Responses of neurons in the caudal medullary raphe nuclei of the cat to stimulation of the vestibular nerve

Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats

Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Further, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH vs. 2 weeks of dAIH preconditioning on spontaneous and evoked responses recorded in anesthetized, paralyzed (with pancuronium bromide) and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at C2 delivered prior to- and 60 min post-AIH (or an equivalent time in controls). Charge-balanced biphasic pulses (100 µs/phase) of progressively increasing intensity (100 to 700 µA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (~60% from baseline) was observed after a single exposure to moderate AIH (3 x 5 min; 5 min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.

Descending Influences on Vestibulospinal and Vestibulosympathetic Reflexes

This review considers the integration of vestibular and other signals by the central nervous system pathways that participate in balance control and blood pressure regulation, with an emphasis on how this integration may modify posture-related responses in accordance with behavioral context. Two pathways convey vestibular signals to limb motoneurons: the lateral vestibulospinal tract and reticulospinal projections. Both pathways receive direct inputs from the cerebral cortex and cerebellum, and also integrate vestibular, spinal, and other inputs. Decerebration in animals or strokes that interrupt corticobulbar projections in humans alter the gain of vestibulospinal reflexes and the responses of vestibular nucleus neurons to particular stimuli. This evidence shows that supratentorial regions modify the activity of the vestibular system, but the functional importance of descending influences on vestibulospinal reflexes acting on the limbs is currently unknown. It is often overlooked that the vestibulospinal and reticulospinal systems mainly terminate on spinal interneurons, and not directly on motoneurons, yet little is known about the transformation of vestibular signals that occurs in the spinal cord. Unexpected changes in body position that elicit vestibulospinal reflexes can also produce vestibulosympathetic responses that serve to maintain stable blood pressure. Vestibulosympathetic reflexes are mediated, at least in part, through a specialized group of reticulospinal neurons in the rostral ventrolateral medulla that project to sympathetic preganglionic neurons in the spinal cord. However, other pathways may also contribute to these responses, including those that dually participate in motor control and regulation of sympathetic nervous system activity. Vestibulosympathetic reflexes differ in conscious and decerebrate animals, indicating that supratentorial regions alter these responses. However, as with vestibular reflexes acting on the limbs, little is known about the physiological significance of descending control of vestibulosympathetic pathways.

The Vestibular System: A Newly Identified Regulator of Bone Homeostasis Acting Through the Sympathetic Nervous System

The vestibular system is a small bilateral structure located in the inner ear, known as the organ of balance and spatial orientation. It senses head orientation and motion, as well as body motion in the three dimensions of our environment. It is also involved in non-motor functions such as postural control of blood pressure. These regulations are mediated via anatomical projections from vestibular nuclei to brainstem autonomic centers and are involved in the maintenance of cardiovascular function via sympathetic nerves. Age-associated dysfunction of the vestibular organ contributes to an increased incidence of falls, whereas muscle atrophy, reduced physical activity, cellular aging, and gonadal deficiency contribute to bone loss. Recent studies in rodents suggest that vestibular dysfunction might also alter bone remodeling and mass more directly, by affecting the outflow of sympathetic nervous signals to the skeleton and other tissues. This review will summarize the findings supporting the influence of vestibular signals on bone homeostasis, and the potential clinical relevance of these findings.

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.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

Bone remodeling is regulated by inner ear vestibular signals

Bone remodeling allows the conservation of normal bone mass despite constant changes in internal and external environments. The adaptation of the skeleton to these various stimuli leads credence to the notion that bone remodeling is a true homeostatic function, and as such is under the control of specific centers in the central nervous system (CNS). Hypothalamic and brainstem centers, as well as the sympathetic nervous system (SNS), have been identified as regulators of bone remodeling. However, the nature of the afferent CNS stimuli that may modulate CNS centers involved in the control of bone remodeling, with the exception of leptin, remains unclear. Based on the partial efficacy of exercise and mechanical stimulation regimens to prevent microgravity-induced bone loss and the known alterations in vestibular functions associated with space flights, we hypothesized that inner ear vestibular signals may contribute to the regulation of bone remodeling. Using an established model of bilateral vestibular lesions and microtomographic and histomorphometric bone analyses, we show here that induction of bilateral vestibular lesion in rats generates significant bone loss, which is restricted to weight-bearing bones and associated with a significant reduction in bone formation, as observed in rats under microgravity conditions. Importantly, this bone loss was not associated with reduced locomotor activity or metabolic abnormalities, was accompanied with molecular signs of increased sympathetic outflow, and could be prevented by the β-blocker propranolol. Collectively, these data suggest that the homeostatic process of bone remodeling has a vestibulosympathetic regulatory component and that vestibular system pathologies might be accompanied by bone fragility.

