Neck afferents and muscle sympathetic activity in humans: implications for the vestibulosympathetic reflex

Vestibular-autonomic interactions: beyond orthostatic dizziness

PURPOSE OF REVIEW

This review aims to summarize the current literature describing vestibular-autonomic interactions and to describe their putative role in various disorders' clinical presentations, including orthostatic dizziness and motion sensitivity.

RECENT FINDINGS

The vestibular-autonomic reflexes have long been described as they relate to cardiovascular and respiratory function. Although orthostatic dizziness may be in part related to impaired vestibulo-sympathetic reflex (orthostatic hypotension), there are various conditions that may present similarly. A recent clinical classification aims to improve identification of individuals with hemodynamic orthostatic dizziness so that appropriate recommendations and management can be efficiently addressed. Researchers continue to improve understanding of the underlying vestibular-autonomic reflexes with recent studies noting the insular cortex as a cortical site for vestibular sensation and autonomic integration and modulation. Work has further expanded our understanding of the clinical presentation of abnormal vestibular-autonomic interactions that may occur in various conditions, such as aging, peripheral vestibular hypofunction, traumatic brain injury, and motion sensitivity.

SUMMARY

The vestibular-autonomic reflexes affect various sympathetic and parasympathetic functions. Understanding these relationships will provide improved identification of underlying etiology and drive improved patient management.

Neck movement but not neck position modulates skin sympathetic nerve activity supplying the lower limbs of humans

We previously showed that dynamic, but not static, neck displacement modulates muscle sympathetic nerve activity (MSNA) to lower limbs of humans. However, it is not known whether dynamic neck displacement modulates skin sympathetic nerve activity (SSNA). Tungsten microelectrodes inserted into the common peroneal nerve were used to record SSNA in 5 female and 4 male subjects lying supine on a table that fixed their head in space but allowed trapezoidal ramp (8.1 ± 1.2°/s) and hold (17.5° for 53 s) or sinusoidal (35° peak to peak at 0.33-0.46 Hz) horizontal displacement of the body about the head. SSNA recordings were made before, during, and after trapezoidal and sinusoidal displacements of the body. Spike frequency analysis of trapezoidal displacements and cross-correlation analysis during sinusoidal displacements revealed that SSNA was not changed by trapezoid body-only displacement but was cyclically modulated during sinusoidal angular displacements (median, 95% CI: 27.9%, 19.6-48.0%). The magnitude of this modulation was not statistically ( P > 0.05) different from that of cardiac and respiratory modulation at rest (47.1%, 18.7-56.3% and 48.6%, 28.4-59.3%, respectively) or during sinusoidal displacement (10.3%, 6.2-32.1% and 26.9%, 13.6-43.3%, respectively). Respiratory frequency was entrained above its resting rate (0.26 Hz, 0.2-0.29 Hz) during sinusoidal neck displacement; there was no significant difference ( P > 0.05) between respiratory frequency (0.38 Hz, 0.25-0.49 Hz) and sinusoidal displacement frequency (0.39 Hz, 0.35-0.42 Hz). This study provides evidence that SSNA is modulated during neck movement, raising the possibility that neck mechanoreceptors may contribute to the cutaneous vasoconstriction and sweat release associated with motion sickness. NEW & NOTEWORTHY This study demonstrates that dynamic, but not static, stretching of the neck modulates skin sympathetic nerve activity in the lower limbs.

Dual control of the vestibulosympathetic reflex following hypotension in rats

Orthostatic hypotension (OH) is associated with symptoms including headache, dizziness, and syncope. The incidence of OH increases with age. Attenuation of the vestibulosympathetic reflex (VSR) is also associated with an increased incidence of OH. In order to understand the pathophysiology of OH, we investigated the physiological characteristics of the VSR in the disorder. We applied sodium nitroprusside (SNP) to conscious rats with sinoaortic denervation in order to induce hypotension. Expression of pERK in the intermediolateral cell column (IMC) of the T4~7 thoracic spinal regions, blood epinephrine levels, and blood pressure were evaluated following the administration of glutamate and/or SNP. SNP-induced hypotension led to increased pERK expression in the medial vestibular nucleus (MVN), rostral ventrolateral medullary nucleus (RVLM) and the IMC, as well as increased blood epinephrine levels. We co-administered either a glutamate receptor agonist or a glutamate receptor antagonist to the MVN or the RVLM. The administration of the glutamate receptor agonists, AMPA or NMDA, to the MVN or RVLM led to elevated blood pressure, increased pERK expression in the IMC, and increased blood epinephrine levels. Administration of the glutamate receptor antagonists, CNQX or MK801, to the MVN or RVLM attenuated the increased pERK expression and blood epinephrine levels caused by SNP-induced hypotension. These results suggest that two components of the pathway which maintains blood pressure are involved in the VSR induced by SNP. These are the neurogenic control of blood pressure via the RVLM and the humoral control of blood pressure via epinephrine release from the adrenal medulla.

Vestibular Modulation of Sympathetic Nerve Activity to Muscle and Skin in Humans

We review the existence of vestibulosympathetic reflexes in humans. While several methods to activate the human vestibular apparatus have been used, galvanic vestibular stimulation (GVS) is a means of selectively modulating vestibular afferent activity via electrodes over the mastoid processes, causing robust vestibular illusions of side-to-side movement. Sinusoidal GVS (sGVS) causes partial entrainment of sympathetic outflow to muscle and skin. Modulation of muscle sympathetic nerve activity (MSNA) from vestibular inputs competes with baroreceptor inputs, with stronger temporal coupling to the vestibular stimulus being observed at frequencies remote from the cardiac frequency; “super entrainment” was observed in some individuals. Low-frequency (<0.2 Hz) sGVS revealed two peaks of modulation per cycle, with bilateral recordings of MSNA or skin sympathetic nerve activity, providing evidence of lateralization of sympathetic outflow during vestibular stimulation. However, it should be noted that GVS influences the firing of afferents from the entire vestibular apparatus, including the semicircular canals. To identify the specific source of vestibular input responsible for the generation of vestibulosympathetic reflexes, we used low-frequency (<0.2 Hz) sinusoidal linear acceleration of seated or supine subjects to, respectively, target the utricular or saccular components of the otoliths. While others had discounted the semicircular canals, we showed that the contributions of the utricle and saccule to the vestibular modulation of MSNA are very similar. Moreover, that modulation of MSNA occurs at accelerations well below levels at which subjects are able to perceive any motion indicates that, like vestibulospinal control of posture, the vestibular system contributes to the control of blood pressure through potent reflexes in humans.

Prone position results in enhanced pressor response to ephedrine compared with supine position during general anesthesia

STUDY OBJECTIVE

To elucidate and compare the pressor response to ephedrine in the prone or supine position during general anesthesia (GA).

DESIGN

Prospective cohort study.

SETTING

Department of General Surgery or Spine Surgery, Zhongda Hospital, Southeast University, Nanjing, China.

PATIENTS

Fifty-six patients who were scheduled to undergo elective surgery in the supine or prone position (n = 28 each) and using a generic GA protocol.

INTERVENTIONS

During surgery, the patients received intravenous (IV) ephedrine when their systolic blood pressure (SBP) decreased to 90 to 110 mm Hg.

MEASUREMENTS

Hemodynamic changes were measured at 1-minute intervals for 10 minutes and were compared with baseline.

MAIN RESULTS

Forty-nine patients (23 in the prone position and 26 in the supine position) completed the study. There were no significant differences between the groups with regard to demographic characteristics, hemodynamic parameters, end-tidal concentration of sevoflurane, and dose of propofol and remifentanil (all P> .05). After the bolus injection of ephedrine, a significant increase in SBP was observed in both groups compared to baseline, but the duration and magnitude of the increase in SBP were longer and greater in the prone position than in the supine position. The magnitude of increase of the mean blood pressure was significantly greater in the prone position compared to the supine position at 2 to 7 minutes after ephedrine injection. Ephedrine could cause significant increase in diastolic blood pressure 2 minutes after IV injection, which could last until at least 9 minutes in the prone position group compared to only for 5 minutes in the supine position group (all P< .05).

