Vestibular effects on respiratory outflow in the decerebrate cat

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.

Hemodynamic orthostatic dizziness/vertigo: Diagnostic criteria

This paper presents the diagnostic criteria for hemodynamic orthostatic dizziness/vertigo to be included in the International Classification of Vestibular Disorders (ICVD). The aim of defining diagnostic criteria of hemodynamic orthostatic dizziness/vertigo is to help clinicians to understand the terminology related to orthostatic dizziness/vertigo and to distinguish orthostatic dizziness/vertigo due to global brain hypoperfusion from that caused by other etiologies. Diagnosis of hemodynamic orthostatic dizziness/vertigo requires: A) five or more episodes of dizziness, unsteadiness or vertigo triggered by arising or present during upright position, which subsides by sitting or lying down; B) orthostatic hypotension, postural tachycardia syndrome or syncope documented on standing or during head-up tilt test; and C) not better accounted for by another disease or disorder. Probable hemodynamic orthostatic dizziness/vertigo is defined as follows: A) five or more episodes of dizziness, unsteadiness or vertigo triggered by arising or present during upright position, which subsides by sitting or lying down; B) at least one of the following accompanying symptoms: generalized weakness/tiredness, difficulty in thinking/concentrating, blurred vision, and tachycardia/palpitations; and C) not better accounted for by another disease or disorder. These diagnostic criteria have been derived by expert consensus from an extensive review of 90 years of research on hemodynamic orthostatic dizziness/vertigo, postural hypotension or tachycardia, and autonomic dizziness. Measurements of orthostatic blood pressure and heart rate are important for the screening and documentation of orthostatic hypotension or postural tachycardia syndrome to establish the diagnosis of hemodynamic orthostatic dizziness/vertigo.

Nondestructive and objective assessment of the vestibular function in rodent models: A review

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

Influence of graviceptor stimulation initiated by off-vertical axis rotation on ventilation

NEW FINDINGS

What is the central question of this study? It is known that respiration is affected by graviceptors, but it remains unclear to what extent labyrinthine and non-labyrinthine graviceptors are involved in this process. What is the main finding and its importance? Our results suggest that the modulation of respiration is not a result of a simple reflex arc, but that it involves a higher integration of different types of receptors with variable contributions of either type of graviceptor among subjects.

ABSTRACT

It has been suggested that the otolith system is involved in the physiological response to changes in body orientation with respect to gravity. In studies on animals, an otolith-respiratory reflex has been observed, but data on humans are scarce and inconclusive, mainly because pure otolithic stimulation is difficult to produce in humans. To assess the otolithic-respiratory reflex in humans, we used an off-vertical axis rotation (OVAR) that produces periodic and pure stimulation of graviceptors. The inspiratory flow was measured during earth vertical axis rotation (EVAR, control conditions) and OVAR in 21 subjects. To distinguish the effects of the labyrinthine and non-labyrinthine graviceptors on ventilation, these measurements were repeated with two different static head positions: head turned leftward and rightward in yaw. The velocity of rotation was individually selected to match spontaneous breathing rate (mean 11.4 cycles min^-1 , 0.19 Hz). Average ventilatory flow was higher in OVAR than in EVAR, as was tidal volume. In OVAR, the transition between inspiration and expiration occurred mainly in the forward pitch position. The phase of this transition in most subjects was driven mostly by the body position rather than by the head position, suggesting that respiratory modifications during OVAR mainly involved non-labyrinthine receptors. However, the study demonstrated a high intersubject variability both in the ability of OVAR to synchronize breathing and in the influence of labyrinthine stimulation. We conclude that the respiratory response to changes in orientation of the body with respect to the vertical involves labyrinthine and non-labyrinthine stimulation, with the gain of each signal varying individually.

The effect of body position on esophageal reflexes in cats: a possible mechanism of SIDS?

BackgroundIt has been hypothesized that life-threatening events are caused by supraesophageal reflux (SER) of gastric contents that activates laryngeal chemoreflex-stimulated apnea. Placing infants supine decreases the risk of sudden infant death syndrome (SIDS). The aim of this study was to determine whether body position affects esophageal reflexes that control SER.MethodsWe instrumented the pharyngeal and esophageal muscles of decerebrate cats (N=14) to record EMG or manometry, and investigated the effects of body position on the esophago-upper esophageal sphincter (UES) contractile reflex (EUCR), esophago-UES relaxation reflex (EURR), esophagus-stimulated pharyngeal swallow response (EPSR), secondary peristalsis (SP), and pharyngeal swallow (PS). EPSR, EUCR, and SP were activated by balloon distension, EURR by air pulse, and PS by nasopharyngeal water injection. The esophagus was stimulated in the cervical, proximal thoracic, and distal thoracic regions. The threshold stimulus for activation of EUCR, EURR, and PS, and the chance of activation of EPSR and SP were quantified.ResultsWe found that only EPSR was significantly more sensitive in the supine vs. prone position regardless of the stimulus or the position of the stimulus in the esophagus.ConclusionWe hypothesize that the EPSR may contribute to the protection of infants from SIDS by placement in the supine position.

Medullary Astrogliosis in Sudden Infant Death Syndrome Varies With Sleeping Environment: Evidence for Different Mechanisms of Death in Alone Versus Co-sleepers?

Sudden infant death syndrome remains the leading cause of death in infants under 1 year, and underlying pathophysiological mechanisms are poorly understood. The current study investigated the hypothesis that co-sleepers die more rapidly from causes such as suffocation from overlaying by comparing levels of reactive astrogliosis in the medulla of infants who died sleeping alone to those who died co-sleeping. The amount of glial fibrillary acidic protein (GFAP) staining in alone sleepers was significantly higher than shared sleepers in 3 specific areas of the medulla, the inferior vestibular nucleus, the medial vestibular nucleus and the cochlear nucleus. Given that glial fibrillary acidic protein elevations follow a delayed time course, this suggests that death in co-sleepers was more rapid, not allowing for reactive gliosis to occur. This provides evidence of pathological differences in mechanisms of death in infants who are classified as having died from sudden infant death syndrome, suggesting potential need for refinement of categorization of these cases.

