Characteristics of laryngeal cold receptors

Outdoor Cold Air Versus Room Temperature Exposure for Croup Symptoms: A Randomized Controlled Trial

OBJECTIVES

Croup is the most common cause of acute upper airway obstruction in children. The benefits of treating croup with steroids are well established, with an onset of effect 30 minutes after administration. We investigated whether a 30-minute exposure to outdoor cold air might improve mild to moderate croup symptoms before the onset of action of steroids.

METHODS

This open-label, single-center, randomized controlled trial, enrolled children aged 3 months to 10 years with croup and a Westley Croup Score (WCS) ≥2 attending a tertiary pediatric emergency department. Participants were randomized (1:1) to either a 30-minute exposure to outdoor cold (<10°C) atmospheric air or to indoor ambient room air immediately after triage and administration of a single-dose oral dexamethasone. The primary endpoint was a decrease in WCS ≥2 points from baseline at 30 minutes. Analyses were intention to treat.

RESULTS

A total of 118 participants were randomly assigned to be exposed to outdoor cold air (n = 59) or indoor room temperature (n = 59). Twenty-nine of 59 children (49.2%) in the outdoor group and 14 of 59 (23.7%) in the indoor group showed a decrease in WCS ≥2 points from baseline at 30 minutes after triage (risk difference 25.4% [95% confidence interval 7.0-43.9], P = .007). Patients with moderate croup benefited the most from the intervention at 30 minutes (risk difference 46.1% [20.6-71.5], P < .001).

CONCLUSIONS

A 30-minute exposure to outdoor cold air (<10°C), as an adjunct to oral dexamethasone, is beneficial for reducing the intensity of clinical symptoms in children with croup, especially when moderate.

Mechanisms underlying the sensation of dyspnea

Dyspnea is defined as a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity. It is a common symptom among patients with respiratory diseases that reduces daily activities, induces deconditioning, and is self-perpetuating. Although clinical interventions are needed to reduce dyspnea, its underlying mechanism is poorly understood depending on the intertwined peripheral and central neural mechanisms as well as emotional factors. Nonetheless, experimental and clinical observations suggest that dyspnea results from dissociation or a mismatch between the intended respiratory motor output set caused by the respiratory neuronal network in the lower brainstem and the ventilatory output accomplished. The brain regions responsible for detecting the mismatch between the two are not established. The mechanism underlying the transmission of neural signals for dyspnea to higher sensory brain centers is not known. Further, information from central and peripheral chemoreceptors that control the milieu of body fluids is summated at higher brain centers, which modify dyspneic sensations. The mental status also affects the sensitivity to and the threshold of dyspnea perception. The currently used methods for relieving dyspnea are not necessarily fully effective. The search for more effective therapy requires further insights into the pathophysiology of dyspnea.

Body thermal responses and the vagus nerve

While thermosensation from external environment has been extensively studied, physiological responses to temperature changes inside the body and the underlying regulatory mechanisms are less understood. As a critical link between body and brain that relays visceral organ information and regulates numerous physiological functions, the vagus nerve has been proposed to mediate diverse visceral thermal reflexes and indirectly regulate body temperature. However, the precise role of the vagus nerve in body thermal responses or visceral organ-related thermoregulation is still under debate due to extensive contradictory results. This data discrepancy is likely due to the high cell heterogeneity in the vagus nerve, as diverse vagal neuron types mediate numerous and sometimes opposite physiological functions. Here, we will review evidences that support and against the role of the vagus nerve in body thermosensation and thermoregulation and discuss potential future approaches for better understanding of this critical issue.

TRPM8 and dyspnea: from the frigid and fascinating past to the cool future?

The transient receptor potential melastatin 8 (TRPM8) ion channel is gated by cool and noxious cold temperatures. The activation threshold is in the range of ≈25-28°C, which aligns well with the discharge of airway afferents. TRPM8 is widely expressed across species and evolutionary changes in the TRPM8 amino acid sequence may tune the temperatures at which it is gated. The discovery of TRPM8 and its molecular/biophysical characterization provides a robust candidate for airway afferents responding to cool/cold temperatures. TRPM8 may provide a mechanistic link for the manipulation of respiratory sensations such as dyspnea or mechanisms leading to cold-induced asthma and cough.