CONCLUSION

Compared to the supine position, the prone position could augment the pressor response to IV ephedrine during GA. Further studies are recommended to identify its association with other confounding factors such as surgery type or duration, patient history of cardiovascular disease, or patient hydration status.

Body height and arterial pressure in seated and supine young males during +2 G centrifugation

It is known that arterial pressure correlates positively with body height in males, and it has been suggested that this is due to the increasing vertical hydrostatic gradient from the heart to the carotid baroreceptors. Therefore, we tested the hypothesis that a higher gravito-inertial stress induced by the use of a human centrifuge would increase mean arterial pressure (MAP) more in tall than in short males in the seated position. In short (162–171 cm; n = 8) and tall (194–203 cm; n = 10) healthy males (18–41 yr), brachial arterial pressure, heart rate (HR), and cardiac output were measured during +2G centrifugation, while they were seated upright with the legs kept horizontal (+2Gz). In a separate experiment, the same measurements were done with the subjects supine (+2Gx). During +2Gz MAP increased in the short (22 ± 2 mmHg, P < 0.0001) and tall (23 ± 2 mmHg, P < 0.0001) males, with no significant difference between the groups. HR increased more ( P < 0.05) in the tall than in the short group (14 ± 2 vs. 7 ± 2 bpm). Stroke volume (SV) decreased in the short group (26 ± 4 ml, P = 0.001) and more so in the tall group (39 ± 5 ml, P < 0.0001; short vs. tall, P = 0.047). During +2Gx, systolic arterial pressure increased ( P < 0.001) and SV ( P = 0.012) decreased in the tall group only. In conclusion, during +2Gz, MAP increased in both short and tall males, with no difference between the groups. However, in the tall group, HR increased more during +2Gz, which could be caused by a larger hydrostatic pressure gradient from heart to head, leading to greater inhibition of the carotid baroreceptors.

The function of the autonomic nervous system during spaceflight

INTRODUCTION

Despite decades of study, a clear understanding of autonomic nervous system activity in space remains elusive. Differential interpretation of fundamental data has driven divergent theories of sympathetic activation and vasorelaxation.

METHODS

This paper will review the available in-flight autonomic and hemodynamic data in an effort to resolve these discrepancies. The NASA NEUROLAB mission, the most comprehensive assessment of autonomic function in microgravity to date, will be highlighted. The mechanisms responsible for altered autonomic activity during spaceflight, which include the effects of hypovolemia, cardiovascular deconditioning, and altered central processing, will be presented.

RESULTS

The NEUROLAB experiments demonstrated increased sympathetic activity and impairment of vagal baroreflex function during short-duration spaceflight. Subsequent non-invasive studies of autonomic function during spaceflight have largely reinforced these findings, and provide strong evidence that sympathetic activity is increased in space relative to the supine position on Earth. Others have suggested that microgravity induces a state of relative vasorelaxation and increased vagal activity when compared to upright posture on Earth. These ostensibly disparate theories are not mutually exclusive, but rather directly reflect different pre-flight postural controls.

CONCLUSION

When these results are taken together, they demonstrate that the effectual autonomic challenge of spaceflight is small, and represents an orthostatic stress less than that of upright posture on Earth. In-flight countermeasures, including aerobic and resistance exercise, as well short-arm centrifugation, have been successfully deployed to counteract these mechanisms. Despite subtle changes in autonomic activity during spaceflight, underlying neurohumoral mechanisms of the autonomic nervous system remain intact and cardiovascular function remains stable during long-duration flight.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Neck proprioceptors contribute to the modulation of muscle sympathetic nerve activity to the lower limbs of humans

Several different strategies have now been used to demonstrate that the vestibular system can modulate muscle sympathetic nerve activity (MSNA) in humans and thereby contribute to the regulation of blood pressure during changes in posture. However, it remains to be determined how the brain differentiates between head-only movements that do not require changes in vasomotor tone in the lower limbs from body movements that do require vasomotor changes. We tested the hypothesis that neck movements modulate MSNA in the lower limbs of humans. MSNA was recorded in 10 supine young adult subjects, at rest, during sinusoidal stretching of neck muscles (100 cycles, 35° peak to peak at 0.37 ± 0.02 Hz) and during a ramp-and-hold (17.5° for 54 ± 9 s) static neck muscle stretch, while their heads were held fixed in space. Cross-correlation analysis revealed cyclical modulation of MSNA during sinusoidal neck muscle stretch (modulation index 45.4 ± 5.3 %), which was significantly less than the cardiac modulation of MSNA at rest (78.7 ± 4.2 %). Interestingly, cardiac modulation decreased significantly during sinusoidal neck displacement (63.0 ± 9.3 %). By contrast, there was no significant difference in MSNA activity during static ramp-and-hold displacements of the neck to the right or left compared with that with the head and neck aligned. These data suggest that dynamic, but not static, neck movements can modulate MSNA, presumably via projections of muscle spindle afferents to the vestibular nuclei, and may thus contribute to the regulation of blood pressure during orthostatic challenges.

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.

Evidence for vestibular dysfunction in orthostatic hypotension

There is little definitive evidence of the clinical significance of the vestibular-cardiovascular reflex in humans, despite the fact that the vestibular system is known to contribute to cardiovascular control in animals. The present study involved 248 dizzy patients (127 male patients and 121 female patients) aged 65 years and younger. We classified all participants into three groups based on their vestibular evoked myogenic potential (VEMP) responses; absent VEMP, asymmetry VEMP and normal VEMP. To investigate the effect of the otolith disorder, which was estimated by the VEMP, on the orthostatic blood pressure responses, the subjects' systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate were monitored during the orthostatic test after they actively stood up. The male patients in the absent VEMP group had a significant drop in their DBP at 1 min after active standing up (P < 0.05) without any change in their SBP. Conversely, male patients in the asymmetry VEMP and normal VEMP groups showed a significant increase in the SBP at 1 min after active standing up (P < 0.05). Female patients in the absent VEMP group did not show any significant drop in their blood pressure after standing up (P > 0.05). In the entire group of participants, a total of 19.6% of the patients in the absent VEMP group fulfilled the criteria for orthostatic hypotension (OH), which was significantly > the 8.6% of patients in the normal VEMP group and the 7.2% in the asymmetry VEMP group (P < 0.05). Our results suggest that vestibular disorders due to the dysfunction of otolith organs provoke OH.

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.

Low-frequency sinusoidal galvanic stimulation of the left and right vestibular nerves reveals two peaks of modulation in muscle sympathetic nerve activity

Studies previously performed in our laboratory have shown that sinusoidal galvanic vestibular stimulation (sGVS), a means of selectively modulating vestibular input without affecting other inputs, can cause partial entrainment of muscle sympathetic nerve activity (MSNA) at frequencies ranging from 0.2 to 2.0 Hz. Here we test the effect of sGVS on sympathetic outflow when stimulating the vestibular system at lower frequencies. MSNA was recorded via tungsten microelectrodes inserted into the left common peroneal nerve in 12 awake, seated subjects. Bipolar binaural sinusoidal GVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08, 0.13 and 0.18 Hz. Cross-correlation analysis revealed two bursts of modulation of MSNA for each cycle of stimulation. We believe the primary peak is related to the positive phase of the sinusoid, in which the right vestibular nerve is hyperpolarised and the left vestibular nerve depolarised. Furthermore, we believe the secondary peak is related to the negative phase of the sinusoid (depolarisation of the right vestibular nerve and hyperpolarisation of the left vestibular nerve). This was never observed at higher frequencies of stimulation, presumably because at such frequencies there is insufficient time for a second peak to be expressed. The incidence of double peaks of MSNA was highest at 0.08 Hz and lowest at 0.18 Hz. These observations emphasise the role of the vestibular apparatus in the control of blood pressure, and further suggest convergence of bilateral inputs from vestibular nuclei onto the output nuclei from which MSNA originates, the rostral ventrolateral medulla (RVLM).