Vestibular Stimulation for ADHD: randomized controlled trial of Comprehensive Motion Apparatus

OBJECTIVE

This research evaluates effects of vestibular stimulation by Comprehensive Motion Apparatus (CMA) in ADHD.

METHOD

Children ages 6 to 12 (48 boys, 5 girls) with ADHD were randomized to thrice-weekly 30-min treatments for 12 weeks with CMA, stimulating otoliths and semicircular canals, or a single-blind control of equal duration and intensity, each treatment followed by a 20-min typing tutorial.

RESULTS

In intent-to-treat analysis (n = 50), primary outcome improved significantly in both groups (p = .0001, d = 1.09 to 1.30), but treatment difference not significant (p = .7). Control children regressed by follow-up (difference p = .034, d = 0.65), but overall difference was not significant (p = .13, d = .47). No measure showed significant treatment differences at treatment end, but one did at follow-up. Children with IQ-achievement discrepancy > or = 1 SD showed significantly more CMA advantage on three measures.

CONCLUSION

This study illustrates the importance of a credible control condition of equal duration and intensity in trials of novel treatments. CMA treatment cannot be recommended for combined-type ADHD without learning disorder.

Persistent respiratory changes following intermittent hypoxic stimulation in cats and human beings

Repeated intermittent hypoxia or other stimulation of carotid chemoreceptors produces a consistent long-term increase in respiratory nerve activity in vagotomized, artificially ventilated anesthetized or decerebrate animals, but variable results have been reported in more intact preparations. We sought additional variables that could be measured to help gain an understanding of persistent respiratory responses to intermittent hypoxia. The variance of respiratory phases decreased in 10 of 11 recordings from vagotomized anesthetized cats during long-term facilitation induced by carotid chemoreceptor stimulation. The variance of expiratory time was reduced in 10 awake human beings exposed to repetitive, brief episodes of isocapnic hypoxia (6% O(2) in N(2), 60s). Respiratory frequency was increased in humans and tidal volume decreased so that minute ventilation remained unchanged. The results suggest that there are persistent changes in the output of the respiratory central pattern generator following intermittent peripheral chemoreceptor stimulation or hypoxia.

5-HT at hypoglossal motor nucleus and respiratory control of genioglossus muscle in anesthetized rats

Serotonin (5-HT) from medullary raphe neurons excites hypoglossal motoneurons innervating genioglossus (GG) muscle. Since some raphe neurons also show increased activity in hypercapnia, we tested the hypothesis that serotonergic mechanisms at the hypoglossal motor nucleus (HMN) modulate GG activity and responses to CO2. Seventeen urethane-anesthetized, tracheotomized and vagotomized rats were studied. Microdialysis probes were used to deliver mianserin (5-HT receptor antagonist, 0 and 0.1 mM) or 5-HT (eight doses, 0-50 mM) to the HMN during room air or CO2-stimulated breathing. Mianserin decreased respiratory-related GG activity during room air and CO2-stimulated breathing (P<0.001), and also suppressed GG responses to CO2 (P=0.05). In contrast, GG activity was increased by 5-HT at the HMN, and was further increased in hypercapnia (P<0.02). However, 5-HT increased respiratory-related GG activity at levels lower (1 mM) than those eliciting tonic GG activity (10-30 mM 5-HT). The results show that 5-HT at the HMN contributes to the respiratory control of GG muscle.

