Reflex respiratory response to changes in upper airway pressure in the anaesthetized rat

Muscarinic Inhibition of Hypoglossal Motoneurons: Possible Implications for Upper Airway Muscle Hypotonia during REM Sleep

Proper function of pharyngeal dilator muscles, including the genioglossus muscle of the tongue, is required to maintain upper airway patency. During sleep, the activity of these muscles is suppressed, and as a result individuals with obstructive sleep apnea experience repeated episodes of upper airway closure when they are asleep, in particular during rapid-eye-movement (REM) sleep. Blocking cholinergic transmission in the hypoglossal motor nucleus (MoXII) restores REM sleep genioglossus activity, highlighting the importance of cholinergic transmission in the inhibition of hypoglossal motor neurons (HMNs) during REM sleep. Glutamatergic afferent input from neurons in the parahypoglossal (PH) region to the HMNs is critical for MoXII respiratory motor output. We hypothesized that state-dependent cholinergic regulation may be mediated by this pathway. Here we studied the effects of cholinergic transmission in HMNs in adult male and female mice using patch-clamp recordings in brain slices. Using channelrhodopsin-2-assisted circuit mapping, we first demonstrated that PH glutamatergic neurons directly and robustly activate HMNs (PH^Glut → HMNs). We then show that carbachol consistently depresses this input and that this effect is presynaptic. Additionally, carbachol directly affects HMNs by a variable combination of muscarinic-mediated excitatory and inhibitory responses. Altogether, our results suggest that cholinergic signaling impairs upper airway dilator muscle activity by suppressing glutamatergic input from PH premotoneurons to HMNs and by directly inhibiting HMNs. Our findings highlight the complexity of cholinergic control of HMNs at both the presynaptic and postsynaptic levels and provide a possible mechanism for REM sleep suppression of upper airway muscle activity.SIGNIFICANCE STATEMENT Individuals with obstructive sleep apnea can breathe adequately when awake but experience repeated episodes of upper airway closure when asleep, in particular during REM sleep. Similar to skeletal postural muscles, pharyngeal dilator muscles responsible for maintaining an open upper airway become hypotonic during REM sleep. Unlike spinal motoneurons controlling postural muscles that are inhibited by glycinergic transmission during REM sleep, hypoglossal motoneurons that control the upper airway muscles are inhibited in REM sleep by the combination of monoaminergic disfacilitation and cholinergic inhibition. In this study, we demonstrated how cholinergic signaling inhibits hypoglossal motoneurons through presynaptic and postsynaptic muscarinic receptors. Our results provide a potential mechanism for upper airway hypotonia during REM sleep.

Thyroxin and calcitonin secretion into thyroid venous blood is regulated by pharyngeal mechanical stimulation in anesthetized rats

The effects of the pharyngeal non-noxious mechanical stimulation on the secretion of immunoreactive thyroxin (iT4), immunoreactive calcitonin (iCT), and immunoreactive parathyroid hormone (iPTH) into thyroid venous blood were examined in anesthetized rats. Secretion rates of iT4, iCT, and iPTH were calculated from their concentration in thyroid venous plasma and the plasma flow rate. A mechanical stimulation was delivered to the pharynx by a rubber balloon placed on the tongue that was intermittently pushed into the pharyngeal cavity. Pharyngeal stimulation increased iT4 and iCT secretion, but iPTH secretion was unchanged. The secretion responses were abolished by transecting the superior laryngeal nerves (SLNs) bilaterally. The activities of the thyroid parasympathetic efferent nerves and the afferent nerves in the SLN increased significantly during pharyngeal stimulation. These results indicate that pharyngeal mechanical stimulation promotes thyroxin and calcitonin secretion from the thyroid gland by a reflex increase in SLN parasympathetic efferent activity, triggered by excitation of SLN mechanoreceptive afferents.

An acute exposure to intermittent negative airway pressure elicits respiratory long-term facilitation in awake humans

BACKGROUND

A sustained elevation in respiratory drive following removal of the inducing stimulus is known as respiratory long-term facilitation (rLTF). We investigated whether an acute exposure to intermittent negative airway pressure (INAP) elicits rLTF in humans.

METHOD

13 healthy males (20.9 ± 2.8 years) undertook two trials (INAP and Control). In the INAP trial participants were exposed to one hour of 30-second episodes of breathing against negative pressure (-10 cmH₂O) interspersed by 60-second intervals of breathing at atmospheric pressure. In the Control trial participants breathed at atmospheric pressure for one hour. Ventilation following INAP (recovery phase) was compared to that during baseline.

