Head-down Rotation Research Articles

Central Hemodynamic Response to Activation of the Vestibular Sympathetic Reflex with Head Down Rotation in Healthy Young Adults.

The purpose of this study was to assess the effect of vestibular-sympathetic activation on estimated measures of central (aortic) hemodynamic load in young adults. Methods: 31 participants (n=14 female, 17 male) underwent radial tonometry and popliteal Doppler-ultrasound in the prone position with the head in a neutral position and during 10 minutes of head-down rotation (HDR) as a means of activating the vestibular sympathetic reflex. Subjective orthostatic intolerance was assessed utilizing a 10-item orthostatic hypotension questionnaire. Results: There was a reduction in brachial systolic BP during HDR (111±10 vs. 109±9 mmHg, p<0.05), but there was no change in aortic systolic BP (100±9 vs. 99±8 mmHg, p>0.05). Aortic augmentation index (-5±11 vs. -12±12%, p<0.05) and reservoir pressure (28±8 vs. 26±8mmHg, p<0.05) were reduced with a concomitant reduction in popliteal conductance (5.6±0.7 vs. 4.5±0.7 ml/min∙mmHg, p<0.05). Change in aortic systolic BP was associated with subjective orthostatic intolerance score (r = -0.39, p<0.05). Conclusions: Activation of the vestibular sympathetic reflex via HDR resulted in slight reductions in brachial BP concomitant with preservation of aortic BP. Despite peripheral vascular constriction during HDR, there was a reduction in pressure from wave reflections and reservoir pressure. Finally, there was an association between change in aortic systolic BP during HDR and orthostatic intolerance score suggesting that individuals that cannot defend against drops in aortic BP during vestibular sympathetic reflex activation may be more likely to experience higher subjective symptomatology of orthostatic intolerance.

Hand and Foot Selection in Mental Body Rotations Involves Motor-Cognitive Interactions.

Action imagery involves the mental representation of an action without overt execution, and can contribute to perspective taking, such as that required for left-right judgments in mental body rotation tasks. It has been shown that perspective (back view, front view), rotational angle (head-up, head-down), and abstractness (abstract, realistic) of the stimulus material influences speed and correctness of the judgement. The present studies investigated whether left-right judgements are more difficult on legs than on arms and whether the type of limb interacts with the other factors. Furthermore, a combined score for speed and accuracy was explored to eliminate possible tradeoffs and to obtain the best possible measure of subjects' individual ability. Study 1 revealed that the front view is more difficult than the back view because it involves a vertical rotation in perspective taking. Head-down rotations are more difficult than head-up rotations because they involve a horizontal rotation in perspective taking. Furthermore, leg stimuli are more difficult than hand stimuli, particularly in head-down rotations. In Study 2, these findings were replicated in abstract stimuli as well as in realistic stimuli. In addition, perspective taking for realistic stimuli in the back view is easier than realistic stimuli in the front view or abstract stimuli (in both perspectives). We conclude that realistic stimulus material facilitates task comprehension and amplifies the effects of perspective. By replicating previous findings, the linear speed-accuracy score was shown to be a valid measure to capture performance in mental body rotations.

The functional morphology of the postpulmonary septum of the American alligator (Alligator mississippiensis).

The American alligator (Alligator mississippiensis) has a postpulmonary septum (PPS) that partitions the intracoelomic cavity. The PPS adheres to the capsule of the liver caudally and to the visceral pleura of the lung cranially; the ventrolateral portions of the PPS are invested with smooth muscle, the remainder is tendinous. Differential pressure transducers were used to record the intrathoracic (ITP) and intraperitoneal (IPP) pressures, and determine the transdiaphragmatic pressure (TDP). Each ventilatory pulse resulted in a pulse in ITP and a significantly lower pulse in IPP; meaning that a TDP was established, and that the pleural and peritoneal cavities were functionally isolated. The anesthetized alligators were tilted 30° head-up or head-down in order to displace the liver. Head-up rotations caused a significant increase in IPP, and a significant decrease in ITP (which became negative); head-down rotations produced the opposite effect. During these rotations, the PPS maintained opposite pressures (positive or negative) in the pleural and peritoneal cavities, and established TDPs greater than have been reported for some mammals. Two types of "breaths" were recorded during these experiments. The first was interpreted as a contraction of the diaphragmaticus muscle, which displaces the liver caudally; these breaths had the same effect as the head-up rotations. The second type of breath was interpreted as constriction of the thoracic and abdominal body walls; this type of breath produced pronounced, long-duration, roughly parallel, increases in ITP and IPP. The smooth muscle within the PPS is suggestive of higher-order adjustment or tuning of the PPS's tensile state.

