Hypoglossal motoneuron responses to pulmonary and superior laryngeal afferent inputs

In decerebrate, paralyzed cats ventilated with a cycle-triggered pump, the inspiratory discharges of the hypoglossal (whole nerve or single fibers), phrenic, and recurrent laryngeal nerves were compared, and the effects of pulmonary and superior laryngeal afferent inputs were observed. During lung inflations in phase with neural inspiration, hypoglossal and recurrent laryngeal activities differed from phrenic with respect to (a) burst onset times: both preceded the phrenic; (b) overall pattern: phrenic, augmenting; hypoglossal, decrementing; recurrent laryngeal, plateau-like. When inflation was withheld, the phrenic pattern was not markedly changed, but both hypoglossal and recurrent laryngeal became augmenting; the marked increase of hypoglossal activity (both whole nerve and single fiber) indicated strong inhibition by lung afferents. Superior laryngeal electrical stimulation evoked excitation of the contralateral phrenic (latency 4.1 msec) and the ipsilateral whole hypoglossal (latency 5.3 msec), followed by bilateral inhibitions (durations 20-30 msec); most hypoglossal fibers showed only inhibition. We conclude that, although both hypoglossal and phrenic outputs are driven by the inspiratory pattern generator(s), their promotor systems differ with respect to influences from central and peripheral inputs.     

Activities of pulmonary stretch receptors during ventilatory cycles without lung inflation

When lung inflation is temporarily withheld in paralyzed, ventilated cats with intact vagi, the activities of inspiratory motor nerves are greater during the second cycle without inflation than during the first. This response is not easily attributable to increasing drive from chemoreceptors as it is abolished by vagotomy. We examined the hypothesis that the increasing inspiratory activity is the result of decreasing inhibitory feedback from pulmonary stretch receptors (PSRs). Decerebrate, paralyzed cats were ventilated by a servo-respirator in accordance with their own phrenic nerve activity. Afferent activities from individual PSRs were recorded from a few cut fibers of one vagus nerve; the vagi were otherwise intact. When lung inflation was withheld, phrenic and hypoglossal nerve activities and the durations of inspiration and expiration all increased and were significantly greater during the second cycle without inflation than during the first. The frequency of PSR discharge was also greater during the second cycle and thus did not account for the responses recorded from the motor nerves. We conclude that the latter responses probably reflect neural processes within the brain stem, involving a persistent inhibitory influence from lung inflation, which outlasts the inflation itself.     

Respiratory-modulated activities of motor units of the facial nerve

The purpose of this work was to characterize the influence of activity of vagal pulmonary receptors upon the discharge pattern of motor units of the facial nerve. Decerebrate and paralyzed cats were ventilated with a servo-respirator which produced pulmonary inflations in parallel with activity of the phrenic nerve. At normocapnia, facial units discharged phasically during neural inspiration, expiration or across both phases or discharged tonically throughout the respiratory cycle. When pulmonary inflation was withheld, the tonic discharge of some units became phasic; others changed the pattern of phasic discharge. In hypercapnia, the number of tonic fiber activities increased and, again, some phasic discharge patterns were altered. Withholding inflation caused similar alterations as in normocapnia. Activities of facial fibers in vagotomized animals differed in that no tonic activities were recorded, and no change in phasic discharge patterns was induced by hypercapnia. We conclude that afferents from pulmonary stretch receptors influence ventilatory activity throughout the entire respiratory cycle. The concept is discussed that the tonic, as well as phasic discharge of these receptors, is important for the regulation of activity of motoneurons to upper airway muscles.     

Properties of inspiratory termination by superior laryngeal and vagal stimulation.