RESULTS

Ventilation increased from baseline to recovery in the INAP trial (14.9 ± 0.9 vs 19.1 ± 0.7 L/min, P = 0.002). This increase was significantly greater than the equivalent during the Control trial (P = 0.019). Data shown as mean ± SEM.

CONCLUSION

In this study INAP elicited rLTF in awake, healthy humans. Further research is required to investigate the responsible mechanisms.

Effect of chronic intermittent hypoxia on the reflex recruitment of the genioglossus during airway obstruction in the anesthetized rat

We sought to test the hypothesis that chronic intermittent hypoxia (CIH)-a feature of sleep-disordered breathing in humans-impairs reflex recruitment of the genioglossus (GG, pharyngeal dilator) during obstructive airway events. Adult male Wistar rats were exposed to 20 cycles of normoxia and hypoxia (5% O2 at nadir) per hour, 8h a day for 7 days (CIH, N=7). The sham group (N=7) were exposed to normoxia in parallel. Following gas treatments, rats were anesthetized with an i.p. injection of urethane (1.5g/kg; 20%, w/v). Fine concentric needle electrodes were inserted into the GG and the costal diaphragm. Discriminated GG motor unit potentials and whole electromyograph (EMG), together with arterial blood pressure and arterial O2 saturation, were recorded during quiet basal breathing and during nasal airway occlusion. Airway occlusion significantly increased GG EMG activity in all animals; but there was no difference in the reflex response to airway occlusion between sham and CIH-treated animals (+105±22% vs. +105±17%, mean±SEM for area under the curve of integrated GG EMG, % increase from baseline, p=0.99). Occluded breaths were characterized by a significant increase in the firing frequency of phasically active units and the recruitment of large motor units that were quiescent under basal conditions. Though there are reports of impaired control of the upper airway following CIH in the rat, we conclude that reflexly evoked motor discharge to the GG is not affected by 7 days of CIH, a paradigm that we have shown increases apnea index in sleeping rats.

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.

Activation of upper airway muscles during breathing and swallowing

The upper airway is a complex muscular tube that is used by the respiratory and digestive systems. The upper airway is invested with several small and anatomically peculiar muscles. The muscle fiber orientations and their nervous innervation are both extremely complex, and how the activity of the muscles is initiated and adjusted during complex behaviors is poorly understood. The bulk of the evidence suggests that the entire assembly of tongue and laryngeal muscles operate together but differently during breathing and swallowing, like a ballet rather than a solo performance. Here we review the functional anatomy of the tongue and laryngeal muscles, and their neural innervation. We also consider how muscular activity is altered as respiratory drive changes, and briefly address upper airway muscle control during swallowing.

State-dependent and reflex drives to the upper airway: basic physiology with clinical implications

The root cause of the most common and serious of the sleep disorders is impairment of breathing, and a number of factors predispose a particular individual to hypoventilation during sleep. In turn, obstructive hypopneas and apneas are the most common of the sleep-related respiratory problems and are caused by dysfunction of the upper airway as a conduit for airflow. The overarching principle that underpins the full spectrum of clinical sleep-related breathing disorders is that the sleeping brain modifies respiratory muscle activity and control mechanisms and diminishes the ability to respond to respiratory distress. Depression of upper airway muscle activity and reflex responses, and suppression of arousal (i.e., "waking-up") responses to respiratory disturbance, can also occur with commonly used sedating agents (e.g., hypnotics and anesthetics). Growing evidence indicates that the sometimes critical problems of sleep and sedation-induced depression of breathing and arousal responses may be working through common brain pathways acting on common cellular mechanisms. To identify these state-dependent pathways and reflex mechanisms, as they affect the upper airway, is the focus of this paper. Major emphasis is on the synthesis of established and recent findings. In particular, we specifically focus on 1) the recently defined mechanism of genioglossus muscle inhibition in rapid-eye-movement sleep; 2) convergence of diverse neurotransmitters and signaling pathways onto one root mechanism that may explain pharyngeal motor suppression in sleep and drug-induced brain sedation; 3) the lateral reticular formation as a key hub of respiratory and reflex drives to the upper airway.

