This study has led to a number of new findings in awake humans: (i) relatively physiological (i.e. slow and phasic) changes in upper airway mechanoreceptor stimuli throughout inspiration result in phasic activation of an upper airway dilator muscle; (ii) throughout inspiration the magnitude of intra-pharyngeal negative pressure and inspiratory airflow changes were both highly correlated with the degree of GG activation; and (iii) the slopes of the relationships between intra-pharyngeal negative pressure and GG activity and between flow and GG activity were unchanged across virtually all conditions, both within a breath and between breaths. Thus, there was a robust relationship between upper airway mechanoreceptor stimuli and pharyngeal muscle activation despite greatly altered degrees of central drive to the respiratory pump muscles (i.e. diminished or absent drive during passive mechanical ventilation and hypocapnia, and increased drive during hypercapnia and spontaneous breathing). These data provide strong evidence that upper airway dilator muscles can be activated throughout inspiration via ongoing upper airway mechanoreceptor reflexes. Such a feedback mechanism may naturally protect upper airway patency within each breath in awake humans.
Evidence that mechanoreceptive reflexes contribute to upper airway dilator muscle control during spontaneous breathing
Several studies both in humans and animals have demonstrated the existence of an upper airway reflex in response to the application of relatively non-physiological, sudden onset, large negative airway pressure pulses (
Horner et al. 1991a,
b;
Wheatley et al. 1993). We found that a mechanoreceptive reflex is still present at normal breathing rates, and that the timing and magnitude of this reflex is adequate to account for the GG activity that occurs throughout normal breathing, stimulated breathing and all conditions of mechanical ventilation used. Several additional studies suggest that this mechanoreceptive reflex may be active during spontaneous breathing. First, breathing through a tracheotomy (absence of upper airway stimuli) immediately reduces upper airway dilator muscle activity in patients with obstructive sleep apnoea (
Malhotra et al. 2000) and in vagotomised rabbits (
Mathew et al. 1982a). Second, nasopharyngeal anaesthesia reduces spontaneous GG activation during inspiration both in healthy subjects (
White et al. 1998) and in patients with OSA (
Fogel et al. 2000). These observations, together with the results of the current experiment, strongly suggest that ongoing mechanoreceptive reflexes contribute significantly to the phasic activation of upper airway dilator muscles throughout spontaneous breathing in awake humans.
The specific airway mechanoreceptive stimulus to genioglossus activation
The precise airway mechanoreceptive stimulus that is responsible for activation of the upper airway muscles is unknown. Numerous receptors exist in the upper airway, including superficial receptors responsive to pressure, flow (or temperature) and mechanical deformation, and deeper receptors responsive to muscle stretch and motion (
Widdicombe et al. 1988). Any number of these receptors could contribute to the mechanoreceptor reflex control of upper airway muscles. Studies in humans using topical nasopharyngeal anaesthesia and local anaesthetic block of internal branches of the superior laryngeal nerves (SLN) (
Horner et al. 1991b), and denervation studies in anaesthetized rabbits (
Mathew et al. 1982b) and unanaesthetized dogs (
Issa et al. 1988;
Curran et al. 1997) have demonstrated that mucosal and/or submucosal laryngeal receptors mediate an important component of this reflex response to upper airway pressure. However, some proportion of the reflex response remains after SLN blockade or section (
Horner et al. 1991b;
Curran et al. 1997), suggesting that other nerve afferents are involved to some degree (e.g. trigeminal or glossopharyngeal afferents). Our results demonstrate equally robust relationships between negative
Pepi and GG EMG, and between airflow and GG EMG in individual within-breath analyses and across all conditions. While we could not distinguish the actions of pressure and flow receptors in our study, pressure receptors may be the more likely candidates for producing the mechanoreceptive reflexes for several reasons (
Widdicombe et al. 1988). Pressure receptors constitute the largest proportion of the mechanosensitive endings in the larynx (
Sant'Ambrogio et al. 1983), and a correlation exists between peak phasic GG EMG and oesophageal pressure swings generated by the respiratory pump muscles during obstructive apnoeas when there is no airflow (
Berry et al. 1997).
We note that, as well as upper airway receptors, pulmonary and chest wall mechanoreceptors (muscle spindles, Golgi tendon organs and body surface receptors) are stimulated in synchrony with ventilatory movements and could have affected GG activity in the current study. However, we doubt that pulmonary and chest wall mechanoreceptors explain the robust relationship between upper airway mechanoreceptor stimuli and GG activation that we observed because pulmonary stretch receptors activated during inspiration are more likely to inhibit rather than facilitate GG activity (Gauda et al. 1994), and the pattern of activation of muscle spindles and body surface receptors would differ between spontaneous and mechanical ventilation.