Functional morphology of the Crocodilian Diaphragm

In the American alligator (Alligator mississippiensis) there is an intracoelomic septum, herein termed the diaphragm, which anatomically partitions the intracoelomic cavity. The diaphragm adheres to the capsule of the liver caudally and to the visceral pleura of the lung cranially; the ventrolateral portions of the diaphragm are invested with smooth muscle, the remainder is tendinous. Differential pressure transducers were implanted into the intrathoracic and intraperitoneal cavities; these allowed for the direct recording of intrathoracic (ITP) and intraperitoneal (IPP) pressures, and the determination of transdiaphragmatic pressure (TDP). Sub-adult specimens were anesthetized with isoflurane, then a mechanical ventilator delivered discrete pulses of Oxygen (and isoflurane) to the lungs. Each ventilatory pulse resulted in a pulse in ITP and in IPP; however, the IPP pulses had significantly lower amplitude, meaning that a TDP was established, and that the diaphragm was functionally capable of isolating the pleural and peritoneal cavities. Power spectral analysis of the ITP and IPP pressure traces revealed the same dominant frequency (the frequency of the ventilatory pulses) and similar harmonics which appear to be oscillations of the diaphragm. Crocodilians, including Alligator, have a highly mobile liver that can function in ventilation. The anesthetized alligators were tilted 30° head-up or head-down in order to displace the liver caudally or cranially. Head-up rotations caused a significant increase in IPP, and a significant decrease in ITP (which became negative). Head-down rotations caused a significant increase in ITP, and a significant decrease in IPP (which became negative). These rotations were maintained for a duration of 30s during which the ITP and IPP were always stable. During these rotations, the diaphragm maintained opposite pressures (positive or negative) in the pleural and peritoneal cavities, and established TDPs greater than have been reported for some mammals. Analysis of the IPP traces before and after rotation revealed shifts in the power spectral density in opposite directions depending on the direction of rotation; these shifts appear to reflect changes in the tensile state of the diaphragm. During these experiments the specimens would occasional make voluntary movements similar to a breath. Two types of “breaths” were seen. The first was interpreted as a contraction of the diaphragmaticus muscle, which displaces the liver caudally; these breaths had the same effect as the head-up rotations. The second type of breath was interpreted as constriction of the thoracic and abdominal body walls; this type of breath produced pronounced, long-duration, roughly parallel, increases in ITP and IPP. These experiments demonstrate that the coelomic cavity of Alligator mississippiensis is functionally separated by a dynamic diaphragm. The presence of smooth muscle within the diaphragm indicates the possibility of higher-order adjustment or tuning of the tensile state of the diaphragm. These experiments focused on the ability of the diaphragm to maintain separate pressures in the peritoneal and pleural cavities, they were not intended to document the ventilatory, or non-ventilatory, functional role(s) of the diaphragm.

Regulatory effects of cervical sympathetic trunk and renal sympathetic nerve activities on cerebral blood flow during head-down postural rotations.