Pulmonary C-fiber activation attenuates respiratory-related tongue movements

The functional impact of pulmonary C-fiber activation on upper airway biomechanics has not been evaluated. Here, we tested the hypothesis that pulmonary C-fiber activation alters the respiratory-related control of tongue movements. The force produced by tongue movements was quantified in spontaneously breathing, anesthetized adult rats before and after stimulation of pulmonary C fibers via intrajugular delivery of capsaicin (0.625 and 1.25 μg/kg). Brief occlusion of the trachea was used to increase the respiratory drive to the tongue muscles, and hypoglossal (XII) nerve branches were selectively sectioned to denervate the protrusive and retrusive tongue musculature. Tracheal occlusion triggered inspiratory-related tongue retrusion in rats with XII nerves intact or following section of the medial XII nerve branch, which innervates the genioglossus muscle. Inspiratory-related tongue protrusion was only observed after section of the lateral XII branch, which innervates the primary tongue retrusor muscles. The tension produced by inspiratory-related tongue movement was significantly attenuated by capsaicin, but tongue movements remained retrusive, unless the medial XII branch was sectioned. Capsaicin also significantly delayed the onset of tongue movements such that tongue forces could not be detected until after onset of the inspiratory diaphragm activity. We conclude that altered neural drive to the tongue muscles following pulmonary C-fiber activation has a functionally significant effect on tongue movements. The diminished tongue force and delay in the onset of tongue movements following pulmonary C-fiber activation are potentially unfavorable for upper airway patency.

Phasic respiratory modulation of pharyngeal collapsibility via neuromuscular mechanisms in rats

Obstructive sleep apnea patients experience recurrent upper airway (UA) collapse due to decreases in the UA dilator muscle activity during sleep. In contrast, activation of UA dilators reduces pharyngeal critical pressure (Pcrit, an index of pharyngeal collapsibility), suggesting an inverse relationship between pharyngeal collapsibility and dilator activity. Since most UA muscles display phasic respiratory activity, we hypothesized that pharyngeal collapsibility is modulated by respiratory drive via neuromuscular mechanisms. Adult male Sprague-Dawley rats were anesthetized, vagotomized, and ventilated (normocapnia). In one group, integrated genioglossal activity, Pcrit, and maximal airflow (V(max)) were measured at three expiration and five inspiration time points within the breathing cycle. Pcrit was closely and inversely related to phasic genioglossal activity, with the value measured at peak inspiration being the lowest. In other groups, the variables were measured during expiration and peak inspiration, before and after each of five manipulations. Pcrit was 26% more negative (-15.0 ± 1.0 cmH(2)O, -18.9 ± 1.2 cmH(2)O; n = 23), V(max) was 7% larger (31.0 ± 1.0 ml/s, 33.2 ± 1.1 ml/s), nasal resistance was 12% bigger [0.49 ± 0.05 cmH(2)O/(ml/s), 0.59 ± 0.05 cmH(2)O/(ml/s)], and latency to induced UA closure was 14% longer (55 ± 4 ms, 63 ± 5 ms) during peak inspiration vs. expiration (all P < 0.005). The expiration-inspiration difference in Pcrit was abolished with neuromuscular blockade, hypocapnic apnea, or death but was not reduced by the superior laryngeal nerve transection or altered by tracheal displacement. Collectively, these results suggest that pharyngeal collapsibility is moment-by-moment modulated by respiratory drive and this phasic modulation requires neuromuscular mechanisms, but not the UA negative pressure reflex or tracheal displacement by phasic lung inflation.

A secondary reflex suppression phase is present in genioglossus but not tensor palatini in response to negative upper airway pressure

On the basis of recent reports, the genioglossus (GG) negative-pressure reflex consists initially of excitation followed by a secondary state-dependent suppression phase. The mechanistic origin and functional role of GG suppression is unknown but has been hypothesized to arise from transient inhibition of respiratory active neurons as a protective reflex to prevent aspiration, as observed in other respiratory muscles (e.g., diaphragm) during airway occlusion. Unlike GG, tensor palatini (TP) is a tonic muscle with minimal respiratory phasic activation during relaxed breathing, although both muscles are important in preserving pharyngeal patency. This study aimed to compare GG vs. TP reflex responses to the same negative-pressure stimulus. We hypothesized that reflex suppression would be present in GG, but not TP. Intramuscular GG and TP EMGs were recorded in 12 awake, healthy subjects (6 female). Reflex responses were generated via 250-ms pulses of negative upper airway pressure (approximately -16 cmH2O mask pressure) delivered in early inspiration. GG and TP demonstrated reflex activation in response to negative pressure (peak latency 31+/-4 vs. 31+/-6 ms and peak amplitude 318+/-55 vs. 314+/-26% baseline, respectively). A secondary suppression phase was present in 8 of 12 subjects for GG (nadir latency 54+/-7 ms, nadir amplitude 64+/-6% baseline), but not in any subject for TP. These data provide further support for the presence of excitatory and inhibitory components of GG (phasic muscle) in response to brief upper airway negative-pressure pulses. Conversely, no reflex suppression below baseline was present in TP (tonic muscle) in response to the same stimuli. These differential responses support the hypothesis that GG reflex suppression may be mediated via inhibition of respiratory-related premotor input.

