Influence of pulmonary inflations on discharge of pontile respiratory neurons

The purpose of this study was to characterize the influence of pulmonary inflations on the discharge patterns of rostral pontile respiratory neurons. Decerebrate and paralyzed cats were ventilated with a servo-respirator which produced patterns of pulmonary inflation, assessed by tracheal pressure, which paralleled alterations in integrated activity of the phrenic nerve. Neurons with respiratory-modulated neuronal activities were recorded in the pneumotaxic region of the nucleus parabrachialis medialis and Kolliker-Fuse nucleus, as well as in the trigeminal motor nucleus. Approximately equal numbers of neurons had phasic and tonic respiratory-modulated discharge patterns. The discharge patterns of most neurons were not qualitatively altered when pulmonary inflation was prevented. However, withholding inflation did cause the recruitment of some respiratory-modulated neuronal activities. Similar findings were obtained in normocapnia and hypercapnia. Results support the concept that the discharge of neurons in the pneumotaxic region may exert phasic, as well as tonic, influences on ventilatory activity.     

Characterization of expiratory intercostal activity to triangularis sterni in cats

Our purpose was to characterize activity of the intercostal nerve branch innervating the triangularis sterni muscle and the motoneuronal activities comprising this nerve discharge. In decerebrate, vagotomized, paralyzed, and ventilated cats, phasic triangularis sterni neural activity was evident in normocapnia. In most cats, activity did not commence until midexpiration. Activity then rose progressively to terminate at end expiration. Peak neural activities increased in parallel with phrenic activity in hypercapnia and fell in hypocapnia. The progressive increase in triangularis sterni neural activity within each respiratory cycle resulted from recruitment of motoneuronal activities throughout expiration. Once recruited, many motoneurons had a decrementing or constant discharge frequency. In hypercapnia, motoneuronal discharge frequencies increased, and additional activities were recruited. The number of active motoneurons and their discharge frequencies fell in hypocapnia. A similar pattern of motoneuronal activities and responses to stimuli was observed in cats with intact vagi. Factors are considered that may underlie the recruitment pattern of triangularis sterni motoneuronal activities and the inhibition of these in early expiration.     

Tonic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

The perfused in situ juvenile rat preparation produces phrenic discharge patterns comparable to eupnea and gasping in vivo. These ventilatory patterns of eupnea and gasping differ in multiple aspects, including most prominently the rate of rise of inspiratory activity. Because gasping, but not eupnea, appeared similar after vagotomy in spontaneous breathing preparations, it has been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, efferent activity of the phrenic nerve was recorded during eupnea and gasping in the in situ juvenile rat preparation. Gasping was induced in hypoxic-hypercapnia or ischemia. An increase in the pressure of tonic lung inflation from 1 to 10 cmH2O caused a prolongation of the duration between phrenic bursts in both eupnea or gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation and that the brain stem circuitry generating the respiratory patterns responds to tonic activation of pulmonary stretch receptors in a similar manner in eupnea and gasping. These findings support the homology of eupnea-like phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.     

Changes in strength of lung inflation reflex during prolonged inflation.

Recovery from respiratory inhibition produced by the lung inflation reflex was studied in anesthetized dogs, paralyzed and ventilated with a respiratory pump. During constant ventilation the lungs were periodically inflated using positive end-expiratory pressure, while the respiratory motor output was monitored in the phrenic nerve. Inhibition of the phrenic discharge was followed by gradual recovery throughout 8-min inflation periods despite constant blood gases. Recording afferent potentials in a vagus nerve indicated that adaptation of pulmonary stretch receptors contributed to the initial recovery of the phrenic discharge, but this recovery continued after the receptor discharge had stabilized. The phrenic discharge also recovered after initial inhibition in two situations which avoided stretch receptor adaptation: a) when the stretch receptor discharge from the separate lungs was alternated in an overlapping manner by asynchronous pulmonary ventilation, and b) during continuous electrical stimulation of a vagus nerve. Phrenic activity was temporarily increased above its control value after periods of lung inflation, asynchronous ventilation and vagal stimulation. It is concluded that the lung inflation reflex gradually attenuates during prolonged stimulation due to both stretch receptor adaptation and changes within the central pathways.     