This study attempts to clarify the neural control of cerebral blood flow (CBF) during head-down postural rotation, which induces a cephalad fluid shift in urethane-anesthetized rats. The animals were placed on a table, tilted to a 45° head-down position over 5s and maintained in that position. Head-down rotation (HDR) induced a transient decrease (8±3mmHg; mean±SE) in mean arterial blood pressure (ABP) at 7.3±0.3s after the onset of HDR. The pressure returned to the pre-HDR level within 1min in the head-down position. Pretreatment with hexamethonium bromide suppressed the HDR-elicited decrease in ABP, suggesting that the decrease in ABP was induced by the suppression of autonomic neural outflow. The administration of phenoxybenzamine (PB), an α-adrenergic antagonist, also eliminated the HDR-elicited decrease in ABP, suggesting that this decrease was elicited by the suppression of α-adrenergic vascular tone. To test sympathetic outflow during HDR, renal sympathetic nerve activity (RSNA) and cervical sympathetic trunk (CST) activity (CSTA) were recorded. RSNA was transiently suppressed at 2.3±0.4s after HDR onset, followed by a decrease in ABP, suggesting that this decrease was, at least in part, induced by the suppression of sympathetic nerves. CSTA did not change significantly during HDR. These results suggest that HDR suppresses sympathetic nerves in the lower body rather than in the head, which might result in a decrease in ABP. To test the effect of the decrease in ABP due to sympathetic activity on CBF during HDR, changes in CBF during HDR were measured. CBF did not change significantly during HDR in the control group after the administration of an α-receptor blocker or after denervation of the CSTs. These results suggest that the impact of the CSTs on CBF is likely to be limited by a rapid increase in CBF due to HDR-elicited cephalad fluid shift and that CBF autoregulation proceeds through an alternative mechanism involving the myogenic properties of cerebral vessels.

Prominent facial feature and hybrid learning method-based advanced face detector robust to head-up, head-down, and arbitrary rotation cases

Face detection is important for computer vision. Until now, the Viola and Jones algorithm is still the most popular face detector embedded in digital cameras. Although it has good performance in frontal face detection, it may not perform well in detecting rotated, head-up, and head-down faces. In this paper, we propose a more robust face detector based on prominent feature extraction. We find that, in addition to using eyes and mouth, which have been adopted by other algorithms, noses and ears are also important clues for face detection. Particularly, for a profile face, only a single eye can be detected. For head-up or head-down faces, eyes and mouth may not be well detected. However, in these cases, noses and ears can still be detected well and we can apply these features to improve the accuracy of rotated, head-up, and head-down face detection. To well detect these prominent features, in addition to the Viola–Jones detector, we also apply edge and color information and use the refocus algorithm based on the convolutional neural network. Simulations conducted on several popular multi-view face databases show that the proposed face detector can attain higher detection rates and is robust to rotated, head-up, and head-down cases.

The Influence of Gravitational Gradients on the American Alligator (Alligator mississippiensis)

The cardiovascular systems of terrestrial and aquatic snakes cannot accommodate gravitational gradients, while blood flow in arboreal snakes is not disrupted by tilting. Alligators have a very dynamic cardiovascular system, with multiple means of active regulation; however, the response of alligators to gravitational gradients has never been documented. Gravitational gradients were induced by rotating sub-adult specimens of the American alligator (Alligator mississippiensis) between 45° head-up to 45° head-down in 15° increments. Intracranial pressure was assayed by quantifying the diameter of the optic nerve sheath using ocular ultrasonography; each increment of head-up rotation produced a significant decrease in nerve sheath diameter, while head-down rotations resulted in corresponding significant increases in nerve sheath diameter. Vascular ultrasonography revealed a consistent pattern. Head-down rotation resulted in vasodilation of the carotid artery and jugular vein, and head-up rotation resulted in a decrease in the luminal area of these vessels. In contrast to these manifestations of orthostatic pressure gradients, instantaneous heart rate (determined by EKG) revealed no evidence for a barostatic reflex in A. mississippiensis. This is the first report from a non-mammalian vertebrate of how intracranial pressure varies under gravitational gradients. The alligator has a unique response to gravitational gradients, characterized by the lack of a barostatic reflex.

