Breathing pattern in rats with chronic section of the superior laryngeal nerves

A review of the peripheral proprioceptive apparatus in the larynx

The larynx is an organ of the upper airway that participates in breathing, glutition, voice production, and airway protection. These complex functions depend on vocal fold (VF) movement, facilitated in turn by the action of the intrinsic laryngeal muscles (ILM). The necessary precise and near-instantaneous modulation of each ILM contraction relies on proprioceptive innervation of the larynx. Dysfunctional laryngeal proprioception likely contributes to disorders such as laryngeal dystonia, dysphagia, vocal fold paresis, and paralysis. While the proprioceptive system in skeletal muscle derived from somites is well described, the proprioceptive circuitry that governs head and neck structures such as VF has not been so well characterized. For over two centuries, researchers have investigated the question of whether canonical proprioceptive organs, muscle spindles, and Golgi tendon organs, exist in the ILM, with variable findings. The present work is a state-of-the-art review of the peripheral component of laryngeal proprioception, including current knowledge of canonical and possible alternative proprioceptive circuitry elements in the larynx.

Superior laryngeal and hypoglossal afferents tonically influence upper airway motor excitability in anesthetized rats

Upper airway (UA) muscle activity is stimulated by changes in UA transmural pressure and by asphyxia. These responses are reduced by muscle relaxation. We hypothesized that this is due to a change in afferent feedback in the ansa hypoglossi and/or superior laryngeal nerve (SLN). We examined 1) the glossopharyngeal motor responses to UA transmural pressure and asphyxia and 2) how these responses were changed by muscle relaxation in animals where one or both of these afferent pathways had been sectioned bilaterally. Experiments were performed in 24 anesthetized, thoracotomized, artificially ventilated rats. Baseline glossopharyngeal activity and its response to UA transmural pressure and asphyxia were moderately reduced after bilateral section of the ansa hypoglossi (P < 0.05). Conversely, bilateral SLN section increased baseline glossopharyngeal activity, augmented the response to asphyxia, and abolished the response to UA transmural pressure. Muscle relaxation reduced resting glossopharyngeal activity and the response to asphyxia (P < 0.001). This occurred whether or not the ansa hypoglossi, the SLN, or both afferent pathways had been interrupted. We conclude that ansa hypoglossi afferents tonically excite and SLN afferents tonically inhibit UA motor activity. Muscle relaxation depressed UA motor activity after section of the ansa hypoglossi and SLN. This suggests that some or all of the response to muscle relaxation is mediated by alterations in the activity of afferent fibers other than those in the ansa hypoglossi or SLN.

Ventilatory responses to changes in temperature in mammals and other vertebrates

This article reviews the relationship between pulmonary ventilation (VE) and metabolic rate (oxygen consumption) during changes in ambient temperature. The main focus is on mammals, although for comparative purposes the VE responses of ectothermic vertebrates are also discussed. First, the effects of temperature on pulmonary mechanics, chemoreceptors, and airway receptors are summarized. Then we review the main VE responses to cold and warm stimuli and their interaction with exercise, hypoxia, or hypercapnia. In these cases, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation. Finally, we consider the VE responses of mammals when body temperature changes, as during torpor, fever, sleep, and hypothermia. In ectotherms, during changes in temperature, VE control becomes part of a general strategy to maintain constant relative alkalinity and ensure a constancy of pH-dependent protein functions (alphastat regulation). In mammals on the other hand, VE control is aimed to balance metabolic needs with homeothermy. Therefore, alphastat regulation in mammals seems to have a low priority, and it may be adopted only in exceptional cases.

