter (1.6 ± 0.1 vs. 1.3 ± 0.1 s, young vs. elderly; p < 0.05) in the elderly. At 15 mmHg-LBNP, VDI was decreased and T1/2 was shortened significantly in the young group, but only slightly in the elderly group. Neither blood pressure nor heart rate changed significantly in either group. The reduction in peripheral venous pressure, which was recorded from the left antecubital vein at the cubital fossa, was less in the elderly, indicating a smaller decrease in central blood volume during LBNP; however, the enhancement of muscle sympathetic nerve activity was nearly the same as that in the young. We conclude that leg vein filling and emptying functions are impaired in elderly people, producing less blood pooling in the legs and smaller reduction in peripheral venous pressure during LBNP; the maintained sympathetic reflex response might be attributable to the well-preserved baroreflex function control of sympathetic outflow to the muscle in the elderly.
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age related changes in vasomotor reflex control of calf venous capacitance response to lower body negative pressure in humans
Japanese Journal of Physiology, 2002Co-Authors: Satoshi Iwase, Yuki Niimi, Daisaku Michikami, Tadaaki Mano, Astunori Kamiya, Akio SuzumuraAbstract:The present study was performed to test the hypothesis that calf venous capacitance would be reduced by mild Gravitational Stress through a vasomotor reflex in humans, and this response could be diminished with advancing age. Nine young (31 +/- 1 years, mean +/- SE) and 9 elderly (69 +/- 1 years) healthy males were exposed to a lower body negative pressure (LBNP) of 15 mmHg. Venous occlusion plethysmography was used to measure calf venous capacitance and calf blood flow. Muscle sympathetic nerve activity (MSNA) was recorded microneurographically from the tibial nerve along with cardiovascular variables. It was found that baseline MSNA was higher [21 +/- 4 (mean +/- SE) vs. 37 +/- 5 bursts x min(-1), young vs. elderly; p < 0.05] and calf venous capacitance was lower (1.71 +/- 0.12 vs. 1.44 +/- 0.10, ml x 100 ml(-1), young vs. elderly; p < 0.05) in the elderly group. At 15 mmHg-LBNP, heart rate and mean arterial pressure both remained unchanged, MSNA was enhanced, and calf blood flow was reduced in all subjects. Calf venous capacitance during LBNP decreased in the young, but did not change in the elderly. A significant negative correlation between percent changes in MSNA and percent changes in calf venous capacitance existed in the young group (y = 20.171x-11.863, r = 20.682; p = 0.0432), but disappeared in the elderly group. The ratio of percent changes in calf venous capacitance to percent changes in MSNA was markedly lower in the elderly (p < 0.01). In conclusion, these results substantiate our hypothesis that calf venous capacitance is reduced by mild LBNP through the vasomotor reflex, and this response is diminished in the elderly.
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muscle sympathetic nerve activity in blood pressure control against Gravitational Stress
Journal of Cardiovascular Pharmacology, 2001Co-Authors: Tadaaki ManoAbstract:Muscle sympathetic nerve activity (MSNA) can be directly recorded from human peripheral nerves in situ using microneurography. MSNA plays an essential role to control systemic blood pressure against Gravitational Stress. MSNA was enhanced by changing posture against terrestrial gravity from lying to sitting, and from sitting to standing. This activity was enhanced by head-up tilt depending on the Gravitational input from the head to the leg (+Gz) in the human body. Orthostatic hypotension occurred when MSNA response to Gravitational Stress was impaired both in high and low responders of this sympathetic outflow. Syncope was preceded and/or associated by a withdrawal of MSNA. MSNA was suppressed by short-term exposure to microgravity but was enhanced after long-term exposure to microgravity. Orthostatic intolerance after exposure to prolonged microgravity was associated with a reduction of increased MSNA response to Gravitational Stress. Aging influenced gravity-related responses of MSNA.