Effect of Ultrasonic Bone Stimulation of the Mastoid on Renal and Coronary Blood Flow in Humans

We have previously demonstrated that external stimulation of the otolith organs by ultrasonic bone stimulation (UBS) of the mastoid increases muscle sympathetic nerve activity. These responses are similar to that observed by head‐down rotation. Additionally, stimulation of the vestibulosympathetic reflex by head‐down rotation elicits leg and renal vasoconstriction. The purpose of this study was to determine if UBS elicits vascular changes to the renal and coronary arteries. We hypothesized that UBS would elicit vasoconstriction in the renal but not the coronary vascular bed. Ten subjects (3 F, 7 M; 25±1 yr; 174.0±3.2 cm; 71.8±4.3 kg) performed UBS in the supine position for 3 min during measurement of arterial blood pressure, heart rate, renal blood flow, and coronary blood flow. Blood flow was measured using Doppler ultrasound. UBS did not change mean arterial blood pressure (Δ‐1±1 mmHg) or heart rate (Δ‐1±1 beats/min) from baseline. Renal vascular conductance was significantly decreased from baseline during UBS (0.63±0.03 and 0.60±0.04 cm/s/mmHg, respectively; P=0.02). However, there were no changes in coronary vascular conductance during UBS (0.18±0.01 and 0.18±0.02, for baseline and UBS, respectively). In conclusion, UBS can elicit vasoconstriction of the renal vascular bed similar to that observed during head‐down rotation. This finding suggests that external stimulation of the otolith organs might assist in postural blood pressure regulation in people with decreased orthostatic tolerance.Supported by: NIH R21HL109952

Ultrasonic bone stimulation increases orthostatic tolerance after 24 hours of bed rest (1171.6)

Our studies indicate that stimulation of the otolith organs by gravitational (head‐down rotation) and non‐gravitational (ultrasonic bone stimulation of mastoid; UBS) activation increases muscle sympathetic activity in humans. These findings suggest that otolith stimulation might assist in maintaining vascular resistance and arterial blood pressure during orthostatic challenges. To test this hypothesis, 16 subjects were placed in head‐down bed rest for 24 hours and then tested twice with an 80° head‐up tilt test once with and without UBS. The order of the two tilt tests was randomized with a maximum test duration of 30 min. Eight subjects were able to complete both tests after bed rest. In the remaining 8 subjects (2 M, 6 F, 26 ± 2 yr, 172.4 ± 3.0 cm, 74.6 ± 5.8 kg) who could not complete the tilt tests, tilt time was significantly greater in 7 of the 8 subjects with UBS compared to without (743 ± 180 vs. 574 ± 158 s, P = 0.002). Mean arterial pressure was 60±6 mmHg at the stoppage of the control tilt test and 90±5 mmHg at the same time point during UBS trial (P = 0.006). However, heart rate was unaffected by UBS during head‐up tilt (95 ± 5 and 93 ± 11 bpm for UBS and control, respectively). These findings indicate that UBS can improve orthostatic tolerance in subjects following bed rest. Similar results might be expected in subjects that experience orthostatic intolerance due to other reasons.Grant Funding Source: Supported by NIH (HL109952), NSBRI (CA00404)

Is there diurnal variation of the vestibulosympathetic reflex: implications for orthostatic hypotension

Incidences of adverse cardiac events and orthostatic hypotension are associated with diurnal variations. The primary purpose of the present study was to determine if the vestibulosympathetic reflex (VSR) follows a diurnal variation in humans. We hypothesized that the VSR would be attenuated at night based on the relation between melatonin and the VSR. Arterial blood pressure, heart rate, calf blood flow, and muscle sympathetic nerve activity (MSNA) were measured in nine healthy subjects (28 ± 1 yr, 5 men and 4 women) at rest and during head-down rotation. Each subject was tested during the day at 11:34 ± 13 and again at night 22:10 ± 5. MSNA was significantly decreased at night compared with day (8 ± 1 vs. 11 ± 2 bursts/min, respectively, P < 0.02). Heart rate and arterial blood pressure at rest were significantly increased at night compared with day (heart rate: 70 ± 4 vs. 66 ± 4 beats/min and mean arterial blood pressure: 91 ± 2 vs. 87 ± 1 mmHg, respectively). MSNA and hemodynamic responses to head-down rotation were not significantly altered at night compared with day (changes of 3 ± 1 bursts/min and 25 ± 6% for MSNA and calf blood flow, respectively). The data indicate that MSNA at rest decreases during the late evening hours and exhibits a diurnal variation, whereas the VSR does not. In summary, diurnal variation of orthostatic hypotension in humans does not appear to be associated with changes in the VSR and MSNA at rest.