Long-term facilitation of upper airway muscle activity induced by episodic upper airway negative pressure and hypoxia in spontaneously breathing anaesthetized rats

Obstruction of the upper airway (UA) is associated with episodes of hypoxia and upper airway negative pressure (UANP). In the companion paper it is shown that episodic hypoxia elicits long-term facilitation (LTF) of tongue protrudor, retractor and respiratory pump muscle activity. However, whether repeated exposure to UANP also induces LTF is unknown. We hypothesized that repetitive exposure to UANP would induce LTF of UA and respiratory pump muscle activity and when coupled with hypoxia, as occurs when the UA obstructs, would lead to an even greater facilitation of muscle activity and the response to UANP. Experiments were performed in 24 anaesthetized, spontaneously breathing rats with intact vagi. To induce LTF, UANP stimuli (-10 cmH(2)O) of 5 s duration were delivered every 30 s for 3 min (+/- hypoxia). This was repeated eight times over 1 h, each 3 min episode separated by 5 min of normoxia. Genioglossus (GG), hyoglossus (HG) and diaphragm (Dia) muscle activity was recorded before, during and for 1 h following the last exposure to episodic UANP alone (n = 8), UANP and hypoxia together (n = 8) or normoxia alone (n = 8). During the final hour, single pulses of UANP were applied at 1 min and every 10 min thereafter to determine whether LTF of the response to UANP had been induced. Our results show that LTF of GG muscle activity and its response to UANP was induced following exposure to episodic UANP stimuli alone and UANP applied during hypoxia. However, there was no significant difference between these responses. Episodic UANP alone also induced LTF of HG muscle activity but this effect did not manifest until 40 min following the last episode of repeated UANP stimulation. In the presence of hypoxia, no LTF of HG muscle response to UANP was found. In conclusion, episodic UANP stimulation induces LTF of UA dilator and retractor tongue muscles, but no further facilitation occurs when coupled with hypoxia. This response may serve as an important protective mechanism of respiratory homeostasis during sleep, particularly in patients who suffer from obstructive sleep apnoea.

Episodic hypoxia induces long-term facilitation of upper airway muscle activity in spontaneously breathing anaesthetized rats

We performed these experiments to determine if repeated exposure to episodic hypoxia induces long term facilitation (LTF) in anaesthetized spontaneously breathing rats. A previous study in spontaneously breathing rats was unable to demonstrate evidence of LTF with repeated hypoxia, but this may have been due to the low number of hypoxic episodes used. We hypothesized that with sufficient exposure, episodic hypoxia LTF of genioglossus (GG), hyoglossus (HG) and diaphragm (Dia) activities would be elicited. Experiments were performed in 24 anaesthetized spontaneously breathing rats with intact vagi. Peak and tonic GG, HG and Dia EMG activities were recorded before, during and for 1 h following exposure to eight (n = 8) or three (n = 8) episodes of isocapnic hypoxia ( = 0.1) each of 3 min duration. A third time control series was also performed with exposure to normoxia alone ( = 0.28, n = 8). Short-term potentiation of GG and HG muscle activity developed during the early period after repeated exposure to eight and three hypoxic episodes. LTF, however, occurred only after eight hypoxic episodes. This manifested as an increase in peak GG and Dia inspiratory muscle activity and tonic HG activity. LTF of respiratory breathing frequency was also induced, reflected by a reduction in inspiratory and expiratory time. These findings support our initial hypothesis that LTF in the anaesthetized, spontaneously breathing rat is dependent on the number of exposures to hypoxia and show that the responses to repetitive hypoxia are composed of both short and long-term facilitatory changes.

Upper airway pressure-flow relationships and pharyngeal constrictor EMG activity during prolonged expiration in awake goats

We undertook the present investigation to establish whether narrowing/closure of the upper airway occurs during spontaneous and provoked respiratory rhythm disturbances and whether pharyngeal constrictor muscle recruitment occurs coincident with upper airway occlusion during prolonged expiratory periods. Upper airway pressure-flow relationships and middle pharyngeal constrictor (mPC) EMG activities were recorded in 11 adult female goats during spontaneous and provoked prolongations in expiratory time (Te). A total of 213 spontaneous prolongations of expiration were recorded. Additionally, 169 prolonged expiratory events preceded by an augmented breath were included in the analyses. In separate trials on different days, Te was prolonged by systemic administration of dopamine, by raising the inspired fraction of O2 from 0.10 to 1.00 during poikilocapnic conditions or by systemic administration of clonidine. Continuous tonic activation of the mPC EMG was observed during each prolonged Te period regardless of the duration or initiating cause. However, significant increases in subglottic tracheal pressure, with expiratory airflow braking indicative of upper airway narrowing or closure, was only observed during spontaneous events without a preceding augmented breath and during clonidine-induced events. Tonic mPC activation proved an unreliable indicator of airway occlusion. Furthermore, mPC muscle activation alone is not sufficient to induce pharyngeal occlusion during prolonged expiration. Our data suggest that airway closure is not a common occurrence during provoked respiratory disturbances in awake goats. We propose that airway closure, when present during prolonged Te, is more likely dependent on activation of laryngeal adductor muscles with glottic braking independent of pharyngeal narrowing.