Effect of Rocking Movements on Respiration

For centuries, rocking has been used to promote sleep in babies or toddlers. Recent research suggested that relaxation could play a role in facilitating the transition from waking to sleep during rocking. Breathing techniques are often used to promote relaxation. However, studies investigating head motions and body rotations showed that vestibular stimulation might elicit a vestibulo-respiratory response, leading to an increase in respiration frequency. An increase in respiration frequency would not be considered to promote relaxation in the first place. On the other hand, a coordination of respiration to rhythmic vestibular stimulation has been observed. Therefore, this study aimed to investigate the effect of different movement frequencies and amplitudes on respiration frequency. Furthermore, we tested whether subjects adapt their respiration to movement frequencies below their spontaneous respiration frequency at rest, which could be beneficial for relaxation. Twenty-one healthy subjects (24–42 years, 12 males) were investigated using an actuated bed, moving along a lateral translation. Following movement frequencies were applied: +30%, +15%, -15%, and -30% of subjects’ rest respiration frequency during baseline (no movement). Furthermore, two different movement amplitudes were tested (Amplitudes: 15 cm, 7.5 cm; movement frequency: 0.3 Hz). In addition, five subjects (25–28 years, 2 males) were stimulated with their individual rest respiration frequency. Rocking movements along a lateral translation caused a vestibulo-respiratory adaptation leading to an increase in respiration frequency. The increase was independent of the applied movement frequencies or amplitudes but did not occur when stimulating with subjects’ rest respiration frequency. Furthermore, no synchronization of the respiration frequency to the movement frequency was observed. In particular, subjects did not lower their respiration frequency below their resting frequency. Hence, it was not feasible to influence respiration in a manner that might be considered beneficial for relaxation.

Dysfunctional vestibular system causes a blood pressure drop in astronauts returning from space

It is a challenge for the human body to maintain stable blood pressure while standing. The body’s failure to do so can lead to dizziness or even fainting. For decades it has been postulated that the vestibular organ can prevent a drop in pressure during a position change – supposedly mediated by reflexes to the cardiovascular system. We show – for the first time – a significant correlation between decreased functionality of the vestibular otolith system and a decrease in the mean arterial pressure when a person stands up. Until now, no experiments on Earth could selectively suppress both otolith systems; astronauts returning from space are a unique group of subjects in this regard. Their otolith systems are being temporarily disturbed and at the same time they often suffer from blood pressure instability. In our study, we observed the functioning of both the otolith and the cardiovascular system of the astronauts before and after spaceflight. Our finding indicates that an intact otolith system plays an important role in preventing blood pressure instability during orthostatic challenges. Our finding not only has important implications for human space exploration; they may also improve the treatment of unstable blood pressure here on Earth.

Termination of vestibulospinal fibers arising from the spinal vestibular nucleus in the mouse spinal cord

The present study investigated the vestibulospinal system which originates from the spinal vestibular nucleus (SpVe) with both retrograde and anterograde tracer injections. We found that fluoro-gold (FG) labeled neurons were found bilaterally with a contralateral predominance after FG injections into the upper lumbar cord. Anterogradely labeled fibers from the rostral SpVe traveled in the medial part of the ventral funiculus ipsilaterally and the dorsolateral funiculus bilaterally in the cervical cord. They mainly terminated in laminae 5-8, and 10 of the ipsilateral spinal cord. The contralateral side had fewer fibers and they were found in laminae 6-8, and 10. In the thoracic cord, fibers were also found to terminate in bilateral intermediolateral columns. In the lumbar and lower cord, fibers were mainly found in the dorsolateral funiculus bilaterally and they terminated predominantly in laminae 3-7 contralaterally. Anterogradely labeled fibers from the caudal SpVe did not travel in the medial part of the ventral funiculus but in the dorsolateral funiculus bilaterally. They mainly terminated in laminae 3-8 and 10 contralaterally. The present study is the first to describe the termination of vestibulospinal fibers arising from the SpVe in the spinal cord. It will lay the anatomical foundation for those who investigate the physiological role of vestibulospinal fibers and potentially target these fibers during rehabilitation after stroke, spinal cord injury, or vestibular organ injury.

Does locomotor training improve pulmonary function in patients with spinal cord injury?

OBJECTIVES

The aim of this study was to compare the effects of a locomotor training (LT) combined rehabilitation program with a rehabilitation-only program on pulmonary function in spinal cord injury (SCI) patients by investigating spirometric analyses of the patients.

SETTING

Rehabilitation center in Ankara, Turkey.

METHODS

Fifty-two patients (40 male, 12 female) with SCI enrolled in the study. The subjects were divided into two groups: the first group (group A) received both LT and a rehabilitation program and the second group (group B) received only the rehabilitation program for 4 weeks. The LT program was prescribed as three 30-min sessions per week. Pulmonary function was evaluated spirometrically in both groups before and after the rehabilitation program.

RESULTS

The spirometric values of the SCI patients, including forced vital capacity, forced expiratory volume in 1 second, forced expiratory flow rate and vital capacity (VC) and VC%, increased significantly with LT in the first group (all P<0.05). Maximum voluntary ventilation values increased significantly in both groups (both P<0.05).

CONCLUSION

These findings suggest that LT is effective for improving pulmonary function in SCI patients. We also highlight the useful effects of LT, which are likely the result of erect posture, gait and neuroplastic changes that prevent potential complications in SCI patients.

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.

Hypothalamic orexin-A (hypocretin-1) neuronal projections to the vestibular complex and cerebellum in the rat

Immunohistochemistry combined with retrograde tract-tracing techniques were used to investigate the distribution of orexin-A (OX-A)- and OX-A receptor-like (OX1) immunoreactivity within the vestibular complex and cerebellum, and the location of hypothalamic OX-A neurons sending axonal projections to these regions in the Wistar rat. OX-A immunoreactive fibers and presumptive terminals were found throughout the medial (MVe) and lateral (LVe) vestibular nuclei. Light fiber labeling was also observed in the spinal and superior vestibular nuclei. Within the cerebellum, dense fiber and presumptive terminal labeling was observed in the medial cerebellar nucleus (Med; fastigial nucleus), with less dense labeling in the interposed (Int) and lateral cerebellar nuclei (Lat; dentate nucleus). A few scattered OX-A immunoreactive fibers were also observed throughout the cortex of the paraflocculus. OX1-like immunoreactivity was found densely concentrated within LVe, moderate in MVe, and scattered within the spinal and superior vestibular nuclei. Within the cerebellum, OX1-like immunoreactivity was also observed densely within Med and in the dorsolateral aspects of Int. Additionally, OX1 like-labeling was found in Lat, and within the granular layer of the caudal paraflocculus cerebellar cortex. Fluorogold (FG) microinjected into these vestibular and cerebellar regions resulted in retrogradely labeled neurons throughout the ipsilateral hypothalamus. Retrogradely labeled neurons containing OX-A like immunoreactivity were observed dorsal and caudal to the anterior hypothalamic nucleus and extending laterally into the lateral hypothalamic area, with the largest number clustered around the dorsal aspects of the fornix in the perifornical area. A few FG OX-A like-immunoreactive neurons were also observed scattered throughout the dorsomedial, and posterior hypothalamic nuclei. These data indicate that axons from OX-A neurons terminate within the vestibular complex and deep cerebellar nuclei of the cerebellum and although the function of these pathways is unknown, they likely represent pathways by which hypothalamic OX-A containing neurons co-ordinate vestibulo-cerebellar motor and autonomic functions associated with ingestive behaviors.