Autonomic Reflexes May Contribute to the Symptomatology of Mal de Debarquement Syndrome

Mal de debarquement syndrome (MdDS) is the phantom perception of self‐motion triggered by a passive motion event such as sea or air travel. To test the hypothesis that autonomic reflexes are altered in MdDS, head‐down rotation (HDR) was used to test vestibular reflexes and 60° head‐up tilt (HUT) was used to test combined baroreceptor and vestibular reflexes in 9 female MdDS patients and 9 age‐ and sex‐matched controls. Heart rate (HR, ECG), arterial blood pressure (ABP, finger photoplethysmography), and cardiac output (CO), stroke volume and systemic vascular resistance (SVR) were calculated (Modelflow) in 6 min bins for steady‐state and dynamic (FFT) analysis during baseline, perturbation (HDR or HUT), and recovery. HDR decreased stroke volume and CO in both groups, but SVR increased in MdDS. No changes in the ABP power spectrum were observed during HDR. HUT increased mean ABP, HR, and SVR while decreasing pulse pressure, stroke volume, and CO in both groups. Increases in mean ABP and SVR in MdDS were augmented compared to controls. HUT increased the ABP low frequency spectral power and R‐to‐R interval low frequency/high frequency ratios in both groups. These pilot data identified exaggerated vascular resistances to a postural change in MdDS possibly due to increased sympathetic reflex gain. Thus, altered autonomic reflexes appear to be a part of MdDS symptomatology.Funding: MdDS Balance Disorder Foundation

Reply to Sauder and Ray

to the editor: Ray's group (see Ref. [5][1]) has performed elegant experiments that demonstrate a potential interaction between the vestibulosympathetic reflex and the baroreflex in humans. In their experiments, head-down rotation (HDR) was performed to stimulate the vestibulosympathetic reflex, and

Effects of short-term and prolonged bed rest on the vestibulosympathetic reflex

The mechanism(s) for post-bed rest (BR) orthostatic intolerance is equivocal. The vestibulosympathetic reflex contributes to postural blood pressure regulation. It was hypothesized that muscle sympathetic nerve responses to otolith stimulation would be attenuated by prolonged head-down BR. Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and peripheral vascular conductance were measured during head-down rotation (HDR; otolith organ stimulation) in the prone posture before and after short-duration (24 h; n = 22) and prolonged (36 ± 1 day; n = 8) BR. Head-up tilt at 80° was performed to assess orthostatic tolerance. After short-duration BR, MSNA responses to HDR were preserved (Δ5 ± 1 bursts/min, Δ53 ± 13% burst frequency, Δ65 ± 13% total activity; P < 0.001). After prolonged BR, MSNA responses to HDR were attenuated ∼50%. MSNA increased by Δ8 ± 2 vs. Δ3 ± 2 bursts/min and Δ83 ± 12 vs. Δ34 ± 22% total activity during HDR before and after prolonged BR, respectively. Moreover, these results were observed in three subjects tested again after 75 ± 1 days of BR. This reduction in MSNA responses to otolith organ stimulation at 5 wk occurred with reductions in head-up tilt duration. These results indicate that prolonged BR (∼5 wk) unlike short-term BR (24 h) attenuates the vestibulosympathetic reflex and possibly contributes to orthostatic intolerance following BR in humans. These results suggest a novel mechanism in the development of orthostatic intolerance in humans.