Sensory input pathways and mechanisms in swallowing: a review

Over the past 20 years, research on the physiology of swallowing has confirmed that the oropharyngeal swallowing process can be modulated, both volitionally and in response to different sensory stimuli. In this review we identify what is known regarding the sensory pathways and mechanisms that are now thought to influence swallowing motor control and evoke its response. By synthesizing the current state of research evidence and knowledge, we identify continuing gaps in our knowledge of these mechanisms and pose questions for future research.

TRPA1 channels mediate cold temperature sensing in mammalian vagal sensory neurons: pharmacological and genetic evidence

Cold thermoreceptors have been described in different territories of the vagus nerve. Application of cold temperature to these visceral afferents can evoke major protective reflexes and thermoregulatory responses. However, virtually nothing is known about the transduction mechanisms underlying cold sensitivity in vagal afferents. Here, we investigated the effects of cold stimulation on intracellular calcium responses and excitability of cultured vagal sensory neurons in the rat nodose ganglion. A large fraction of vagal neurons were activated by cold, with a mean threshold of approximately 24 degrees C. Cooling was accompanied by development of a small inward current and the firing of action potentials. Most cold-sensitive neurons were also activated by heat and capsaicin, suggesting a nociceptive function. The pharmacological response to TRPM8 and TRPA1 agonists and antagonists suggested that, unlike results observed in somatic tissues, TRPA1 is the major mediator of cold-evoked responses in vagal visceral neurons. Thus, most cold-evoked responses were potentiated by cinnamaldehyde, menthol, icilin, and BCTC [4-(3-chloro-pyridin-2-yl)-piperazine-1-carboxylic acid (4-tert-butyl-phenyl)-amide], agonists of TRPA1, and were inhibited by ruthenium red, camphor, and HC03001 [2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopropylphenyl)acetamide]. Results in mouse nodose neurons revealed a similar pharmacological profile of cold-evoked responses. Furthermore, experiments in TRPA1 knock-out mice showed a large reduction in the percentage of cold-sensitive neurons compared with wild-type animals. Together, these results support an important role of TRPA1 channels in visceral thermosensation and indicate major differences in the transduction of temperature signals between somatic and visceral sensory neurons.

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.

Calbindin D28k-immunoreactive afferent nerve endings in the laryngeal mucosa

The distribution of the calbindin D28k in the laryngeal sensory structures was studied by immunohistochemistry, immunoelectronmicroscopy, and double immunofluorescence with calretinin-immunoreactivity. Moreover, origin of the nerve endings were observed using retrograde tracer, fast blue. Immunoreactivity for calbindin D28k was found in the various types of nerve endings in the larynx, namely, laminar nerve endings, nerve endings associated with the taste buds, intraepithelial nerve endings, and endocrine cells. The laminar endings with calbindin D28k-immunoreactivity were observed in the subepithelial connective tissue. In some endings, terminals were expanded. The laminar endings were also observed in the perichondrium of the epiglottic cartilage. In the epiglottic and arytenoid epithelia, thick nerve fibers with calbindin D28k-immunoreactivity ascending to taste buds and intragemmal nerve fibers were also observed. Within the epithelial layer, intraepithelial free nerve endings with calbindin D28k-immunoreactivity were observed. Furthermore, diffuse endocrine cells were observed within the laryngeal epithelium. By immunoelectron microscopy, immunoreaction products in the endings mentioned above were localized in the cytoplasm of the axon terminals and nerve fibers which contained with numerous mitochondria. Out of the 100 laminar endings, 18 endings were immunopositive for both calbindin D28k and calretinin, 33 were positive for calbindin D28k but negative for calretinin, and 49 were positive for only calretinin in the double immunofluorescence microscopy. The nerve fibers associated with the taste buds and the free nerve endings, which immunostained for calbindin D28k, were not stained with antibody against calretinin. After injection of the fast blue in the laryngeal mucosa, fast blue-labeled cells were mainly observed in the nodose ganglia. Of the total number of labeled cell in the nodose and dorsal root ganglia at the level C1 to Th2, 65.1% occurred in nodose ganglia (572/879, n = 6). In the nodose ganglia, 79.7% of labeled cells (456/572) were immunoreacted for calbindin D28k. The distribution of calbindin D28k-immunoreactivity may be differnt from that of calretinin. It is suggested that calbindin D28k have regulatory role on intracellular calcium concentration in the laryngeal sensory corpuscles.

