The position of the hyoid arch suggests that it supports soft tissue surrounding the upper airway (UA) and can act to maintain UA patency. We also suspected that muscles inserting on the hyoid arch might show respiratory patterns of activity that could be affected by respiratory stimuli. To test these possibilities, we moved the hyoid arch ventrally in six anesthetized dogs either by traction on it or by stimulation of hyoid muscles. UA resistance was decreased 73 +/- (SE) 6% and 72 +/- 6% by traction and stimulation during expiration and 57 +/- 15% and 52 +/- 8% during inspiration. Moving averages of the geniohyoid (GH) and thyrohyoid (TH) obtained in six other dogs breathing 100% O2 showed phasic respiratory activity while the sternohyoid (SH) showed phasic respiratory activity in only two of these animals and no activity in four. With progressive hypercapnia, GH and TH increased as did SH when activity was already present. Airway occlusion at end expiration augmented and prolonged inspiratory activity in the hyoid muscles but did not elicit SH activity if not already present. Occlusion at end inspiration suppressed phasic activity in hyoid muscles for as long as in the diaphragm. After vagotomy activity increased and became almost exclusively inspiratory. Activity appeared in SH when not previously present. Duration and amplitude of hyoid muscle activity were increased with negative UA pressure and augmented breaths. We conclude that the hyoid arch and muscles can strongly affect UA flow resistance. Hyoid muscles show responses to chemical, vagal, and negative pressure stimuli similar to other UA muscles.

The effects of negative pressure applied to just the upper airway on nasal and laryngeal muscle activity were studied in 14 spontaneously breathing anesthetized dogs. Moving average electromyograms were recorded from the alae nasi (AN) and posterior cricoarytenoid (PCA) muscles and compared with those of the genioglossus (GG) and diaphragm. The duration of inspiration and the length of inspiratory activity of all upper airway muscles was increased in a graded manner proportional to the amount of negative pressure applied. Phasic activation of upper airway muscles preceded inspiratory activity of the diaphragm under control conditions; upper airway negative pressure increased this amount of preactivation. Peak diaphragm activity was unchanged with negative pressure, although the rate of rise of muscle activity decreased. The average increases in peak upper airway muscle activity in response to all levels of negative pressure were 18 +/- 4% for the AN, 27 +/- 7% for the PCA, and 122 +/- 31% for the GG (P less than 0.001). Rates of rise of AN and PCA electrical activity increased at higher levels of negative pressure. Nasal negative pressure affected the AN more than the PCA, while laryngeal negative pressure had the opposite effect. The effects of nasal negative pressure could be abolished by topical anesthesia of the nasal passages, while the effects of laryngeal negative pressure could be abolished by either topical anesthesia of the larynx or section of the superior laryngeal nerve. Electrical stimulation of the superior laryngeal nerve caused depression of AN and PCA activity, and hence does not reproduce the effects of negative pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of upper airway resistance suggest that the activity of cranial nerves supplying upper airway muscles changes with chemical drive and that imbalances in the activation of these nerves as compared to the phrenic play a role in causing upper airway obstruction. We assessed the effect of hypoxia and hypercapnia on the activity of the hypoglossal nerve, the recurrent laryngeal nerve, and phrenic nerve in paralyzed anesthetized artificially ventilated dogs. Comparison of hypoglossal and phrenic nerves were also repeated after vagotomy. Both hypoglossal and recurrent laryngeal nerves exhibited increased activity with inspiration. Hypoxia and hypercapnia increased phrenic nerve activity as well as the activity of the two cranial nerves. While linear increases occurred in phrenic and recurrent laryngeal nerve activity with both chemical stimuli, the relationship between hypoglossal and phrenic nerve activity was curvilinear. At lower levels of chemical drive, changes in hypoglossal nerve were less than in the phrenic, and the reverse was true at higher levels of chemical stimulation. There were also differences in the response of both cranial nerves and the phrenic to changing vagal stimulation. The dissimilarities observed in the cranial response of the nerves (versus the phrenic) could potentially affect the forces developed during inspiration and lead to obstruction in the upper airway.

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SUMMARY1. The effects of oesophageal distension on respiratory patterns and the moving average electromyogram (e.m.g.) activity of three upper airway muscles -the alae nasi, the genioglossus, and the posterior cricoarytenoid -and four chest wall muscles -the costal and crural diaphragm and the inspiratory and expiratory intercostals -were examined in ten anaesthetized, tracheostomized, spontaneously breathing dogs. Distension was produced by inflations of a balloon placed in the middle part of the thoracic oesophagus with volumes of air ranging from 50 to 200 ml.2. Esophageal distension increased respiratory frequency, mainly due to a significant shortening of the expiratory time.3. Activity of both the costal and crural parts of the diaphragm was inhibited with oesophageal distension, whereas that of the inspiratory intercostal muscles increased, tending to maintain a near-normal tidal volume and end-tidal CO. Phasic inspiratory activity of all three upper airway muscles increased in response to oesophageal distension, as did the activity of the expiratory intercostal muscles.4. The changes in the breathing pattern and the electrical activity of all muscles in response to oesophageal distension were immediate, occurring during the first breath after the balloon was inflated.5. The responses were graded, so that increases in the volume of the oesophageal balloon progressively increased the activity of the upper airway and intercostal muscles, and decreased diaphragm activity.6. Bilateral vagotomy abolished the effects of oesophageal distension on upper airway and chest wall muscle activity, suggesting that vagal afferents constitute the major pathway for the reflex.