Differential effects of isoflurane and propofol on upper airway dilator muscle activity and breathing

BACKGROUND

Anesthesia impairs upper airway integrity, but recent data suggest that low doses of some anesthetics increase upper airway dilator muscle activity, an apparent paradox. The authors sought to understand which anesthetics increase or decrease upper airway dilator muscle activity and to study the mechanisms mediating the effect.

METHODS

The authors recorded genioglossus electromyogram, breathing, arterial blood pressure, and expiratory carbon dioxide in 58 spontaneously breathing rats at an estimated ED50 (median effective dose) of isoflurane or propofol. The authors further evaluated the dose-response relations of isoflurane under different study conditions: (1) normalization of mean arterial pressure, or end-expiratory carbon dioxide; (2) bilateral lesion of the Kölliker-Fuse nucleus; and (3) vagotomy. To evaluate whether the markedly lower inspiratory genioglossus activity during propofol could be recovered by increasing flow rate, a measure of respiratory drive, the authors performed an additional set of experiments during hypoxia or hypercapnia.

RESULTS

In vagally intact rats, tonic and phasic genioglossus activity were markedly higher with isoflurane compared with propofol. Both anesthetics abolished the genioglossus negative pressure reflex. Inspiratory flow rate and anesthetic agent predicted independently phasic genioglossus activity. Isoflurane dose-dependently decreased tonic and increased phasic genioglossus activity, and increased flow rate, and its increasing effects were abolished after vagotomy. Impairment of phasic genioglossus activity during propofol anesthesia was reversed during evoked increase in respiratory drive.

CONCLUSION

Isoflurane compared with propofol anesthesia yields higher tonic and phasic genioglossus muscle activity. The level of respiratory depression rather than the level of effective anesthesia correlates closely with the airway dilator muscle function during anesthesia.

Obesity and obstructive sleep apnea: pathogenic mechanisms and therapeutic approaches

Obstructive sleep apnea is a common disorder whose prevalence is linked to an epidemic of obesity in Western society. Sleep apnea is due to recurrent episodes of upper airway obstruction during sleep that are caused by elevations in upper airway collapsibility during sleep. Collapsibility can be increased by underlying anatomic alterations and/or disturbances in upper airway neuromuscular control, both of which play key roles in the pathogenesis of obstructive sleep apnea. Obesity and particularly central adiposity are potent risk factors for sleep apnea. They can increase pharyngeal collapsibility through mechanical effects on pharyngeal soft tissues and lung volume, and through central nervous system-acting signaling proteins (adipokines) that may affect airway neuromuscular control. Specific molecular signaling pathways encode differences in the distribution and metabolic activity of adipose tissue. These differences can produce alterations in the mechanical and neural control of upper airway collapsibility, which determine sleep apnea susceptibility. Although weight loss reduces upper airway collapsibility during sleep, it is not known whether its effects are mediated primarily by improvement in upper airway mechanical properties or neuromuscular control. A variety of behavioral, pharmacologic, and surgical approaches to weight loss may be of benefit to patients with sleep apnea, through distinct effects on the mass and activity of regional adipose stores. Examining responses to specific weight loss strategies will provide critical insight into mechanisms linking obesity and sleep apnea, and will help to elucidate the humoral and molecular predictors of weight loss responses.