A subpopulation of dorsal raphe nucleus neurons retrogradely labeled with cholera toxin-B injected into the inner ear

Previous studies have shown that: (1) raphe neurons respond to acoustic and vestibular stimuli, some with a latency of 10-15 ms; (2) alterations of the raphe nuclei alter the acoustic startle reflex; (3) the dorsal raphe nucleus (DRN) is the major source of serotonergic neurons; and (4) approximately 57% of the DRN neurons are nonserotonergic. In the present study, cholera toxin subunit-B (CTB) was injected into cat cochleas, and the brain tissue was examined after a survival period of 5-7 days. Aside from neurons which were known to project to the inner ear, i.e., olivocochlear and vestibular efferent neurons, a surprising new finding was made that somata of a subpopulation of DRN neurons were intensely labeled with CTB. These CTB-labeled neurons were densely distributed in a dorsomedian part of the DRN with some in a surrounding area outside the DRN. The present results suggest that a novel raphe-labyrinthine projection may exist. A future study of anterograde labeling with injections of a tracer in the DRN will be needed to establish the existence of a raphe-labyrinthine projection more thoroughly. A raphe-labyrinthine descending input, together with an ascending input from the inner ear to the DRN through intervening neurons, such as the juxta-acousticofloccular raphe neurons (JAFRNs) described by Ye and Kim, may mediate a brain stem reflex whereby a salient multisensory (including auditory and vestibular) stimulus may alter the sensitivity of the inner ear. As a mammal responds to a biologically important auditory-vestibular multisensory event, the raphe projections to the inner ear and other auditory and vestibular structures may enhance the mammal's ability to localize and recognize the sound and respond properly. The raphe-labyrinthine projection may also modulate the inner ear's sensitivity as a function of the sleep-wake arousal state of an organism on a slower time course.

Effects of acute hypotension on expression of cFos-like protein in the vestibular nuclei of rats

The expression and regional distribution of cFos protein, which is an oncogene product and metabolic marker of neural excitation, were investigated in the vestibular nuclear complex following acute hypotension in adult Sprague-Dawley rats. Intravenous administration of nitroprusside elicited a 10-50% reduction in mean blood pressure for 10 min. Unilateral or bilateral chemical labyrinthectomies were performed 14 days before the start of the experiment to eliminate afferent signals from the peripheral vestibular receptors in the inner ear. All of the animals were sacrificed and the tissues were fixed 2 h after the onset of acute hypotension using the cardiac perfusion method for c-Fos immunohistochemical staining. The cFos-like immunoreactive (cFLI) neurons were expressed selectively in the central area of the medial vestibular nucleus following a 10% reduction in blood pressure. Once the blood pressure had fallen by 30%, bilateral expression of cFLI neurons was observed in the superior, medial, and spinal vestibular nuclei, but not in the lateral vestibular nucleus, of control rats with intact labyrinths. The expression of cFLI neurons increased proportionately with reductions in blood pressure. In unilaterally labyrinthectomized rats, acute hypotension induced the expression of cFLI neurons in vestibular nuclei contra lateral to the injured labyrinth, but not in the ipsilateral vestibular nuclei. However, cFLI neurons were not expressed in bilateral vestibular nuclei following acute hypotension in bilateral labyrinthectomized rats. These results suggest that afferent signals from the peripheral vestibular receptors are essential for cFos protein expression in the vestibular nuclei following acute hypotension.

Intermittent hypoxia induces phrenic long-term facilitation in carotid-denervated rats

Episodic hypoxia elicits a long-lasting augmentation of phrenic inspiratory activity known as long-term facilitation (LTF). We investigated the respective contributions of carotid chemoafferent neuron activation and hypoxia to the expression of LTF in urethane-anesthetized, vagotomized, paralyzed, and ventilated Sprague-Dawley rats. One hour after three 5-min isocapnic hypoxic episodes [arterial Po(2) (Pa(O(2))) = 40 +/- 5 Torr], integrated phrenic burst amplitude was greater than baseline in both carotid-denervated (n = 8) and sham-operated (n = 7) rats (P < 0.05), indicating LTF. LTF was reduced in carotid-denervated rats relative to sham (P < 0.05). In this and previous studies, rats were ventilated with hyperoxic gas mixtures (inspired oxygen fraction = 0.5) under baseline conditions. To determine whether episodic hyperoxia induces LTF, phrenic activity was recorded under normoxic (Pa(O(2)) = 90-100 Torr) conditions before and after three 5-min episodes of isocapnic hypoxia (Pa(O(2)) = 40 +/- 5 Torr; n = 6) or hyperoxia (Pa(O(2)) > 470 Torr; n = 6). Phrenic burst amplitude was greater than baseline 1 h after episodic hypoxia (P < 0.05), but episodic hyperoxia had no detectable effect. These data suggest that hypoxia per se initiates LTF independently from carotid chemoafferent neuron activation, perhaps through direct central nervous system effects.