Glycerol-induced fluid shifts attenuate the vestibulosympathetic reflex in humans

The glycerol dehydration test (GDT) has been used to test for the presence of Ménière's disease and elicits acute alterations in vestibular reflexes in both normal and pathological states. Activation of the vestibulosympathetic reflex (VSR) increases muscle sympathetic nerve activity (MSNA) and peripheral vascular resistance. We hypothesized that the GDT would attenuate the VSR through fluid shifts of the inner ear. Sixteen male subjects (26 ± 1 yr) were randomly assigned to be administered either glycerol mixed with cranberry juice (97 ± 3 ml glycerol + equal portion of cranberry juice; n = 9) or a placebo control [water + cranberry juice (100 ml each); n = 7]. Subjects in both groups performed head-down rotation (HDR), which engages the VSR, before and after administration of either the glycerol or placebo. MSNA (microneurography), arterial blood pressure, and leg blood flow (venous occlusion plethysmography) were measured during HDR. Before glycerol administration, HDR significantly increased MSNA burst frequency (Δ8 ± 1 bursts/min; P < 0.01) and total activity (Δ77 ± 18%; P < 0.01) and decreased calf vascular conductance (-Δ20 ± 3%; P < 0.01). However, HDR performed postadministration of glycerol resulted in an attenuated MSNA increase (Δ3 ± 1 bursts/min, Δ22 ± 3% total activity) and decrease in calf vascular conductance (-Δ7 ± 4%). HDR significantly increased MSNA burst frequency (Δ5 ± 1 and Δ5 ± 2 bursts/min) and total activity (Δ58 ± 13% and Δ52 ± 18%) in the placebo group before and after placebo, respectively (P < 0.01). Likewise, decreases in calf vascular conductance during HDR before and after placebo were not different (-Δ13 ± 4% and -Δ14 ± 2%, respectively; P < 0.01). These results suggest that fluid shifts of the inner ear via glycerol dehydration attenuate the VSR. These data provide support that inner ear fluid dynamics can have a significant impact on blood pressure regulation via the VSR in humans.

Melatonin attenuates the vestibulosympathetic but not vestibulocollic reflexes in humans: selective impairment of the utricles

Melatonin has been reported to decrease nerve activity of medial vestibular nuclei in the rat and is associated with attenuated muscle sympathetic nerve activity (MSNA) responses to baroreceptor unloading in humans. The purpose of this study was to determine if melatonin alters the vestibulosympathetic reflex (VSR) and vestibulocollic reflex (VCR) in humans. In study 1, MSNA, arterial blood pressure, and heart rate were measured in 12 healthy subjects (28 ± 1 yr; 6 men, 6 women) during head-down rotation (HDR) before and 45 min after ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the opposite treatment (melatonin or placebo). Melatonin significantly attenuated MSNA responses during HDR compared with placebo (burst frequency Δ 4 ± 1 vs. Δ 7 ± 1 bursts/min, and total MSNA Δ 51 ± 20 and Δ 96 ± 15%, respectively; P < 0.02). In study 2, vestibular evoked myogenic potentials (VEMP) were measured in 10 healthy subjects (26 ± 1 yr; 4 men and 6 women) before and after ingestion of 3 mg melatonin. Melatonin did not alter the timing of the p13 and n23 peaks (pre-melatonin 13.2 ± 0.4 and 21.3 ± 0.6 ms vs. post-melatonin 13.5 ± 0.4 and 21.4 ± 0.7 ms, respectively) or the p13-n23 interpeak amplitudes [pre-melatonin 22.5 ± 4.6 arbitrary units (au) and post-melatonin 22.7 ± 4.6 au]. In summary, melatonin attenuates the VSR and supports the concept that melatonin negatively affects orthostatic tolerance. However, melatonin does not alter the VCR in humans suggesting melatonin's effect on the VSR appears to be mediated by the utricles.