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microneurography as a tool to investigate sympathetic nerve responses to environmental Stress
Aerospace and environmental medicine, 1997Co-Authors: Tadaaki ManoAbstract:Microneurography is an electrophysiological method to record impulse traffic in human peripheral nerve in situ. Using this method, not only sensory afferent nerve activity, but also postganglionic sympathetic efferent outflow leading to muscle (muscle sympathetic nerve activity-MSNA) and skin (skin sympathetic nerve activity-SSNA) can be recorded in human subjects. In this paper, the methodology of microneurography and following findings obtained by microneurography on sympathetic nerve responses to environmental Stress in humans are reviewed. 1. MSNA is enhanced by Gravitational Stress, while being suppressed by simulated weightlessness through baroreflex mechanism to maintain hemodynamic homeostasis. 2. MSNA is enhanced by simulated high altitude through chemoreflex mechanism. 3. SSNA, which is composed of sudomotor and vasomotor discharges, plays an essential role in thermoregulation. There exist regional differences between SSNA responses to ambient temperature in the nerves innervating hairy and glabrous skin. The function of the sympathetic nervous system in humans has been so far generally analyzed indirectly by observing the effector organ activity or by measuring the plasma noradrenaline level. Meanwhile, a more direct method to approach the sympathetic nervous function in man, called microneurography, has been developed. By applying this technique, it is possible to investigate how the human sympathetic nervous system responds to different kinds of environmental Stress (Mano, 1990, 1994). In this paper, the usefulness of microneurography as a research tool in environmental physiology is shown with a review of microneurographic findings on sympathetic nerve responses to environmental Stress in human subjects.
Akio Suzumura - One of the best experts on this subject based on the ideXlab platform.
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Age-related influences of leg vein filling and emptying on blood volume redistribution and sympathetic reflex during lower body negative pressure in humans.
Japanese Journal of Physiology, 2002Co-Authors: Qi Fu, Yuki Niimi, Daisaku Michikami, Atsunori Kamiya, Satoshi Iwase, Tadaaki Mano, Akio SuzumuraAbstract:To test the hypothesis that leg vein filling and emptying functions could be impaired with advancing age, which would produce less blood volume redistribution toward the lower body and smaller sympathetic reflex response during mild Gravitational Stress, 9 young and 10 elderly healthy males were exposed to a lower body negative pressure (LBNP) of 15 mmHg. Venous occlusion plethysmography was used to determine the functions of the leg veins. We found that the baseline venous distensibility index (VDI) was lower (0.057 ± 0.004 vs. 0.048 ± 0.003 ml·100 ml−1·mmHg−1, young vs. elderly; p < 0.05), and half-emptying time (T1/2) was shorter (1.6 ± 0.1 vs. 1.3 ± 0.1 s, young vs. elderly; p < 0.05) in the elderly. At 15 mmHg-LBNP, VDI was decreased and T1/2 was shortened significantly in the young group, but only slightly in the elderly group. Neither blood pressure nor heart rate changed significantly in either group. The reduction in peripheral venous pressure, which was recorded from the left antecubital vein at the cubital fossa, was less in the elderly, indicating a smaller decrease in central blood volume during LBNP; however, the enhancement of muscle sympathetic nerve activity was nearly the same as that in the young. We conclude that leg vein filling and emptying functions are impaired in elderly people, producing less blood pooling in the legs and smaller reduction in peripheral venous pressure during LBNP; the maintained sympathetic reflex response might be attributable to the well-preserved baroreflex function control of sympathetic outflow to the muscle in the elderly.