Influence of pulmonary inflations on discharge patterns of phrenic motoneurons

The purpose of this study was to assess the influence of pulmonary inflations on activities of single phrenic motoneurons. Studies were performed in decerebrate and paralyzed cats; activities of phrenic nerve and single phrenic motoneurons were recorded. Animals were ventilated with a servo-respirator which produced alterations in tracheal pressure in parallel with changes in integrated activity of the phrenic nerve. At end-tidal fractional concentrations of CO2 of 0.05, phrenic motoneurons were distributed into "early" and "late" populations, depending on time of onset of activity. During the late stages of neural inspiration, differences in levels of integrated activity of the phrenic nerve became evident between cycles with and without lung inflations. At a time approximating 90% of the inspiratory duration during inflations, integrated phrenic activity was higher for cycles with inflation. Concomitantly, with lung inflations, the discharge frequencies of early phrenic motoneurons were lower, and late motoneurons began to discharge sooner than when inflations were withheld. Similar results were obtained in hypercapnia. We conclude that reflexes activated by pulmonary inflations may produce augmentation, as well as inhibition of phrenic motoneuronal activities. Factors responsible for eliciting these reflex augmentations and inhibitions are discussed.     

Triangularis sterni: a primary muscle of breathing in the dog

The isolated action, pattern of neural activation, and mechanical contribution to eupnea of the triangularis sterni (transversus thoracis) muscle were studied in supine anesthetized dogs. Linear displacement transducers were used to measure the axial displacements of the ribs and sternum. Tetanic stimulation of the triangularis sterni in the apneic animal caused a marked caudal displacement of the ribs, a moderate cranial displacement of the sternum, and a decrease in lung volume. During quiet breathing, there was invariably a rhythmic activation of the muscle in phase with expiration that was independent of the presence or absence of activity in the abdominal and internal interosseous intercostal muscles. This phasic expiratory activity in the triangularis sterni was of large amplitude and caused the ribs to be more caudal and the sternum to be more cranial during the spontaneous expiratory pause than during relaxation. Additional studies on awake animals showed that rhythmic activation of the triangularis sterni occurs in all body positions and is not caused by anesthesia. These findings indicate that expiration in the dog is not a passive process and that the end-expiratory volume of the rib cage is not determined by an equilibrium of static forces alone. Rather, it is actively determined and maintained below its relaxation volume by contraction of the triangularis sterni throughout expiration. The use of this muscle is likely to facilitate inspiration by increasing the length of the parasternal intercostals and taking on a portion of their work.     

Expiratory neural activities in gasping

The purpose was to characterize expiratory-related neural activities in eupnea and gasping. In decerebrate and vagotomized cats, activities were recorded from the phrenic nerve, spinal intercostal and abdominal nerves, and recurrent laryngeal nerve and its branches. Neural inspiration was defined by phrenic discharge. The spinal and laryngeal nerves discharged in inspiration, expiration, or during both phases. Gasping was induced by freezing the brain stem at the pontomedullary junction, exposure to asphyxia or anoxia, or ligation of the basilar artery and its branches. In gasping, peak phrenic activity typically increased as did inspiratory-related activities of laryngeal and spinal nerves. Expiratory activities were greatly reduced in gasping, with some activities being completely eliminated. Reductions of expiratory activity were more prominent for spinal than laryngeal nerves. Similar results were obtained in cats having intact vagi that were ventilated with a servo-respirator so that lung inflation paralleled phrenic activity. The concept that gasping differs fundamentally form other ventilatory patterns is discussed.     

Apnoeic responses to intracarotid nicotine challenge in anaesthetized cats.

