Carbon dioxide-responsive laryngeal receptors in the dog

Effect of inhaled carbon dioxide on laryngeal abduction

Hypercapnia produces a profound effect on respiratory drive and upper airway function to maintain airway patency. Previous work has evaluated the effects of hypercapnia on the sole arytenoid abductor, the posterior cricoarytenoid (PCA), using indirect measures of function, such as electromyography and direct nerve recording. Here we describe a novel method to evaluate PCA function in anesthetized animals and use this method to determine the effects of hypercapnia on PCA function. Eight dogs were anesthetized, and a laryngeal mask airway was used, in combination with high-speed videoendoscopy, to evaluate laryngeal function. A stepwise increase in inspired partial pressure of CO2 produced marked arytenoid abduction above 70-mmHg end-tidal CO2 (ETCO2) ( P < 0.001). Glottic length increased above 80-mmHg ETCO2 ( P < 0.02), and this lead to underrepresentation of changes in glottic area, if standard measures of glottic area (normalized glottic gap area) were used. Use of a known scale to determine absolute glottic area demonstrated no plateau with increasing ETCO2 up to 120 mmHg. Ventilatory parameters also continued to increase with no evidence of a maximal response. In a second anesthetic episode, repeated bursts of transient hypercapnia for 60 s with an ETCO2 of 90 mmHg produced a 43–55% increase in glottic area ( P < 0.001) at or shortly after the end of the hypercapnic burst. A laryngeal mask airway can be used in combination with videoendoscopy to precisely determine changes in laryngeal dimensions with high temporal resolution. Absolute glottic area more precisely represents PCA function than normalized glottic gap area at moderate levels of hypercapnia.

Ventilatory response to high inspired carbon dioxide concentrations in anesthetized dogs

Background:

The ventilation ( ) response to inspired CO₂ has been extensively studied, but rarely with concentrations >10%.

Aims:

These experiments were performed to determine whether would increase correspondingly to higher concentrations and according to conventional chemoreceptor time delays.

Materials and Methods:

We exposed anesthetized dogs acutely, with and without vagotomy and electrical stimulation of the right vagus, to 20-100% CO₂-balance O₂ and to 0 and 10% O₂-balance N₂.

Results:

The time delays decreased and response magnitude increased with increasing concentrations (p<0.01), but at higher concentrations the time delays were shorter than expected, i.e., 0.5 s to double at 100% CO₂, with the response to 0% O₂ being ~3 s slower. Right vagotomy significantly reduced baseline breathing frequency (fR), increased tidal volume (VT) and increased the time delay by ~3 s. Bilateral vagotomy further reduced baseline fR and , and reduced the response to CO₂ and increased the time delay by ~12 s. Electro-stimulation of the peripheral right vagus while inspiring CO₂ caused a 13 s asystole and further reduced and delayed the response, especially after bilateral vagotomy, shifting the mode from VT to fR.

Conclusions:

Results indicate that airway or lung receptors responded to the rapid increase in lung H⁺ and that vagal afferents and unimpaired circulation seem necessary for the initial rapid response to high CO₂ concentrations by receptors upstream from the aortic bodies.

The reflex effects on the respiratory regulation of the CO2 at the different flow rate and concentration

PURPOSE

The purpose of this study was to investigate the activation of the respiratory centers during insufflation of the larynx with CO2 at different flow rates and concentrations.

MATERIALS AND METHODS

The experiments were carried out in spontaneous air breathing rabbits, anesthetized with thiopental sodium (25 mg kg(-1) i.v.). The larynx was separated from the oropharyngeal cavity and the trachea. The tidal volume (VT) and respiratory frequency (f min(-1)) were recorded from the lower tracheal cannula. The respiratory minute volume (VE) was calculated, the action potentials from the right phrenic nerve were recorded and the inspiratory (TI) and expiratory (TE) periods and the mean inspiratory flow rate (VT/TI) were calculated. The larynx was insufflated at flow rates of 500 mL min(-1) and 750 mL min(-1), with 7 and 12% CO2-Air by means of a respiratory pump.