Respiratory motor control disrupted by spinal cord injury: mechanisms, evaluation, and restoration

Pulmonary complications associated with persistent respiratory muscle weakness, paralysis, and spasticity are among the most important problems faced by patients with spinal cord injury when lack of muscle strength and disorganization of reciprocal respiratory muscle control lead to breathing insufficiency. This review describes the mechanisms of the respiratory motor control and its change in individuals with spinal cord injury, methods by which respiratory function is measured, and rehabilitative treatment used to restore respiratory function in those who have experienced such injury.

Locomotor step training with body weight support improves respiratory motor function in individuals with chronic spinal cord injury

This prospective case-controlled clinical study was undertaken to investigate to what extent the manually assisted treadmill stepping locomotor training with body weight support (LT) can change respiratory function in individuals with chronic spinal cord injury (SCI). Pulmonary function outcomes (forced vital capacity /FVC/, forced expiratory volume one second /FEV1/, maximum inspiratory pressure /PImax/, maximum expiratory pressure /PEmax/) and surface electromyographic (sEMG) measures of respiratory muscles activity during respiratory tasks were obtained from eight individuals with chronic C3-T12 SCI before and after 62±10 (mean±SD) sessions of the LT. FVC, FEV1, PImax, PEmax, amount of overall sEMG activity and rate of motor unit recruitment were significantly increased after LT (p<0.05). These results suggest that these improvements induced by the LT are likely the result of neuroplastic changes in spinal neural circuitry responsible for the activation of respiratory muscles preserved after injury.

Vestibular-mediated synaptic inputs and pathways to sympathetic preganglionic neurons in the neonatal mouse

To assess when vestibulosympathetic projections become functional postnatally, and to establish a preparation in which vestibulosympathetic circuitry can be characterized more precisely, we used an optical approach to record VIIIth nerve-evoked synaptic inputs to thoracic sympathetic preganglionic neurons (SPNs) in newborn mice. Stimulation of the VIIIth nerve was performed in an isolated brainstem-spinal cord preparation after retrogradely labelling with the fluorescent calcium indicator Calcium Green 1-conjugated dextran amine, the SPNs and the somatic motoneurons (MNs) in the thoracic (T) segments T2, 4, 6, 8, 10 and 12. Synaptically mediated calcium responses could be visualized and recorded in individual SPNs and MNs, and analysed with respect to latency, temporal pattern, magnitude and synaptic pharmacology. VIIIth nerve stimulation evoked responses in all SPNs and MNs investigated. The SPN responses had onset latencies from 90 to 200 ms, compared with much shorter latencies in MNs, and were completely abolished by mephenesin, a drug that preferentially reduces polysynaptic over monosynaptic transmission. Bicuculline and picrotoxin, but not strychnine, increased the magnitudes of the SPN responses without changing the onset latencies, suggesting a convergence of concomitant excitatory and inhibitory synaptic inputs. Lesions strategically placed to test the involvement of direct vestibulospinal pathways versus indirect pathways within the brainstem showed that vestibulosympathetic inputs in the neonate are mediated predominantly, if not exclusively, by the latter. Thus, already at birth, synaptic connections in the vestibulosympathetic reflex are functional and require the involvement of the ventrolateral medulla as in adult mammals.

Compensation following bilateral vestibular damage

Bilateral loss of vestibular inputs affects far fewer patients than unilateral inner ear damage, and thus has been understudied. In both animal subjects and human patients, bilateral vestibular hypofunction (BVH) produces a variety of clinical problems, including impaired balance control, inability to maintain stable blood pressure during postural changes, difficulty in visual targeting of images, and disturbances in spatial memory and navigational performance. Experiments in animals have shown that non-labyrinthine inputs to the vestibular nuclei are rapidly amplified following the onset of BVH, which may explain the recovery of postural stability and orthostatic tolerance that occurs within 10 days. However, the loss of the vestibulo-ocular reflex and degraded spatial cognition appear to be permanent in animals with BVH. Current concepts of the compensatory mechanisms in humans with BVH are largely inferential, as there is a lack of data from patients early in the disease process. Translation of animal studies of compensation for BVH into therapeutic strategies and subsequent application in the clinic is the most likely route to improve treatment. In addition to physical therapy, two types of prosthetic devices have been proposed to treat individuals with bilateral loss of vestibular inputs: those that provide tactile stimulation to indicate body position in space, and those that deliver electrical stimuli to branches of the vestibular nerve in accordance with head movements. The relative efficacy of these two treatment paradigms, and whether they can be combined to facilitate recovery, is yet to be ascertained.

Current Emotion Research in Psychophysiology: The Neurobiology of Evaluative Bivalence

Evaluative processes have their roots in early evolutionary history, as survival is dependent on an organism’s ability to identify and respond appropriately to positive, rewarding or otherwise salubrious stimuli as well as to negative, noxious, or injurious stimuli. Consequently, evaluative processes are ubiquitous in the animal kingdom and are represented at multiple levels of the nervous system, including the lowest levels of the neuraxis. While evolution has sculpted higher level evaluative systems into complex and sophisticated information-processing networks, they do not come to replace, but rather to interact with more primitive lower level representations. Indeed, there are basic features of the underlying neuroarchitectural plan for evaluative processes that are common across levels of organization—including that of evaluative bivalence.

Sinusoidal galvanic vestibular stimulation (sGVS) induces a vasovagal response in the rat

Blood pressure (BP) and heart rate (HR) were studied in isoflurane-anesthetized Long-Evans rats during sinusoidal galvanic vestibular stimulation (sGVS) and sinusoidal oscillation in pitch to characterize vestibular influences on autonomic control of BP and HR. sGVS was delivered binaurally via Ag/AgCl needle electrodes inserted over the mastoids at stimulus frequencies 0.008-0.4 Hz. Two processes affecting BP and HR were induced by sGVS: 1) a transient drop in BP (≈15-20 mmHg) and HR (≈3 beat*s(-1)), followed by a slow recovery over 1-6 min; and 2) inhibitory modulations in BP (≈4.5 mmHg/g) and HR (≈0.15 beats*s(-1)/g) twice in each stimulus cycle. The BP and HR modulations were approximately in-phase with each other and were best evoked by low stimulus frequencies. A wavelet analysis indicated significant energies in BP and HR at scales related to twice and four times the stimulus frequency bands. BP and HR were also modulated by oscillation in pitch at frequencies 0.025-0.5 Hz. Sensitivities at 0.025 Hz were ≈4.5 mmHg/g (BP) and ≈0.17 beat*s(-1)/g (HR) for pitches of 20-90°. The tilt-induced BP and HR modulations were out-of-phase, but the frequencies at which responses were elicited by tilt and sGVS were the same. The results show that the sGVS-induced responses, which likely originate in the otolith organs, can exert a powerful inhibitory effect on both BP and HR at low frequencies. These responses have a striking resemblance to human vasovagal responses. Thus, sGVS-activated rats can potentially serve as a useful experimental model of the vasovagal response in humans.

Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Research on animal models as well as human subjects has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body through effects on the sympathetic nervous system. Elimination of vestibular inputs results in increased blood flow to the hindlimbs during vestibular stimulation, because it attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. Additionally, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers and a decrease in hindlimb vascular resistance, but an initial excitation of most upper body vasoconstrictor fibers accompanied by an increase in upper body vascular resistance. The present study tested the hypothesis that neurons in the principal vasomotor region of the brainstem, the rostral ventrolateral medulla (RVLM), whose projections extended past the T10 segment, to spinal levels containing sympathetic preganglionic neurons regulating lower body blood flow, respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminate rostral to T10. Contrary to our hypothesis, the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection in the spinal cord. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.

Integration of nonlabyrinthine inputs by the vestibular system: role in compensation following bilateral damage to the inner ear

Inputs from the skin and muscles of the limbs and trunk as well as the viscera are relayed to the medial, inferior, and lateral vestibular nuclei. Vestibular nucleus neurons very quickly regain spontaneous activity following a bilateral vestibular neurectomy, presumably due to the presence of such nonlabyrinthine inputs. The firing of a small fraction of vestibular nucleus neurons in animals lacking labyrinthine inputs can be modulated by whole-body tilts; these responses are eliminated by a spinal transection, showing that they are predominantly elicited by inputs from the trunk and limbs. The ability to adjust blood distribution in the body and maintain stable blood pressure during movement is diminished following a bilateral vestibular neurectomy, but compensation occurs within a week. However, bilateral lesions of the caudal portions of the vestibular nuclei produce severe and long-lasting cardiovascular disturbances during postural alterations, suggesting that the presence of nonlabyrinthine signals to the vestibular nuclei is essential for compensation of posturally-related autonomic responses to occur. Despite these observations, the functional significance of nonlabyrinthine inputs to the central vestibular system remains unclear, either in modulating the processing of vestibular inputs or compensating for their loss.

Vestibular afferents to the motoneurons of glossopharyngeal and vagus nerves in the frog, Rana esculenta

The aim of this work was to study whether the vestibular afferent fibers establish direct connections with the motoneurons of glossopharyngeal and vagus nerves of the frog, Rana esculenta. In anaesthetized animals the vestibulocochlear nerve and the common root of glossopharyngeal-vagus and accessory (IX-X-XI) nerves were simultaneously labeled with fluorescein dextran amine (vestibulocochlear nerve) and tetramethylrhodamine dextran amine (IX-X-XI). With a confocal laser scanning microscope we could detect close appositions between the vestibular afferent fibers and somatodendritic components of the general and special visceral motoneurons of the ambiguus nucleus of IX-X nerves. The direct impulse transmission may provide a quick and immediate response of cardiovascular and gastrointestinal system upon body displacement.

Responses of thoracic spinal interneurons to vestibular stimulation

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 ms (minimum of approximately 3 ms), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

Neural circuits controlling diaphragm function in the cat revealed by transneuronal tracing

Although a number of studies have considered the neural circuitry that regulates diaphragm activity, these pathways have not been adequately discerned, particularly in animals such as cats that utilize the respiratory muscles during a variety of different behaviors and movements. The present study employed the retrograde transneuronal transport of rabies virus to identify the extended neural pathways that control diaphragm function in felines. In all animals deemed to have successful rabies virus injections into the diaphragm, large, presumed motoneurons were infected in the C(4)-C(6) spinal segments. In addition, smaller presumed interneurons were labeled bilaterally throughout the cervical and upper thoracic spinal cord. While in short and intermediate survival cases, infected interneurons were concentrated in the vicinity of phrenic motoneurons, in late survival cases, the distribution of labeling was more expansive. Within the brain stem, the earliest infected neurons included those located in the classically defined pontine and medullary respiratory groups, the medial and lateral medullary reticular formation, the region immediately ventral to the spinal trigeminal nucleus, raphe pallidus and obscurus, and the vestibular nuclei. At longer survival times, infection appeared in the midbrain, which was concentrated in the lateral portion of the periaqueductal gray, the region of the tegmentum that contains the locomotion center, and the red nucleus. Considerable labeling was also present in the fastigial nucleus of the cerebellum, portions of the posterior and lateral hypothalamus and the adjacent fields of Forel known to contain hypocretin-containing neurons and the precruciate gyrus of cerebral cortex. These data raise the possibility that several parallel pathways participate in regulating the activity of the feline diaphragm, which underscores the multifunctional nature of the respiratory muscles in this species.

Responses of caudal vestibular nucleus neurons of conscious cats to rotations in vertical planes, before and after a bilateral vestibular neurectomy

Although many previous experiments have considered the responses of vestibular nucleus neurons to rotations and translations of the head, little data are available regarding cells in the caudalmost portions of the vestibular nuclei (CVN), which mediate vestibulo-autonomic responses among other functions. This study examined the responses of CVN neurons of conscious cats to rotations in vertical planes, both before and after a bilateral vestibular neurectomy. None of the units included in the data sample had eye movement-related activity. In labyrinth-intact animals, some CVN neurons (22%) exhibited graviceptive responses consistent with inputs from otolith organs, but most (55%) had dynamic responses with phases synchronized with stimulus velocity. Furthermore, the large majority of CVN neurons had response vector orientations that were aligned either near the roll or vertical canal planes, and only 18% of cells were preferentially activated by pitch rotations. Sustained head-up rotations of the body provide challenges to the cardiovascular system and breathing, and thus the response dynamics of the large majority of CVN neurons were dissimilar to those of posturally-related autonomic reflexes. These data suggest that vestibular influences on autonomic control mediated by the CVN are more complex than previously envisioned, and likely involve considerable processing and integration of signals by brainstem regions involved in cardiovascular and respiratory regulation. Following a bilateral vestibular neurectomy, CVN neurons regained spontaneous activity within 24 h, and a very few neurons (<10%) responded to vertical tilts <15 degrees in amplitude. These findings indicate that nonlabyrinthine inputs are likely important in sustaining the activity of CVN neurons; thus, these inputs may play a role in functional recovery following peripheral vestibular lesions.