To determine the effects of an intraarterial administration of nicotine on the occurrence of apnoea and the activity of rib cage respiratory muscles, we studied 31 anaesthetized, spontaneously breathing cats. Phrenic activity was used as an index of neural inspiratory drive. Activity of parasternal intercostal (PIM) and triangularis sterni (TS) muscles was recorded. Nicotine in a dose of 65 microg/kg was injected into the left common carotid artery prior to and after midcervical vagotomy, preceded by section of the superior laryngeal nerves (SLNs). In eight additional cats, initially neurotomized as mentioned, nicotine was injected after bilateral disruption of the carotid sinus nerves (CSNs). Nicotine induced prompt expiratory apnoea of mean duration of 5.4+/-0.3s in 19 non-vagotomized and of 5.92+/-0.51 s (mean+/-S.E.M.) in 13 vagotomized cats. The occurrence and duration of the temporary arrest of breathing were reduced by midcervical vagotomy but not by subsequent CSNs neurotomy, which abolished post-apnoeic acceleration of breathing.In post-nicotine breathing of increased tidal volume and respiratory rate, peak activity of the parasternal intercostal muscles increased from baseline of 3.2+/-1.2 to 9.5+/-2.0 arbitrary units (p<0.001). The peak height of the phrenic nerve elevated from 7.9+/-0.9 to 14.5+/-1.7 arbitrary units (p<0.001). That of the triangularis sterni showed no change.The response of the respiratory effectors elicited by nicotine was independent of the vagal integrity and may be attributed to activation of nicotine receptors within the brainstem respiratory neurones.     

Phasic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

The perfused in situ juvenile rat preparation produces patterns of phrenic discharge comparable to eupnea and gasping in vivo. These ventilatory patterns differ in multiple aspects, including most prominently the rate of rise of inspiratory activity. Although we have recently demonstrated that both eupnea and gasping are similarly modulated by a Hering-Breuer expiratory-promoting reflex to tonic pulmonary stretch, it has generally been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, we recorded eupneic and gasplike efferent activity of the phrenic nerve in the in situ juvenile rat perfused brain stem preparation, with and without phrenic-triggered phasic pulmonary inflation. We tested the hypothesis that phasic pulmonary inflation produces reflex responses in situ akin to those in vivo and that both eupnea and gasping are similarly modulated by phasic pulmonary stretch. In eupnea, we found that phasic pulmonary inflation decreases inspiratory burst duration and the period of expiration, thus increasing burst frequency of the phrenic neurogram. Phasic pulmonary inflation also decreases the duration of expiration and increases the burst frequency during gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation and that the brain stem circuitry generating the respiratory patterns respond to phasic activation of pulmonary stretch receptors in both eupnea and gasping. These findings support the homology of eupneic phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.     

Inspiratory rhythm in airway smooth muscle tone

In anesthetized paralyzed open-chested cats ventilated with low tidal volumes at high frequency, we recorded phrenic nerve activity, transpulmonary pressure (TPP), and either the tension in an upper tracheal segment or the impulse activity in a pulmonary branch of the vagus nerve. The TPP and upper tracheal segment tension fluctuated with respiration, with peak pressure and tension paralleling phrenic nerve activity. Increased end-tidal CO2 or stimulation of the carotid chemoreceptors with sodium cyanide increased both TPP and tracheal segment tension during the increased activity of the phrenic nerve. Lowering end-tidal CO2 or hyperinflating the lungs to achieve neural apnea (lack of phrenic activity) caused a decrease in TPP and tracheal segment tension and abolished the inspiratory fluctuations. During neural apnea produced by lowering end-tidal CO2, lung inflation caused no further decrease in tracheal segment tension and TPP. Likewise, stimulation of the cervical sympathetics, which caused a reduction in TPP and tracheal segment tension during normal breathing, caused no further reduction in these parameters when the stimulation occurred during neural apnea. During neural apnea the tracheal segment tension and TPP were the same as those following the transection of the vagi or the administration of atropine (0.5 mg/kg). Numerous fibers in the pulmonary branch of the vagus nerve fired in synchrony with the phrenic nerve. Only these fibers had activity which paralleled changes in TPP and tracheal tension. We propose that the major excitatory input to airway smooth muscle arises from cholinergic nerves that fire during inspiration, which have preganglionic cell bodies in the ventral respiratory group in the region of the nucleus ambiguus and are driven by the same pattern generators that drive the phrenic and inspiratory intercostal motoneurons.     

Effect of phrenic afferent stimulation on pattern of respiratory muscle activation.

