Giuseppe Sant'Ambrogio

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.

Laryngeal pressure receptors

We studied the response characteristics of laryngeal pressure receptors in anesthetized dogs, breathing through a tracheal cannula, by recording single unit action potentials from the peripheral cut end of the internal branch of the superior laryngeal nerve. The larynx, with the rest of the upper airway, was isolated and cannulated separately for the application of distending and collapsing pressures. We identified receptors responding to either negative or positive pressure and a few responding to both. All these receptors showed a marked dynamic sensitivity and had the characteristics of slowly adapting mechanoreceptors. The majority of pressure receptors were active at zero transmural pressure and the gain of their response to pressure was higher at lower values, suggesting a role for these receptors in eupnea. Reflex alterations in breathing pattern and upper airway muscle activity during upper airway pressure changes, previously reported, are presumably mediated by the receptors described here. Moreover, these receptors may play a role in certain pathological states, such as obstructive sleep apnea, in which the upper airway is transiently subjected to large collapsing pressure.

Localization of rapidly adapting receptors in the trachea and main stem bronchus of the dog

Abstract Fast-adapting tracheo-bronchial receptors were identified by their rapidly adapting dicharge of action potentials, recorded from small filaments in the peripheral cut end of the right vagus nerve, to either a maintained inflation or deflation of the lungs. Their location was visually determined in 19 dogs by probing the airways through a fiberoptic bronchoscope, and thereby eliciting their response to light touch. Only receptors placed in the trachea and in main stem bronchus were considered. Thirty-five receptors were studied, 24 in the trachea and 11 in the right main stem bronchus. The receptive fields to light touch were roughly circular and approximately 1 cm in diameter. Receptors were distributed throughout the ipsilateral hemicircumference of the trachea (4 in the contralateral hemicircumference) and throughout the circumference of the right main stem bronchus. In 9 receptors resection of the mucosal receptive field eliminated the response to local probing, but left intact the response to deflation and inflation in 7 of them. The results indicate that the sensory endings of an ‘irritant’ receptor lie both in the epithelial and in deeper structures of the airway. We infer that mucosal endings provide sensitivity to light touch and that deeper endings mediate the response to gross mechanical deformation.

Water-responsive laryngeal receptors in the dog are not specialized endings.

The primary purpose of this study was to ascertain whether laryngeal receptors activated by water are specialized endings or whether they also respond to other stimuli, such as pressure, temperature and laryngeal motion as they occur during the breathing cycle. In 35 anesthetized mongrel dogs, breathing spontaneously through a lower cervical tracheostomy, water and other test solutions at approximately 37 degrees C were injected into the functionally isolated larynx with a small catheter. Of the 130 receptors studied, none of the cold receptors (N = 13) responded to water, whereas approximately 60% of all laryngeal mechanoreceptors (72 of 117) responded with either a short delay, short duration or a long delay, long duration response. In general the former pattern of response was exhibited by nonrespiratory-modulated receptors, whereas the latter was typical of respiratory-modulated receptors. The specific nature of the stimulus (hypotonicity or lack of chloride ion) of the water response was further studied in 53 receptors with isoosmotic solutions of dextrose and sodium gluconate. The long delay, long duration response was dependent on a decreased osmolality, while the short delay, short duration response was dependent on the lack of chloride ion of the test solutions. All water-responsive receptors tested (N = 17) were blocked within 50 sec by topically applied 2% lidocaine and thus presumed to be superficial. However, 10 receptors which did not respond to water were also blocked within 50 sec, suggesting that not all superficial receptors are stimulated by water. Based on these observations, we propose that changes in osmolality or ionic composition of the laryngeal surface liquid could play an important role in modifying reflexes involved in the maintenance of upper airway patency.

Respiratory afferent activity in the superior laryngeal nerves

This study evaluates the afferent activity in the superior laryngeal nerve (SLN) during breathing as well as during occluded inspiratory efforts. Experiments were performed in 11 anesthetized and spontaneously breathing dogs. Electroneurographic activity was recorded from the peripheral cut end of the SLN and, in 3 dogs, also from the contralateral vagus nerve. A tracheal cannula with a side arm allowed the bypass of the larynx during breathing and occluded efforts. A clear inspiratory modulation was present in all experimental conditions. Both peak and duration of the SLN activity decreased (87% and 89%) when breathing was diverted from the upper airway to the tracheostomy. Peak and duration of the SLN activity (as % of upper airway breathing) increased during occluded efforts; however, the increase was greater when the larynx was not by-passed (peak = 118% vs 208%, duration = 143% vs 178%). Section of the ipsilateral recurrent laryngeal nerve reduced the inspiratory modulation. Vagal afferent activity increased equally during tracheostomy and upper airway breathing and decreased markedly during tracheal and upper airway occlusions. Our results indicate that collapsing pressure in the larynx is the major stimulus in activating laryngeal afferents.

Circulatory accessibility of nervous receptors localized in the tracheobronchial tree.