Consequences of postural changes and removal of vestibular inputs on the movement of air in and out of the lungs of conscious felines

A variety of experimental approaches in human subjects and animal models established that the vestibular system contributes to regulation of respiration. In cats, the surgical elimination of labyrinthine signals produced changes in the spontaneous activity and posturally related responses of a number of respiratory muscles. However, these effects were complex and sometimes varied between muscle compartments, such that the physiological role of vestibulo-respiratory responses is unclear. The present study determined the functional significance of vestibulo-respiratory influences by examining the consequences of a bilateral labyrinthectomy on breathing rate and the pressure, volume, and flow rate of air exchanged during inspiration and expiration as body orientation with respect to gravity was altered. Data were collected from conscious adult cats acclimated to breathing through a facemask connected to a pneuomotach during 60 degrees head-up pitch and ear-down roll body rotations. Removal of vestibular inputs resulted in a 15% reduction in breathing rate, a 13% decrease in minute ventilation, a 16% decrease in maximal inspiratory airflow rate, and a 14% decrease in the maximal expiratory airflow rate measured when the animals were in the prone position. However, the lesions did not appreciably affect phasic changes in airflow parameters related to alterations in posture. These results suggest that the role of the vestibular system in the control of breathing is to modify baseline respiratory parameters in proportion to the general intensity of ongoing movements, and not to rapidly alter ventilation in accordance with body position.

Visually induced postural sway in anxiety disorders

Postural sensitivity to moving visual environments in patients with anxiety disorders was studied. We hypothesized that patients with anxiety disorders would have greater sway in response to a moving visual environment compared to healthy adults, especially if they have space and motion discomfort (SMD). Twenty-one patients with generalized anxiety without panic (NPA) and 38 patients with panic and agoraphobia (PAG) were compared to 22 healthy controls. SMD was evaluated in all subjects via questionnaire. Subjects stood on a force platform that was either fixed or rotating with the subject (i.e., sway referenced) during exposure to a sinusoidally moving visual surround. Center of pressure (COP) data were computed from force transducers in the platform as a measure of sway. Results showed that patients swayed significantly more in response to the moving visual scene compared to control subjects, with no differences between the NPA and PAG groups. SMD was a predictor of sway response in the patients: patients with high SMD swayed significantly more than both Controls and anxiety patients with low SMD. These results indicate that patients with anxiety disorders, particularly those with SMD, are more visually dependent for balance. This subgroup of patients may be amenable to treatment used for patients with balance disorders (i.e., vestibular rehabilitation) that focuses on sensory re-integration processes that address visual sensitivity.

Vestibular inputs elicit patterned changes in limb blood flow in conscious cats

Previous experiments have demonstrated that the vestibular system contributes to regulating sympathetic nervous system activity, particularly the discharges of vasoconstrictor fibres. In the present study, we examined the physiological significance of vestibulosympathetic responses by comparing blood flow and vascular resistance in the forelimb and hindlimb during head-up tilt from the prone position before and after the removal of vestibular inputs through a bilateral vestibular neurectomy. Experiments were performed on conscious cats that were trained to remain sedentary on a tilt table during rotations up to 60 deg in amplitude. Blood flow through the femoral and brachial arteries was recorded during whole-body tilt using perivascular probes; blood pressure was recorded using a telemetry system and vascular resistance was calculated from blood pressure and blood flow measurements. In vestibular-intact animals, 60 deg head-up tilt produced approximately 20% decrease in femoral blood flow and approximately 37% increase in femoral vascular resistance relative to baseline levels before tilt; similar effects were also observed for the brachial artery ( approximately 25% decrease in blood flow and approximately 38% increase in resistance). Following the removal of vestibular inputs, brachial blood flow and vascular resistance during head-up tilt were almost unchanged. In contrast, femoral vascular resistance increased only approximately 6% from baseline during 60 deg head-up rotation delivered in the first week after elimination of vestibular signals and approximately 16% in the subsequent 3-week period (as opposed to the approximately 37% increase in resistance that occurred before lesion). These data demonstrate that vestibular inputs associated with postural alterations elicit regionally specific increases in vascular resistance that direct blood flow away from the region of the body where blood pooling may occur. Thus, the data support the hypothesis that vestibular influences on the cardiovascular system serve to protect against the occurrence of orthostatic hypotension.

Effects of postural changes and removal of vestibular inputs on blood flow to the head of conscious felines

Prior studies have shown that removal of vestibular inputs produces lability in blood pressure during orthostatic challenges (Holmes MJ, Cotter LA, Arendt HE, Cass SP, and Yates BJ. Brain Res 938: 62-72, 2002; Jian BJ, Cotter LA, Emanuel BA, Cass SP, and Yates BJ. J Appl Physiol 86: 1552-1560, 1999). Furthermore, these studies led to the prediction that the blood pressure instability results in susceptibility for orthostatic intolerance. The present experiments tested this hypothesis by recording common carotid blood flow (CCBF) in conscious cats during head-up tilts of 20, 40, and 60 degrees amplitudes, before and after the surgical elimination of labyrinthine inputs through a bilateral vestibular neurectomy. Before vestibular lesions in most animals, CCBF remained stable during head-up rotations. Unexpectedly, in five of six animals, the vestibular neurectomy resulted in a significant increase in baseline CCBF, particularly when the laboratory was illuminated; on average, basal blood flow measured when the animals were in the prone position was 41 +/- 17 (SE) % higher after the first week after the lesions. As a result, even when posturally related lability in CCBF occurred after removal of vestibular inputs, blood supply to the head was not lower than when labyrinthine inputs were present. These data suggest that vestibular influences on cardiovascular regulation are more complex than previously appreciated, because labyrinthine signals appear to participate in setting basal rates of blood flow to the head in addition to triggering dynamic changes in the circulation to compensate for orthostatic challenges.

Vestibular inputs to propriospinal interneurons in the feline C1-C2 spinal cord projecting to the C5-C6 ventral horn

The resting length of respiratory muscles must be altered during changes in posture in order to maintain stable ventilation. Prior studies showed that although the vestibular system contributes to these adjustments in respiratory muscle activity, the medullary respiratory groups receive little vestibular input. Additionally, previous transneuronal tracing studies demonstrated that propriospinal interneurons in the C(1)-C(2) spinal cord send projections to the ipsilateral diaphragm motor pool. The present study tested the hypothesis that C(1)-C(2) interneurons mediate vestibular influences on diaphragm activity. Recordings were made from 145 C(1)-C(2) neurons that could be antidromically activated from the ipsilateral C(5)-C(6 )ventral horn, 60 of which had spontaneous activity, during stimulation of vestibular receptors using electric current pulses or whole-body rotations in vertical planes. The firing of 19 of 31 spontaneously active neurons was modulated by vertical vestibular stimulation; the response vector orientations of many of these cells were closer to the pitch plane than the roll plane, and their response gains remained relatively constant across stimulus frequencies. Virtually all spontaneously active neurons responded robustly to electrical vestibular stimulation, and their response latencies were typically shorter than those for diaphragm motoneurons. Nonetheless, respiratory muscle responses to vestibular stimulation were still present after inactivation of the C(1)-C(2) cord using large injections of either muscimol or ibotenic acid. These data suggest that C(1)-C(2) propriospinal interneurons contribute to regulating posturally related responses of the diaphragm, although additional pathways are also involved in generating this activity.