Electrical stimulation of two respiratory afferent nerves, the vagus and the internal branch of the superior laryngeal, was used to terminate inspiration. The short latency responses of phrenic motoneurones to these stimuli were studied to determine if inspiratory termination was preceded by a characteristic phrenic motoneurone discharge pattern, reflecting changes in brainstem inspiratory neurone discharge and inspiratory terminating mechanisms. Stimulus trains of sufficient intensity delivered to the superior laryngeal nerve terminated inspiration within 50 ms and were preceded by a stereotyped pattern of phrenic motoneurone discharge. This consisted of a short latency (disynaptic), predominantly contralateral excitation in response to the first shock of the train, followed by a marked and long lasting inhibition. In contrast, vagal stimulation typically terminated inspiration hundreds of milliseconds after the onset of the stimulus train and was not preceded by a stereotyped pattern of phrenic motoneurone responses to single shocks. Transient short latency responses were obtained but were extremely small, requiring considerable excitation followed by a moderate bilateral depression of activity. Inspiration could be terminated with or without the presence of these short latency responses. These results indicate that superior laryngeal and vagal (presumably pulmonary stretch receptor) afferents have different projections to brainstem inspiratory neurones and may exert their effects on inspiratory duration through different, but as yet undefined, neural mechanisms.     

Influence of lung volume on phrenic, hypoglossal and mylohyoid nerve activities

In decerebrate, paralyzed cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, we examined the influence of lung volume on the activities of the phrenic, hypoglossal and mylohyoid nerves. When lung inflation was briefly withheld, the durations of inspiration (TI) and expiration (TE) and the activities of all three nerves increased. The relative increase in hypoglossal activity greatly exceeded that of phrenic activity and was apparent earlier in the course of inspiration. This hypoglossal response was enhanced by hypercapnia and isocapnic hypoxia. The responses of mylohyoid activity were quite variable: withholding lung inflation augmented inspiratory activity in some cats, but expiratory discharge in others. Sustained increases in end-expiratory lung volume were induced by application of 3-4 cm H2O of positive end-expiratory pressure (PEEP). Steady-state PEEP did not influence nerve activities or the breathing pattern. Bilateral vagotomy increased TI, TE, and the activities of all three nerves. No response to withoholding lung inflation could be discerned after vagal section. The results provide further definition of the influence of vagally mediated, lung volume dependent reflexes on the control of upper airway muscles. These reflexes are well suited to relieve or prevent upper airway obstruction.     

Responses of early and late onset phrenic motoneurons to lung inflation

In anesthetized or decerebrate cats that were paralyzed and ventilated with a cycle-triggered pump, we produced changes in activity of the whole phrenic nerve and of individual phrenic motoneurons (fibers or cells in the spinal cord) by withholding lung inflation during the inspiratory (I) phase. The neurons were classified into early- and late-onset types (discharge onset less or greater than 80 msec, respectively, after whole phrenic onset). Both unit and whole phrenic activity exhibited a variety of responses to inflation (excitation, depression, or no effect); but there were no consistent differences between responses of early- and late-onset neurons. The distribution of responses was quite different from that of dorsal respiratory group (DRG) I neurons (Cohen and Feldman, 1984); in particular there was no group of phrenic neurons corresponding to the late-onset I-beta neurons (I neurons excited by inflation). We conclude that the inputs to late-onset phrenic neurons are not predominantly or exclusively from late-onset DRG neurons.     

Influence of phasic volume feedback on abdominal expiratory nerve activity

Our purpose was to examine the influence of phasic lung volume feedback on the activities of motor nerves innervating the diaphragm and transversus abdominis muscles during hypercapnia and hypoxia. We studied seventeen decerebrate cats that were paralyzed and ventilated with a servo-respirator controlled by the integrated phrenic neurogram. The effects of phasic lung volume feedback were assessed by withholding pulmonary inflation during the central inspiratory period. Withholding lung inflation for a single respiratory cycle under hyperoxic, normocapnic conditions consistently prolonged the durations of the inspiratory and expiratory periods, and caused marked increases in the peak electrical activities of both phrenic and abdominal nerves. Hyperoxic hypercapnia (PaCO2 50-80 mmHg) and isocapnic hypoxia (PaO2 60-35 mmHg) increased peak phrenic and abdominal neural activities, and withholding pulmonary inflation under these conditions caused even greater augmentations of inspiratory and expiratory motor output. The augmentation of expiratory activity by withholding lung inflation was proportionately greater than the concomitant prolongation of the central expiratory period. All responses to non-inflation maneuvers were abolished following bilateral cervical vagotomy. The results indicate that vagally mediated volume feedback during inspiration can attenuate the output of abdominal motoneurons in the subsequent expiratory period. Moreover, hypoxia, which attenuates abdominal motor activity in vagotomized animals, enhances this activity when the vagi are intact.     