Menstrual cycle elicits divergent forearm vascular responses to vestibular activation in humans

The menstrual cycle has been reported to alter mean arterial pressure (MAP), but not muscle sympathetic nerve activity (MSNA), during vestibular activation. Specifically, MAP responses to head-down rotation (HDR) are augmented during the mid-luteal (ML) phase compared to the early follicular (EF) phase in young, eumenorrheic women. The purpose of the present study was to determine if the menstrual cycle influences vestibular-mediated changes in limb blood flow. MSNA, MAP, heart rate, and limb blood flow responses to HDR were measured in 12 healthy women. Resting MSNA, MAP, heart rate, forearm blood flow and calf blood flow were not altered by the menstrual cycle. HDR elicited similar increases in MSNA during the EF (Δ3 ± 1 bursts/min; P < 0.05) and ML (Δ2 ± 1 bursts/min; P < 0.05) phase, but only increased MAP during the ML phase (Δ4 ± 2 mm Hg; P < 0.05). HDR did not change heart rate during either the EF or ML phase. HDR elicited similar increases in calf vascular resistance during the EF (Δ6 ± 2 mm Hg/mL/100 mL/min; P < 0.05) and ML (Δ7 ± 2 mm Hg/mL/100 mL/min; P < 0.05) phases of the menstrual cycle. In contrast, HDR increased forearm vascular resistance during the ML phase (Δ4 ± 2 mm Hg/mL/100 mL/min; P < 0.05), but not the EF phase (Δ0 ± 2 mm Hg/mL/100 mL/min). These findings suggest an increased transduction of sympathetic nerve activity into forearm vascular resistance during the ML phase, and reveal the first recorded divergent vascular response to vestibular excitation in human limbs.

Vestibular control of arterial blood pressure during head-down postural change in anesthetized rabbits

This study was undertaken to elucidate neural control of the arterial blood pressure (ABP) in head-down postural change which causes both stimulation to the vestibular system and head-ward fluid shift. Experiments were carried out with urethane-anesthetized rabbits. The animal was mounted on a tilting table, tilted to 45 degrees head-down in 5 s, and kept at the position for 5 min. The head-down rotation (HDR) induced a transient decrease in ABP (10 +/- 3 mmHg; mean +/- SE), and then the pressure gradually recovered toward the pre-HDR level during the 5 min at the head-down position. Pretreatment with hexamethonium bromide, a ganglionic transmission blocker, suppressed the HDR-induced drop of ABP, suggesting that the ABP drop was induced by an inhibition of autonomic neural outflows. Renal sympathetic nerve activity (RSNA) decreased considerably after 1.6 +/- 0.2 s from the onset of HDR, which was followed by the ABP drop. Aortic depressor nerve activity (ADNA), an afferent for baroreceptor reflex, increased significantly during the rotation, but the peak of ADNA increase was 3.2 +/- 0.5 s after the initiation of the HDR. Therefore, the suppression of RSNA seems to be induced mainly by a quicker mechanism than baroreceptor reflex. In order to test the possibility, we examined changes in ABP and RSNA during HDR using vestibular-lesioned rabbits. In these rabbits, RSNA and ABP did not change significantly during HDR. These results suggest that vestibular organs play a role in the transient drop in ABP induced by HDR through the suppression of sympathetic nerve outflows.

Otolithic activation on visceral circulation in humans: effect of aging

Engagement of the otolith organs elicits differential activation of sympathetic nerve activity and vascular responses to muscle and skin in humans. Additionally, aging attenuates the otolith organ-mediated increases in muscle sympathetic nerve activity in older adults. In this study, we hypothesized that 1) the vestibulosympathetic reflex (VSR) would elicit visceral vascular vasoconstriction and 2) visceral vascular response to the VSR would be attenuated in older subjects compared with young. To test these hypotheses, heart rate, mean arterial blood pressure, and renal, celiac trunk, and superior mesenteric arterial blood velocity (Doppler ultrasound) were measured in 22 young (25+/-1 yr) and 18 older (65+/-2 yr) healthy subjects during head-down rotation (HDR), which selectively activates the otolith organs. Mean arterial pressure and heart rate did not change from baseline during HDR in young or older subjects. Renal blood velocity (Delta -2+/-1 cm/s) and vascular conductance (Delta -0.03+/-0.01 cm.s(-1).mmHg(-1)) significantly decreased from baseline during HDR (P<0.05) in young subjects. In contrast, renal blood velocity and conductance did not change in older subjects (Delta -0.2+/-1 cm/s and Delta0.02+/-0.08 mmHg.cm(-1).s(-1), respectively) during HDR. Superior mesenteric and celiac blood velocity and vascular conductance did not change in response to HDR in either the young or older subjects. These data suggest that renal vasoconstriction occurs during otolith organ activation in young but not older humans. Together with our previous studies, we conclude that the VSR elicits a diverse patterning of sympathetic outflow that results in heterogeneous vascular responses in humans and that these responses are significantly attenuated in older humans.

Greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans

Otolith organs have been shown to activate the sympathetic nervous system in the prone position by head-down rotation (HDR) in humans. To date, otolithic stimulation by HDR has not been comprehensively studied in the upright posture. The purpose of the present study was to determine whether otolithic stimulation increases muscle sympathetic nerve activity (MSNA) in the upright posture. It was hypothesized that stimulation of the otolith organs would increase MSNA in the upright posture, despite increased baseline sympathetic activation due to unloading of the baroreceptors. MSNA, arterial blood pressure, heart rate, and degree of head rotation were measured during HDR in 18 volunteers (23 +/- 1 yr) in different postures. Study 1 (n = 11) examined HDR in the prone and sitting positions and study 2 (n = 7) examined HDR in the prone and 60 degrees head-up tilt positions. Baseline MSNA was 8 +/- 4, 15 +/- 4, and 33 +/- 2 bursts/min for prone, sitting, and head-up tilt, respectively. HDR significantly increased MSNA in the prone (Delta4 +/- 1 and Delta105 +/- 37% for burst frequency and total activity, respectively), sitting (Delta5 +/- 1 and Delta43 +/- 12%), and head-up tilt (Delta7 +/- 1 and Delta110 +/- 41%; P < 0.05). Sensitivity of the vestibulosympathetic reflex (%DeltaMSNA/DeltaHDR; degree of head rotation) was significantly greater in the sitting and head-up tilt than prone position (prone = 74 +/- 22; sitting = 109 +/- 30; head-up tilt = 276 +/- 103; P < 0.05). These data indicate that stimulation of the otolith organs can mediate increases in MSNA in the upright posture and suggest a greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans.

Vestibulosympathetic reflex during the early follicular and midluteal phases of the menstrual cycle

Evidence suggests that both the arterial baroreflex and vestibulosympathetic reflex contribute to blood pressure regulation, and both autonomic reflexes integrate centrally in the medulla cardiovascular center. A previous report indicated increased sympathetic baroreflex sensitivity during the midluteal (ML) phase of the menstrual cycle compared with the early follicular (EF) phase. On the basis of this finding, we hypothesize an augmented vestibulosympathetic reflex during the ML phase of the menstrual cycle. Muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP), and heart rate responses to head-down rotation (HDR) were measured in 10 healthy females during the EF and ML phases of the menstrual cycle. Plasma estradiol (Delta72 +/- 13 pg/ml, P < 0.01) and progesterone (Delta8 +/- 2 ng/ml, P < 0.01) were significantly greater during the ML phase compared with the EF phase. The menstrual cycle did not alter resting MSNA, MAP, and heart rate (EF: 13 +/- 3 bursts/min, 80 +/- 2 mmHg, 65 +/- 2 beats/min vs. ML: 14 +/- 3 bursts/min, 81 +/- 3 mmHg, 64 +/- 3 beats/min). During the EF phase, HDR increased MSNA (Delta3 +/- 1 bursts/min, P < 0.02) but did not change MAP or heart rate (Delta0 +/- 1 mmHg and Delta1 +/- 1 beats/min). During the ML phase, HDR increased both MSNA and MAP (Delta4 +/- 1 bursts/min and Delta3 +/- 1 mmHg, P < 0.04) with no change in heart rate (Delta0 +/- 1 beats/min). MSNA and heart rate responses to HDR were not different between the EF and ML phases, but MAP responses to HDR were augmented during the ML phase (P < 0.03). Our results demonstrate that the menstrual cycle does not influence the vestibulosympathetic reflex but appears to alter MAP responses to HDR during the ML phase.