Afferent pathways to the region of the vestibular nuclei that participates in cardiovascular and respiratory control

Prior experiments have shown that a region of the medial and inferior vestibular nuclei contributes to cardiovascular and respiratory regulation. In addition to labyrinthine inputs, the majority of neurons in this region of the vestibular nuclei receive signals from the skin, muscle, and viscera, although the pathways conveying these nonlabyrinthine inputs to the vestibular nucleus neurons are unknown. To gain further insight into the afferent pathways to this functionally distinct subdivision of the vestibular complex, we combined monosynaptic mapping with viral transneuronal tracing in the ferret. First order afferent projections were defined by retrograde transport of the beta-subunit of cholera toxin (CTbeta), and the extended polysynaptic circuitry was defined in the same animals by injection of a recombinant of pseudorabies virus Bartha (PRV) into the contralateral vestibular nuclei. Neurons containing CTbeta or infected by retrograde transneuronal transport and replication of PRV were distributed throughout the spinal cord, but were 10 times more prevalent in the cervical cord than the lumbar cord. The labeled spinal neurons were most commonly observed in Rexed's laminae IV-VI and the dorsal portions of laminae VII-VIII. Both the CTbeta and PRV injections also resulted in labeling of neurons in all four vestibular nuclei, the prepositus hypoglossi, the reticular formation, the inferior olivary nucleus, the medullary raphe nuclei, the spinal and principal trigeminal nuclei, the facial nucleus, and the lateral reticular nucleus. Following survival times >/=3 days, PRV-infected neurons were additionally present in nucleus solitarius and the gracile and cuneate nuclei. These data show that an anatomical substrate is present for somatosensory and visceral inputs to influence the activity of cells in the autonomic region of the vestibular nuclei and suggest that these signals are primarily transmitted through brainstem relay neurons.

Medial vestibular nucleus mediates the cardiorespiratory responses to fastigial nuclear activation and hypercapnia

Electrical stimulation of the cerebellar fastigial nucleus (FN) evokes hyperventilation and hypertension responses that are similar to those induced by stimulation of the medial region of the vestibular nucleus (VNM). Because there are mutual projections between these two nuclei morphologically, we hypothesized that the FN-mediated cardiorespiratory responses were related to the integrity of the VNM. Experiments were conducted on 21 anesthetized, tracheotomized, and spontaneously breathing rats. Electrical stimulation (approximately 10 s) of the FN was used to evoke cardiorespiratory responses, and the same stimulus was repeated 30-45 min after bilateral lesions of the VNM by local microinjection of ibotenic acid (100 mM, 100 nl). We found that FN stimulation-induced hyperventilation and hypertension were attenuated significantly by the lesions. The role of the VNM in the ventilatory responses to chemical challenges was subsequently defined. The animals were exposed to hypercapnia (10% CO2) and hypoxia (10% O2) for 1-2 min randomly before and after VNM lesions. The results showed that VNM lesions significantly attenuated the cardiorespiratory responses to hypercapnia but not to hypoxia, with little effect on baseline respiratory variables. These findings suggest that the VNM is required for full expression of the cardiorespiratory responses to electrical stimulation of the FN as well as to hypercapnia. However, neurons within the VNM do not appear to be critical for maintaining eupneic breathing and the cardiorespiratory responses to hypoxia.

Effects of practice on cardiorespiratory responses during postural control

The present study examined the effects of practice of a balance test on cardiorespiratory changes in response to a 1-min balance test performed by standing on one leg with eyes closed (SOLEC) in 30 females (n=15, 21+/-4 years, mean+/-SD, for the experimental group; n=15, 22+/-4 years for the control group). Blood pressure (BP), heart rate (HR), minute ventilation (VE), respiratory rate (RR), tidal volume (VT), expiratory duration (Te), inspiratory duration (Ti), and oxygen uptake (VO(2)) were measured during the balance test before and after 2 wk of daily practice. The experimental group was given a daily 15-min practice session for the balance test. In contrast, the control group was instructed not to do any special practice for the balance test. In both the experimental and control groups, SOLEC induced significant increases in BP, HR, VE, RR, and VO(2), and decreases in Te and Ti. Following the practice sessions, the balance time increased significantly in the experimental group (P<0.01). In addition, 2 wk of practice reduced the increases in BP (P<0.01), VE (P<0.05), and RR (P<0.01), and prolonged Te (P<0.01) during the SOLEC test. These results suggest that practice of a postural task affects cardiorespiratory responses to the balance test in addition to postural control.

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.

Responses of feline medial medullary reticular formation neurons with projections to the C5–C6 ventral horn to vestibular stimulation

Prior studies have shown that the vestibular system contributes to adjusting respiratory muscle activity during changes in posture, and have suggested that portions of the medial medullary reticular formation (MRF) participate in generating vestibulo-respiratory responses. However, there was previously no direct evidence to demonstrate that cells in the MRF relay vestibular signals monosynaptically to respiratory motoneurons. The present study tested the hypothesis that the firing of MRF neurons whose axons could be antidromically activated from the vicinity of diaphragm motoneurons was modulated by whole-body rotations in vertical planes that stimulated vestibular receptors, as well as by electrical current pulses delivered to the vestibular nerve. In total, 171 MRF neurons that projected to the C5-C6 ventral horn were studied; they had a conduction velocity of 34+/-15 (standard deviation) m/sec. Most (135/171 or 79%) of these MRF neurons lacked spontaneous firing. Of the subpopulation of units with spontaneous discharges, only 3 of 20 cells responded to vertical rotations up to 10 degrees in amplitude, whereas the activity of 8 of 14 neurons was affected by electrical stimulation of the vestibular nerve. These data support the hypothesis that the MRF participates in generating vestibulo-respiratory responses, but also suggest that some neurons in this region have other functions.