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age related changes in vasomotor reflex control of calf venous capacitance response to lower body negative pressure in humans
Japanese Journal of Physiology, 2002Co-Authors: Satoshi Iwase, Yuki Niimi, Daisaku Michikami, Tadaaki Mano, Astunori Kamiya, Akio SuzumuraAbstract:The present study was performed to test the hypothesis that calf venous capacitance would be reduced by mild Gravitational Stress through a vasomotor reflex in humans, and this response could be diminished with advancing age. Nine young (31 +/- 1 years, mean +/- SE) and 9 elderly (69 +/- 1 years) healthy males were exposed to a lower body negative pressure (LBNP) of 15 mmHg. Venous occlusion plethysmography was used to measure calf venous capacitance and calf blood flow. Muscle sympathetic nerve activity (MSNA) was recorded microneurographically from the tibial nerve along with cardiovascular variables. It was found that baseline MSNA was higher [21 +/- 4 (mean +/- SE) vs. 37 +/- 5 bursts x min(-1), young vs. elderly; p < 0.05] and calf venous capacitance was lower (1.71 +/- 0.12 vs. 1.44 +/- 0.10, ml x 100 ml(-1), young vs. elderly; p < 0.05) in the elderly group. At 15 mmHg-LBNP, heart rate and mean arterial pressure both remained unchanged, MSNA was enhanced, and calf blood flow was reduced in all subjects. Calf venous capacitance during LBNP decreased in the young, but did not change in the elderly. A significant negative correlation between percent changes in MSNA and percent changes in calf venous capacitance existed in the young group (y = 20.171x-11.863, r = 20.682; p = 0.0432), but disappeared in the elderly group. The ratio of percent changes in calf venous capacitance to percent changes in MSNA was markedly lower in the elderly (p < 0.01). In conclusion, these results substantiate our hypothesis that calf venous capacitance is reduced by mild LBNP through the vasomotor reflex, and this response is diminished in the elderly.
Jean-pierre De Villiers - One of the best experts on this subject based on the ideXlab platform.
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Evolution of Self-Gravitating Magnetized Disks. II. Interaction between Magnetohydrodynamic Turbulence and Gravitational Instabilities
The Astrophysical Journal, 2004Co-Authors: Sébastien Fromang, Steven A. Balbus, Caroline Terquem, Jean-pierre De VilliersAbstract:We present three-dimensional magnetohydrodynamic (MHD) numerical simulations of the evolution of self-gravitating and weakly magnetized disks with an adiabatic equation of state. Such disks are subject to the development of both the magnetorotational and Gravitational instabilities, which transport angular momentum outward. As in previous studies, our hydrodynamic simulations show the growth of a strong m = 2 spiral structure. This spiral disturbance drives matter toward the central object and disappears when the Toomre parameter, Q, has increased well above unity. When a weak magnetic field is present as well, the magnetorotational instability grows and leads to turbulence. In that case, the strength of the Gravitational Stress tensor is lowered by a factor of ~2 compared with the hydrodynamic run and oscillates periodically, reaching very small values at its minimum. We attribute this behavior to the presence of a second spiral mode with higher pattern speed than the one that dominates in the hydrodynamic simulations. It is apparently excited by the high-frequency motions associated with MHD turbulence. The nonlinear coupling between these two spiral modes gives rise to a Stress tensor that oscillates with a frequency that is a combination of the frequencies of each of the modes. This interaction between MHD turbulence and Gravitational instabilities therefore results in a smaller mass accretion rate onto the central object.
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evolution of self gravitating magnetized disks ii interaction between mhd turbulence and Gravitational instabilities
arXiv: Astrophysics, 2004Co-Authors: Sébastien Fromang, Steven A. Balbus, Caroline Terquem, Jean-pierre De VilliersAbstract:We present 3D magnetohydrodynamic (MHD) numerical simulations of the evolution of self--gravitating and weakly magnetized disks with an adiabatic equation of state. Such disks are subject to the development of both the magnetorotational and Gravitational instabilities, which transport angular momentum outward. As in previous studies, our hydrodynamical simulations show the growth of strong m=2 spiral structure. This spiral disturbance drives matter toward the central object and disappears when the Toomre parameter Q has increased well above unity. When a weak magnetic field is present as well, the magnetorotational instability grows and leads to turbulence. In that case, the strength of the Gravitational Stress tensor is lowered by a factor of about~2 compared to the hydrodynamical run and oscillates periodically, reaching very small values at its minimum. We attribute this behavior to the presence of a second spiral mode with higher pattern speed than the one which dominates in the hydrodynamical simulations. It is apparently excited by the high frequency motions associated with MHD turbulence. The nonlinear coupling between these two spiral modes gives rise to a Stress tensor that oscillates with a frequency which is a combination of the frequencies of each of the modes. This interaction between MHD turbulence and Gravitational instabilities therefore results in a smaller mass accretion rate onto the central object.