Adaptive plasticity in vestibular influences on cardiovascular control

Data collected in both human subjects and animal models indicate that the vestibular system influences the control of blood pressure. In animals, peripheral vestibular lesions diminish the capacity to rapidly and accurately make cardiovascular adjustments to changes in posture. Thus, one role of vestibulo-cardiovascular influences is to elicit changes in blood distribution in the body so that stable blood pressure is maintained during movement. However, deficits in correcting blood pressure following vestibular lesions diminish over time, and are less severe when non-labyrinthine sensory cues regarding body position in space are provided. These observations show that pathways that mediate vestibulo-sympathetic reflexes can be subject to plastic changes. This review considers the adaptive plasticity in cardiovascular responses elicited by the central vestibular system. Recent data indicate that the posterior cerebellar vermis may play an important role in adaptation of these responses, such that ablation of the posterior vermis impairs recovery of orthostatic tolerance following subsequent vestibular lesions. Furthermore, recent experiments suggest that non-labyrinthine inputs to the central vestibular system may be important in controlling blood pressure during movement, particularly following vestibular dysfunction. A number of sensory inputs appear to be integrated to produce cardiovascular adjustments during changes in posture. Although loss of any one of these inputs does not induce lability in blood pressure, it is likely that maximal blood pressure stability is achieved by the integration of a variety of sensory cues signaling body position in space.

Effects of galvanic stimulation of the mastoid process on the gastric motility induced by caloric stimulation

The effects of galvanic stimulation to the mastoid portion on the vestibuloautonomic symptoms induced by caloric stimulation, such as nausea, vomiting, and vertigo, were evaluated in this study. Gastric motility was measured by electrogastrography (EGG) in 20 healthy volunteers (11 male and nine female) aged 20-30 (average: 25.4) years. Electrical stimulation of the mastoid process with 1.0-3.0 mA, 1.0 ms, 100 Hz was applied using a bipolar-biaural method during caloric stimulation of the external auditory canal. The dominant frequency and power of EGG were determined using running spectral frequency analysis and the time-course of EGG was evaluated in a pseudo three dimensional graphic. Frequency of EGG was classified into normogastria with 3 cpm, bradygastria with lower than 3 cpm, and tachygastria with higher than 3 cpm. At quiescent period, normogastria was 78.7 +/- 3.7%, bradygastria 5.0 +/- 1.1%, and tachygastria 16.4 +/- 3.7%. Caloric stimulation with warm water in the unilateral ear and cold water in the contralateral ear elicited vestibuloautonomic symptoms, with accompanied decreases in normogastria (57.7 +/-4.6%, P < 0.01) and increases in tachygastria (34.8 +/- 4.8%, P < 0.01). Cathodal stimulation to the mastoid process ipsilateral to cold water irrigation during caloric stimulation restored normal pattern of gastric motility: normogastria in 77.1 + 5.3% and tachygastria in 19.3 +/- 4.7%, and relieved vestibuloautonomic symptoms. Cathodal stimulation to the inhibited vestibular system ameliorates the vestibular symptoms induced by caloric stimulation.