Superior laryngeal nerve section alters responses to upper airway distortion in sleeping dogs

We investigated the effect of superior laryngeal nerve (SLN) section on expiratory time (TE) and genioglossus electromyogram (EMGgg) responses to upper airway (UA) negative pressure (UANP) in sleeping dogs. The same dogs used in a similar intact study (C. A. Harms, C. A., Y.-J. Zeng, C. A. Smith, E. H. Vidruk, and J. A. Dempsey. J. Appl. Physiol. 80: 1528-1539, 1996) were bilaterally SLN sectioned. After recovery, the UA was isolated while the animal breathed through a tracheostomy. Square waves of negative pressure were applied to the UA from below the larynx or from the mask (nares) at end expiration and held until the next inspiratory effort. Section of the SLN increased eupneic respiratory frequency and minute ventilation. Relative to the same dogs before SLN section, sublaryngeal UANP caused less TE prolongation while activation of the genioglossus required less negative pressures. Mask UANP had no effect on TE or EMGgg activity. We conclude that the SLN 1) is not obligatory for the reflex prolongation of TE and activation of EMGgg activity produced by UANP and 2) plays an important role in the maintenance of UA stability and the pattern of breathing in sleeping dogs.

Effect of 2,4-dinitrophenol on the hypometabolic response to hypoxia of conscious adult rats

During acute hypoxia, a hypometabolic response is commonly observed in many newborn and adult mammalian species. We hypothesized that, if hypoxic hypometabolism were entirely a regulated response with no limitation in O2 availability, pharmacological uncoupling of the oxidative phosphorylation should raise O2 consumption (VO2) by similar amounts in hypoxia and normoxia. Metabolic, ventilatory, and cardiovascular measurements were collected from conscious rats in air and in hypoxia, both before and after intravenous injection of the mitochondrial uncoupler 2,4-dinitrophenol (DNP). In hypoxia (10% O2 breathing, 60% arterial O2 saturation), VO2, as measured by an open-flow technique, was less than in normoxia (approximately 80%). Successive DNP injections (6 mg/kg, 4 times) progressively increased VO2 in both normoxia and hypoxia by similar amounts. Body temperature slightly increased in normoxia, whereas it did not change in hypoxia. The DNP-stimulated VO2 during hypoxia could even exceed the control normoxic value. A single DNP injection (17 mg/kg iv) had a similar metabolic effect; it also resulted in hypotension and a drop in systemic vascular resistance. We conclude that pharmacological stimulation of VO2 counteracts the VO2 drop determined by hypoxia and stimulates VO2 not dissimilarly from normoxia. Hypoxic hypometabolism is likely to reflect a regulated process of depression of thermogenesis, with no limitation in cellular O2 availability.

The ventilatory and metabolic response to hypercapnia in newborn mammalian species

Conscious newborns of 12 species from 4 mammalian orders, ranging in body mass (M) from 1 g (mouse) to 5 kg (deer), were studied during air and during 5% CO2 breathing. The interspecies relationship between oxygen consumption (VO2) and M was the same in air and hypercapnia, in both cases VO2 alpha M 0.90; on average, hypercapnic VO2 was 101% of the air value. In 5% CO2, ventilation (VE) increased in all newborns, mostly because of the increase in tidal volume (178%), whereas breathing rates averaged 98% of the air values. The hyperpnea during CO2 was slightly greater in the larger newborns. Body temperature was not altered by CO2 breathing. We conclude that the average respiratory response of the newborn to moderate hypercapnia is a hyperventilation different from that of the neonatal mammal in acute hypoxia (Mortola et al., Respir. Physiol. 78: 31-43, 1989). In fact, hypercapnic hyperventilation resulted only from the hyperpnea, with no hypometabolic contribution, and the hyperpnea reflected the increase in tidal volume, with no change in rate. It is also concluded that the neonatal hypometabolic response is specific to hypoxia, and not an undifferentiated response to chemoreceptors stimulation.

Metabolism and ventilation in hypoxic rats: effect of body mass

Oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured by the flow-through method, and ventilation (VE) by the barometric technique in post-weaning age rats of 50, 100, 250 and 400 g, (5 males and 5 females in each group), at ambient temperature congruent to 24 degrees C. In normoxia, VO2, VCO2 and VE decreased with the increase in body weight (BW), whether normalization was by BW or by BW minus the weights of fat and skeleton; VE/VO2 and rectal temperature remained constant. In hypoxia (10% inspired O2), VE VO2 increased in all groups, to 2-2.5 times the normoxic values, because of a significant increase in VE (hyperpnea) and decrease in VO2 (hypometabolism); arterial PCO2, measured in some 100 g and 400 g rats, dropped similarly. However, the hyperpnea was about twice as large, and metabolism and body temperature decreased significantly less, in the 400 g than in the 50 g rats. The cost (ml O2) of breathing, computed in the paralysed animal artificially ventilated, averaged approximately 0.7% (normoxia) and 2% of VO2 (hypoxia), with no systematic differences with BW. The results agree with the concept that the metabolic response to hypoxia can be an important determinant of the magnitude of the hyperpnea.