The effects of hypercapnia and hypoxia on single hypoglossal nerve fiber activity

The respiratory related modulation of hypoglossal nerve activity has been studied at the single fiber level in cats under hyperoxic hypercapnia and hypoxic conditions and their conduction velocities determined. Changes in fiber activity were compared to simultaneous changes occurring in phrenic activity. Three different kinds of discharge patterns were observed: (a) inspiratory, (b) phasic activity during both inspiration and expiration, and (c) continuous random activity with no respiratory modulation. These fibers could be grouped into three categories according to their pattern of discharge during CO2 breathing. Type I fibers, mean conduction velocity of 30.0 m/sec, exhibited only an inspiratory phasic discharge during 100% O2 breathing. Their discharge frequency increased rapidly with higher levels of CO2 and hypoxia. Type II fibers, mean conduction velocity of 36.7 m/sec, had three different kinds of inspiratory-expiratory discharge patterns during 100% O2 breathing. With increasing hypercapnia or hypoxia fibers of this group discharged phasically during inspiration and discharge at low frequency during expiration. Type III fibers had a non phasic discharge pattern at 100% O2 breathing and at all levels of CO2 tested (up to 10%). Discharge frequency rose during CO2 rebreathing and hypoxia, but the rate of increase was much less than Type I and Type II fibers. Their mean conduction velocity was 41.3 m/sec. The inspiratory activity of Type I and II fibers increased their activity more than the phrenic during hypercapnia and hypoxia. Type II and Type III fibers are responsible at least in part for the tonic activity of the nerve.     

Comparison of phrenic motoneuron activity in eupnea and apneusis

Our purpose was to characterize activities of phrenic motoneurons during apneusis. In decerebrate, cerebellectomized, vagotomized, paralyzed and ventilated cats, we recorded activities of phrenic nerve and single phrenic fibers during eupnea and apneusis. Reversible apneusis was obtained by cooling the rostral pons with a fork thermode. Phrenic motoneurons were defined as 'early' or 'late' during eupnea. Early units commenced activity before or during the first 20% of neural inspiration. The onset of discharge of late units extended throughout the rest of inspiration. In apneusis, some late units ceased activity entirely; others commenced activity at the end of the rising phase of phrenic activity or during the apneustic plateau. Early units commenced activities at the same time as in eupnea and generally maintained the same discharge frequency. Hence, the ramp phase of phrenic discharge in apneusis is generated largely by activities of early motoneurons. Our results imply that the level of bulbospinal activity impinging upon the phrenic nucleus is reduced in apneusis. The integration of efferent activity within the phrenic nucleus is discussed.     

Characterization of respiratory-related activity of the facial nerve

Activities of the facial, hypoglossal and phrenic nerves were recorded in decerebrate and paralyzed cats. These animals were ventilated with a servo-respirator which produced lung inflations in parallel with phrenic activity. Peak inspiratory phrenic, hypoglossal and facial activities increased in hypercapnia or hypoxia. When pulmonary inflation was prevented, hypoglossal and facial activities increased more than phrenic. Responses to withholding lung inflation differed from those following vagotomy. These differences were observed in expiratory facial and hypoglossal activities and in hypercapnia- and hypoxia-induced changes in facial activity. Administration of pentobarbital or hyperventilation to hypocapnia caused greater suppressions of hypoglossal than facial activity; the latter declined more than phrenic activity. The results support the hypothesis that influences from the brainstem reticular formation and from pulmonary stretch receptors are differentially distributed to motoneurons innervating upper airway muscles compared to those of the bulbospinal-phrenic system. The concept that ventilatory activity is influenced by tonic, as well as phasic discharge of pulmonary receptors is discussed.     