Central vestibular system: vestibular nuclei and posterior cerebellum

The vestibular nuclei and posterior cerebellum are the destination of vestibular primary afferents and the subject of this review. The vestibular nuclei include four major nuclei (medial, descending, superior and lateral). In addition, smaller vestibular nuclei include: Y-group, parasolitary nucleus, and nucleus intercalatus. Each of the major nuclei can be subdivided further based primarily on cytological and immunohistochemical histological criteria or differences in afferent and/or efferent projections. The primary afferent projections of vestibular end organs are distributed to several ipsilateral vestibular nuclei. Vestibular nuclei communicate bilaterally through a commissural system that is predominantly inhibitory. Secondary vestibular neurons also receive convergent sensory information from optokinetic circuitry, central visual system and neck proprioceptive systems. Secondary vestibular neurons cannot distinguish between sources of afferent activity. However, the discharge of secondary vestibular neurons can distinguish between "active" and "passive" movements. The posterior cerebellum has extensive afferent and efferent connections with vestibular nuclei. Vestibular primary afferents are distributed to the ipsilateral uvula-nodulus as mossy fibers. Vestibular secondary afferents are distributed bilaterally. Climbing fibers to the cerebellum originate from two subnuclei of the contralateral inferior olive; the dorsomedial cell column and beta-nucleus. Vestibular climbing fibers carry information only from the vertical semicircular canals and otoliths. They establish a coordinate map, arrayed in sagittal zones on the surface of the uvula-nodulus. Purkinje cells respond to vestibular stimulation with antiphasic modulation of climbing fiber responses (CFRs) and simple spikes (SSs). The modulation of SSs is out of phase with the modulation of vestibular primary afferents. Modulation of SSs persists, even after vestibular primary afferents are destroyed by a unilateral labyrinthectomy, suggesting that an interneuronal network, triggered by CFRs is responsible for SS modulation. The vestibulo-cerebellum, imposes a vestibular coordinate system on postural responses and permits adaptive guidance of movement.

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.

Neurochemical phenotypes of MRF neurons influencing diaphragm and rectus abdominis activity

In prior studies that used transneuronal transport of isogenic recombinants of pseudorabies virus, we established that medial medullary reticular formation (MRF) neurons sent collateralized projections to both diaphragm and abdominal muscle motoneurons. Furthermore, inactivation of MRF neurons in cats and ferrets increased the excitability of diaphragm and abdominal motoneurons, suggesting that MRF neurons controlling respiratory activity are inhibitory. To test this hypothesis, the present study determined the neurochemical phenotypes of MRF premotor respiratory neurons in the ferret by using immunohistochemical procedures. Dual-labeling immunohistochemistry combining pseudorabies virus injections into respiratory muscles with the detection of glutamic acid decarboxylase-like immunoreactive and glutamate-like immunoreactive cells showed that both GABAergic and glutamatergic MRF neurons project to respiratory motoneurons, although the latter are more common. These data suggest that the role of the MRF in respiratory regulation is multifaceted, as this region provides both inhibitory and excitatory influences on motoneuron activity.

Activation of different vestibular subnuclei evokes differential respiratory and pressor responses in the rat

Activation of the vestibular system can either increase or decrease ventilation. The objectives of the present study were to clarify whether these different responses are the result of activating different vestibular subnuclei, by addressing three questions. Do neurones within the medial, lateral and spinal vestibular nuclei (VN(M), VN(L) and VN(S), respectively) function differently in respiratory modulation? Is the ventral medullary nucleus gigantocellularis (NGC) required to fully express the VN-mediated respiratory responses? Is glutamate, by acting on N-methyl-D-aspartic acid (NMDA) receptors in the vestibular subnuclei, capable of modulating respiration? In anaesthetized, tracheotomized and spontaneously breathing rats, electrical stimuli (< 10 s) applied in the VN(L) and VN(S) significantly elevated ventilation by 35 % and 30 % (P < 0.05), respectively. However, VN(M) stimulation produced statistically significant (P < 0.05) changes that differed depending upon the stimulation site: either ventilatory inhibition (by 40 % in 57 % of the trials) or excitation (by 55 % in 43 % of trials), and which were often accompanied by a pressor response. These electrical-stimulation-evoked cardiorespiratory responses were almost eliminated following microinjection of ibotenic acid into the stimulation sites (P < 0.05) or bilaterally into the NGC (P < 0.05). As compared to vehicle, microinjection of NMDA into the unilateral VN(M), VN(L) and VN(S) significantly increased ventilation to 74 %, 58 % and 60 % (P < 0.05), respectively, with no effect on arterial blood pressure. These data suggest that neurones within the vestibular subnuclei play different roles in cardiorespiratory modulation, and that the integrity of the NGC is essential for the full expression of these VN-mediated responses. The evoked respiratory excitatory responses are probably mediated by glutamate acting on NMDA receptors, whereas the neurotransmitters involved in VN(M)-mediated respiratory inhibition and hypertension remain unknown.

Contributions from rostral medullary nuclei to coordination of swallowing and breathing in awake goats

The purpose of this study was to determine whether neurons in the facial (FN), gigantocellularis reticularis (RGN), and vestibular (VN) nuclei contribute to the regulation of breathing, swallowing, and the coordination of these two functions. Microtubules were chronically implanted bilaterally in goats. Two weeks later during wakefulness, 100-nl unilateral injections were made of mock cerebral spinal fluid or an excitatory amino acid receptor agonist or antagonists. When the agonist, N-methyl-D-aspartic acid, was injected into any nuclei, breathing and swallowing increased transiently (15-30%; P < 0.05), whereas only injections of the antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo-(f)quinoxaline into VN increased swallowing (20%; P < 0.05). The phase of breathing in which the swallows occurred was not altered by any injections. However, more importantly, injections of the agonist and the antagonists significantly altered (P < 0.05) by 5-50% the respiratory phase-dependent timing and tidal volume effect of swallows on breathing relative to mock cerebral spinal fluid injections. In addition, these effects were not uniform for all three nuclei. We conclude that the FN, RGN, and VN are part of a neural circuit in the rostral medulla that regulates and/or modulates breathing, swallowing, and their coordination in the awake state.

Long-term modulation of respiratory network activity following anoxia in vitro

Neural networks that produce rhythmic behaviors require flexibility to respond to changes in the internal and external state of the animal. It is important to not only understand how a network responds during such perturbations but also how the network recovers. For example, the respiratory network needs to respond to and recover from temporary changes in oxygen level that can occur during sleep, exercise, and respiratory disorders. During a temporary decrease in oxygen level, there is an increase in respiratory frequency followed by a depression that can lead to complete apnea. Here we used a mouse brain stem slice preparation as a model system to examine the recovery of respiratory network activity after brief episodes of anoxia. We found the respiratory network recovers from a single anoxic episode with a transient increase in fictive respiratory frequency. Although repetitive anoxia does not elicit a greater frequency increase, it does elicit a longer lasting frequency increase persisting < or =90 min. Thus there is a centrally mediated long-lasting influence on the respiratory network elicited by decreased oxygen levels. This modulation occurs as a prolonged facilitation of fictive respiratory frequency after brief repetitive but not single anoxic exposure. These data are important to consider in the context of disorders such as sleep apnea in which brief periodic anoxic episodes are experienced.