RESULTS

Insufflation of the larynx, with both gas mixtures, decreased the f and VT significantly. The TI and TE were found to increase significantly due to the decreasing in f. There was a significant decrease in VT/TI ratio. Following bilateral midcervical vagotomy, on the passing of both gas mixtures, significant decreases were observed in the VT, and the responses of f, TI and TE were abolished. After cutting the superior laryngeal nerve, the responses of the VT to both gas mixtures were abolished.

CONCLUSION

In conclusion, the results of this study purpose that the stimulation of the laryngeal mechanoreceptors by the effect of hypercapnia decreases the activation of the respiratory center.

Effects of hypoxia and hypercapnia on nonnutritive swallowing in newborn lambs

The aim of the present study was to investigate the effect of hypercapnia and hypoxia on apnea and nonnutritive swallowing (NNS) frequency, as well as on the coordination between NNS and phases of the respiratory cycle in newborn lambs, while taking into account the potential effects of states of alertness. Six lambs were chronically instrumented for recording electroencephalogram, eye movements, diaphragm and thyroarytenoid muscle (a glottal adductor) activity, nasal airflow, and electrocardiogram. Polysomnographic recordings were performed in nonsedated lambs exposed to air (control), 10% O(2), and 5% CO(2) in a random order at 3, 4, and 5 days of age. Although hypercapnia decreased apnea frequency in wakefulness and active sleep (P = 0.002 vs. air and hypoxia), hypoxia had no significant effect on apnea. In addition, although hypercapnia increased NNS frequency during wakefulness and quiet sleep (P < 0.005 vs. air and hypoxia), hypoxia tended to decrease NNS frequency. Finally, only hypercapnia altered NNS-breathing coordination by increasing NNS at the transition from inspiration to expiration (ie-type NNS; P < 0.001 vs. air and hypoxia). In conclusion, whereas hypercapnia increases overall NNS frequency by specifically increasing ie-type NNS, hypoxia has the inverse tendency. Results were identical in all three states of alertness.

Serotonergic modulation of inspiratory hypoglossal motoneurons in decerebrate dogs

Inspiratory hypoglossal motoneurons (IHMNs) maintain upper airway patency. However, this may be compromised during sleep and by sedatives, potent analgesics, and volatile anesthetics by either depression of excitatory or enhancement of inhibitory inputs. In vitro data suggest that serotonin (5-HT), through the 5-HT2A receptor subtype, plays a key role in controlling the excitability of IHMNs. We hypothesized that in vivo 5-HT modulates IHMNs activity through the 5-HT2A receptor subtype. To test this hypothesis, we used multibarrel micropipettes for extracellular single neuron recording and pressure picoejection of 5-HT or ketanserin, a selective 5-HT2A receptor subtype antagonist, onto single IHMNs in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs. Drug-induced changes in neuronal discharge frequency (F(n)) and neuronal discharge pattern were analyzed using cycle-triggered histograms. 5-HT increased the control peak F(n) to 256% and the time-averaged F(n) to 340%. 5-HT increased the gain of the discharge pattern by 61% and the offset by 34 Hz. Ketanserin reduced the control peak F(n) by 68%, the time-averaged F(n) by 80%, and the gain by 63%. These results confirm our hypothesis that in vivo 5-HT is a potent modulator of IHMN activity through the 5-HT2A receptor subtype. Application of exogenous 5-HT shows that this mechanism is not saturated during hypercapnic hyperoxia. The two different mechanisms, gain modulation and offset change, indicate that 5-HT affects the excitability as well as the excitation of IHMNs in vivo.