Neurochemical markers in the nervous plexus of the canine glottis

The structure of the nervous network and the distribution of tyrosine hydroxylase (TH)- and various neuropeptide-containing nerves were immunohistochemically studied in the glottis of the dog. The nervous network in the glottis revealed apparent regional differences in morphology. The nervous network in the cartilaginous vocal fold of the posterior glottis consisted of nerve bundles running parallel to the edge of the vocal fold. Only a small number of nerve bundles were observed in the anterior glottis, specifically in membranous vocal fold. In the subepithelial layer of the posterior glottis, a moderate number of galanin (GAL)-immunoreactive nerve fibers were observed, while only a few fibers were present in the anterior glottis. Numerous vasoactive intestinal peptide (VIP)-, GAL-, methionine-enkephalin (ENK)- and TH-immunoreactive nerve fibers were observed within and around the laryngeal submucosal seromucous gland. Many TH- and neuropeptide Y (NPY)-immunoreactive fibers were arranged around the blood vessels. In the epithelia, free nerve endings with immunoreactivity for substance P (SP) and calcitonin gene-related peptide (CGRP) was observed. Furthermore, nerve cell bodies with SP-, VIP-, GAL-, ENK-, and NPY-immunoreactivity were observed in the deep region of the submucosal layer. The results from the present study suggest that there is autonomic regulation of the glottis. Regional structural differences in the nervous network of the glottis may reflect functional differences.

Nasal inhalation of l-menthol reduces respiratory discomfort associated with loaded breathing

To test the hypothesis that stimulation of cold receptors in the upper airway may alleviate the sensation of respiratory discomfort, we investigated the effects of nasal inhalation of l-menthol (a specific stimulant of cold receptors) on the respiratory sensation and ventilation during the loaded breathing in 11 normal subjects. Subjects were asked to rate their sensation of respiratory discomfort using a visual analog scale (VAS) while breathing on a device with a flow-resistive load (180 cm H2O/L/s) or with an elastic load (75.5 cm H2O/L). The effects of inhalation of l-menthol on ventilation and respiratory sensation were evaluated by comparing the steady-state values of ventilatory variables and VAS scores obtained before, during, and after l-menthol inhalation. In 8 of 11 subjects inhalation of strawberry-flavored air instead of l-menthol was performed during loaded breathing. Both during the flow-resistive loading and the elastic loading, inhalation of l-menthol caused a significant reduction in sensation of respiratory discomfort (flow-resistive loading: 62 +/- 14 [mean +/- SD] VAS units before inhalation versus 36 +/- 16 during inhalation, p < 0.01; elastic loading: 68 +/- 13 before inhalation versus 55 +/- 17 during inhalation, p < 0.01) without a significant change in breathing pattern and ventilation. Comparison of the effects between the flow-resistive loading and the elastic loading also revealed that the reduction in VAS score was more during the flow-resistive loading than during the elastic loading (p < 0.01). Inhalation of strawberry-flavored air caused neither changes in VAS score nor changes in breathing pattern and ventilation, indicating that olfaction is not a contributing factor in the relief of respiratory discomfort. We concluded that stimulation of cold receptors in the upper airway with nasal inhalation of l-menthol reduces the sensation of respiratory discomfort associated with loaded breathing. This effect is more effective during the flow-resistive loading than during the elastic loading.