Role of the vestibular system in the arterial pressure response to parabolic-flight-induced gravitational changes in human subjects

Arterial pressure (AP) is known to fluctuate during parabolic-flight-induced gravitational changes in human subjects, increasing during hypergravity and decreasing during microgravity. In this study, we examined whether the vestibular system participates in the AP response to the gravitational changes induced by parabolic flight in human subjects. Eight subjects performed parabolic flights in a supine position as their AP was measured. Their vestibular inputs during the gravitational changes were reversibly masked by artificial electrical stimulation (galvanic vestibular stimulation, GVS). The AP responses during the parabolas were then compared between the GVS-off and GVS-on conditions. AP increased during hypergravity and decreased during microgravity. The AP responses at the onset of hypergravity and microgravity were abolished by GVS. These results indicate that the vestibular system elicits pressor and depressor responses during parabolic-flight-induced hypergravity and microgravity, respectively.

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.

Frequency-dependent modulation of muscle sympathetic nerve activity by sinusoidal galvanic vestibular stimulation in human subjects

We have previously demonstrated that selective modulation of vestibular inputs, via sinusoidal galvanic vestibular stimulation (GVS) delivered at 0.5-0.8 Hz, can cause partial entrainment of muscle sympathetic nerve activity (MSNA). Given that we had seen interaction between the dynamic vestibular input and the normal cardiac-locked MSNA rhythm, we tested the hypothesis that frequencies of GVS remote from the cardiac frequency would cause a greater modulation of MSNA than those around the cardiac frequency. Bipolar binaural sinusoidal GVS (+/-2 mA, 200 cycles) was applied to the mastoid processes in 11 seated subjects at frequencies of 0.2, 0.5, 0.8, 1.1, 1.4, 1.7 and 2.0 Hz. In all subjects, the stimulation evoked robust vestibular illusions of "rocking in a boat" or "swinging from side to side." Cross-correlation analysis revealed a cyclic modulation of MSNA at all frequencies, with the modulation index being similar between 1.1 Hz (78.5 +/- 3.7%) and 2.0 Hz (77.0 +/- 4.3%). However, vestibular modulation of MSNA was significantly stronger at 0.2 Hz (93.1 +/- 1.7%) and significantly weaker at 0.8 Hz (67.2 +/- 1.8%). The former suggests that low-frequency changes in vestibular input, such as those associated with postural changes, preferentially modulate MSNA; the latter suggests that vestibular inputs compete with the stronger baroreceptor inputs operating at the cardiac rhythm (approximately 0.8 Hz), with vestibular modulation of MSNA being greater when this competition with the baroreceptors is reduced.

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.

Sympathetic responses to vestibular activation in humans

Activation of sympathetic neural traffic via the vestibular system is referred to as the vestibulosympathetic reflex. Investigations of the vestibulosympathetic reflex in humans have been limited to the past decade, and the importance of this reflex in arterial blood pressure regulation is still being determined. This review provides a summary of sympathetic neural responses to various techniques used to engage the vestibulosympathetic reflex. Studies suggest that activation of the semicircular canals using caloric stimulation and yaw rotation do not modulate muscle sympathetic nerve activity (MSNA) or skin sympathetic nerve activity (SSNA). In contrast, activation of the otolith organs appear to alter MSNA, but not SSNA. Specifically, head-down rotation and off-vertical axis rotation increase MSNA, while sinusoidal linear accelerations decrease MSNA. Galvanic stimulation, which results in a nonspecific activation of the vestibule, appears to increase MSNA if the mode of delivery is pulse trained. In conclusion, evidence strongly supports the existence of a vestibulosympathetic reflex in humans. Furthermore, attenuation of the vestibulosympathetic reflex is coupled with a drop in arterial blood pressure in the elderly, suggesting this reflex may be important in human blood pressure regulation.

Linear acceleration-evoked cardiovascular responses in awake rats

It has been well documented that vestibular-mediated cardiovascular regulation plays an important role in maintaining stable blood pressure (BP) during postural changes. But the underlying neural mechanisms remain to be elucidated. In particular, because the vestibular stimulation employed in previous animal studies activated both semicircular canals and otolith organs, the contributions of the otolith system has not been studied selectively. The goal of the present study was to characterize cardiovascular responses to natural otolith stimulation in awake rats that were subjected to pure linear motion. In any of the four directions tested, transient linear motion produced a short-latency (∼520 ms) increase in mean BP with a peak of 8.27 ± 0.66 mmHg and was followed by a decrease in BP. There was an initial small biphasic response in heart rate (HR) that was followed by a longer duration increase. The short-latency increase in BP was absent in rats that were pentobarbital sodium anesthetized or that were labyrinthectomized bilaterally, but it was unaffected by baroreceptor denervation, indicating that it was of otolith origin. The increase in BP was linear acceleration intensity dependent and was not affected by absence of visual cues. Furthermore, the BP response was attenuated by inactivation of the medial and inferior vestibular nuclei by microinjections of muscimol, indicating that the otolith-driven cardiovascular responses are mediated by the neurons in these areas. These results not only demonstrate the otolith specific influences on the cardiovascular system but also they establish the first rodent model for examining the neural mechanisms underlying the otolith-mediated cardiovascular regulation.

Effect of baroreflex loading on the responsiveness of the vestibulosympathetic reflex in humans

Activation of the vestibular otolith organs with head-down rotation (HDR) increases muscle sympathetic nerve activity (MSNA) in humans. Previously, we demonstrated this vestibulosympathetic reflex (VSR) elicits increases in MSNA during baroreflex unloading (i.e., lower body negative pressure) in humans. Whether such an effect persists during baroreflex loading is unknown. We tested the hypothesis that the ability of the VSR to increase MSNA is preserved during baroreflex unloading and inhibited during baroreflex loading. Ten subjects (26 +/- 1 yr) performed three trials of HDR to activate the VSR. These trials were performed after a period of sustained saline (control), nitroprusside (baroreflex unloading: 0.8-1.0 microg.kg(-1).min(-1)), and phenylephrine (baroreflex loading: 0.6-0.8 microg.kg(-1).min(-1)) infusion. Nitroprusside infusion decreased (Delta7 +/- 1 mmHg, where Delta is change; P < 0.001) and phenylephrine infusion increased mean arterial pressure (Delta8 +/- 1 mmHg; P < 0.001) at rest. HDR performed during the control [Delta3 +/- 2 bursts/min, Delta314 +/- 154 arbitrary units (au) total activity, Delta41 +/- 18% total activity; P < 0.05] and nitroprusside trials [Delta5 +/- 2 bursts/min, Delta713 +/- 241 au total activity, Delta49 +/- 20% total activity; P < 0.05] increased MSNA similarly despite significantly elevated levels at rest (13 +/- 2 to 26 +/- 3 bursts/min) in the latter. In contrast, HDR performed during the phenylephrine trial failed to increase MSNA (Delta0 +/- 1 bursts/min, Delta-15 +/- 33 au total activity, Delta-8 +/- 21% total activity). These results confirm previous findings that the ability of the VSR to increase MSNA is preserved during baroreflex unloading. In contrast, the ability of the VSR to increase MSNA is abolished during baroreflex loading. These results provide further support for the concept that the VSR may act primarily to defend against hypotension in humans.

Modulation of muscle sympathetic bursts by sinusoidal galvanic vestibular stimulation in human subjects

There is controversy as to whether the vestibulosympathetic reflexes demonstrated in experimental animals actually exist in human subjects. While head-down neck flexion and off-vertical axis rotation can increase muscle sympathetic nerve activity (MSNA) in awake subjects, we recently showed that bipolar galvanic vestibular stimulation (GVS) does not. However, it is possible that our stimuli (2 mA, 1 s)-although capable of causing strong postural and occulomotor responses-were too brief. To address this issue we activated vestibular afferents using continuous sinusoidal (0.5-0.8 Hz, 60-100 cycles, +/-2 mA) bipolar binaural GVS in 11 seated subjects. Sinusoidal GVS evoked robust vestibular illusions of "rocking in a boat" or "swinging from side to side." Cross-correlation analysis revealed a cyclic modulation of MSNA ranging from 31 to 86% across subjects (mean +/- SE 58 +/- 5%), with total MSNA increasing by 156 +/- 19% (P = 0.001). Furthermore, we documented de novo synthesis of sympathetic bursts that were coupled to the sinusoidal input, such that two bursts-rather than the obligatory single burst-could be generated within a cardiac interval. This demonstrates that the human vestibular apparatus exerts a potent facilitatory influence on MSNA that potentially operates independently of the baroreceptor system.