Upper airway dynamic responses in children with the obstructive sleep apnea syndrome

Normal children have a smaller upper airway than adults, but, nevertheless, snore less and have less apnea. We have previously shown that normal children have an upper airway that is resistant to collapse during sleep. We hypothesized that this resistance to collapse is due to preservation of upper airway neuromotor responses during sleep. Furthermore, we hypothesized that upper airway responses would be diminished in children with the obstructive sleep apnea syndrome (OSAS). We therefore compared the upper airway pressure-flow relationship during sleep between children with OSAS and controls. Measurements were made by correlating maximal inspiratory airflow with the level of nasal pressure applied via a mask. Neuromotor upper airway activation was assessed by evaluating the upper airway response to 1) hypercapnia and 2) intermittent, acute negative pressure. We found that children with OSAS had no significant response to either hypercapnia or negative pressure during sleep, compared with the normal children. After treatment of OSAS by tonsillectomy and adenoidectomy, there was a trend for normalization of upper airway responses. We conclude that upper airway dynamic responses are decreased in children with OSAS but recover after treatment. We speculate that the pharyngeal airway neuromotor responses present in normal children are a compensatory response for a relatively narrow upper airway. Further, we speculate that this compensatory response is lacking in children with OSAS, most likely due to either habituation to chronic respiratory abnormalities during sleep or to mechanical damage to the upper airway.

Developmental changes in upper airway dynamics

Normal children have a less collapsible upper airway in response to subatmospheric pressure administration (P(NEG)) during sleep than normal adults do, and this upper airway response appears to be modulated by the central ventilatory drive. Children have a greater ventilatory drive than adults. We, therefore, hypothesized that children have increased neuromotor activation of their pharyngeal airway during sleep compared with adults. As infants have few obstructive apneas during sleep, we hypothesized that infants would have an upper airway that was resistant to collapse. We, therefore, compared the upper airway pressure-flow (V) relationship during sleep between normal infants, prepubertal children, and adults. We evaluated the upper airway response to 1). intermittent, acute P(NEG) (infants, children, and adults), and 2). hypercapnia (children and adults). We found that adults had a more collapsible upper airway during sleep than either infants or children. The children exhibited a vigorous response to both P(NEG) and hypercapnia during sleep (P < 0.01), whereas adults had no significant change. Infants had an airway that was resistant to collapse and showed a very rapid response to P(NEG). We conclude that the upper airway is resistant to collapse during sleep in infants and children. Normal children have preservation of upper airway responses to P(NEG) and hypercapnia during sleep, whereas responses are diminished in adults. Infants appear to have a different pattern of upper airway activation than older children. We speculate that the pharyngeal airway responses present in normal children are a compensatory response for a relatively narrow upper airway.

Contribution of free nerve endings in the laryngeal epithelium to CO2 reception in rats

The superior laryngeal nerve (SLN) contains CO2-sensitive fibers. In the laryngeal epithelium, two candidates for CO2 reception have been identified, namely the intraepithelial free nerve endings and the taste buds. To elucidate the contribution of free nerve endings to CO2 reception, electrophysiological activities were recorded during various stages of regeneration of nerve endings following SLN-crush in rats. The left SLN was crushed surgically and maintained from 4 to 40 days for regeneration of nerve endings. Laryngeal sections were processed for immunohistochemical staining of protein gene product 9.5 to observe regeneration of free nerve endings and taste buds in the epithelium. By day 4 after SLN-crush, both the free nerve endings and taste buds had disappeared. Regeneration of the free nerve endings was recognized from day 8, while that of the taste buds started at day 16. On day 40, the number of taste buds on SLN-crush side was similar to that on the untreated side. Electrophysiological recording of SLN throughout the regeneration period (excluding day 4), showed response to intralaryngeal 9% CO2 (stimulation or inhibition) whether or not taste buds were present. Our results showed intralaryngeal CO2 reception without taste bud involvement, indicating that the free nerve endings in the laryngeal epithelium are receptive to intralaryngeal CO2.

Effects of subglottal air pressure on velopharyngeal muscle activity in dogs

OBJECTIVE

To analyze the effects of airflow in the larynx on activity of the levator veli palatini and pterygopharyngeal muscles.

DESIGN

Ten adult beagle dogs were anesthetized with sodium pentobarbital. In each dog, two tracheal tubes were inserted subsequent to tracheotomy, one in the direction of the vocal folds and the other toward the lungs for respiration. In the first of three experiments, the effect of artificial airflow on electromyographic activity of the levator and pterygopharyngeal muscles was studied. In the second experiment, the effect of air pressure beneath the vocal folds on the activity of these muscles was studied. For the third experiment, the larynx was isolated surgically without cutting the bilateral superior laryngeal nerves and the effect of airflow through it examined.

