Superior laryngeal nerve section alters responses to upper airway distortion in sleeping dogs

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.

Power spectral analysis of hypoglossal nerve activity during intermittent hypoxia-induced long-term facilitation in mice

Power spectral analyses of electrical signals from respiratory nerves reveal prominent oscillations above the primary rate of breathing. Acute exposure to intermittent hypoxia can induce a form of neuroplasticity known as long-term facilitation (LTF), in which inspiratory burst amplitude is persistently elevated. Most evidence indicates that the mechanisms of LTF are postsynaptic and also that high-frequency oscillations within the power spectrum show coherence across different respiratory nerves. Since the most logical interpretation of this coherence is that a shared presynaptic mechanism is responsible, we hypothesized that high-frequency spectral content would be unchanged during LTF. Recordings of inspiratory hypoglossal (XII) activity were made from anesthetized, vagotomized, and ventilated 129/SVE mice. When arterial O2 saturation (SaO2) was maintained >96%, the XII power spectrum and burst amplitude were unchanged for 90 min. Three, 1-min hypoxic episodes (SaO2 = 50 ± 10%), however, caused a persistent (>60 min) and robust (>400% baseline) increase in burst amplitude. Spectral analyses revealed a rightward shift of the signal content during LTF, with sustained increases in content above ∼125 Hz following intermittent hypoxia and reductions in power at lower frequencies. Changes in the spectral content during LTF were qualitatively similar to what occurred during the acute hypoxic response. We conclude that high-frequency content increases during XII LTF in this experimental preparation; this may indicate that intermittent hypoxia-induced plasticity in the premotor network contributes to expression of XII LTF.

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.

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.

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

1. We examined the upper airway (UA) motor response to upper airway negative pressure (UANP) in the rat. We hypothesized that this response is mediated by superior laryngeal nerve (SLN) afferents and is not confined to airway dilator muscles but also involves an increase in motor drive to tongue retractor and pharyngeal constrictor muscles, reflecting a role for these muscles in stabilizing the UA. 2. Experiments were performed in 49 chloralose-anaesthetized, tracheostomized rats. Subatmospheric pressure in the range 0 to -30 cmH(2)O was applied to the isolated UA. Motor activity was recorded in separate experiments from the main trunk of the hypoglossal nerve (XII, n = 8), the pharyngeal branch of the glossopharyngeal nerve (n = 8), the medial and lateral branches of the XII (n = 8) and the pharyngeal branch of the vagus (n = 8). Afferent activity was recorded from the whole SLN in six experiments. 3. All UA motor outflows exhibited phasic inspiratory activity and this was significantly (P < 0.05) increased by UANP. Tonic end-expiratory activity increased significantly in response to pressures more negative than -20 cmH(2)O. Bilateral section of the SLN also increased (P < 0.05) motor activity and abolished the responses to UANP. Electrical stimulation of the SLN inhibited inspiratory XII activity. SLN afferents were tonically active and inhibited by UANP. 4. We conclude that UANP causes a reflex increase in motor drive to pharyngeal dilator, tongue retractor and pharyngeal constrictor muscles via afferent fibres in the SLN. Tonic activity in SLN afferent fibres at zero transmural pressure exerts a marked inhibitory effect on UA motor outflow.

Phasic mechanoreceptor stimuli can induce phasic activation of upper airway muscles in humans

1. Upper airway dilator muscles are phasically activated throughout breathing by respiratory pattern generator neurons. Studies have shown that non-physiological upper airway mechanoreceptive stimuli (e.g. rapidly imposed pulses of negative pressure) also activate these muscles. Such reflexes may become activated during conditions that alter airway resistance in order to stabilise airway patency. 2. To determine the contribution of ongoing mechanoreceptive reflexes to phasic activity of airway dilators, we assessed genioglossal electromyogram (GG EMG: rectified with moving time average of 100 ms) during slow (physiological) oscillations in negative pressure generated spontaneously and passively (negative pressure ventilator). 3. Nineteen healthy adults were studied while awake, during passive mechanical ventilation across normal physiological ranges of breathing rates (13-19 breaths min-1) and volumes (0.5-1.0 l) and during spontaneous breathing across the physiological range of end-tidal carbon dioxide (PET,CO2; 32-45 mmHg). 4. Within-breath phasic changes in airway mechanoreceptor stimuli (negative pressure or flow) were highly correlated with within-breath phasic genioglossal activation, probably representing a robust mechanoreceptive reflex. These reflex relationships were largely unchanged by alterations in central drive to respiratory pump muscles or the rate of mechanical ventilation within the ranges studied. A multivariate model revealed that tonic GG EMG, PET,CO2 and breath duration provided no significant independent information in the prediction of inspiratory peak GG EMG beyond that provided by epiglottic pressure, which alone explained 93 % of the variation in peak GG EMG across all conditions. The overall relationship was: Peak GG EMG = 79.7 - (11.3 X Peak epiglottic pressure), where GG EMG is measured as percentage of baseline, and epiglottic pressure is in cmH2O. 5. These data provide strong evidence that upper airway dilator muscles can be activated throughout inspiration via ongoing mechanoreceptor reflexes. Such a feedback mechanism is likely to be active on a within-breath basis to protect upper airway patency in awake humans. This mechanism could mediate the increased genioglossal activity observed in patients with obstructive sleep apnoea (i.e. reflex compensation for an anatomically smaller airway).