Phrenic afferents and ventilatory control

It has been understood since the late 1800s that the diaphragm has significant sensory innervation. The role of phrenic afferents in the control of breathing, however, has been obscure. The phrenic nerve has been shown to contain a full array of afferent fibers. However, proprioceptive (group 1 fibers) afferents are few compared to postural muscles or the intercostals. The diaphragm, unlike the inspiratory intercostal muscles, has a small complement of spindle afferents and not all of these spindles are supplied with fusorial fibers. Reduced spindle afferents under gamma control help to explain previous studies of the diaphragm that have failed to reveal autogenic facilitation, that is, a reflex-mediated increase in drive during inspiratory loading. Nevertheless, some clinical studies have revealed increased activation of the diaphragm when its length is reduced. Group 1 fibers, which are predominantly tendon organ afferents in the diaphragm, have been shown to have a phasic inhibitory function. A reduction in this inhibition brought about by a reduction in diaphragmatic length during lung inflation may explain the increased diaphragmatic activation reported in clinical studies. Phrenic afferents have been shown to have multiple spinal and supraspinal projections. Recent studies have explored the ventilatory effects of thin fiber afferents (group III and IV fibers) in the phrenic nerve. Stimulation of these afferents has been shown both to inhibit and excite ventilation. These afferents arise from polymodal receptors that respond to both mechanical and chemical stimulation. Activation of these receptors may occur in a variety of conditions and the ventilatory response may be determined by the specific receptor activated.     

Phrenic afferents and ventilatory control

It has been understood since the late 1800s that the diaphragm has significant sensory innervation. The role of phrenic afferents in the control of breathing, however, has been obscure. The phrenic nerve has been shown to contain a full array of afferent fibers. However, proprioceptive (group 1 fibers) afferents are few compared to postural muscles or the intercostals. The diaphragm, unlike the inspiratory intercostal muscles, has a small complement of spindle afferents and not all of these spindles are supplied with fusorial fibers. Reduced spindle afferents under gamma control help to explain previous studies of the diaphragm that have failed to reveal autogenic facilitation, that is, a reflex-mediated increase in drive during inspiratory loading. Nevertheless, some clinical studies have revealed increased activation of the diaphragm when its length is reduced. Group 1 fibers, which are predominantly tendon organ afferents in the diaphragm, have been shown to have a phasic inhibitory function. A reduction in this inhibition brought about by a reduction in diaphragmatic length during lung inflation may explain the increased diaphragmatic activation reported in clinical studies. Phrenic afferents have been shown to have multiple spinal and supraspinal projections. Recent studies have explored the ventilatory effects of thin fiber afferents (group III and IV fibers) in the phrenic nerve. Stimulation of these afferents has been shown both to inhibit and excite ventilation. These afferents arise from polymodal receptors that respond to both mechanical and chemical stimulation. Activation of these receptors may occur in a variety of conditions and the ventilatory response may be determined by the specific receptor activated.     

The respiratory activity of the superior laryngeal nerve in the rat.

The aim of this study was to characterize the laryngeal afferent activity of the rat. The animals were anesthetized and breathing spontaneously. Laryngeal afferent activity was recorded from both the whole superior laryngeal nerve (SLN) and from single fibers isolated from this nerve. An overall inspiratory augmenting activity was observed in the whole SLN during tracheostomy breathing, tracheal occlusion and upper airway breathing, but an expiratory augmenting activity was present during upper airway occlusion. The inspiratory modulated activity was abolished by bilateral section of the hypoglossal nerves but not the recurrent laryngeal nerves. A great number of receptors (46/80, 58%) were identified as 'drive' receptors, and others as 'pressure' (22/80, 28%) and 'irritant' type receptors (9/80, 11%). Nineteen pressure receptors were stimulated by positive transmural pressure, while only three stimulated by negative pressure. Nine drive receptors were also stimulated by positive pressure and inhibited by negative pressure. Such response to pressure was further evaluated by applying maintained pressures to the functionally isolated upper airway. These results are essentially consistent with findings obtained in the rabbit, but differ from those reported for the dog.     