RESULTS AND CONCLUSION

Both outward airflow and higher pressure enhanced expiratory activity of the levator and pterygopharyngeal muscles. Receptors in the subglottal area play major roles in this enhancement. Furthermore, an increase in air pressure during expiration enhances closure of the velopharynx.

Effects of C fiber blockade on cardiorespiratory responses to laryngeal stimulation in concious lambs

The primary aim of the study was to explore cardiorespiratory reflexes originating from laryngeal C fiber endings in the neonatal period. Seventeen lambs were instrumented for recording glottal adductor and diaphragm EMG, heart rate, systemic arterial pressure and respiratory movements. C fiber blockade was induced in eight lambs by 30 mg/kg capsaicin, the remaining nine lambs serving as controls. Cardiorespiratory reflexes were induced in non-sedated lambs by flowing air, menthol or 13% CO2, or by injecting water or 50 microg capsaicin in the laryngeal inlet through an endoscope. Responses to all stimuli but capsaicin were similar between the two groups. While cardiorespiratory responses were induced by capsaicin in control lambs, the responses were significantly inhibited in lambs with C fiber blockade. We conclude that laryngeal C fiber endings are functional and responsible for laryngeal chemoreflexes in newborn lambs.

Reflexes from airway rapidly adapting receptors

Rapidly adapting receptors (RARs) occur throughout the respiratory tract from the nose to the bronchi. They have thin myelinated nerve fibres, an irregular discharge and adapt rapidly to a maintained volume stimulus, but often slowly to a chemical stimulus. They are polymodal, responding to mechanical and chemical irritant stimuli, and to many inflammatory and immunological mediators. RARs show very varied sensitivities to different stimuli, and diverse reflex responses. Those in the larynx are usually called 'irritant' receptors. They probably cause cough, the expiration reflex and other laryngeal reflexes: cardiovascular, mucus secretion, bronchoconstrictor and laryngoconstrictor. Those in the trachea and larger bronchi are very mechanosensitive; they cause cough, bronchoconstriction and airway mucus secretion. Those in the larger bronchi are more chemosensitive; they may cause cough, but also stimulate hyperventilation, augmented breaths, mucus secretion, bronchoconstriction and laryngeal closure. Most of the stimuli to RARs also affect other airway receptors, especially those with C-fibre afferents, and the total reflex response will be the additive affect of all these reflexes.

Effects of intralaryngeal carbon dioxide and acetazolamide on the laryngeal chemoreflex

Sudden infant death syndrome is the leading cause of death in infants in the United States. The laryngeal chemoreflex (LCR) is thought to contribute to its pathogenesis. In adult animals, increasing levels of intralaryngeal CO2 result in a decrease in ventilatory activity. Intravenous acetazolamide (AZ) abolishes this response. The purpose of this study was to determine the effects of intralaryngeal CO2 and AZ on the LCR and respiratory physiology of piglets under normoxic and hypoxic conditions. We applied 0% or 10% CO2 in a randomized order to the larynx of 26 piglets. Intubation via tracheotomy prevented inhalation of the gas mixtures. Laryngeal stimulation was performed under normoxic conditions (PaO2 of >70 mm Hg) in 15 animals and under hypoxic conditions (PaO2 of 50 to 65 mm Hg) in 11 animals both with and without intravenous AZ (5 mg/kg). Respiratory and cardiovascular response data were recorded. Ten percent intralaryngeal CO2 has no significant effect on mean baseline respiratory rate, systemic PaCO2 or PaO2 levels, or apnea duration (p > .05). The use of AZ (versus no AZ) resulted in significantly higher baseline respiratory rates (64 versus 51 breaths per minute; p = .016), a decreased baseline systemic PaCO2 level (38.8 versus 45.9 mm Hg; p < .001), a higher baseline PaO2 level (97.9 versus 82.8 mm Hg; p < .001), shorter mean apnea durations (15.5 versus 24.8 seconds; p = .001), a higher lowest O2 saturation level after the stimulus (78.0% versus 68.4%; p = .003), and fewer profound apneas (10 of 90 versus 41 of 90 trials; p < .001). We conclude that 10% intralaryngeal CO2 does not decrease ventilatory activity in piglets and has no significant effect on the LCR. Acetazolamide, however, appears to have a protective effect against the LCR, resulting in shorter and less severe apneas. The protective effect of AZ against the LCR appears to be related to its ability to stimulate the respiratory drive and increase oxygenation at baseline.