Hemodynamic effects of pressures applied to the upper airway during sleep

The increase in systemic blood pressure after an obstructive apnea is due, in part, to sympathetically mediated vasoconstriction. We questioned whether upper airway (UA) receptors could contribute reflexly to this vasoconstriction. Four unanesthetized dogs were studied during wakefulness and non-rapid-eye-movement (NREM) sleep. The dogs breathed via a fenestrated tracheostomy tube sealed around the tracheal stoma. The snout was sealed with an airtight mask, thereby isolating the UA when the fenestration was closed and exposing the UA to negative inspiratory intrathoracic pressure when it was open. The blood pressure response to three UA perturbations was studied: 1) square-wave negative pressures sufficient to cause UA collapse with the fenestration closed during a mechanical hyperventilation-induced central apnea; 2) tracheal occlusion with the fenestration open vs. closed; and 3) high-frequency pressure oscillations (HFPO) with the fenestration closed. During NREM sleep, 1) blood pressure response to tracheal occlusion was similar with the fenestration open or closed; 2) collapsing the UA with negative pressures failed to alter blood pressure during a central apnea; and 3) application of HFPO to the UA during eupnea and resistive-loaded breaths increased heart rate and blood pressure. However, these changes were likely to be secondary to the effects of HFPO-induced reflex changes on prolonging expiratory time. These findings suggest that activation of UA pressure-sensitive receptors does not contribute directly to the pressor response associated with sleep-disordered breathing events.

Effects of volatile anesthetics on the activity of laryngeal 'drive' receptors in anesthetized dogs

Effects of halothane, isoflurane and sevoflurane on laryngeal drive receptor activity were studied in the afferent activity of the superior laryngeal nerve in anesthetized spontaneously breathing dogs. Of 40 single units recorded, most of them (65%) responded to the volatile anesthetics applied to the isolated larynx at a concentration of 5%. The exposure to the anesthetics resulted in either an inspiratory increase (15%), both inspiratory and expiratory decrease (54%), or both inspiratory increase and expiratory decrease (31%) responses. The average discharge frequency of the receptors tended to be decreased on inhalation of the anesthetics, where significant decreases were observed in both respiratory phases for halothane and at expiration for isoflurane, but in neither respiratory phase for sevoflurane. These results support an advantage of sevoflurane over halothane and isoflurane for induction of anesthesia to minimize the influence of the activity of laryngeal drive receptors on the breathing pattern and airway stability.

Effect of upper airway negative pressure on inspiratory drive during sleep

To determine the effect of upper airway (UA) negative pressure and collapse during inspiration on regulation of breathing, we studied four unanesthetized female dogs during wakefulness and sleep while they breathed via a fenestrated tracheostomy tube, which was sealed around the permanent tracheal stoma. The snout was sealed with an airtight mask, thereby isolating the UA when the fenestration (Fen) was closed and exposing the UA to intrathoracic pressure changes, but not to flow changes, when Fen was open. During tracheal occlusion with Fen closed, inspiratory time (TI) increased during wakefulness, non-rapid-eye-movement (NREM) sleep and rapid-eye-movement (REM) sleep (155 +/- 8, 164 +/- 11, and 161 +/- 32%, respectively), reflecting the removal of inhibitory lung inflation reflexes. During tracheal occlusion with Fen open (vs. Fen closed): 1) the UA remained patent; 2) TI further increased during wakefulness and NREM (215 +/- 52 and 197 +/- 28%, respectively) but nonsignificantly during REM sleep (196 +/- 42%); 3) mean rate of rise of diaphragm EMG (EMGdi/TI) and rate of fall of tracheal pressure (Ptr/TI) were decreased, reflecting an additional inhibitory input from UA receptors; and 4) both EMGdi/TI and Ptr/TI were decreased proportionately more as inspiration proceeded, suggesting greater reflex inhibition later in the effort. Similar inhibitory effects of exposing the UA to negative pressure (via an open tracheal Fen) were seen when an inspiratory resistive load was applied over several breaths during wakefulness and sleep. These inhibitory effects persisted even in the face of rising chemical stimuli. This inhibition of inspiratory motor output is alinear within an inspiration and reflects the activation of UA pressure-sensitive receptors by UA distortion, with greater distortion possibly occurring later in the effort.