The mild degree of hysteresis in the relationship between flow and GG and between negative intra-pharyngeal pressure and GG activity (Fig. 5A and B) suggest that there may be non-linearities in this reflex pathway. This hysteresis could have resulted from the summation of inputs to the GG motoneurons from various mechanoreceptors. For instance, airflow and negative intra-pharyngeal pressure receptors in the upper airway may facilitate GG while ongoing pulmonary stretch receptor activity throughout inspiration may inhibit GG. Furthermore, such receptors could have different thresholds for activation and may undergo different degrees of adaptation throughout a breath (Bartlett et al. 1976). Also, pharyngeal wall surface tension may differ across inspiration and affect the relationship between receptor activation and muscle activity. In addition, these mechanoreceptive inputs will summate with any central respiratory drive to the diaphragm related to ongoing activity of the central respiratory pattern generator. Despite the multiple potential sources of such non-linearities, we were most impressed by the relatively stable mean linear relationship between mechanoreceptive stimuli and GG activation. Moreover, negative pressure and flow always changed together, and we found no significant difference in airway resistance across conditions (Table 2), and no correlation between airway resistance and GG activation (data not shown). Thus, it seems reasonable to hypothesise that the integration of receptor information concerning pressure differences across regions of the airway could result in the active regulation of airway resistance. While the airway sensors are evidently in place, the central integration of such a mechanism is hypothetical at this stage.
Central respiratory drive versus mechanoreceptive reflexes as the principal stimulus to phasic muscle activation
Although our data suggest that upper airway dilator muscles can be activated throughout inspiration via ongoing upper airway mechanoreceptor reflexes, a large body of evidence in humans and animals demonstrates that chemoreceptive stimuli can also drive these muscles. Two studies in humans (
Redline et al. 1987;
Innes et al. 1995) have demonstrated that laryngectomised patients who can only breathe through a tracheotomy retain some inspiratory phasic GG activity during spontaneous breathing, and that this phasic activity increases during imposed hypercapnia and while breathing against a resistive load (despite an absence of intra-pharyngeal negative pressure swings). In the present study we demonstrated that the linear relationship between peak negative
Pepi and peak phasic GG EMG was largely unaffected by alterations in central ventilatory drive across the normal physiological range (during mild hypocapnia, eucapnia and mild hypercapnia). These new data are consistent with the results of a separate study (
Shea et al. 2000) in which spontaneous ventilation was stimulated across wider ranges of blood gases including combined hypercapnia and hypoxia (arterial oxygen saturation was 87 % combined with +4 mmHg hypercapnia, resulting in spontaneous ventilation of 23 l min
−1). Interestingly, the GG response in those more stimulated conditions could also be fully explained based upon the mechanoreceptive stimuli alone. These observations suggest that local mechanoreceptive reflexes are more potent than chemoreceptive mechanisms in mediating upper airway dilator muscle activation across a wide physiological range. Thus, in the current experiment it appears likely that much of the relationship between central respiratory drive and phasic GG activation occurs indirectly via mechanoreceptor reflex mechanisms.
Integration of spontaneous respiratory rhythm and mechanoreceptive reflex effects on genioglossus activity
We remain uncertain of the specific pathways involved in the integration of spontaneous respiratory rhythm and mechanoreceptive reflex effects on GG activation. Spontaneous respiratory phasic activity can be recorded from central pattern generator neurons in the brainstem that activate the pump muscles (e.g. pre-Botzinger complex) and from hypoglossal motor nuclei which activate the genioglossus muscle (
Bianchi et al. 1995). However, the functional link between these nuclei has not been well studied. In particular, we do not know whether the premotor neurons that activate respiratory pump muscles are the same as those premotor neurons that activate upper airway muscles, how differences in these pathways result in the pre-activation of upper airway muscles relative to pump muscles, and where in this pathway the mechanoreceptive stimuli exert their effect (i.e. via central pattern generator neurons or directly onto hypoglossal motoneurons). While chemoreceptive stimuli concurrently activate both pump and airway dilator muscles, specific mechanoreceptive stimuli have been shown to have differential effects on these muscles. For instance, airway deformation can activate airway dilator muscles and simultaneously inhibit respiratory pump muscles (
Harms et al. 1996). Nonetheless, in the current study we found the same relationship between mechanoreceptive stimuli and upper airway dilator muscle activity during spontaneous breathing and during passive mechanical ventilation when the spontaneous respiratory rhythm output was diminished or absent. This dissociation between pump muscle and airway muscle activation suggests that phasic mechanoreceptive stimuli exerted their influence on upper airway dilator muscles by directly stimulating hypoglossal motoneurons (rather than by facilitating the central respiratory drive to the diaphragm which is absent or diminished during passive ventilation). A possible caveat to this last interpretation is that subjects may have volitionally suppressed their pump muscles during mechanical ventilation, or even voluntarily activated their GG muscle. Although we do not have any direct evidence to this effect, we note that our subjects were instructed to remain completely relaxed (i.e. to not actively suppress breathing), and they reported that they were not voluntarily controlling breathing. Whether or not spontaneous breathing can be suppressed in awake humans during mechanical ventilation is the subject of controversy (e.g.