Ventilation and electromyogram (EMG) activities of the right hemidiaphragm, parasternal intercostal, triangularis sterni, transversus abdominis, genioglossus, and alae nasi muscles were measured before and during central stimulation of the left thoracic phrenic nerve in 10 alpha-chloralose anesthetized vagotomized dogs. Pressure in the carotid sinuses was fixed to maintain baroreflex activity constant. The nerve was stimulated for 1 min with a frequency of 40 Hz and stimulus duration of 1 ms at voltages of 5, 10, 20, and 30 times twitch threshold (TT). At five times TT, no change in ventilation or EMG activity occurred. At 10 times TT, neither tidal volume nor breathing frequency increased sufficiently to reach statistical significance, although the change in their product (minute ventilation) was significant (P less than 0.05). At 20 and 30 times TT, increases in both breathing frequency and tidal volume were significant. At these stimulus intensities, the increases in ventilation were accompanied by approximately equal increases in the activity of the diaphragm, parasternal, and alae nasi muscles. The increase in genioglossus activity was much greater than that of the other inspiratory muscles. Phrenic nerve stimulation also elicited inhomogeneous activation of the expiratory muscles. The transversus abdominis activity increased significantly at intensities from 10 to 30 times TT, whereas the activity of the triangularis sterni remained unchanged. The high stimulation intensities required suggest that the activation of afferent fiber groups III and IV is involved in the response. We conclude that thin-fiber phrenic afferent activation exerts a nonuniform effect on the upper airway, rib cage, and abdominal muscles and may play a role in the control of respiratory muscle recruitment.     

Breuer-Hering reflexes in ketamine-induced apneustic breathing in the rabbit

The effects of ketamine on the activities of the mylohyoid nerve (a branch of the Vth nerve) and of both phrenic nerves were investigated in rabbits anaesthetized with halothane, paralyzed and artificially ventilated. Intravenous administration of ketamine elicited a marked prolongation of the phrenic inspiratory discharge (without significantly affecting its amplitude) and a depression of the mylohyoid expiratory activity. An elimination of the volume-related input from the lungs ("no-inflation manoeuvre") or deflation elicited under these conditions typical apneustic pattern of breathing. The response to tracheal occlusion at peak-inspiration was "classical". We conclude that ketamine inhibits the Vth nerve motor nucleus which is not only an important component of the central inspiratory-inhibitory neurones but also a "relay station" between the vagal and the central inspiratory "off-switch" mechanisms.     

Characterization of respiratory-modulated activities of hypoglossal motoneurons

In decerebrate, vagotomized, paralyzed, and ventilated cats, activities of the phrenic nerve and single hypoglossal nerve fibers were monitored. The great majority of hypoglossal neuronal activities were inspiratory (I), discharging during a period approximating that of phrenic. Many were not active at normocapnia but were recruited in hypercapnia or hypoxia. Once recruited, discharge frequencies, which rose quickly to near maximal levels in early to midinspiration, significantly increased with further augmentations of drive. Also, the onset of activities became progressively earlier, compared with phrenic discharge, in hypercapnia or hypoxia. Smaller numbers of hypoglossal fiber activities, having inspiratory-expiratory (I-E), expiratory (E), expiratory-inspiratory (E-I), or tonic discharge patterns, were also recorded. Activities of E, I-E, and those I fibers that became I-E in high drive may underlie the early burst of expiratory activity of the hypoglossal nerve. It is concluded that the firing and recruitment patterns of hypoglossal neurons differ from those of phrenic motoneurons. However, responses to chemoreceptor stimuli are similar among the two neuronal groups.     

Vagal modulation of respiratory muscle activity in awake dogs during exercise and hypercapnia.