Effects of CO2 and H+ on laryngeal receptor activity in the perfused larynx in anaesthetized cats

1. Intralaryngeal CO2 reflexly decreases ventilation and increases upper airway muscle activity. Topical anaesthesia of the laryngeal mucosa or cutting the superior laryngeal nerves (SLNs) abolishes these reflexes, indicating that the receptors responsible are superficially located and that their afferent fibres are in the SLN. Intralaryngeal CO2 affects the activity of receptors recorded from the SLN. 2. An isolated, luminally perfused laryngeal preparation was developed in anaesthetized, paralysed cats in order to compare the effects of solutions with varying levels of pH and PCO2 on pressure-sensitive laryngeal receptor activity. Since the pH of tracheal surface fluid is reported to be approximately 7.0, two neutral (pH 7.4 and 7.0) and two acidic (pH 6.8 and 6.3) solutions were used. 3. Compared with neutral acapnic control solutions, neutral hypercapnic (PCO2 64 mmHg) solutions either excited or inhibited the discharge of 113 out of 211 pressure-sensitive SLN afferents. In 24 receptors, the effects of hypercapnic solutions with either neutral or acidic pH were similar in both direction and magnitude. In 50 receptors affected by neutral hypercapnic solutions, acidic acapnic solutions had no effect on 66 % of units and significantly smaller effects in the remaining units. In 17 receptors, the effects of neutral solutions with a PCO2 of 35 mmHg were significantly less than for neutral solution with a PCO2 of 64 mmHg. 4. These results show that the effects of CO2 on laryngeal pressure-sensitive receptors are independent of the pH of the perfusing media, and suggest that acidification of the receptor cell or its microenvironment is the main mechanism of CO2 chemoreception.

Laryngeal-receptor responses to phasic CO2 in anesthetized cats

We compared the effects of CO2 applied continuously and during expiration on laryngeal-receptor activity in paralyzed, artificially ventilated and nonparalyzed, spontaneously breathing cats by using an isolated larynx, artificially ventilated to approximate a normal respiratory cycle. The majority of quiescent negative-pressure and all cold receptors were excited by 5 and 9% CO2 applied both continuously and during expiration. In general, quiescent positive-pressure, tonic negative-pressure, and tonic positive-pressure receptors were inhibited by 5 and 9% CO2 applied continuously and during expiration. There were no significant differences between responses to 5 and 9% CO2 or to continuous and expired CO2 or between paralyzed and nonparalyzed preparations. In conclusion, laryngeal receptors respond to changes in CO2 concentration occurring during a normal respiratory cycle. Because laryngeal-receptor stimulation exerts reflex effects on ventilation and upper airway muscle activity, these results suggest that airway CO2 plays a role in reflex regulation of breathing and upper airway patency.

