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Results for “Reflex/physiology”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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25 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Spatial orientation perception and reflexive eye movements--a perspective, an overview, and some clinical implications

When head motion includes a linear velocity component, eye velocity required to track an earth-fixed target depends upon: a) angular and linear head velocity, b) target distance, and c) direction of gaze relative to the motion trajectory. Recent research indicates that eye movements (LVOR), presumably otolith-mediated, partially compensate for linear velocity in small head excursions on small devices. Canal-mediated eye velocity (AVOR), otolith-mediated eye velocity (LVOR), and Ocular Torsion (OT) can be measured, one by one, on small devices. However, response dynamics that depend upon the ratio of linear to angular velocity in the motion trajectory and on subject orientation relative to the trajectory are present in a centrifuge paradigm. With this paradigm, two 3-min runs yields measures of: LVOR differentially modulated by different subject orientations in the two runs; OT dynamics in four conditions; two directions of "steady-state" OT, and two directions of AVOR. Efficient assessment of the dynamics (and of the underlying central integrative processes) may require a centrifuge radius of 1.0 meters or more. Clinical assessment of the spatial orientation system should include evaluation of central integrative processes that determine the dynamics of these responses.

Review, Tutorial↗
NASA NTRS1993

Influence of cigarette smoking on human autonomic function

BACKGROUND. Although cigarette smoking is known to lead to widespread augmentation of sympathetic nervous system activity, little is known about the effects of smoking on directly measured human sympathetic activity and its reflex control. METHODS AND RESULTS. We studied the acute effects of smoking two research-grade cigarettes on muscle sympathetic nerve activity and on arterial baroreflex-mediated changes of sympathetic and vagal neural cardiovascular outflows in eight healthy habitual smokers. Measurements were made during frequency-controlled breathing, graded Valsalva maneuvers, and carotid baroreceptor stimulation with ramped sequences of neck pressure and suction. Smoking provoked the following changes: Arterial pressure increased significantly, and RR intervals, RR interval spectral power at the respiratory frequency, and muscle sympathetic nerve activity decreased. Plasma nicotine levels increased significantly, but plasma epinephrine, norepinephrine, and neuropeptide Y levels did not change. Peak sympathetic nerve activity during and systolic pressure overshoots after Valsalva straining increased significantly in proportion to increases of plasma nicotine levels. The average carotid baroreceptor-cardiac reflex relation shifted rightward and downward on arterial pressure and RR interval axes; average gain, operational point, and response range did not change. CONCLUSIONS. In habitual smokers, smoking acutely reduces baseline levels of vagal-cardiac nerve activity and completely resets vagally mediated arterial baroreceptor-cardiac reflex responses. Smoking also reduces muscle sympathetic nerve activity but augments increases of sympathetic activity triggered by brief arterial pressure reductions. This pattern of autonomic changes is likely to influence smokers' responses to acute arterial pressure reductions importantly.

NASA Discipline Cardiopulmonary↗
NASA NTRS1993

Changes in plasma volume and baroreflex function following resistance exercise

The dynamics of change in plasma volume (PV) and baroreflex responses have been reported over 24 h immediately following maximal cycle exercise. The purpose of this study was to determine if PV and baroreflex showed similar changes for 24 h after resistance exercise. Eight men were studied on 2 test days, 1 week apart. On 1 day, per cent change (% delta) in PV was estimated at 0,3, and 6 h after resistance exercise using haematocrit and haemoglobin. Baseline PV was measured 24 h after exercise using Evans blue dye. The carotid baroreceptor-cardiac reflex response was measured before, and 3, 6, 9, 12, and 24 h post-exercise. Each subject performed six sets of the bench press and leg press with 10 repetitions per set with a load that induced failure within each set. On a control day, the protocol was used without exercise. Plasma volume did not change during the control day. There was a 20% decrease in PV immediately post-exercise; the recovery of the PV was rapid and complete within 3 h. PV was 20% greater 24 h post-exercise than on the control day. There were no differences in any of the baroreflex measurements. Therefore, it is suggested that PV shifts may occur without altering baroreflex sensitivity.