Lonnie G Petersen - One of the best experts on this subject based on the ideXlab platform.
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lower body negative pressure to safely reduce intracranial pressure
The Journal of Physiology, 2019Co-Authors: Justin S Lawley, Lonnie G Petersen, Satyam Sarma, Erin J Howden, Alexander Liljacyron, Johan C G Petersen, William K CornwellAbstract:KEY POINTS: During long-term missions, some astronauts experience structural and functional changes of the eyes and brain which resemble signs/symptoms experienced by patients with intracranial hypertension. Weightlessness prevents the normal cerebral volume and pressure 'unloading' associated with upright postures on Earth, which may be part of the cerebral and ocular pathophysiology. By placing the lower body in a negative pressure device (LBNP) that pulls fluid away from cranial compartments, we simulated effects of gravity and significantly lowered pressure within the brain parenchyma and ventricle compartments. Application of incremental LBNP demonstrated a non-linear dose-response curve, suggesting 20 mmHg LBNP as the optimal level for reducing pressure in the brain without impairing cerebral perfusion pressure. This non-invasive method of reducing pressure in the brain holds potential as a countermeasure in space as well as having treatment potential for patients on Earth with traumatic brain injury or other pathology leading to intracranial hypertension. ABSTRACT: Patients with elevated intracranial pressure (ICP) exhibit neuro-ocular symptoms including headache, papilloedema and loss of vision. Some of these symptoms are also present in astronauts during and after prolonged space-flight where lack of Gravitational Stress prevents daily lowering of ICP associated with upright posture. Lower body negative pressure (LBNP) simulates the effects of gravity by displacing fluid caudally and we hypothesized that LBNP would lower ICP without compromising cerebral perfusion. Ten cerebrally intact volunteers were included: six ambulatory neurosurgical patients with parenchymal ICP-sensors and four former cancer patients with Ommaya-reservoirs to the frontal horn of a lateral ventricle. We applied LBNP while recording ICP and blood pressure while supine, and during simulated intracranial hypertension by 15° head-down tilt. LBNP from 0 to 50 mmHg at increments of 10 mmHg lowered ICP in a non-linear dose-dependent fashion; when supine (n = 10), ICP was decreased from 15 ± 2 mmHg to 14 ± 4, 12 ± 5, 11 ± 4, 10 ± 3 and 9 ± 4 mmHg, respectively (P < 0.0001). Cerebral perfusion pressure (CPP), calculated as mean arterial blood pressure at midbrain level minus ICP, was unchanged (from 70 ± 12 mmHg to 67 ± 9, 69 ± 10, 70 ± 12, 72 ± 13 and 74 ± 15 mmHg; P = 0.02). A 15° head-down tilt (n = 6) increased ICP to 26 ± 4 mmHg, while application of LBNP lowered ICP (to 21 ± 4, 20 ± 4, 18 ± 4, 17 ± 4 and 17 ± 4 mmHg; P < 0.0001) and increased CPP (P < 0.01). An LBNP of 20 mmHg may be the optimal level to lower ICP without impairing CPP to counteract spaceflight-associated neuro-ocular syndrome in astronauts. Furthermore, LBNP holds clinical potential as a safe, non-invasive method for lowering ICP and improving CPP for patients with pathologically elevated ICP on Earth.