Influence of innocuous cervical vertebral movement on the efferent innervation of the adrenal gland in the rat

In general, in central nervous system intact anesthetized animals, adrenal sympathetic efferent nerve activity and catecholamine secretion increase in response to noxious somatic stimulation, and decrease in response to innocuous somatic stimulation. In anesthetized rats, noxious chemical stimulation of the thoracic and lumbar interspinous tissues is associated with large increases in adrenal sympathetic efferent nerve activity and catecholamine secretion, with a clear segmental organization to the reflex apparent in spinalized animals. However, the adrenal sympathetic nerve responses to mechanical stimulation in the form of pressure applied laterally to the lower thoracic and lower lumbar vertebrae do not display segmental organization, and the depressor response is more characteristic of responses to innocuous somatic stimulation despite the use of large forces (up to 3.0 kg). Therefore, we sought to determine whether innocuous movements of the mechanoreceptor-rich deep tissues of the neck modulate the sympathetic outflow to the adrenal gland. We performed experiments in 14 anaesthetised (Urethane 1 g/kg and Chloralose 0.1 g/kg) adult rats. Rats were intubated and breathed spontaneously. A computer driven small animal manipulator was used to impose ramp and hold rotational displacements (12 degrees /s, hold duration 2 s) of the 2nd cervical vertebra (range 2-30 degrees ) while recording multi-unit activity from sympathetic nerves innervating the adrenal gland. While noxious forepaw pinch elicited an increase in sympathetic nerve activity to the adrenal gland, there was no significant change in sympathetic nerve activity with small (2 degrees or 6 degrees ) rotations. Significant changes (P<0.05) in sympathetic activity were observed in only 7% (n=21) of all trials at larger displacements (12 degrees , 20 degrees , 25 degrees , 30 degrees n=287 trials). Our data suggest that although noxious stimuli may modulate sympathetic outflow, it is rare for afferents signalling innocuous cervical vertebral movements to modulate sympathetic nerves innervating the adrenal gland.

Whole-Body Heating Decreases Skin Vascular Response to Low Orthostatic Stress in the Lower Extremities

To elucidate the influence of heat stress on cutaneous vascular response in the lower extremities during orthostatic stress, a head-up tilt (HUT) test at angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees for 4 min each was conducted under normothermic control conditions followed by whole-body heat stress produced by a hot water-perfused suit in healthy volunteers. Skin blood flows (SkBF) in the forearm, thigh, and calf were monitored using laser-Doppler flowmetry throughout the experiment. Furthermore, to elucidate the effects of increased core and local skin temperatures on the local vascular response in calf skin under increasing orthostatic stress, the thigh was occluded at 20, 30, 50, 70, and 80 mmHg with a cuff in both the normothermic condition and the whole-body or local heating condition. Significant decreases in forearm SkBF during HUT were observed at an angle of 60 degrees during normothermia and at 30 degrees or more during heating. SkBF in the thigh and calf was decreased significantly by HUT at 15 degrees and above during normothermia, and there was no significant reduction of SkBF in these sites during HUT at the lower angles (15 degrees -45 degrees ) during whole-body heating. Significant decreases of calf SkBF were observed at cuff pressures of 20 mmHg and above during normothermia and of 30 mmHg and above during whole-body and local heating, respectively. These results suggest that SkBF in the lower extremities shows a marked reduction compared with the upper extremities during low orthostatic stress in normothermia, and the enhanced skin vasoconstrictor response in the lower extremities is diminished by both whole-body and local heat stress.

P6 acupressure increases tolerance to nauseogenic motion stimulation in women at high risk for PONV

PURPOSE

In a previous study we noticed that P6 acupressure decreased postoperative nausea and vomiting (PONV) more markedly after discharge. As motion sickness susceptibility is increased by, for example, opioids we hypothesized that P6 acu-pressure decreased PONV by decreasing motion sickness susceptibility. We studied time to nausea by a laboratory motion challenge in a group of volunteers, during P6 and placebo acupressure.

METHODS

60 women with high and low susceptibilities for motion sickness participated in a randomized and double-blind study with an active P6 acupressure, placebo acupressure, and a control group (n = 20 in each group). The risk score for PONV was over 50%. The motion challenge was by eccentric rotation in a chair, blindfolded and with chin to chest movements of the head. The challenge was stopped when women reported moderate nausea. Symptoms were recorded.

RESULTS

Mean time to moderate nausea was longer in the P6 acu-pressure group compared to the control group. P6 acupressure = 352 (259-445), mean (95% confidence interval) in seconds, control = 151 (121-181) and placebo acupressure = 280 (161-340); (P = 0.006). No difference was found between P6 and placebo acupressure or placebo acupressure and control groups. Previous severity of motion sickness did not influence time to nausea (P = 0.107). The cumulative number of symptoms differed between the three groups (P < 0.05). Fewer symptoms were reported in the P6 acupressure compared to the control group P < 0.009. Overall, P6 acupressure was only marginally more effective than placebo acupressure on the forearms.

CONCLUSION

In females with a history of motion sickness P6 acu-pressure increased tolerance to experimental nauseogenic stimuli, and reduced the total number of symptoms reported.

Effects of bilateral vestibular nucleus lesions on cardiovascular regulation in conscious cats

The vestibular system participates in cardiovascular regulation during postural changes. In prior studies (Holmes MJ, Cotter LA, Arendt HE, Cas SP, and Yates BJ. Brain Res 938: 62-72, 2002, and Jian BJ, Cotter LA, Emanuel BA, Cass SP, and Yates BJ. J Appl Physiol 86: 1552-1560, 1999), transection of the vestibular nerves resulted in instability in blood pressure during nose-up body tilts, particularly when no visual information reflecting body position in space was available. However, recovery of orthostatic tolerance occurred within 1 wk, presumably because the vestibular nuclei integrate a variety of sensory inputs reflecting body location. The present study tested the hypothesis that lesions of the vestibular nuclei result in persistent cardiovascular deficits during orthostatic challenges. Blood pressure and heart rate were monitored in five conscious cats during nose-up tilts of varying amplitude, both before and after chemical lesions of the vestibular nuclei. Before lesions, blood pressure remained relatively stable during tilts. In all animals, the blood pressure responses to nose-up tilts were altered by damage to the medial and inferior vestibular nuclei; these effects were noted both when animals were tested in the presence and absence of visual feedback. In four of the five animals, the lesions also resulted in augmented heart rate increases from baseline values during 60 degrees nose-up tilts. These effects persisted for longer than 1 wk, but they gradually resolved over time, except in the animal with the worst deficits. These observations suggest that recovery of compensatory cardiovascular responses after loss of vestibular inputs is accomplished at least in part through plastic changes in the vestibular nuclei and the enhancement of the ability of vestibular nucleus neurons to discriminate body position in space by employing nonlabyrinthine signals.

Aging, opioid-receptor agonists and antagonists, and the vestibulosympathetic reflex in humans

Animal studies indicate that opioids inhibit the firing rate of vestibular neurons, which are important in mediating the vestibulosympathetic reflex. Furthermore, this inhibition appears to be greater in more mature rats. In the present study, we tested the hypotheses that opioids inhibit the vestibulosympathetic reflex in humans and that endogenous opioids contribute to the age-related impairment of the vestibulosympathetic reflex. These hypotheses were tested by measuring muscle sympathetic nerve activity (MSNA), arterial blood pressure, and heart rate responses to otolith organ engagement during head-down rotation (HDR) in young (24 ± 2 yr old) and older (63 ± 2 yr) subjects before and after administration of either an opioid-receptor antagonist (16 mg naloxone in 9 young and 8 older subjects) or an opioid-receptor agonist (60 mg codeine in 7 young and 7 older subjects). Naloxone did not augment the reflex increase in MSNA during HDR in young (Δ7 ± 2 vs. Δ4 ± 2 bursts/min and Δ81 ± 23 vs. Δ60 ± 24% change in burst frequency and total MSNA before and after naloxone, respectively) or older subjects (Δ2 ± 2 vs. Δ1 ± 2 burst/min and Δ8 ± 7 vs. Δ8 ± 9% before and after naloxone). Similarly, codeine did not attenuate the increase in MSNA during HDR in young (Δ8 ± 1 vs. Δ7 ± 2 bursts/min and Δ53 ± 4 vs. Δ64 ± 16% before and after codeine) or older subjects (Δ6 ± 4 vs. Δ3 ± 3 bursts/min and Δ38 ± 21 vs. Δ33 ± 20%). Mean arterial blood pressure and heart rate responses to HDR were not altered by either naloxone or codeine. These data do not provide experimental support for the concept that opioids modulate the vestibulosympathetic reflex in humans. Moreover, endogenous opioids do not appear to contribute the age-associated impairment of the vestibulosympathetic reflex.