NASA Program Space Physiology and Countermeasures↗
NASA NTRS1992

Vasopressin and angiotensin II in reflex regulation of ACTH, glucocorticoids, and renin: effect of water deprivation

Angiotensin II (ANG II) and vasopressin participate in baroreflex regulation of adrenocorticotropic hormone (ACTH), glucocorticoid, and renin secretion. The purpose of this study was to determine whether this participation is enhanced in water-deprived dogs, with chronically elevated plasma ANG II and vasopressin levels, compared with water-replete dogs. The baroreflex was assessed by infusing increasing doses of nitroprusside (0.3, 0.6, 1.5, and 3.0 micrograms.kg-1.min-1) in both groups of animals. To quantitate the participation of ANG II and vasopressin, the dogs were untreated or pretreated with the competitive ANG II antagonist saralasin, a V1-vasopressin antagonist, or combined V1/V2-vasopressin antagonist, either alone or in combination. The findings were as follows. 1) Larger reflex increases in ANG II, vasopressin, and glucocorticoids, but not ACTH, were produced in water-deprived dogs compared with water-replete dogs. 2) ANG II blockade blunted the glucocorticoid and ACTH responses to hypotension in water-deprived dogs, but not water-replete dogs. In contrast, vasopressin blockade reduced the ACTH response only in water-replete dogs. 3) Vasopressin or combined vasopressin and ANG II blockade reduced the plasma level of glucocorticoids related either to the fall in arterial pressure or to the increase in plasma ACTH concentration in water-replete dogs, and this effect was enhanced in water-deprived dogs. 4) In both water-deprived and water-replete animals, saralasin and/or a V1-antagonist increased the renin response to hypotension, but a combined V1/V2-antagonist did not. These results reemphasize the importance of endogenous ANG II and vasopressin in the regulation of ACTH, glucocorticoid, and renin secretion.(ABSTRACT TRUNCATED AT 250 WORDS).

NASA Discipline Cardiopulmonary↗
NASA NTRS1992

Mathematical modeling of human cardiovascular system for simulation of orthostatic response

This paper deals with the short-term response of the human cardiovascular system to orthostatic stresses in the context of developing a mathematical model of the overall system. It discusses the physiological issues involved and how these issues have been handled in published cardiovascular models for simulation of orthostatic response. Most of the models are stimulus specific with no demonstrated capability for simulating the responses to orthostatic stimuli of different types. A comprehensive model incorporating all known phenomena related to cardiovascular regulation would greatly help to interpret the various orthostatic responses of the system in a consistent manner and to understand the interactions among its elements. This paper provides a framework for future efforts in mathematical modeling of the entire cardiovascular system.

NASA Discipline Cardiopulmonary↗
NASA NTRS1991

Enhanced vagal baroreflex response during 24 h after acute exercise

We evaluated carotid-cardiac baroreflex responses in eight normotensive men (25-41 yr) on two different test days, each separated by at least 1 wk. On one day, baroreflex response was tested before and at 3, 6, 12, 18, and 24 h after graded supine cycle exercise to volitional exhaustion. On another day, this 24-h protocol was repeated with no exercise (control). Beat-to-beat R-R intervals were measured during external application of graded pressures to the carotid sinuses from 40 to -65 mmHg; changes of R-R intervals were plotted against carotid pressure (systolic pressure minus neck chamber pressure). The maximum slope of the response relationship increased (P less than 0.05) from preexercise to 12 h (3.7 +/- 0.4 to 7.1 +/- 0.7 ms/mmHg) and remained significantly elevated through 24 h. The range of the R-R response was also increased from 217 +/- 24 to 274 +/- 32 ms (P less than 0.05). No significant differences were observed during the control 24-h period. An acute bout of graded exercise designed to elicit exhaustion increases the sensitivity and range of the carotid-cardiac baroreflex response for 24 h and enhances its capacity to buffer against hypotension by increasing heart rate. These results may represent an underlying mechanism that contributes to blood pressure stability after intense exercise.

Vagus Nerve/physiology↗
NASA NTRS1986

M.I.T./Canadian vestibular experiments on the Spacelab-1 mission: 3. Effects of prolonged weightlessness on a human otolith-spinal reflex

Reflex responses that depend on human otolith organ sensitivity were measured before, during and after a 10 day space flight. Otolith-spinal reflexes were elicited by means of sudden, unexpected falls. In weightlessness, "falls" were achieved using elastic cords running from a torso harness to the floor. Electromyographic (EMG) activity was recorded from gastrocnemius-soleus. The EMG response occurring in the first 100-120 ms of a fall, considered to be predominantly otolith-spinal in origin, decreased in amplitude immediately upon entering weightlessness, and continued to decline throughout the flight, especially during the first two mission days. The response returned to normal before the first post-flight testing session. The results suggest that information coming from the otolith organs is gradually ignored by the nervous system during prolonged space flight, although the possibility that otolith-spinal reflexes are decreased independent of other otolith output pathways cannot be ruled out.

NASA Discipline Neuroscience↗