Cervical dorsal rhizotomy enhances serotonergic innervation of phrenic motoneurons and serotonin-dependent long-term facilitation of respiratory motor output in rats

We tested the hypothesis that spinal plasticity elicited by chronic bilateral cervical dorsal rhizotomy (C3-C5; CDR) has functional implications for respiratory motor control. Surgery was performed on rats (CDR or sham-operated) 26 d before phrenic motoneurons were retrogradely labeled with cholera toxin. Rats were killed 2 d later, and their spinal cords were harvested and processed to reveal the cholera toxin-labeled phrenic motoneurons and serotonin-immunoreactive terminals. The number of serotonin-immunoreactive terminals within 5 micrometer of labeled phrenic motoneuron soma and primary dendrites increased 2.1-fold after CDR versus sham-operation. Time-dependent phrenic motor responses to hypoxia were compared among CDR, sham-operated, and control rats. Anesthetized, paralyzed, vagotomized, and artificially ventilated rats were exposed to three, 5 min episodes of isocapnic hypoxia (FiO2 = 0.11), separated by 5 min hyperoxic intervals (FiO2 = 0.5). One hour after hypoxia, a long-lasting, serotonin-dependent enhancement of phrenic motor output (long-term facilitation) was observed in both sham and control rats. After CDR, long-term facilitation was 108 and 163% greater than control and sham responses, respectively. Pretreatment of CDR rats with a 5-HT2 receptor antagonist (ketanserin tartrate, 2 mg/kg, i.v.) before episodic hypoxia prevented long-term facilitation and revealed a modest (-28 +/- 13%; p < 0.05) long-lasting depression of phrenic motor output. The results indicate that CDR: (1) increases serotonergic innervation of the phrenic motor nucleus; and (2) augments serotonin-dependent long-term facilitation of phrenic motor output. These results further suggest a form of plasticity based on changes in the capacity for neuromodulation.

Autonomic reaction to vestibular damage

The vestibular system provides inputs to many neurons in the brain stem that participate in autonomic control. This multiplicity of vestibular-autonomic connections plays a variety of roles. Whereas it has been known for decades that unilateral vestibular lesions can result in motion sickness, recent data suggest that the vestibular system participates in making adjustments in blood pressure and respiration that are necessary to maintain homeostasis during movement and changes in posture. Animals with bilateral vestibular lesions are more susceptible to posturally related hypotension than vestibularly intact animals, and it is also possible that orthostatic hypotension after space flight is caused in part by microgravity-related changes in otolith function. Patients with vestibular lesions could also be more vulnerable to respiratory disturbances related to posture, such as obstructive apnea. Vestibular dysfunction has additionally been linked with anxiety disorders, such as agoraphobia, which may result from alteration of vestibular inputs to brain stem monoaminergic neurons (which are known to process these signals). Even sleep disturbances might be connected with vestibular disorders because neurons in the pontine reticular formation that are critical in switching between sleep states may be influenced by labyrinthine inputs. Thus it is likely that vestibular damage will result in a number of parallel disturbances in autonomic function.

Modulation of vomiting by the medullary midline

Studies were conducted to investigate the possible role of the brainstem midline region as a source of facilitatory input for the vomiting process. Experiments were conducted using the "fictive vomiting' model in decerebrate, paralysed cats. Dysfunction of the medullary midline region produced by localized injections of the neurotoxin kainic acid abolished or greatly attenuated fictive vomiting. In addition, some respiratory-related midline neurons were found to fire in synchrony with co-active phrenic and abdominal nerve discharge during fictive vomiting. These experiments demonstrate the importance of the medullary midline for the normal occurrence of the vomiting process. An explanation for the post-lesion elimination of vomiting is that the lesions remove an important source of facilitatory input to spinal respiratory motoneurons and/or to the brainstem circuitry that mediates vomiting.

Hypoxia-induced long-term facilitation of respiratory activity is serotonin dependent

Repeated isocapnic hypoxia evokes long-term facilitation (LTF) of phrenic nerve activity in rats. We wished to determine: (1) whether hypoxia-induced LTF is serotonin dependent; and (2) whether hypoxia-induced LTF is a property of upper airway motoneurons. Phrenic and hypoglossal nerve activities were recorded in urethane anesthetized, vagotomized, paralyzed and artificially ventilated rats (n = 7). Rats were exposed to three, 5-min hypoxic episodes (FIo2 = 0.10) separated by 5 min of hyperoxia (FIo2 = 0.50). One hour after the final hypoxic episode, integrated phrenic and hypoglossal amplitudes and burst frequency were increased above control values (63 +/- 17%, 78 +/- 26% and 9.6 +/- 2.1 bursts/min, respectively: p < 0.05). In rats pretreated with methysergide (n = 7; 4 mg/kg), no changes in phrenic or hypoglossal activity from pre-stimulus control values were observed at any time post-stimulation. The results indicate that hypoxia-induced LTF requires 5-HT receptors and is characteristic of both hypoglossal and phrenic motor output.