Human cerebrovascular and autonomic rhythms during vestibular activation

Otolith activation increases muscle sympathetic nerve activity (MSNA), and MSNA activation may alter associations among autonomic oscillators, including those modulating cerebral hemodynamics. The purpose of this study was to determine the influence of vestibulosympathetic activation on cerebral and autonomic rhythms. We recorded the ECG, finger arterial pressure, end-tidal CO2, respiration, cerebral blood flow velocity, and MSNA in eight subjects. Subjects breathed at 0.25 Hz for 5 min in the prone and head-down positions. We analyzed data in time and frequency domains and performed cross-spectral analyses to determine coherence and transfer function magnitude. Head-down rotation increased MSNA from 7 ± 1.3 to 12 ± 1.5 bursts/min ( P = 0.001) but did not affect R-R intervals, arterial pressures, mean cerebral blood flow velocities ( Vmean), or their power spectra. Vestibular activation with head-down rotation had no effect on mean arterial pressure and Vmean transfer function magnitude. The two new findings from this study are 1) head-down rotation independently activates the sympathetic nervous system with no effect on parasympathetic activity or Vmean; and 2) frequency-dependent associations between arterial pressures and Vmean are independent of vestibular activation. These findings support the concept that vestibular-autonomic interactions independently and redundantly serve to maintain steady-state hemodynamics.

Roles of baroreflex and vestibulosympathetic reflex in controlling arterial blood pressure during gravitational stress in conscious rats

Gravity acts on the circulatory system to decrease arterial blood pressure (AP) by causing blood redistribution and reduced venous return. To evaluate roles of the baroreflex and vestibulosympathetic reflex (VSR) in maintaining AP during gravitational stress, we measured AP, heart rate (HR), and renal sympathetic nerve activity (RSNA) in four groups of conscious rats, which were either intact or had vestibular lesions (VL), sinoaortic denervation (SAD), or VL plus SAD (VL + SAD). The rats were exposed to 3 G in dorsoventral axis by centrifugation for 3 min. In rats in which neither reflex was functional (VL + SAD group), RSNA did not change, but the AP showed a significant decrease (-8 +/- 1 mmHg vs. baseline). In rats with a functional baroreflex, but no VSR (VL group), the AP did not change and there was a slight increase in RSNA (25 +/- 10% vs. baseline). In rats with a functional VSR, but no baroreflex (SAD group), marked increases in both AP and RSNA were observed (AP 31 +/- 6 mmHg and RSNA 87 +/- 10% vs. baseline), showing that the VSR causes an increase in AP in response to gravitational stress; these marked increases were significantly attenuated by the baroreflex in the intact group (AP 9 +/- 2 mmHg and RSNA 38 +/- 7% vs. baseline). In conclusion, AP is controlled by the combination of the baroreflex and VSR. The VSR elicits a huge pressor response during gravitational stress, preventing hypotension due to blood redistribution. In intact rats, this AP increase is compensated by the baroreflex, resulting in only a slight increase in AP.

Absence of short-term vestibular modulation of muscle sympathetic outflow, assessed by brief galvanic vestibular stimulation in awake human subjects

There is evidence in experimental animals for a potent vestibulosympathetic reflex, but its existence in humans is controversial. Static head-down neck flexion and off-vertical axis rotation have been shown to increase muscle sympathetic nerve activity (MSNA), but not skin sympathetic nerve activity (SSNA), whereas horizontal linear acceleration decreases MSNA in humans. However, both forms of stimuli also activate other receptors. To examine the effects of a pure vestibular stimulus on MSNA and SSNA, and its potential interaction with the baroreceptors, we used galvanic vestibular stimulation (GVS) in 12 healthy seated subjects. MSNA was recorded in ten subjects via a percutaneous microelectrode in the peroneal nerve; ECG, blood pressure, respiration, skin blood flow and sweating were also recorded. GVS (2 mA, 1 s pulse) was delivered via surface electrodes over the mastoid processes at unexpected times, triggered from the R-wave with a delay of 0, 200, 400 or 600 ms. In addition to causing robust postural illusions, GVS caused cutaneous vasoconstriction and sweat release in all subjects (due to a short-latency increase in SSNA, three subjects), but no significant change in MSNA. The failure of GVS to elicit a change in muscle sympathetic nerve activity, as documented by averaging, suggests that the vestibular system is not engaged in short-term modulation of muscle sympathetic activity. Conversely, phasic vestibular inputs do excite cutaneous sympathetic neurones, consistent with the observation that motion sickness is accompanied by pallor and sweating.

Aging attenuates the vestibulorespiratory reflex in humans

Activation of the vestibular system changes ventilation in humans. The purpose of the present study was to investigate whether aging alters the vestibulorespiratory reflex in humans. Because aging attenuates the vestibulosympathetic reflex, it was hypothesized that aging would attenuate the vestibulorespiratory reflex. Changes in ventilation during engagement of the semicircular canals and/or the otolith organs were measured in fourteen young (26 +/- 1 years) and twelve older subjects (66 +/- 1 years). In young subjects, natural engagement of the semicircular canals and the otolith organs by head rotation increased breathing frequency during dynamic upright pitch at 0.25 Hz (15 cycles min-1) and 0.5 Hz (30 cycles min-1) (delta2 +/- 1 and delta4 +/- 1 breaths min-1, respectively; P < 0.05) and during dynamic upright roll (delta2 +/- 1 and delta4 +/- 1, respectively; P < 0.05). In older subjects, the only significant changes in breathing frequency occurred during dynamic pitch and roll at 0.5 Hz (delta2 +/- 1 and delta2 +/- 1 for pitch and roll, respectively). Stimulation of the horizontal semicircular canals by yaw rotation increased minute ventilation in young but not older subjects. Selective engagement of the otolith organs during static head-down rotation did not alter breathing frequency in either the young or older subjects. The results of this study indicate that the vestibulorespiratory reflex is attenuated in older humans, with greater vestibular stimulation needed to activate the reflex.

Sympathetic nerve activity in hypotension and orthostatic intolerance

AIM

The present paper reviews how changes in sympathetic nerve activity are related to hypotensive episodes and orthostatic intolerance in humans.

RESULTS

It has been well documented that sympathetic neural traffic to skeletal muscles (muscle sympathetic nerve activity; MSNA) plays an essential role in maintaining blood pressure homeostasis mainly through baroreflex. The MSNA responded to gravitational loading from the head to the leg (+Gz) during passive head-up tilt (HUT). Patients who suffered from orthostatic hypotension with or without syncope were classified into at least two groups; low and high responders of MSNA to orthostatic loading. The typical examples belonging to the former group were patients of multiple system atrophy who had very low basal sympathetic outflow to muscle which responded extremely poorly to HUT. Patients of multiple system atrophy presented also postprandial hypotension in which muscle sympathetic response to oral glucose administration was absent. The latter group was represented by subjects who manifested vasovagal syncope with normal or even higher muscle sympathetic response to HUT, which was suddenly withdrawn concomitantly with bradycardia and hypotension. Similar withdrawal of sympathetic nerve traffic to muscle was encountered in a rare case of idiopathic non-orthostatic episodic hypotension which accompanied bradycardia. The MSNA was suppressed by short-term exposure to microgravity but was enhanced after long-term exposure to microgravity. Orthostatic intolerance after long-term exposure to microgravity was related to progressive reduction of muscle sympathetic response to orthostatic loading with impaired arterial baroreflex.