Sensory information from the upper airway: role in the control of breathing

The functional integrity of extrathoracic airways critically depends on the proper orchestration of the activities of a set of patency-maintaining muscles. Recruitment and control of these muscles is regulated by a laryngeal and trigeminal affects that originate from pressure sensing endings. These sensors are particularly numerous among laryngeal receptors and, indeed, they constitute the main element in the respiration-modulated activity of the superior laryngeal nerve. Considering that the most compliant region of the upper airway, and thus more vulnerable to inspiratory collapse, lies cranially to the larynx, the laryngeal pressure-sensing endings seem to be ideally located for detecting collapsing forces and initiating reflex mechanisms for the preservation of patency. This process operates by activating upper airway dilating muscles and by decreasing inspiratory drive: both actions limit t he effect of the collapsing forces. Cold reception is differently represented in various mammalian species within nasal and laryngeal segments. Cooling of the upper airway has an inhibitory influence on breathing, especially in newborns, and a depressive effect on upper airway dilating muscles. The latter response is presumably mediated through the inhibitory effect of cooling on laryngeal pressure endings. These responses could be harmful during occlusive episodes. Powerful defensive responses with distinct characteristics can be elicited through the simulation of laryngeal and nasal irritant type receptors. Sneezing is elicited through the stimulation of trigeminal afferents, cough through the stimulation of laryngeal vagal endings. Changes in osmolality and ionic composition of the mucosal surface liquid can lead to conspicuous alterations in receptor activity and related reflexes.

Ventilatory and upper-airway resistance responses to upper-airway cooling and CO2 in anaesthetised rats

The effects of upper airway (UA) cool air and CO2 on breathing and on laryngeal and supraglottic resistances were studied in anaesthetised rats breathing spontaneously through a tracheostomy. Warm, humidified air containing 0, 5 and 9-10% CO2 and cool, room-humidity air were delivered at constant flow to either the isolated larynx to exit through a pharyngotomy or to the supraglottic UA to exit through the mouth and/or nose (nose open or sealed). Spontaneous tracheal airflow and UA airflows, temperatures and pressures were recorded. CO2 had no effect on breathing but caused a slight increase in laryngeal resistance which was abolished by cutting the superior laryngeal nerves (SLN). Cool air caused a decrease in respiratory frequency and/or peak inspiratory flow when applied to the isolated larynx or to the supraglottic airway with the nose closed. These effects were abolished by SLN section. With the nose open, the ventilatory inhibition was not abolished by SLN section. Cool air also caused substantial decreases in laryngeal and supraglottic resistances which were attenuated by SLN section and which persisted following recurrent laryngeal nerve section. In conclusion, whilst UA cooling inhibits breathing and decreases UA resistances, UA CO2 has minimal effects.

A developmental study of the dose-response curve of the respiratory sensory reflex

We have shown previously that inhalation of high concentration of CO2 (about 8%) inhibits breathing in preterm infants, presumably through an upper airway sensory reflex. To study the developmental aspects and the dose-response curve of this reflex, we studied eight preterm infants (body weight, 1.6 +/- 0.1 kg mean +/- SE; gestational age, 31 +/- 1 wk; postnatal age, 22 +/- 5 days) and eight term infants (body weight, 3.2 +/- 0.1 kg; gestational age, 39 +/- 1 wk; postnatal age, 8 +/- 6 days) using a flow-through system; eight adult subjects (weight, 67 +/- 5 kg; age, 30 +/- 4 yr) were studied during quiet sleep using a nasal mask. We gave 2, 4, 6, and 8% CO2 in 21+ O2 randomly for 20 to 30 s. A clear inhibition of breathing typically occurred during inhalation of 8% CO2 only in preterm infants, as reflected by the presence of an apnea of 11 +/- 1 s occurring at 7 +/- 2 s after the beginning of CO2 inhalation. Short apneas were occasionally observed with lower concentrations of CO2, but they were significantly fewer and shorter than with 8% CO2. No clear inhibition was observed in term infants or adult subjects, but pauses of 4 and 6 s were observed in the former group and a pause of 7 s was observed in the latter one. The associated changes in minute ventilation during inhalation of 2, 4, and 6% CO2 were not significantly different between the three groups. During inhalation of 8% CO2, minute ventilation decreased only in preterm infants (-26 +/- 10 compared with +32 +/- 10 in term infants and to +17 +/- 5% in adult subjects; p = 0.003 between groups).(ABSTRACT TRUNCATED AT 250 WORDS)

Acetazolamide specifically inhibits lingual trigeminal nerve responses to carbon dioxide