Using chronically instrumented awake tracheotomized dogs, we examined the contributions of vagal feedback to respiratory muscle activities, both electrical and mechanical, during normoxic hypercapnia (inspired CO2 fraction = 0.03, 0.04, 0.05, and 0.06) and during mild treadmill exercise (3, 4.3, and 6.4 km/h). Cooling exteriorized vagal loops eliminated both phasic and tonic mechanoreceptor input during either of these hyperpneas. At a given chemical or locomotor stimulus, vagal cooling caused a further increase in costal, crural, parasternal, and rib cage expiratory (triangularis sterni) muscles. No further change in abdominal expiratory muscle activity occurred secondary to vagal cooling during these hyperpneas. However, removal of mechanoreceptor input during hypercapnia was not associated with consistent changes in end-expiratory lung volume, as measured by the He-N2 rebreathe technique. We conclude that during these hyperpneas 1) vagal input is not essential for augmentation of expiratory muscle activity and 2) decrements in abdominal expiratory muscle activity may be offset by increments in rib cage expiratory muscle activity and contribute to the regulation of end-expiratory lung volume.     

Influence of lung volume on activities of branches of the recurrent laryngeal nerve

To investigate the influence of inspiratory lung inflation on the respiratory activities of laryngeal motor nerves, vagally intact decerebrate paralyzed cats were ventilated by a servorespirator in accordance with their own phrenic nerve activity. Records were made of the activities of the phrenic nerve, the superior laryngeal nerve (SLN), the recurrent laryngeal nerve (RLN), and the intralaryngeal branches of the RLN serving the thyroarytenoid (TA) and posterior cricoarytenoid (PCA) muscles. Neural activities were assessed in the steady state at different end-tidal O2 and CO2 concentrations. Transient responses to withholding inspiratory lung inflation and to preventing expiratory lung emptying were also studied. Hypercapnia and hypoxia increased the inspiratory activities of the phrenic nerve, SLN, RLN, and its PCA branch. TA inspiratory activity was not changed. Expiratory activities of RLN, PCA, and TA were all increased in hypoxia. When lung inflation was withheld, neural inspiratory duration and the inspiratory activities of all nerves increased. The subsequent period of neural expiration was marked by an exaggerated burst of activity by the TA branch of the RLN. TA expiratory activity was also sharply increased after inspiratory efforts that were reflexly delayed by the prevention of lung emptying. TA activity in expiration was enhanced after vagotomy and was usually more prominent than when lung inflation was withheld before vagal section. The results demonstrate the importance and complexity of the influence of vagal afferents on laryngeal motor activity.     

Preliminary results on the splanchnico-laryngeal reflex in the cat]

Electrical stimulation of low threshold splanchnic afferent nerves in lightly anesthetized cat results in phrenic and recurrent laryngeal nerve responses. Both phrenic and recurrent laryngeal inspiratory nerve activities are inhibited, whereas expiratory recurrent nerve activity is triggered and even increased. The significance of this reflex is discussed in relation to laryngeal adductor muscle contractions and the abdominal pressure increase.     

Influence of the ventral surface of the medulla on tracheal responses to CO2

These studies investigated the role of the intermediate area of the ventral surface of the medulla (VMS) in the tracheal constriction produced by hypercapnia. Experiments were performed in chloralose-anesthetized, paralyzed, and artificially ventilated cats. Airway responses were assessed from pressure changes in a bypassed segment of the rostral cervical trachea. Hyperoxic hypercapnia increased tracheal pressure and phrenic nerve activity. Intravenous atropine pretreatment or vagotomy abolished the changes in tracheal pressure without affecting phrenic nerve discharge. Rapid cooling of the intermediate area reversed the tracheal constriction produced by hypercapnia. Graded cooling produced a progressive reduction in the changes in maximal tracheal pressure and phrenic nerve discharge responses caused by hypercapnia. Cooling the intermediate area to 20 degrees C significantly elevated the CO2 thresholds of both responses. These findings demonstrate that structures near the intermediate area of the VMS play a role in the neural cholinergic responses of the tracheal segment to CO2. It is possible that neurons or fibers in intermediate area influence the motor nuclei innervating the trachea. Alternatively, airway tone may be linked to respiratory motor activity so that medullary interventions that influence respiratory motor activity also alter bronchomotor tone.     