Genioglossus reflex inhibition to upper-airway negative-pressure stimuli during wakefulness and sleep in healthy males

During wakefulness, obstructive sleep apnoea patients appear to compensate for an anatomically narrow upper airway by increasing upper airway dilator muscle activity, e.g. genioglossus, at least partly via a negative-pressure reflex that may be diminished in sleep. Previous studies have assessed the negative-pressure reflex using multi-unit, rectified, moving-time-average EMG recordings during brief pulses of negative upper-airway pressure. However, moving-time averaging probably obscures the true time-related reflex morphology, potentially masking transient excitatory and inhibitory components. This study aimed to re-examine the genioglossus negative-pressure reflex in detail, without moving-time averaging. Bipolar fine-wire electrodes were inserted per orally into the genioglossus muscle in 17 healthy subjects. Two upper airway pressure catheters were inserted per nasally. Genioglossus EMG reflex responses were generated via negative-pressure stimuli (approximately -10 cmH2O at the choanae, 250 ms duration) delivered during wakefulness and sleep. Ensemble-averaged, rectified, genioglossus EMG recordings demonstrated reflex activation (onset latency 26+/-1 ms; peak amplitude 231+/-29% of baseline) followed by a previously unreported suppression (peak latency 71+/-4 ms; 67+/-8% of baseline). Single-motor-unit activity, clearly identifiable in approximately 10% of trials in six subjects, showed a concomitant increase in the interspike interval from baseline (26+/-9 ms, P=0.01). Genioglossus negative-pressure reflex morphology and amplitude of the initial peak were maintained in non-rapid eye movement (NREM) sleep but suppression amplitude was more pronounced during NREM and declined further during REM sleep compared to wakefulness. These data indicate there are both excitatory and inhibitory components to the genioglossus negative-pressure reflex which are differentially affected by state.

Neural drive to tongue protrudor and retractor muscles following pulmonary C-fiber activation

Hypoglossal (XII) nerve recordings indicate that pulmonary C-fiber (PCF) receptor activation reduces inspiratory bursting and triggers tonic discharge. We tested three hypotheses related to this observation: 1) PCF receptor activation inhibits inspiratory activity in XII branches innervating both tongue protrudor muscles (medial branch; XIImed) and retractor muscles (lateral branch; XIIlat); 2) reduced XII neurogram amplitude reflects decreased XII motoneuron discharge rate; and 3) tonic XII activity reflects recruitment of previously silent motoneurons. Phrenic, XIImed, and XIIlat neurograms were recorded in anesthetized, paralyzed, and ventilated rats. Capsaicin delivered to the jugular vein reduced phrenic bursting at doses of 0.625 and 1.25 μg/kg but augmented bursting at 5 μg/kg. All doses reduced inspiratory amplitude in XIImed and XIIlat ( P < 0.05), and these effects were eliminated following bilateral vagotomy. Single-fiber recordings indicated that capsaicin causes individual XII motoneurons to either decrease discharge rate ( n = 101/153) or become silent ( n = 39/153). Capsaicin also altered temporal characteristics such that both XIImed and XIIlat inspiratory burst onset occurred after the phrenic burst ( P < 0.05). Increases in tonic discharge after capsaicin were greater in XIImed vs. XIIlat ( P < 0.05); single-fiber recordings indicated that tonic discharge reflected recruitment of previously silent motoneurons. We conclude that PCF receptor activation reduces inspiratory XII motoneuron discharge and transiently attenuates neural drive to both tongue protrudor and retractor muscles. However, tonic discharge appears to be selectively enhanced in tongue protrudor muscles. Accordingly, reductions in upper airway stiffness associated with reduced XII burst amplitude may be offset by enhanced tonic activity in tongue protrudor muscles.

Genioglossus premotoneurons and the negative pressure reflex in rats

Reflex increases in genioglossus (GG) muscle activity in response to negative pharyngeal pressure are important for maintenance of upper airway patency in humans. However, little is known of the central circuitry that mediates this negative pressure reflex (NPR). We used two approaches to determine which GG premotoneurons relay negative pressure-related information to the hypoglossal motor nucleus. First, to identify GG premotoneurons, we injected pseudorabies virus (PRV152) into the GG muscle. We found that medullary GG premotoneurons were concentrated mainly in the reticular formation adjacent to the hypoglossal motor nucleus. Second, in order to determine whether these perihypoglossal neurons were involved in the NPR, we quantified GG EMG responses to negative pressure applied to the isolated upper airway in anaesthetized rats before and after microinjection of muscimol (9 nl; 0.25 mM), a GABA-A receptor agonist, into the perihypoglossal premotor field. Pressures as low as -4 cm H(2)O increased inspiratory phase-related GG activity. The NPR was abolished following bilateral injections of muscimol into the perihypoglossal premotor field at and up to 500 mum rostral to the obex. Muscimol in this location also increased the amplitude of basal, unstimulated phasic GG activity. By contrast, inhibition of neurons caudal to the obex decreased phasic GG activity but had no impact on the NPR. These results suggest that perihypoglossal GG premotoneurons near the obex mediate the NPR and those caudal to the obex are important mediators of respiratory-related GG activity but are not involved in the NPR.