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.

Hamsters versus rats: ventilatory responses in adults and newborns

Burrowing mammals often demonstrate structural and functional characteristics which could be interpreted as aspects of adaptation to the low PO2 and high PCO2 of their environment. Whether these characteristics are acquired by each individual as the result of life in the burrow or are genetic traits established through evolution is not clear. To examine the latter possibility aspects of ventilatory control were studied in newborns and adults of two rodents, the surface dwelling white rat (Rattus norvegicus, R) and the semifossorial syrian hamster (Mesocricetus auratus, H), both born and raised in normoxia. Adult H, but not R, manifested the burrowing preference whenever offered the opportunity. Adult H presented numerous differences from the adult R, including a higher normoxic oxygen consumption (VO2, +44%), higher hematocrit and heavier right heart. Ventilation (VE) was similar between the two species both in air and hyperoxia, although H had a deeper and slower pattern. Hence, resting VE/VO2 in H was low, probably contributing to the hypoxemia reported in this species. The acute ventilatory response to hypoxia (10 min 10% FIO2), was less in H, because of no increase in tidal volume. Most of the morphological and functional differences between adult hamsters and rats were not apparent between the newborns of the corresponding species. It is concluded that some of the physiological characteristics reported in burrowing species are also observed in adult hamsters which never lived in burrows, but not in the newborns; hence, they could represent genetic traits which are expressed postnatally with no needs of the special environmental conditions of the burrow.

Afferent projections of the superior and recurrent laryngeal nerves

The central afferent projections of the superior and recurrent laryngeal nerves were investigated in the rat, utilizing the transganglionic transport of WGA-HRP. Labelled superior laryngeal nerve terminal fields were found bilaterally in the interstitial and medial subnuclei of the nucleus tractus solitarii with the ipsilateral being more dense. The distribution of the recurrent laryngeal nerve terminals were similar to that of the SLN with two major differences: the projections were ipsilateral, and there was a marked decrease in the terminal field density. The terminal field density differences were confirmed by quantitatively identifying the labelled ganglion cells of the vagus nerve. These findings accurately delineate the first integrative components in the mediation of the complex laryngeal reflexes.

PGF2 alpha and breathing pattern in newborn pigs

The effect of PGF2 alpha has been evaluated in 11 unanaesthetized unrestrained piglets and in 3 anaesthetized piglets (2-3 days old) using a barometric-plethysmographic technique. PGF2 alpha (mg 0.25/pig) was administered as aerosol for 5 min. In 3 of the unanaesthetized newborn pigs the effect of PGF2 alpha aerosol has been evaluated after indomethacin (mg 1/Kg i.v.). The vagal dependent activity of the prostaglandin was also evaluated after atropine (mg 0.08/Kg i.m.). Our results show that PGF2 alpha in newborn pigs causes hypoventilation due to a decrease in respiratory rate and to a lengthening in TE. The changes in TE are due to an increase in the incidence and duration of apneic events characterizing the respiratory activity at birth. After indomethacin PGF2 alpha does not change the breathing pattern. Atropine only partially reduces the effects of PGF2 alpha while, after anaesthesia, prostaglandin does not change the breathing pattern. Consequently our results show that PGF2 alpha in newborn animals similar to other prostaglandins acts as a depressant of respiratory activity.