We studied the accessibility through the systemic and the pulmonary circulations of slowly and rapidly adapting mechanoreceptors having known location within the tracheobronchial tree of the dog. The accessibility was determined by injecting a challenging substance (veratridine and benzonatate) into the two atria and comparing the delays of the receptor response. Eighteen receptors localized in extrapulmonary airways were more accessible from the systemic circulation; they were affected with a shorter delay following left atrium injection. Six receptors in lobar bronchi and another 44 located in more distal intrapulmonary airways were preferentially accessible from the pulmonary circulation (i.e., shorter delays with right atrium injections). Only 4 receptors localized in lobar bronchi were found to be more accessible from the systemic circulation. Therefore the assumption that airway receptors that are more accessible from the pulmonary circulation are located distal to the terminal bronchioles should be re-evaluated. These results may also have important physiological implications since several endogenous substances capable of affecting airway structures have different concentrations in the mixed venous and arterial blood.

Carbon dioxide-responsive laryngeal receptors in the dog

The purpose of this study was to relate the carbon dioxide (CO2) response of laryngeal receptors to their behavior during the breathing cycle (i.e. their response to transmural pressure changes, laryngeal movement or decreases in temperature) or during exposure to irritant stimuli (water or cigarette smoke). In 9 anesthetized mongrel dogs breathing spontaneously through a tracheostomy, unit activity from the superior laryngeal nerve was recorded while warmed and humidified gas mixtures (air or 10% CO2 in O2) were passed, for 1 min, through the functionally isolated upper airway in the expiratory direction. None of the 10 cold receptors studied were affected by CO2. Eleven of 20 laryngeal non-modulated mechano-receptors were stimulated (from 0.3 to 1.6 imp/sec) by exposure to CO2. These CO2-responsive receptors were also stimulated by known irritant stimuli (cigarette smoke, water), although not all receptors which responded to these irritants were stimulated by CO2. Twelve of 33 respiratory-modulated receptors were affected by CO2; 4 were stimulated and 8 inhibited. Receptors inhibited by CO2 were also inhibited by negative pressure while receptors stimulated by CO2 were also stimulated by negative pressure. These results show that CO2-responsive laryngeal receptors are not specialized endings. Although it is not clear to what extent each separate group of laryngeal receptors is involved, each may contribute to the reflex bradypnea which has been observed during exposure of the upper airway to elevated levels of CO2. However, the importance of CO2-responsive laryngeal receptors in physiological conditions remains unclear.

Role of intrinsic muscles and tracheal motion in modulating laryngeal receptors

Recording from the superior laryngeal nerve discloses a respiratory modulated activity even in the absence of airflow and pressure changes in the larynx. The present study evaluates the relative contribution of intrinsic laryngeal muscle activity and transmitted tracheal movement on the respiratory modulation of laryngeal mechanoreceptors. Seventy-four receptors were studied in 22 anesthetized spontaneously breathing dogs. The modulation of 31 receptors depended solely on laryngeal muscle activity since it was abolished by cold block of laryngeal nerves. Twelve receptors were primarily activated by tracheal movement since tracheal stabilization alone reduced or abolished their modulation. The respiratory modulation of the remaining 31 receptors was found to be dependent on both laryngeal muscle activity and tracheal movements. Lidocaine (2%) was applied to the receptor field of 13 endings; the results indicate that while some receptors are located superficially (blocked within 1 min) others are located in deeper structures (not affected in 30 min). These receptors may be involved in the precise coordination of laryngeal muscle activity and could play a role in the regulation of breathing pattern and airway patency due to their pressure sensitivity.

Location and discharge properties of respiratory vagal afferents in the newborn dog

Activity from afferents of respiratory origin was recorded from filaments of the vagus nerve in puppies 1-17 days old. Animals were anesthetized with a mixture of chloralose-urethane and artificially ventilated with the chest open. A total of 256 receptors were studied of which 96% were identified as slowly adapting mechanoreceptors (SARs) and 4% as rapidly adapting mechanoreceptors (RARs). Hence the newborn has fewer spontaneously active RARs than the adult. Thirty-five percent of SARs were located in the trachea and the remainder in the bronchi: a distribution similar to that found in the adult. The transpulmonary pressure (Ptp) threshold required for SAR activation was higher in the newborn than the adult for both tracheal (2.42 cm H2O vs 0.51 cm H2O) and bronchial 4.66 cm H2O vs 3.66 cm H2O) receptors. This resulted in less SAR activity at FRC in the puppy (16% of the total number of SARs in 1-17-day-old animals) compared to the adult (63%). In addition, the discharge frequency of tracheal and bronchial SARs, at any given Ptp, was lower in the newborn than in the adult dog. These data suggest that the newborn has considerably less volume-related feedback during the respiratory cycle compared to the adult.

Behavior of slowly adapting stretch receptors in the extrathoracic trachea of the dog1

We have studied the response of 40 slowly adapting stretch receptors located in the extrathoracic trachea to maintained positive and negative pressures together with their behavior during the breathing cycle and inspiratory efforts against closed airways in anesthetized dogs. Like the other tracheal stretch receptors they are located in the smooth muscle of the posterior wall and are stimulated either by inflation or deflation. Most of these receptors are active at zero transmural pressure and increase their response with positive pressure; with negative pressure their resting discharge decreases at first (-5 to -10 cm H2O) and then increases, but it is generally lower than that of the corresponding positive pressure value. As predicted by their static response, the activity of these receptors decreases during inspiration and increases during expiration and therefore it is out of phase with the discharge of the other airway stretch receptors. Their activity is related to the corresponding transmural pressure and therefore to airflow and upper airway resistance. During inspiratory efforts against the closed airways their activity decreases, unlike that of the intrathoracic airway receptors, which retain essentially their FRC activity.