The goal of this study was to examine the role of the enzyme, carbonic anhydrase, in oral trigeminal chemoreception with particular regard to the reception of CO2. Using both single and multiunit recordings of trigeminal neurons in the lingual nerve of rat, we measured responses to cool (24 degrees C), noxiously hot (55 degrees C) and cold (8 degrees C) H2O, NH4Cl and supersaturated solutions of CO2 (24 degrees C and 33 degrees C). The importance of peripheral carbonic anhydrase was tested by inhibiting enzyme activity with acetazolamide (15 mg/kg b.w.). Single unit responses to CO2 and HCl suggest that neural sensitivity to CO2 is not simply a function of extraepithelial pH. Responses to CO2 were significantly inhibited by acetazolamide while the responses to thermal stimuli and NH4Cl were not. The results support a role for carbonic anhydrase in trigeminal responses to CO2. Furthermore, the results suggest that intraepithelial acidification mediated by carbonic anhydrase may be the basis for sensitivity to CO2.

Alteration of ventilatory activity by intralaryngeal CO2 in the cat

1. We investigated the responses of phrenic and hypoglossal nerve activities to the addition of 3, 5 and 10% CO2 to a constant flow of warm, humidified air through the isolated upper airway in decerebrate, paralysed, artificially ventilated cats. 2. In bilaterally vagotomized animals, intralaryngeal CO2 caused a dose-related decrease in peak integrated phrenic activity. This response became attenuated with time, but was still discernible after 3 min of continuous intralaryngeal CO2. In the same experiments, intralaryngeal CO2 caused a gradual increase in peak integrated hypoglossal nerve activity. 3. Intermittent pulsing of intralaryngeal CO2 during neural inspiration or expiration resulted in similar, but smaller decreases in the phrenic activity of some animals. Hypoglossal activity was not influenced appreciably by this procedure. 4. Systemic hypercapnia attenuated the phrenic responses to intralaryngeal CO2. The hypoglossal responses were greatly reduced or abolished. 5. In vagally intact cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, intralaryngeal CO2 resulted in only a trace of reduction in phrenic discharge. After bilateral vagotomy, the same animals showed typical responses, as described above. 6. All responses to intralaryngeal CO2 were abolished after bilateral section of the superior laryngeal nerves (SLNs). 7. We conclude that intralaryngeal CO2 acts by way of receptors with afferents in the SLNs to decrease phrenic and increase hypoglossal nerve activities. The responses are not importantly gated during neural inspiration or expiration. The responses to intralaryngeal CO2 are most clearly demonstrable after bilateral vagotomy, suggesting that vagal mechanisms serve to stabilize respiratory motor neural activity in intact animals.

Responses of laryngeal receptors to intralaryngeal CO2 in the cat

1. We recorded afferent activities of single fibres in the superior laryngeal nerves of decerebrate or anaesthetized, paralysed cats while 3, 5 and 10% CO2 was added to a constant flow of warm, humidified air through the isolated upper airway. 2. Fifty-three receptors with discharge frequencies modulated by intralaryngeal CO2 were studied. Of these, forty-eight showed CO2-induced attenuation of their firing rates. Pulses of 3, 5 or 10% CO2, alternating with air at intervals ranging from 1.5 to 60 s, also diminished the discharge frequencies. This diminution was greater with higher CO2 concentrations and longer pulse durations. 3. Five of the fifty-three receptors were stimulated by intralaryngeal CO2. The discharge frequencies of these units increased slowly and by only a few impulses per second during CO2 exposure. 4. Thirty-four of the CO2-sensitive receptors were tested with other stimuli, including water, saline, positive and negative intralaryngeal pressures and cold air. The responses to these stimuli varied among receptors, but many of the units that reduced their frequencies with intralaryngeal CO2 were consistently stimulated by positive and/or negative intralaryngeal pressures. 5. Thirty-six of the receptors were anatomically located by probing the upper airway. Twenty-six were in the larynx, and ten were in the rostral trachea, within 5 mm of the cricoid cartilage. 6. The results, which are directly applicable to the investigation of reflex responses reported in the preceding paper, indicate that the predominant initial response to intralaryngeal CO2 under the conditions of these studies is attenuation of laryngeal receptor activity.