Morphological study of the vagal afferent nerve endings in the laryngeal mucosa of the dog

The afferent nerve endings in the laryngeal mucosa of the dog were investigated by immunohistochemical staining specific for neurofilament protein of whole-mount preparation. In the laryngeal mucosa, two kinds of nerve endings, namely, laminar and glomerular endings, were observed. The laminar endings were distributed on the laryngeal side of the epiglottis, the mucosa overlying the arytenoid cartilage and the vocalis muscle. The laminar endings were 100-350 microns long and 60-200 microns wide. Some axons gave rise to a single ending while others continued to two endings. The terminal portions were round, oval or triangular in shape, and occasionally had an immunopositive rim with a negative center. Cryostat sections revealed that the laminar endings were located immediately beneath the laryngeal epithelium. The endings were flattened and extended in two dimensions. The glomerular endings could be divided into two subtypes, large ones and small ones. The large ones were 150-250 microns long and 90-200 microns wide, and they were distributed in the mucosa of the intercartilaginous part of the glottis and the laryngeal side of the epiglottis. Several nerve fibers were gathered in the subepithelial region and arranged in a glomerular pattern. In some large glomerular endings, the terminal portions formed laminar arborizations. The small glomerular endings were 100-200 microns long and 40-120 microns wide, and their distribution was restricted to the mucosa of the corniculate process of the arytenoid cartilage. They were simple in terms of shape and were also located in the subepithelial region. Some small endings were accompanied by mucosal protrusions and were located near taste buds. Dogs that had been subjected to surgical denervation of the internal branch of the cranial (superior) laryngeal nerve suggested that both the laminar and the glomerular endings were mainly derived from this branch of ipsilateral side.

Influences of laryngeal afferent inputs on intralaryngeal muscle activity during vocalization in the cat

The present study was undertaken to elucidate the possible role of the laryngeal afferent inputs in the regulation of intralaryngeal muscle activity during vocalization. We studied the influences of airflow and/or pressure applied to the larynx on intralaryngeal muscle activity during vocalization in ketamine-anesthetized cats. Vocalization was induced by airflow applied to the upper airway, which was isolated from the lower airway, during pontine call site stimulation. When the upper airway was open to the atmosphere through the nostrils and mouth, the airflow increased not only the vocal fold adductor and tensor activities but also the duration of these activities. The adductor and tensor activities were increased suddenly at a critical subglottic pressure level equivalent to the subglottic pressure threshold for vocalization. These effects were significantly reduced by sectioning of the internal branch of the superior laryngeal nerve or by lidocaine application to the laryngeal mucosa. Sustained pressure applied to the isolated upper airway, when the mouth and nostrils were occluded, did not affect adductor or tensor activities. These results indicate that the afferent inputs evoked by vocal fold stretching or vibration play an important role in the motor control of intralaryngeal and respiratory muscles during vocalization.

Thermomechanical facilitation of swallowing evoked by electrical nerve stimulation in cats

Application of a cold metal probe to the anterior faucial pillar has been reported to improve swallowing in some patients with dysphagia. Although a variety of stimuli contribute to the initiation of swallowing, the effects of a controlled, cold-thermal stimulus combined with mechanical stimulation have not been examined. It is known that simultaneous stimulation of the glossopharyngeal nerve (IX) and the superior laryngeal nerve may summate to facilitate swallowing in the cat. The goal of this study was to determine whether thermomechanical stimulation of the mucosa innervated by IX would interact with threshold electrical stimulation of the internal laryngeal nerve (ILN) to augment the swallowing response in cats. Four experimental conditions were tested over 24 trials in 4 pentobarbital-anesthetized cats. These included electrical stimulation of ILN, mechanical stimulation of the anterior faucial pillar with a thermode at ambient (room) temperature, concurrent ambient-mechanical and electrical stimulation, and concurrent cold-mechanical and electrical stimulation. Tissue was cooled to 8.9 degrees C during cold-mechanical-electrical stimulation and 25.3 degrees C during ambient-mechanical-electrical and ambient-mechanical alone stimulation. Ambient-mechanical stimulation alone did not produce swallowing. However, both forms of thermomechanical-electrical stimulation elicited a significantly greater number of swallows than did electrical stimulation alone. Therefore, mechanical stimulation with a thermode was capable of modifying the swallowing response in neurologically intact cats.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Afferent activity in the external branch of the superior laryngeal and recurrent laryngeal nerves