CONCLUSION

It is concluded that hypotensive episodes are closely related to poor or lack of muscle sympathetic outflow, but may depend on various neural mechanisms to induce it.

Vestibular activation of sympathetic nerve activity

AIM

The vestibulosympathetic reflex refers to sympathetic nerve activation by the vestibular system. Animal studies indicate that the vestibular system assists in blood pressure regulation during orthostasis. Although human studies clearly demonstrate activation of muscle sympathetic nerve activity (MSNA) during engagement of the otolith organs, the role of the vestibulosympathetic reflex in maintaining blood pressure during orthostasis is not well-established. Examination of the vestibulosympathetic reflex with other cardiovascular reflexes indicates that it is a powerful and independent reflex. Ageing, which is associated with an increased risk for orthostatic hypotension, attenuates the vestibulosympathetic reflex. The attenuated reflex is associated with a reduction in arterial pressure.

CONCLUSION

These findings suggest that the vestibulosympathetic reflex assists in blood pressure regulation in humans, but future studies examining this reflex in other orthostatically intolerant populations are necessary to address this hypothesis.

Head position modifies cerebrovascular response to orthostatic stress

Previous experiments have shown that the vestibular system participates in cardiovascular control. However, the effects of vestibular activation on cerebrovascular regulation are not known. Therefore, the present experiment tested the hypothesis that specific vestibular activations may be beneficial to cerebral circulation during simulated orthostatic stress. Middle cerebral artery blood flow velocity (CBV; Doppler ultrasound) was measured to examine the effects of head-down neck flexion (HDNF) compared to head-down neck extension (HDNE) with and without lower body negative pressure (LBNP; -40 mmHg) (n=9). The change in CBV (DeltaCBV) during HDNF and HDNE were not different during baseline conditions, however, during LBNP, DeltaCBV was greater in HDNE compared to HDNF (-5.5+/-3.2 cm/s, -11+/-4.6%) vs. (-0.7+/-1.0 cm/s, -1.9+/-1.9%), respectively (P<0.05). Concomitantly, the change in cerebrovascular resistance (DeltaCVR) between rest and LBNP was also greater during HDNE (0.48+/-0.08 mmHg/cm per s, 42.8+/-10.8%) compared with HDNF (0.26+/-0.05 mmHg/cm per s, 22+/-4.1%) (P<0.05). P(ET)CO(2) was greater in HDNE (45+/-2 mmHg) compared to HDNF (42+/-2 mmHg; P<0.05) during LBNP. These results suggest that the vestibular system may affect cerebrovascular tone during simulated postural stress by either constriction or dilation, depending on the vestibular stimulus.

Vestibulosympathetic reflex during orthostatic challenge in aging humans

Aging attenuates the increase in muscle sympathetic nerve activity (MSNA) and elicits hypotension during otolith organ engagement in humans. The purpose of the present study was to determine the neural and cardiovascular responses to otolithic engagement during orthostatic stress in older adults. We hypothesized that age-related impairments in the vestibulosympathetic reflex would persist during orthostatic challenge in older subjects and might compromise arterial blood pressure regulation. MSNA, arterial blood pressure, and heart rate responses to head-down rotation (HDR) performed with and without lower body negative pressure (LBNP) in prone subjects were measured. Ten young (27 +/- 1 yr) and 11 older subjects (64 +/- 1 yr) were studied prospectively. HDR performed alone elicited an attenuated increase in MSNA in older subjects (Delta106 +/- 28 vs. Delta20 +/- 7% for young and older subjects). HDR performed during simultaneous orthostatic stress increased total MSNA further in young (Delta53 +/- 15%; P < 0.05) but not older subjects (Delta-5 +/- 4%). Older subjects demonstrated consistent significant hypotension during HDR performed both alone (Delta-6 +/- 2 mmHg) and during LBNP (Delta-7 +/- 2 mmHg). These data provide experimental support for the concept that age-related impairments in the vestibulosympathetic reflex persist during orthostatic challenge in older adults. Furthermore, these findings are consistent with the concept that age-related alterations in vestibular function might contribute to altered orthostatic blood pressure regulation with age in humans.

Vestibulosympathetic reflex during mental stress

Increases in sympathetic neural activity occur independently with either vestibular or mental stimulation, but it is unknown whether sympathetic activation is additive or inhibitive when both stressors are combined. The purpose of the present study was to investigate the combined effects of vestibular and mental stimulation on sympathetic neural activation and arterial pressure in humans. Muscle sympathetic nerve activity (MSNA), arterial pressure, and heart rate were recorded in 10 healthy volunteers in the prone position during 1) head-down rotation (HDR), 2) mental stress (MS; using arithmetic), and 3) combined HDR and MS. HDR significantly (P < 0.05) increased MSNA (9 +/- 2 to 13 +/- 2 bursts/min). MS significantly increased MSNA (8 +/- 2 to 13 +/- 2 bursts/min) and mean arterial pressure (87 +/- 2 to 101 +/- 2 mmHg). Combined HDR and MS significantly increased MSNA (9 +/- 1 to 16 +/- 2 bursts/min) and mean arterial pressure (89 +/- 2 to 100 +/- 3 mmHg). Increases in MSNA (7 +/- 1 bursts/min) during the combination trial were not different from the algebraic sum of each trial performed alone (8 +/- 2 bursts/min). We conclude that the interaction for MSNA and arterial pressure is additive during combined vestibular and mental stimulation. Therefore, vestibular- and stress-mediated increases of MSNA appear to occur independently in humans.

Do vestibular otolith organs participate in human orthostatic blood pressure control?

We hypothesized that vestibular otolith organ stimulation contributes to human orthostatic responses. Twelve subjects underwent three 60 degrees upright tilts: (1) with the neck flexed from 0 degrees to 30 degrees relative to the body during 60 degrees tilt, such that the head moved from horizontal to 90 degrees above horizontal (0 to 1 Gz otolith stimulation); (2) with the head and body aligned, such that they tilted together to 60 degrees (0 to 0.87 Gz otolith stimulation); and (3) with the neck flexed 30 degrees relative to the body during supine conditions, and the neck then extended to -30 degrees during 60 degrees body tilting, such that the head remained at 30 degrees above horizontal throughout body tilting (constant 0.5 Gz otolith stimulation). All three tilt procedures increased thoracic impedance, sympathetic nerve activity (N = 8 of 12), arterial pressure, and heart rate relative to supine conditions (all P < 0.04). Within the first 20 s of tilt, arterial pressure increased most obviously in the 0 to 1 Gz otolith condition. Thoracic impedance tended to increase more in otolith-constant conditions, but no dependent variable differed significantly between tilt conditions, and no significant time x tilt interactions emerged. Otolith inputs may contribute to early transient adjustments to orthostasis. However, lack of significant main effects of tilt condition and time x tilt interactions suggests that potential otolith effects on the variables we studied are relatively subtle and ephemeral, or that other mechanisms compensate for a lack of change in otolith input with orthostasis.

Influence of vestibular activation on respiration in humans

The purpose of this study was to determine the effects of the semicircular canals and otolith organs on respiration in humans. On the basis of animal studies, we hypothesized that vestibular activation would elicit a vestibulorespiratory reflex. To test this hypothesis, respiratory measures, arterial blood pressure, and heart rate were measured during engagement of semicircular canals and/or otolith organs. Dynamic upright pitch and roll (15 cycles/min), which activate the otolith organs and semicircular canals, increased respiratory rate (Delta2 +/- 1 and Delta3 +/- 1 breaths/min, respectively; P < 0.05). Dynamic yaw and lateral pitch (15 cycles/min), which activate the semicircular canals, increased respiration similarly (Delta3 +/- 1 and Delta2 +/- 1, respectively; P < 0.05). Dynamic chair rotation (15 cycles/min), which mimics dynamic yaw but eliminates neck muscle afferent, increased respiration (Delta3 +/- 1; P < 0.05) comparable to dynamic yaw (15 cycles/min). Increases in respiratory rate were graded as greater responses occurred during upright (Delta5 +/- 2 breaths/min) and lateral pitch (Delta4 +/- 1) and roll (Delta5 +/- 1) performed at 30 cycles/min. Increases in breathing frequency resulted in increases in minute ventilation during most interventions. Static head-down rotation, which activates otolith organs, did not alter respiratory rate (Delta1 +/- 1 breaths/min). Collectively, these data indicate that semicircular canals, but not otolith organs or neck muscle afferents, mediate increased ventilation in humans and support the concept that vestibular activation alters respiration in humans.