Leevers et al. 1993;
Morrell et al. 1993; reviewed by
Shea, 1996). New evidence from a functional magnetic imaging study shows that there are motor cortical regions activated during volitional inspiration which are relatively inactive during mechanical ventilation in awake humans, suggesting that the usual voluntary respiratory motor control regions need not be activated during mechanical ventilation (
Evans et al. 1999). Overall, we do not consider the possibility of volitional control of pump muscles to greatly affect our interpretation because this eventuality would add even one more condition in which the robust relationship between airway mechanoreceptor activity and genioglossus activity is stable, i.e. during basal spontaneous breathing, stimulated spontaneous breathing, passive mechanical ventilation (if present), volitional activation of pump muscles (if present), and volitional suppression of pump muscles (if present). Since we used slow stimuli during mechanical ventilation, it remains possible that these stimuli could result in voluntary activation of GG. However, this seems unlikely as it would require precise moment-to-moment voluntary activation of GG during spontaneous breathing and mechanical ventilation to yield a robust relationship between mechanoreceptor stimuli and GG activity in all conditions.
Relevance to OSA
We chose to study healthy awake subjects rather than patients with OSA as there may be differences in the mechanoreceptive reflex pathways brought on by the disease or the sleep-wake state. For instance, it seems clear from several studies that the pharyngeal airway is smaller in OSA patients compared to normal subjects during wakefulness (
Horner et al. 1989;
Schwab et al. 1995). Also, numerous studies indicate that during wakefulness OSA patients have an abnormally high pharyngeal dilator muscle activity (e.g.
Mezzanotte et al. 1992). Since nasal continuous positive airway pressure diminishes upper airway dilator muscle activity during wakefulness in OSA, it has been proposed that their abnormally high muscle activity is driven by a reflex response to intra-luminal negative pressure (
Mezzanotte et al. 1992). Results from the current study are consistent with these hypotheses. However, the precise stimulus to the neuromuscular compensation in OSA patients during wakefulness is not fully understood, with the greatest conundrum being that augmented pharyngeal muscle activity occurs in the face of relatively normal intra-luminal pressures while awake. This suggests that there may have been adaptation in the mechanoreceptive reflex pathway in OSA patients.
Conclusion
The current study demonstrates that in awake humans relatively physiological (i.e. slow and phasic) changes in upper airway mechanoreceptor stimuli throughout inspiration result in phasic activation of an upper airway dilator muscle. Intra-pharyngeal negative pressure and airflow were equally good predictors of genioglossus activity during spontaneous breathing at various levels of ventilation, and during passive mechanical ventilation at various tidal volumes, breathing frequencies and levels of
PET,CO2. This mechanoreceptive reflex appeared to be much stronger than any chemoreceptive influence on upper airway muscle activation, at least within the physiological range that we studied (from mild hypocapnia to mild hypercapnia). These data collected across the normal physiological ranges of breathing frequency, volume and end-tidal carbon dioxide provide strong evidence that upper airway dilator muscles can be activated throughout inspiration via ongoing upper airway mechanoreceptor reflexes. Such a feedback mechanism may naturally protect upper airway patency within each breath in awake humans. This mechanism may be important in the neuromuscular compensation to upper airway dilator muscles present in the awake patient with obstructive apnoea (
Mezzanotte et al. 1992). Loss of this reflex response during sleep could lead to obstructive sleep apnoea (
Wheatley et al. 1993;
Horner et al. 1994;
Shea et al. 1999).