Bladder contractions alter inspiratory termination by superior laryngeal and intercostal nerve stimulation.

Gradual distension of the urinary bladder evokes spontaneous bladder contractions (SBCs), which are associated with reduced inspiratory activity in the phrenic and other inspiratory motor nerves. We examined the influence of isovolumetric SBCs on the threshold for termination of phrenic inspiration by electrical stimulation of superior laryngeal and/or mid-thoracic intercostal nerves (ICN) in decerebrate, vagotomized, paralyzed, ventilated cats. Although SBCs reduced phrenic inspiratory activity, the threshold for inspiratory termination by nerve stimulation was increased. The results emphasize the complexity of the synaptic connections among brain stem neurons governing micturition and breathing.     

Spectral composition of synchronized discharge of phrenic nerve activity in the rabbit and effects of pulmonary afferents

In anaesthetized rabbits synchronization of efferent phrenic nerve discharge activity (PNA) was investigated by spectrum analysis. During hypercapnia (6-8% end-tidal CO2) in almost all animals the spectrum analysis of PNA during inspiration revealed a bimodal distribution. A broad peak was present at 45.9 +/- 7.7 Hz (MFO peak) whereas a usually well-defined peak with smaller band width was located at 100.3 +/- 17.1 Hz (HFO peak). Increase of hypercapnia promoted the peak amplitude of HFO but had no consistent effect on the amplitude of MFO. Synchronization in the HFO band gradually increased with the progress of inspiration but declined during the last third of inspiration. HFO could be demonstrated to be also present in postinspiratory activity. The amplitude of MFO slowly increased during inspiration becoming strongest at the end of inspiration. The effect of afferent inputs from pulmonary stretch receptors on synchronization of PNA was found to be very weak. Neither withholding inflation during inspiration nor bilateral vagotomy produced substantial changes of MFO and HFO. Tracheal occlusions, however, during inspiration resulted in a slight increase of the power of MFO and HFO, which mainly was due to an overall increase of power. Augmented breaths resulted in a strong shift of the HFO peak to higher frequencies which was accompanied by a dramatic increase of the peak amplitude and a decrease of the peak band width. The presumptive role of the different types of phrenic motoneurons conveying HFO is addressed.     

Influence of laryngeal CO2 on respiratory activities of motor nerves to accessory muscles.

Intralaryngeal CO2 in decerebrate, vagotomized cats decreases phrenic nerve activity and increases the respiratory activity of the hypoglossal (HG) nerve. These responses are mediated by afferents in the superior laryngeal nerves. To explore the responses of other respiratory motor nerves to this stimulus, we have recorded the activities of the nasolabial (NL) branch of the facial nerve, the posterior cricoarytenoid (PCA) and thyroarytenoid (TA) branches of the recurrent laryngeal nerve and the nerve to triangularis sterni (TS) muscle. In response to 5 and 10% CO2 in the surgically isolated upper airway, we found dose-related decreases in phrenic activity, increases in HG and NL activity and characteristic, but intermittent, exaggeration of early expiratory bursts of TA activity. The activities of the PCA and TS nerves showed no consistent responses. These results broaden the definition of the reflex response to intralaryngeal CO2, revealing components that reflect ventilatory inhibition, upper airway dilation and laryngeal protection.     

Nonvagal modulation of hypoglossal neural activity.