Aging attenuates the vestibulosympathetic reflex in humans

BACKGROUND

The vestibular system contributes to sympathetic activation by engagement of the otolith organs. However, there is a significant loss of vestibular function with aging. Therefore, the purpose of the present study was to determine if young and older individuals differ in their cardiovascular and sympathetic responses to otolithic stimulation (ie, head-down rotation, HDR). We hypothesized that responses to otolithic stimulation would be attenuated in older adults because of morphological and physiological alterations that occur in the vestibular system with aging.

METHODS AND RESULTS

Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and head rotation were measured during HDR in 11 young (26 +/- 1 years) and 11 older (64 +/- 1 years) subjects in the prone posture. Five older subjects performed head rotation (chin to chest) in the lateral decubitus position, which simulates HDR but does not alter afferent inputs from the vestibular system. MSNA responses to HDR were significantly attenuated in older as compared with young subjects (P<0.01). MSNA increased in the older subjects by only 12 +/- 5% as compared with 85 +/- 16% in the young. Furthermore, HDR elicited significant reductions in mean arterial blood pressure in older (Delta-6 +/- 1 mm Hg; P<0.01) but not young subjects (Delta1 +/- 1 mm Hg). In contrast to HDR, head rotation performed in the lateral decubitus position did not elicit hypotension. MSNA responses to baroreceptor unloading and the cold pressor test were not different between the age groups.

CONCLUSIONS

These data indicate that aging attenuates the vestibulosympathetic reflex in humans and may contribute to the increased prevalence of orthostatic hypotension with age.

Limb neurovascular control during altered otolithic input in humans

Head-down rotation (HDR), which activates the vestibulosympathetic reflex, increases leg muscle sympathetic nerve activity (MSNA) and produces calf vasoconstriction with no change in either cardiac output or arterial blood pressure. Based on animal studies, it was hypothesized that differential control of arm and leg MSNA explains why HDR does not alter arterial blood pressure. Fifteen healthy subjects were studied. Heart rate, arterial blood pressure, forearm and calf blood flow, and leg MSNA responses were measured during HDR in these subjects. Simultaneous recordings of arm and leg MSNA were obtained from five of the subjects. Forearm and calf blood flow, vascular conductances, and vascular resistances were similar before HDR, as were arm and leg MSNA. HDR elicited similar significant increases in leg (Delta 6 +/- 1 bursts min(-1); 59 +/- 16 % from baseline) and arm MSNA (Delta 5 +/- 1 bursts min(-1); 80 +/- 28 % from baseline). HDR significantly decreased calf (-19 +/- 2 %) and forearm vascular conductance (-12 +/- 2 %) and significantly increased calf (25 +/- 4 %) and forearm vascular resistance (15 +/- 2 %), with ~60 % greater vasoconstriction in the calf than in the forearm. Arterial blood pressure and heart rate were not altered by HDR. These results indicate that there is no differential control of MSNA in the arm and leg during altered feedback from the otolith organs in humans, but that greater vasoconstriction occurs in the calf than in the forearm. These findings indicate that vasodilatation occurs in other vascular bed(s) to account for the lack of increase in arterial blood pressure during HDR.

The vestibulosympathetic reflex in humans: neural interactions between cardiovascular reflexes

1. Over the past 5 years, there has been emerging evidence that the vestibular system regulates sympathetic nerve activity in humans. We have studied this issue in humans by using head-down rotation (HDR) in the prone position. 2. These studies have clearly demonstrated increases in muscle sympathetic nerve activity (MSNA) and calf vascular resistance during HDR. These responses are mediated by engagement of the otolith organs and not the semicircular canals. 3. However, differential activation of sympathetic nerve activity has been observed during HDR. Unlike MSNA, skin sympathetic nerve activity does not increase with HDR. 4. Examination of the vestibulosympathetic reflex with other cardiovascular reflexes (i.e. barorereflexes and skeletal muscle reflexes) has shown an additive interaction for MSNA. 5. The additive interaction between the baroreflexes and vestibulosympathetic reflex suggests that the vestibular system may assist in defending against orthostatic challenges in humans by elevating MSNA beyond that of the baroreflexes. 6. In addition, the further increase in MSNA via otolith stimulation during isometric handgrip, when arterial pressure is elevated markedly, indicates that the vestibulosympathetic reflex is a powerful activator of MSNA and may contribute to blood pressure and flow regulation during dynamic exercise. 7. Future studies will help evaluate the importance of the vestibulosympathetic reflex in clinical conditions associated with orthostatic hypotension.

Influence of long-term experimental orthostatic body position on innervation density in extremity vessels

The aim of this study was to quantitate the density of nerve terminals as well as their synaptic vesicle population in the adventitia of saphenous (SV and SA) and brachial veins and arteries (BV and BA) obtained from rats maintained in a horizontal control or a tilted position. Adult animals were kept individually in tube-like cages in a 45 degrees head-up position. After 2 wk, both tilted and control animals were anesthetized, and the whole body was perfused with fixative solution at physiological pressure. Vessels segments were then excised for electron microscopy and immunohistochemistry. The nerve terminal density (NTD) of SA was 8.20 +/- 1.46 nerve terminals/100 microm(2) cross section of adventitia and that of SV was 4.53 +/- 0.61 nerve terminals/100 microm(2) cross section of adventitia in control rats. Tilting caused a significant increase in NTD of both SA (70%) and SV (52%). The synaptic microvesicle density (SyVD) was larger in SA than SV in control rats (30.48 +/- 4.41 vs. 13.38 +/- 2.61 synaptic vesicles/10 terminal sections), but tilting resulted in more pronounced changes in SyVD of SV (95%) than SA (54%). No significant changes in NTD and SyVD of BA were found after tilt (-3.6% relative to 4.99 +/- 0.33 compared with 0.4% relative to 24.89 +/- 3.7, respectively). Whereas NTD of BV exhibited a tendency to increase (3.73 +/- 0.86 vs. 2.31 +/- 0.29 nerve terminals/100 microm(2) cross section of adventitia), SyVD did not change significantly (18.96 +/- 2.74 vs. 22.85 +/- 3.17 synaptic vesicles/10 terminal sections). A large number of nerve terminals of all vessels were tyrosine hydroxylase immunoreactive (containing norepinephrine). These findings support the hypothesis that long-term gravitational load causes adaptive morphological and functional remodeling of sympathetic innervation in blood vessels of the extremities.

Muscle sympathetic nerve activity in blood pressure control against gravitational stress

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.

Effects of lower body positive pressure on muscle sympathetic nerve activity response to head-up tilt

The present study was performed to test the hypothesis that application of lower body positive pressure (LBPP) during orthostasis would reduce the baroreflex-mediated enhancement in sympathetic activity in humans. Eight healthy young men were exposed to a 70 degrees head-up tilt (HUT) on application of 30 mmHg LBPP. Muscle sympathetic nerve activity (MSNA) was microneurographically recorded from the tibial nerve, along with hemodynamic variables. We found that in the supine position with LBPP, MSNA remained unchanged (13.4 +/- 3.3 vs. 11.8 +/- 2.3 bursts/min, without vs. with LBPP; P > 0.05), mean arterial pressure was elevated, but arterial pulse pressure and heart rate did not alter. At 70 degrees HUT with LBPP, the enhanced MSNA response was reduced (33.8 +/- 5.0 vs. 22.5 +/- 2.2 bursts/min, without vs. with LBPP; P < 0.05), mean arterial pressure was higher, the decreased pulse pressure was restored, and the increased heart rate was attenuated. We conclude that the baroreflex-mediated enhancement in sympathetic activity during HUT was reduced by LBPP. Application of LBPP in HUT induced an obvious cephalad fluid shift as well as a restoration of arterial pulse pressure, which reduced the inhibition of the baroreceptors. However, the activation of the intramuscular mechanoreflexes produced by 30 mmHg LBPP might counteract the effects of baroreflexes.