Discoordination of laryngeal and respiratory movements in aged rats

Voice and swallowing actions require the coordination of multiple motor systems, and this coordination may be impaired with aging. Although recent work has reported impairments in age-related laryngeal kinematics in rats, the temporal relationship of laryngeal excursions to the respiratory cycle is unknown. The goal of this study was to assess laryngeal-respiratory coordination by examining temporal interrelationships between change in laryngeal aperture and chest wall movement during quiet breathing in a rat model. Glottal images were recorded, digitized, and synchronized with respiratory signals, and temporal features were measured. In the young animals, glottal opening began before the onset of inspiration, and glottal and respiratory cycles were phasic and stereotypic. In old animals, however, inspiration often began during the glottal closing phase, and both respiratory signals were asymmetric. Discoordination of laryngeal and respiratory motor actions associated with aging may be caused by a generalized decline in sensorimotor cranial functions and may contribute to age-related swallowing and communication impairment.

Superior laryngeal and hypoglossal afferents tonically influence upper airway motor excitability in anesthetized rats

Upper airway (UA) muscle activity is stimulated by changes in UA transmural pressure and by asphyxia. These responses are reduced by muscle relaxation. We hypothesized that this is due to a change in afferent feedback in the ansa hypoglossi and/or superior laryngeal nerve (SLN). We examined 1) the glossopharyngeal motor responses to UA transmural pressure and asphyxia and 2) how these responses were changed by muscle relaxation in animals where one or both of these afferent pathways had been sectioned bilaterally. Experiments were performed in 24 anesthetized, thoracotomized, artificially ventilated rats. Baseline glossopharyngeal activity and its response to UA transmural pressure and asphyxia were moderately reduced after bilateral section of the ansa hypoglossi (P < 0.05). Conversely, bilateral SLN section increased baseline glossopharyngeal activity, augmented the response to asphyxia, and abolished the response to UA transmural pressure. Muscle relaxation reduced resting glossopharyngeal activity and the response to asphyxia (P < 0.001). This occurred whether or not the ansa hypoglossi, the SLN, or both afferent pathways had been interrupted. We conclude that ansa hypoglossi afferents tonically excite and SLN afferents tonically inhibit UA motor activity. Muscle relaxation depressed UA motor activity after section of the ansa hypoglossi and SLN. This suggests that some or all of the response to muscle relaxation is mediated by alterations in the activity of afferent fibers other than those in the ansa hypoglossi or SLN.

Activation of XII motoneurons and premotor neurons during various oropharyngeal behaviors

Neural control of tongue muscles plays a crucial role in a broad range of oropharyngeal behaviors. Tongue movements must be rapidly and accurately adjusted in response to the demands of multiple complex motor tasks including licking/mastication, swallowing, vocalization, breathing and protective reflexes such as coughing. Yet, central mechanisms responsible for motor and premotor control of hypoglossal (XII) activity during these behaviors are still largely unknown. The aim of this article is to review the functional organization of the XII motor nucleus with particular emphasis on breathing, coughing and swallowing. Anatomical localization of XII premotor neurons is also considered. We discuss results concerned with multifunctional activity of medullary and pontine populations of XII premotor neurons, representing a single network that can be reconfigured to produce different oromotor response patterns. In this context, we introduce new data on swallowing-related activity of XII (and trigeminal) motoneurons, and finally suggest a prominent role for the pontine Kölliker-Fuse nucleus in the control of inspiratory-related activity of XII motoneurons supplying tongue protrusor and retrusor muscles.

Episodic hypoxia induces long-term facilitation of neural drive to tongue protrudor and retractor muscles

Hypoxic episodes can evoke a prolonged augmentation of inspiratory motor output called long-term facilitation (LTF). Hypoglossal (XII) LTF has been assumed to represent increased tongue protrudor muscle activation and pharyngeal airway dilation. However, recent studies indicate that tongue protrudor and retractor muscles are coactivated during inspiration, a behavior that promotes upper airway patency by reducing airway compliance. These experiments tested the hypothesis that XII LTF is manifest as increased inspiratory drive to both tongue protrudor and retractor muscles. Neurograms were recorded in the medial XII nerve branch (XIIMED; contains tongue protrudor motor axons), the lateral XII nerve branch (XIILAT; contains tongue retractor motor axons), and the phrenic nerve in anesthetized, vagotomized, paralyzed, ventilated male rats. Strict isocapnia was maintained for 60 min after five 3-min hypoxic episodes (arterial Po2 = 35 ± 2 Torr) or sham treatment. Peak inspiratory burst amplitude showed a persistent increase in XIIMED, XIILAT, and phrenic nerves during the hour after episodic hypoxia ( P < 0.05 vs. sham). This effect was present regardless of the quantification method (e.g., % baseline vs. percent maximum); however, comparisons of the relative magnitude of LTF between neurograms (e.g., XIIMED vs. XIILAT) varied with the normalization procedure. There was no persistent effect of episodic hypoxia on inspiratory burst frequency ( P > 0.05 vs. sham). These data demonstrate that episodic hypoxia induces LTF of inspiratory drive to both tongue protrudor and retractor muscles and underscore the potential contribution of tongue muscle coactivation to regulation of upper airway patency.