We investigated the presence of respiratory-modulated receptors in the recurrent laryngeal nerve (RLN) and the external branch of the superior laryngeal nerve (ExtSLN) in anesthetized, spontaneously breathing dogs. Of 39 receptors recorded from the ExtSLN, the vast majority responded with a slowly adapting discharge to compression of the cricothyroid muscle, and only 1 responded to probing of the laryngeal mucosa. Ten receptors showed a respiratory modulation. All 30 receptors recorded from the RLN responded to probing of the laryngeal lumen, most of them (60%) with a rapidly adapting response. Seven of the slowly adapting receptors exhibited a respiratory modulation; 38% of the receptors tested were stimulated by water, and only 15% by smoke. No receptors stimulated by laryngeal cooling were identified in either nerve. Our study indicates that in the RLN and the ExtSLN there are relatively few afferents responding to changes in transmural pressure and mechanical irritation, as compared to the internal branch of the SLN. The relative scarcity of receptors responding to transmural pressure and irritant stimuli is consistent with previous observations in dogs that indicate a preponderant role for afferents in the internal branch of the SLN in the reflex responses to laryngeal stimulation.

Trigeminal nasal receptors related to respiration and to various stimuli in cats

In twenty adult cats of either sex under nembutal anaesthesia, we aimed at delineating the sensitive territory of trigeminal nerves innervating the nasal mucosa. The different trigeminal nerves (anterior ethmoidal, posterior nasal and infraorbital nerves) were dissected in the orbit. Activity of these nerves was recorded during spontaneous nasal and tracheal breathing and in response to various stimuli: mechanical (manual probing and air jets) and irritants (ammonia vapours). Multiple and unitary activity recorded in nerve filaments enabled a classification of the receptors on the basis of their discharge pattern as rapidly-, intermediately- or slowly adapting receptors, and as drive or non-drive nasal receptors depending on whether or not the respiratory modulation was preserved during tracheal breathing.

Response of laryngeal receptors to water solutions of different osmolality and ionic composition

Water-responsive laryngeal receptors with fibres in the superior laryngeal nerve were studied to characterize the specific physicochemical properties of aqueous solutions that activate these endings. The responses to water (37 degrees C) of 141 receptors were studied in 39 anaesthetized dogs breathing through a tracheostomy with the larynx functionally isolated. Of the 89 receptors stimulated by water, 53 were also challenged with isosmotic (275-315 mOsm) solutions of dextrose and sodium gluconate at 37 degrees C. Receptors that only responded to water (n = 31) with a long delay, long duration discharge were generally respiratory modulated. On the other hand, laryngeal receptors that responded to all test solutions (n = 22) with a short delay, short duration discharge were generally not respiratory modulated. We conclude that the former type of receptor responds to lower osmolality, whereas the latter responds to the lack of chloride ions in the test solutions. These two types of receptor may be responsible for the cough and bronchoconstriction induced by inhaled aerosols of different osmolalities and ionic compositions.

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.

Nasal 'flow' receptors of the rat

This study was performed to identify trigeminal nasal 'flow' receptors and to investigate their firing characteristics. For this purpose, single unit afferent activity was recorded from the anterior ethmoidal nerve in anesthetized rats breathing through the nose or a tracheostomy. In fourteen rats breathing through the nose, 40 of 73 endings tested were identified as 'flow' receptors for the following characteristics: their spontaneous activity had an inspiratory modulation that disappeared during nasal occlusions, they were markedly stimulated by exposure to cold air and inhibited by warm air. In eleven rats breathing through a tracheostomy, 85 endings were identified as 'flow' receptors being stimulated by a constant nasal airflow (100-300 ml/min) with room air (22-26 degrees C) or cold air (0-15 degrees C), but inhibited with warm air (30-45 degrees C). Fifty-five 'flow' receptors (Type R1 and R2) exhibited a dynamic response to the constant airflow, while the other 30 receptors (Type S) showed a static response. A large proportion of 'flow' receptors (more than 52%) were responsive to tactile stimuli. For all the flow receptors, a decrease in intranasal temperature was the primary factor to excite them. These results suggest that the trigeminal nerve has a number of 'flow' receptors which operate as thermoreceptors.