Muscle sympathetic outflow during horizontal linear acceleration in humans

To elucidate the effects of linear acceleration on muscle sympathetic nerve activity (MSNA) in humans, 16 healthy men were tested in a linear accelerator. Measurements of MSNA, electrocardiogram, blood pressure, and thoracic impedance were undertaken during linear acceleration. Sinusoidal linear acceleration with peak values at +/-0.10, +/-0.15, and +/-0.20 G was applied in anteroposterior (+/-G(x), n = 10) or lateral (+/-G(y), n = 6) directions. The total activity and burst rate of MSNA decreased significantly during forward, backward, left, or right linear accelerations. The total activity of MSNA decreased to 50.5 +/- 6.9, 52.5 +/- 4.4, 71.2 +/- 9.6, and 67.6 +/- 8.2% from the baselines (100%) during linear accelerations with peak values at +/-0.20 G in the four directions, respectively. These results suggest that dynamic stimulation of otolith organs in horizontal directions in humans might inhibit MSNA directly in order to quickly redistribute blood to muscles during postural reflexes induced by passive movement, which supports the concept that the vestibular system contributes to sympathetic regulation in humans.

Influence of head-down and lateral decubitus neck flexion on heart rate variability

The purpose of this study was to examine the response of heart rate variability (HRV), a noninvasive index of autonomic control, to head-down neck flexion (HDNF), which engages both otoliths and neck muscle afferents, and to lateral decubitus neck flexion (LNF), in which neck afferents are activated, whereas otolith afferent input is not. HRV and forearm blood flow were evaluated in participants lying prone, during HDNF, lying in the lateral decubitus position, and during LNF. Compared with the prone position, HDNF resulted in lower high-frequency (46.9 +/- 7.1 vs. 62.3 +/- 6.2) and higher low-frequency (53.1 +/- 7.1 vs. 37.7 +/- 6.2) power, expressed as normalized units, along with higher low-frequency-to-high-frequency ratio (1.65 +/- 0.3 vs. 0.78 +/- 0.2), whereas LNF resulted in no alterations in HRV indexes. Furthermore, there were no significant differences in forearm blood flow or vascular resistance among any of the positions. Our data suggest that otolith organs influence autonomic modulation of the heart, supporting previous studies reporting that HDNF elicits increased sympathetic outflow. These data further suggest that HDNF results in a parasympathetic withdrawal from the heart in addition to sympathetic activation.

Interaction between vestibulosympathetic and skeletal muscle reflexes on sympathetic activity in humans

Evidence from animals indicates that skeletal muscle afferents activate the vestibular nuclei and that both vestibular and skeletal muscle afferents have inputs to the ventrolateral medulla. The purpose of the present study was to investigate the interaction between the vestibulosympathetic and skeletal muscle reflexes on muscle sympathetic nerve activity (MSNA) and arterial pressure in humans. MSNA, arterial pressure, and heart rate were measured in 17 healthy subjects in the prone position during three experimental trials. The three trials were 2 min of 1) head-down rotation (HDR) to engage the vestibulosympathetic reflex, 2) isometric handgrip (IHG) at 30% maximal voluntary contraction to activate skeletal muscle afferents, and 3) HDR and IHG performed simultaneously. The order of the three trials was randomized. HDR and IHG performed alone increased total MSNA by 46 +/- 16 and 77 +/- 24 units, respectively (P < 0.01). During the HDR plus IHG trial, MSNA increased 142 +/- 38 units (P < 0.01). This increase was not significantly different from the sum of the individual trials (130 +/- 41 units). This finding was also observed with mean arterial pressure (sum = 21 +/- 2 mmHg and HDR + IHG = 22 +/- 2 mmHg). These findings suggest that there is an additive interaction for MSNA and arterial pressure when the vestibulosympathetic and skeletal muscle reflexes are engaged simultaneously in humans. Therefore, no central modulation exists between these two reflexes with regard to MSNA output in humans.

Interaction of the vestibular system and baroreflexes on sympathetic nerve activity in humans

Muscle sympathetic nerve activity (MSNA) is altered by vestibular otolith stimulation. This study examined interactive effects of the vestibular system and baroreflexes on MSNA in humans. In study 1, MSNA was measured during 4 min of lower body negative pressure (LBNP) at either -10 or -30 mmHg with subjects in prone posture. During the 3rd min of LBNP, subjects lowered their head over the end of a table (head-down rotation, HDR) to engage the otolith organs. The head was returned to baseline upright position during the 4th min. LBNP increased MSNA above baseline during both trials with greater increases during the -30-mmHg trial. HDR increased MSNA further during the 3rd min of LBNP at -10 and -30 mmHg (Delta32% and Delta34%, respectively; P < 0.01). MSNA returned to pre-HDR levels during the 4th min of LBNP when the head was returned upright. In study 2, MSNA was measured during HDR, LBNP, and simultaneously performed HDR and LBNP. The sum of MSNA responses during individual HDR and LBNP trials was not significantly different from that observed during HDR and LBNP performed together (Delta131 +/- 28 vs. Delta118 +/- 47 units and Delta340 +/- 77 vs. Delta380 +/- 90 units for the -10 and -30 trials, respectively). These results demonstrate that vestibular otolith stimulation can increase MSNA during unloading of the cardiopulmonary and arterial baroreflexes. Also, the interaction between the vestibulosympathetic reflex and baroreflexes is additive in humans. These studies indicate that the vestibulosympathetic reflex may help defend against orthostatic challenges in humans by increasing sympathetic outflow.

Effect of gender on vestibular sympathoexcitation

Studies have suggested that premenopausal women are more prone to orthostatic intolerance than men. Additionally, it has been postulated that the vestibulosympathetic reflex is important in regulating postural-related changes in sympathetic activity. The purpose of the present study was to determine whether men and women differ in their sympathetic and cardiovascular responses to stimulation of the otolith organs elicited by head-down rotation (HDR). Heart rate (HR), arterial pressure, calf blood flow (CBF), and leg muscle sympathetic nerve activity (MSNA) were measured during 3 min of HDR in the prone posture in 33 women and 30 men. With the exception of HR (71 +/- 2 and 63 +/- 1 beats/min for women and men, respectively; P < 0.01), all baseline variables were not different between genders. There were no gender differences in responses to HDR. MSNA increased 72 +/- 33 units (43%) in the men and 88 +/- 15 units (59%) in the women during HDR (P < 0.01). CBF decreased [-0.6 +/- 0.1 (15%) and -0.5 +/- 0.1 (19%) ml. min(-1). 100 ml(-1)] and calf vascular resistance increased [8 +/- 2 (21%) and 11 +/- 3 (25%) units during HDR for men and women, respectively (P < 0.01)]. Both in the men and women, HR increased 2 +/- 1 beats/min (P < 0.01). These results demonstrate that sympathetic activation during HDR in the prone posture is similar in men and women. Therefore, these findings suggest that the vestibulosympathetic reflex is not different between healthy men and women.

Neck afferent involvement in cardiovascular control during movement

It is well established that labyrinth and neck afferent information contributes to the regulation of somatomotor function during movement and changes in posture. There is also convincing evidence that the vestibular system participates in the modulation of sympathetic outflow and cardiovascular function during changes in posture, presumably to prevent orthostatic hypotension. However, the labyrinth organs do not provide any signals concerning body movements with respect to the head. In contrast, the neck receptors, particularly muscle spindles, are well located and suited to provide information about changes in body position with respect to the head and vestibular signals. Studies in the cat suggest that neck afferent information may modulate the vestibulosympathetic reflex responses to head-neck movements. There is some evidence in the cat to suggest involvement of low threshold mechanoreceptors. However, human studies do not indicate that low threshold mechanoreceptors in the neck modulate cardiovascular responses. The human studies are consistent with the studies in the cat in that they demonstrate the importance of otolith activation in mediating cardiovascular and sympathetic responses to changes in posture. This paper briefly reviews the current experimental evidence concerning the involvement of neck afferent information in the modulation of cardiovascular control during movement and changes in posture.