Vestibular influences on the autonomic nervous system

Considerable evidence exists to suggest that both sympathetic and respiratory outflow from the central nervous system are influenced by the vestibular system. Otolith organs that respond to pitch rotations seem to play a predominant role in producing vestibulo-sympathetic and vestibulo-respiratory responses in cats. Because postural changes involving nose-up pitch challenge the maintenance of stable blood pressure and blood oxygenation in this species, vestibular effects on the sympathetic and respiratory systems are appropriate to participate in maintaining homeostasis during movement. Vestibular influences on respiration and circulation are mediated by a relatively small portion of the vestibular nuclear complex comprising regions in the medial and inferior vestibular nuclei just caudal to Deiters' nucleus. Vestibular signals are transmitted to sympathetic preganglionic neurons in the spinal cord through pathways that typically regulate the cardiovascular system. In contrast, vestibular effects on respiratory motoneurons are mediated in part by neural circuits that are not typically involved in the generation of breathing.

Raphe magnus and reticulospinal actions on primary afferent depolarization of group I muscle afferents in the cat

1. In the anaesthetized cat, electrical stimulation of the bulbar reticular formation produced a short latency (2.1 +/- 0.3 ms) positive potential in the cord dorsum. In contrast, stimulation of the nucleus raphe magnus with strengths below 50 microA evoked a slow negative potential with a mean latency of 5.5 +/- 0.6 ms that persisted after sectioning the contralateral pyramid and was abolished by sectioning the ipsilateral dorsolateral funiculus. 2. The field potentials evoked by stimulation of the bulbar reticular formation and of the nucleus raphe magnus had a different intraspinal distribution, suggesting activation of different sets of segmental interneurones. 3. Stimulation of these two supraspinal nuclei produced primary afferent depolarization (PAD) in single Ib fibres and inhibited the PAD elicited by group I volleys in single Ia fibres. The inhibition of the PAD of Ia fibres produced by reticulospinal and raphespinal inputs appears to be exerted on different interneurones along the PAD pathway. 4. It is concluded that, although reticulospinal and raphespinal pathways have similar inhibitory effects on PAD of Ia fibres, and similar excitatory effects on the PAD of Ib fibres, their actions are conveyed by partly independent pathways. This would allow their separate involvement in the control of posture and movement.

Cardiovascular responses to KC-135 hyper-gravity

The present study was designed with two intentions; Are the effects of angular velocity detectable in the cardiovascular responses during the hyper-G? Another is object to examine how the otolith signal could modify the cardiovascular responses provoked by the exposure to the hyper-G. NASA/KC-135 hyper-gravity flight was used to generate high gravito-inertial forces to exclude a possible effect of angular velocity. Six healthy subjects was indicated to make dorsal flexion of the neck to reduce the otolith input. An exposure to +l.8Gz stress resulted in a remarkable increase of systolic and diastolic blood pressure, thereby pulse pressure became a little bit narrower. R-R interval revealed a tachycardia during the hyper-G except one subject. The present experiment bore the similar cardiovascular responses as those observed in the previous studies with a short rotating radius, suggesting that almost no effect of angular velocity acts on their responses. A weaker otolith input could possibly work on them. However a systematical observation can not recognize among the subjects for the vestibular effect on the cardiovascular responses. This fact of vestibular qualification leads us to speculate that it would depend on the subjects or other factors.

Vestibular influences on the sympathetic nervous system

Studies using both electrical and natural stimulation have established that the vestibular system has prominent effects on sympathetic outflow and blood pressure. Preliminary evidence suggests that receptors in both otolith organs and semicircular canals are involved in producing these effects. Furthermore, vestibulosympathetic reflexes appear to be mediated by the medial vestibular nucleus and slowly conducting projections from the rostral ventrolateral medulla and caudal medullary raphe nuclei to preganglionic neurons in the thoracic spinal cord. However, many details are missing from our knowledge and understanding of the functional significance and neural substrate of vestibular influences on the sympathetic nervous system.