Internal intercostal nerve discharges in the cat: influence of chemical stimuli

We studied the influence of central and peripheral chemoreceptor stimulation on the activities of the phrenic and internal intercostal (iic) nerves in decerebrate, vagotomized, and paralyzed cats with bilateral pneumothoraces. Whole iic nerves of the rostral thorax (T2-T5) usually discharged during neural inspiration, whereas those of the caudal thorax (T7-T11) were primarily active during neural expiration. Filaments of rostral iic nerves that terminated in iic muscles generally discharged during expiration, suggesting that inspiratory activity recorded in whole iic nerves may have innervated other structures, possibly parasternal muscles. All nerves were phasically active at hyperoxic normocapnia and increased their activities systematically with hypercapnia. Isocapnic hypoxia or intra-arterial NaCN injection consistently increased phrenic and inspiratory iic nerve activities. In contrast, expiratory iic nerve discharges were either decreased (10 cats) or increased (7 cats) by hypoxia. Furthermore, expiratory responses to NaCN were highly variable and could not be predicted from the corresponding response to hypoxia. The results show that central and peripheral chemoreceptor stimulation can affect inspiratory and expiratory motoneuron activities differentially. The variable effects of hypoxia on expiratory iic nerve activity may reflect a relatively weak influence of carotid body afferents on expiratory bulbospinal neurons. However, the possibility that the magnitude of expiratory motoneuron activity is influenced by the intensity of the preceding centrally generated inspiratory discharge is also discussed.     

Effects of pulse lung inflation on chest wall expiratory motor activity.

The effects of pulse lung inflation (LI) on expiratory muscle activity and phase duration (Te) were determined in anesthetized, spontaneously breathing dogs (n = 20). A volume syringe was used to inflate the lungs at various times during the expiratory phase. The magnitude of lung volume was assessed by the corresponding change in airway pressure (Paw; range 2-20 cmH(2)O). Electromyographic (EMG) activities were recorded from both thoracic and abdominal muscles. Parasternal muscle EMG was used to record inspiratory activity. Expiratory activity was assessed from the triangularis sterni (TS), internal intercostal (IIC), and transversus abdominis (TA) muscles. Lung inflations <7 cmH(2)O consistently inhibited TS activity but had variable effects on TA and IIC activity and expiratory duration. Lung inflations resulting in Paw values >7 cmH(2)O, however, inhibited expiratory EMG activity of each of the expiratory muscles and lengthened Te in all animals. The responses of expiratory EMG and Te were directly related to the magnitude of the lung inflation. The inhibition of expiratory motor activity was independent of the timing of pulse lung inflation during the expiratory phase. The inhibitory effects of lung inflation were eliminated by bilateral vagotomy and could be reproduced by electrical stimulation of the vagus nerve. We conclude that pulse lung inflation resulting in Paw between 7 and 20 cmH(2)O produces a vagally mediated inhibition of expiratory muscle activity that is directly related to the magnitude of the inflation. Lower inflation pressures produce variable effects that are muscle specific.     

Vagal contributions to respiratory muscle activity during eupnea in the awake dog.

We examined the effects of reversible vagal cooling on respiratory muscle activities in awake chronically instrumented tracheotomized dogs. We specifically analyzed electromyographic (EMG) activity and its ventilatory correlates, end-expiratory lung volume (EELV) and diaphragmatic resting length via sonomicrometry. Elimination of phasic and tonic mechanoreceptor activity by vagal cooling doubled the EMG activity of the costal, crural, and parasternal muscles, with activation occurring sooner relative to the onset of inspiratory flow. Diaphragmatic postinspiration inspiratory activity in the intact dog coincided with a brief mechanical shortening of the diaphragm during early expiration; vagal blockade removed both the electrical activity and the mechanical shortening. Vagal blockade also doubled the EMG activity of a rib cage expiratory muscle, the triangularis sterni, but reduced that of an abdominal expiratory muscle, the transversus abdominis. Within-breath electrical activity of both muscles occurred sooner relative to the onset of expiratory flow during vagal blockade. Vagal cooling was also associated with a 12% increase in EELV and a 5% decrease in end-expiratory resting length of the diaphragm. We conclude that vagal input significantly modulates inspiratory and expiratory muscle activities, which help regulate EELV efficiently and optimize diaphragmatic length during eupneic breathing in the awake dog.