Upper airway dilating muscle activity is characterized by an early-peaking pattern which serves to dilate or stiffen the upper airway at the time when the greatest negative intraluminal pressure is generated by contraction of chest wall muscles. This pattern has been attributed to phasic afferent inputs from pulmonary stretch receptors. The present study examines the hypothesis that nonvagal factors may also influence the discharge pattern and coordination of upper airway and chest wall muscle activities. Therefore, in anesthetized, paralyzed, vagotomized and artificially ventilated cats, we examined the effects of changes in respiratory drive produced by activation of cholinergic and GA-BAergic (gamma-aminobutyric acid) receptors at the ventrolateral aspects of the medulla oblongata on phasic intrabreath discharge patterns of hypoglossal and phrenic nerves. Cholinergic agents (acetylcholine, carbachol, methacholine, physostigmine) applied directly to chemoreceptive areas on the ventral medullary surface increased hypoglossal activity, and in addition converted inspiratory discharge from an augmenting to a decrementing pattern of activity. The reverse effect on the discharge pattern of hypoglossal activity was observed with a decrease in respiratory drive. While the amplitude of the phrenic nerve discharge was also affected by these interventions, the augmenting discharge pattern of phrenic nerve activity did not change. These results suggest that the early peaking pattern of hypoglossal nerve discharge in vagotomized cats also depends on the level of respiratory drive, and is not solely dependent on vagal afferent inputs. In addition, the data suggest that structures near the ventral surface of the medulla are influential in shaping the pattern of hypoglossal nerve activity and maintaining balanced activity of upper airway and chest wall muscles.     

Differential elevation by protriptyline and depression by diazepam of upper airway respiratory motor activity

Effects of systemically administered protriptyline and diazepam on the respiratory activity of the phrenic, hypoglossal, and recurrent laryngeal nerves were investigated in vagotomized, decerebrate cats. Both hypoglossal and recurrent laryngeal nerve activities were consistently increased after protriptyline administration, whereas the phrenic nerve discharge was not systematically altered. Similar changes were observed in cats with bilateral carotid sinus nerve sections. Diazepam induced a reduction of hypoglossal and recurrent laryngeal nerve activities at doses that did not alter phrenic nerve discharge. These results with diazepam were the same in carotid chemodenervated cats. We conclude that neural mechanisms controlling upper airway muscles are much more sensitive to protriptyline and diazepam than are those of the bulbospinal-phrenic system. The selective augmentation of hypoglossal and recurrent laryngeal discharges by protriptyline could account for the reported decrease in the frequency of obstructive sleep apneas in patients receiving this antidepressant. In contrast, diazepam, by depressing motor activity to upper airway muscles, may exacerbate oropharyngeal obstruction during sleep.     

The effect of antigen-induced bronchoconstriction on phrenic nerve activity

The effects of acute bronchoconstriction, produced by inhalation of Ascaris suum antigen, on both the amplitude and timing of phrenic nerve activity were studied in anesthetized dogs. Blood gas tensions and inspiratory air flow were maintained constant. Bronchoconstriction resulted in a significant increase in the magnitude and rate of rise of the phrenic neurogram during both normal respiratory cycles and cycles when lung inflation was prevented. The increase in the slope of the phrenic neurogram that results from lung inflation was further increased during bronchoconstriction. In addition, in half the animals, there was a significant increase in the tonic phrenic activity measured during expiration. All of these changes were abolished by bilateral cervical vagotomy. Antigen administration did not affect the timing of the different phases of the cycle equally; inspiratory duration was reduced by 51.8% and expiratory duration by 67.6%. Postinspiratory activity of the phrenic (PIIA) was reduced by only 34.7%; thus, during bronchoconstriction PIIA occupied a proportionally greater fraction of the expiratory phase. Vagotomy abolished the changes in respiratory timing and eliminated all PIIA.     

Phrenic and hypoglossal neural responses to cold airflow in the upper airway.

Cold air flowing through the larynx is known to alter the activities of laryngeal receptors with afferents in the superior laryngeal nerves (SLNs) and to induce reflex apnea in neonatal mammals. To examine the ventilatory response in adult animals and to explore associated upper airway motor responses, we recorded phrenic and hypoglossal neural responses to cooling the isolated larynx with cold air in decerebrate, vagotomized, paralyzed, ventilated cats. The most consistent response was phrenic inhibition, which occurred in all animals tested. Either excitation or inhibition of hypoglossal activity was seen consistently in individual cats, with the result that the group response was not statistically significant. All responses to laryngeal cooling were abolished by section of the SLNs. The findings confirm that directing cold air through the larynx causes reflex inhibition of ventilatory (phrenic) activity, but raise new questions as to how the two, directionally opposite hypoglossal responses are mediated.