Effects of lesions of the caudal cerebellar vermis on cardiovascular regulation in awake cats

The vestibular system is known to participate in cardiovascular regulation during movement and postural alterations. The present study considered whether lesions of two regions of the posterior cerebellar vermis (the nodulus and uvula) that provide inputs to vestibular nucleus regions that affect control of blood pressure would alter cardiovascular responses during changes in posture. Blood pressure and heart rate were monitored in awake cats during nose-up tilts up to 60 degrees in amplitude before and following aspiration lesions of the nodulus or uvula; in most animals, cardiovascular responses were also recorded following the subsequent removal of vestibular inputs. Lesions of the nodulus or uvula did not affect baseline blood pressure or heart rate, although cardiovascular responses during nose-up tilts were altered. Increases in heart rate that typically occurred during 60 degrees nose-up tilt were attenuated in all three animals with lesions affecting both dorsal and ventral portions of the uvula; in contrast, the heart rate responses were augmented in the two animals with lesions mainly confined to the nodulus. Furthermore, following subsequent removal of vestibular inputs, uvulectomized animals, but not those with nodulus lesions, experienced more severe orthostatic hypotension than has previously been reported in cerebellum-intact animals with bilateral labyrinthectomies. These data suggest that the cerebellar nodulus and uvula modulate vestibulo-cardiovascular responses, although the two regions play different roles in cardiovascular regulation.

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.

Role of the vestibular system in regulating respiratory muscle activity during movement

1. Changes in posture can affect the resting length of the diaphragm, which is corrected through increases in both diaphragm and abdominal muscle activity. Furthermore, postural alterations can diminish airway patency, which must be compensated for through increases in firing of particular upper airway muscles. 2. Recent evidence has shown that the vestibular system participates in adjusting the activity of both upper airway muscles and respiratory pump muscles during movement and changes in body position. 3. Vestibulo-respiratory responses do not appear to be mediated through the brainstem respiratory groups; labyrinthine influences on respiratory pump muscles may be relayed through neurons in the medial medullary reticular formation, which have recently been demonstrated to provide inputs to both abdominal and diaphragm motoneurons. 4. Three regions of the cerebellum that receive vestibular inputs, the fastigial nucleus, the nodulus/uvula and the anterior lobe, also influence respiratory muscle activity, although the physiological role of cerebellar regulation of respiratory activity is yet to be determined. 5. It is practical for the vestibular system to participate in the control of respiration, to provide for rapid adjustments in ventilation such that the oxygen demands of the body are continually matched during movement and exercise.

Role of the medial medullary reticular formation in relaying vestibular signals to the diaphragm and abdominal muscles

Changes in posture can affect the resting length of respiratory muscles, requiring alterations in the activity of these muscles if ventilation is to be unaffected. Recent studies have shown that the vestibular system contributes to altering respiratory muscle activity during movement and changes in posture. Furthermore, anatomical studies have demonstrated that many bulbospinal neurons in the medial medullary reticular formation (MRF) provide inputs to phrenic and abdominal motoneurons; because this region of the reticular formation receives substantial vestibular and other movement-related input, it seems likely that medial medullary reticulospinal neurons could adjust the activity of respiratory motoneurons during postural alterations. The objective of the present study was to determine whether functional lesions of the MRF affect inspiratory and expiratory muscle responses to activation of the vestibular system. Lidocaine or muscimol injections into the MRF produced a large increase in diaphragm and abdominal muscle responses to vestibular stimulation. These vestibulo-respiratory responses were eliminated following subsequent chemical blockade of descending pathways in the lateral medulla. However, inactivation of pathways coursing through the lateral medulla eliminated excitatory, but not inhibitory, components of vestibulo-respiratory responses. The simplest explanation for these data is that MRF neurons that receive input from the vestibular nuclei make inhibitory connections with diaphragm and abdominal motoneurons, whereas a pathway that courses laterally in the caudal medulla provides excitatory vestibular inputs to these motoneurons.

Panic disorder: the role of the balance system

Experimental evidences suggest that Panic Disorder (PD) is characterized by abnormalities in respiratory and vestibular functions. We studied balance system function in patients with PD and its relationships with CO(2) reactivity and clinical characteristics. Nineteen patients with PD with/without agoraphobia underwent static posturography and the 35% CO(2) challenge. The severity of clinical symptomatology was measured by standardized psychometric scales. Patients were free of psychotropic medications during the 2 weeks before the study. Different investigators blind to each other carried out the CO(2) challenge, static posturography and clinical assessment. Nineteen age and sex-matched healthy controls underwent static posturography. Body sway velocity and length were significantly higher in panic patients than in controls and patients showed high percentages of abnormal scores. Patients with two or more abnormal scores on static posturography were significantly more agoraphobic than those with less than two. Abnormal posturography scores under the eyes-opened was related to high anticipatory anxiety, whereas those under eyes-closed was related to phobic avoidance. Symptomatological reactivity to CO(2) was significantly correlated to abnormal functions of the balance system in the eyes-closed condition. Our findings suggest that (1) many patients with PD (5-42%) have abnormalities in their balance system function compared with healthy controls (0-5%), (2) symptomatological reactivity to CO(2) and balance system function in patients with PD are correlated only in the eyes-closed condition and (3) there is a significant link between agoraphobic avoidance and subclinical abnormal function of the balance system network.

Multiple rostral medullary nuclei can influence breathing in awake goats

The purpose of this study was to determine the effect on breathing of neuronal dysfunction in the retrotrapezoid (RTN), facial (FN), gigantocellularis reticularis (RGN), or vestibular (VN) nuclei of adult awake goats. Microtubules were chronically implanted to induce neuronal dysfunction by microinjection of an excitatory amino acid (EAA) receptor antagonist or a neurotoxin. The EAA receptor antagonist had minimal effect on eupneic breathing, but 8--10 days after injection of the neurotoxin, 7 of 10 goats hypoventilated (arterial PCO(2) increased 3.2 +/- 0.7 Torr). Overall there were no significant (P > 0.10) effects of the EAA receptor antagonist on CO(2) sensitivity. However, for all nuclei, > or =66% of the antagonist injections altered CO(2) sensitivity by more than the normal 12.7 +/- 1.6% day-to-day variation. These changes were not uniform, inasmuch as the antagonist increased (RTN, n = 2; FN, n = 7; RGN, n = 6; VN, n = 1) or decreased (RTN, n = 2; RGN, n = 3; VN, n = 2) CO(2) sensitivity. Ten days after injection of the neurotoxin into the FN (n = 3) or RGN (n = 5), CO(2) sensitivity was also reduced. Neuronal dysfunction also did not have a uniform effect on the exercise arterial PCO(2) response, and there was no correlation between effects on CO(2) sensitivity and the exercise hyperpnea. We conclude that there is a heterogeneous population of neurons in these rostral medullary nuclei (or adjacent tissue) that can affect breathing in the awake state, possibly through chemoreception or chemoreceptor-related mechanisms.

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.