Ventilation and oxygen consumption during acute hypoxia in newborn mammals: a comparative analysis

We asked whether the lack of sustained hyperventilation during acute hypoxia, often reported to occur in the infant, is a common characteristic among newborn mammalian species, and to which extent inter-species differences may be accounted for by differences in metabolic responses. Ventilation (VE) and breathing pattern have been measured by flow-plethysmography or by the barometric method in normoxia and after 10 min of 10% O2 breathing in newborn mammals of 17 species over a 3 g to 20 kg range in body size. In 14 of these species oxygen consumption (VO2) has also been measured by a manometric technique or by calculation from the changes in chamber O2 pressure. VE and VO2 changed in proportion, among species, both in normoxia and hypoxia. In hypoxia, VE was higher, similar, or even lower than in normoxia, with some relation to the degree of maturity of the species at birth. In general, the small or absent VE responses to hypoxia resulted from small or no increase in tidal volume, while breathing frequency stayed elevated. The few departures from this pattern could be explained by interspecies differences in hypoxic sensitivity, since additional experiments in kittens and puppies indicated that, with more severe hypoxia, the pattern changed from rapid and shallow to deep and slow. In all cases, irrespective of the magnitude of the VE response, the VE/VO2 (and the mean inspiratory flow/VO2) increased during hypoxia, because the drop in VE, when present, was accompanied by an even larger drop in VO2. In fact, VO2 in hypoxia decreased in most species, although to variable degrees. Body temperature either did not change or decreased slightly, possibly indicating a trend toward a decrease of the set point of thermoregulation during hypoxia. In conclusion, the analysis gave further support to the concept that, during acute hypoxia, changes in metabolic rate play a paramount role in the ventilatory response of the newborn mammal.

Ventilation in kittens with chronic section of the superior laryngeal nerves

The breathing pattern of conscious newborn kittens one-to-two weeks old was studied by the barometric method about 5 days after bilateral section of the superior laryngeal nerve (SLN-denervated group) or a sham operation (SLN-sham operated group). None of the ventilatory variables differed between the two groups, whether during normoxia or acute hypoxia (10 min of 10% O2). After anesthesia, delivery of steady airflows in the expiratory direction through the upper airways of the SLN-sham operated had marked inhibitory effects on ventilation which entirely disappeared after SLN section. A small inhibition was still present in the SLN-denervated group, possibly indicating that other non-SLN upper airways receptors developed inhibitory ventilatory effects during the period of chronic denervation. Intermittent expiratory upper airway airflows were much less effective than steady flows and no inhibition was seen with oscillatory flows, indicating that the mode of application of the stimulus to the laryngeal receptors is crucial in determining the magnitude of their reflex response. Under anesthesia, acute bilateral section of the SLN determined a small increase of the integrated peak EMG activity of the diaphragm. We conclude that laryngeal SLN afferents are inhibitory on ventilation in newborn kittens, but this effect is very small during normal conscious conditions. Only under special circumstances, including anesthesia and sustained upper airways flows and pressures, the ventilatory inhibition can be disproportionately magnified.

Metabolic and ventilatory rates in newborn kittens during acute hypoxia

Newborns respond to acute hypoxia with a brief increase in ventilation (VE) followed by a decline toward the normoxic value. We asked to what extent this biphasic pattern is determined by changes in metabolic rate. In newborn awake kittens within the first week after birth we measured VE by the barometric method, oxygen consumption (VO2) and CO2 production (VCO2) by manometric techniques during air breathing and after 10 min of 10% O2 breathing. In hypoxia, VO2 and VCO2 dropped (about 45% and 35%, respectively) with a slight decrease in body temperature. VE was as in normoxia and, from the changes in breathing pattern, a drop in alveolar ventilation was calculated. Since the drop in VCO2 exceeded that in calculated alveolar ventilation (VA), a decrease in PACO2 (estimated at about 5 mm Hg) was expected. This decrement was confirmed by measurements of transcutaneous PCO2, which dropped an average of 3.3 mm Hg in the hypoxic kittens, even when VE was below the normoxic value. Hence, despite the decline in VE and VA during the biphasic response, the newborn continued to hyperventilate with respect to CO2 production, lowering PCO2. The strategy of dropping VO2 instead of maintaining the energetic requirements by increasing VE was not determined by mechanical constraints, since all kittens hyperventilated when exposed to CO2, either in air or hypoxia. Several implications of the tight coupling between ventilation and metabolism are discussed, including a possible role in the genesis of periodic breathing in hypoxia.