Functional organization of the parabrachial complex and intertrigeminal region in the control of breathing

Although the medulla oblongata contains the epicenter for respiratory rhythm generation, many other parts of the neuraxis play significant substratal roles in breathing. Accumulating evidence suggests that the pons contains several groups of neurons that may belong to the central respiratory system. This article will review data from microstimulation mapping and tract-tracing studies of the parabrachial complex (PB) and intertrigeminal region (ITR). Chemical activation of neurons in these areas has distinct effects on ventilatory and airway muscle activity. Tract-tracing experiments from functionally identified sites reveal specific respiratory-related sensory inputs and outputs that are likely anatomical substrates for these effects. The data suggest that an important physiological role for the rostral pons may be reflexive respiratory responses to airway stimuli.

Upper airway muscle paralysis reduces reflex upper airway motor response to negative transmural pressure in rat

The reflex upper airway (UA) motor response to UA negative pressure (UANP) is attenuated by neuromuscular blockade. We hypothesized that this is due to a reduction in the sensitivity of laryngeal mechanoreceptors to changes in UA pressure. We examined the effect of neuromuscular blockade on hypoglossal motor responses to UANP and to asphyxia in 15 anesthetized, thoracotomized, artificially ventilated rats. The activity of laryngeal mechanoreceptors is influenced by contractions of laryngeal and tongue muscles, so we studied the effect of selective denervation of these muscle groups on the UA motor response to UANP and to asphyxia, recording from the pharyngeal branch of the glossopharyngeal nerve (n = 11). We also examined the effect of tongue and laryngeal muscle denervation on superior laryngeal nerve (SLN) afferent activity at different airway transmural pressures (n = 6). Neuromuscular blockade and denervation of laryngeal and tongue muscles significantly reduced baseline UA motor nerve activity (P < 0.05), caused a small but significant attenuation of the motor response to asphyxia, and markedly attenuated the response to UANP. Motor denervation of tongue and laryngeal muscles significantly decreased SLN afferent activity and altered the response to UANP. We conclude that skeletal muscle relaxation reduces the reflex UA motor response to UANP, and this may be due to a reduction in the excitability of UA motor systems as well as a decrease of the response of SLN afferents to UANP.

Difference between hypoglossal and phrenic activities during lung inflation and swallowing in the rat

We aimed in this study to elucidate the discharge properties and neuronal mechanisms of the dissociation between hypoglossal and phrenic inspiratory activities in decerebrate rats, which had been subjected to neuromuscular blockade and artificially ventilated. The discharge of the hypoglossal nerve and the intracellular activity of hypoglossal motoneurones were monitored during respiration and fictive-swallowing evoked by electrical stimulation of the superior laryngeal nerve, and were compared with the activity of the phrenic nerve. The hypoglossal nerve activity was characterized by its onset preceding the phrenic nerve activity ('pre-I' activity). By manipulating artificial respiration, we could augment the 'pre-I' activity, and could elicit another type of hypoglossal activity decoupled from the phrenic-associated inspiratory bursts ('decoupled' activity). We further scrutinized the correlatives of 'pre-I' and 'decoupled' activities in individual hypoglossal motoneurones. Hypoglossal motoneurones consisted of inspiratory (n = 42), expiratory (n = 18) and non-respiratory (n = 1) neurones and were classified by their swallowing activity into depolarized, hyperpolarized, hyperpolarized-depolarized and unresponsive groups. All of the inspiratory neurones were depolarized in accordance with the 'pre-I' and 'decoupled' activities, and all of the expiratory neurones were hyperpolarized during these activities. Fictive swallowing, which was characterized by its frequent emergence just after the phrenic inspiratory activity, was also evoked just after the 'decoupled' hypoglossal activity, suggesting that this activity may have similar effects on swallowing as the 'overt' inspiratory activity. Such a coupling between 'decoupled' and swallowing activities was also revealed in each motoneurone. These findings suggest that the 'pre-I' and 'decoupled' activities may reflect some internal inspiratory activity of the respiratory centre and that hypoglossal motoneurones may be driven by a distinct group of premotor neurones that possibly play a role in the coordination of respiration and swallowing.