W. M. St John

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur ‘automatically’ without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of ‘pacemaker’ neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a ‘superfused’ brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this ‘superfused’ preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is presented that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

Modeling the ponto-medullary respiratory network.

The generation and shaping of the respiratory motor pattern are performed in the lower brainstem and involve neuronal interactions within the medulla and between the medulla and pons. A computational model of the ponto-medullary respiratory network has been developed by incorporating existing experimental data on the medullary neural circuits and possible interactions between the medulla and pons. The model reproduces a number of experimental findings concerning alterations of the respiratory pattern following various perturbations/stimulations applied to the pons and pulmonary afferents. The results of modeling support the concept that eupneic respiratory rhythm generation requires contribution of the pons whereas a gasping-like rhythm (and the rhythm observed in vitro) may be generated within the medulla and involve pacemaker-driven mechanisms localized within the medullary pre-Botzinger Complex. The model and experimental data described support the concept that during eupnea the respiration-related pontine structures control the medullary network mechanisms for respiratory phase transitions, suppress the intrinsic pacemaker-driven oscillations in the pre-BotC and provide inspiration-inhibitory and expiration-facilitatory reflexes which are independent of the pulmonary Hering-Breuer reflex but operate through the same medullary phase switching circuits.

Medullary loci critical for expression of gasping in adult rats.

1. Our purpose was to define whether a region of medulla could be identified that is critical for the expression of gasping. 2. Decerebrate, vagotomized, paralysed and ventilated adult rats were used. The pattern of phrenic activity was reversibly altered from eupnoea to gasping by exposure to hypoxia or anoxia. 3. Gasping was irreversibly eliminated following unilateral electrolytic lesions of the lateral tegmental field of the medulla. The eupnoeic rhythm continued after these lesions. 4. Injections of kainic acid into the lateral tegmental field also eliminated gasping. Phrenic activity in eupnoea was not altered. 5. Lesions outside the lateral tegmental field caused marked changes in the eupnoeic rhythm, including expiratory apnoea. Upon exposure to hypoxia or anoxia, gasping was still induced. 6. This region for the neurogenesis of gasping in rats is identical to the region that serves a comparable function in cats. Moreover, it overlaps with the ‘pre‐Bötzinger’ complex which has been described for the in vitro brainstem preparation of the neonatal rat. Our results raise doubts that this complex plays a role in the neurogenesis of eupnoea.

Characterizations and comparisons of eupnoea and gasping in neonatal rats.

1. Our purpose was to characterize the ventilatory patterns of eupnoea and gasping in the neonatal rat. This study was precipitated by reports, using in vitro brainstem spinal cord preparations, that only a single pattern is present in neonatal rats. 2. In anaesthetized or decerebrate rat pups aged less than 13 days, eupnoea was characterized by a sudden onset of inspiratory activity and then a more gradual rise to peak levels. Following vagotomy, frequency fell and peak phrenic activity and tidal volume increased. The rate of rise of inspiratory activity also rose, but peak levels were still achieved during the latter half of inspiration. Vagal efferent activity exhibited bursts during both inspiration and the early expiration. This basic eupnoeic rhythm was not altered after sectioning of the carotid sinus nerves. 3. Upon exposure to hypoxia or anoxia, phrenic activity, tidal volume and frequency initially increased and then declined. In many animals, ventilatory activity then ceased, but later returned with a gasping pattern. 4. Gasping was characterized by a sudden onset of phrenic activity, which reached a peak intensity during the early portion of inspiration. The expiratory burst of vagal activity was eliminated. 5. Reductions of body temperature from 37 to 27 degrees C resulted in prolongations of inspiration and expiration and decreases of phrenic amplitude; phasic phrenic activity completely disappeared in some animals. Upon exposure to anoxia, gasping was observed, even in animals in which phrenic activity had disappeared in hyperoxia. 6. We conclude that, from the day of birth, rats can exhibit eupnoea and gasping patterns which are very similar to those of adult animals. 7. The rhythmic neural activities of the in vitro brainstem‐spinal cord preparation, reported by others, differ markedly from eupnoea but are identical with gasping. We therefore conclude that this preparation is not suitable for investigation of the mechanisms that generate eupnoeic breathing.

Influence of lung volume on phrenic, hypoglossal and mylohyoid nerve activities

In decerebrate, paralyzed cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, we examined the influence of lung volume on the activities of the phrenic, hypoglossal and mylohyoid nerves. When lung inflation was briefly withheld, the durations of inspiration (TI) and expiration (TE) and the activities of all three nerves increased. The relative increase in hypoglossal activity greatly exceeded that of phrenic activity and was apparent earlier in the course of inspiration. This hypoglossal response was enhanced by hypercapnia and isocapnic hypoxia. The responses of mylohyoid activity were quite variable: withholding lung inflation augmented inspiratory activity in some cats, but expiratory discharge in others. Sustained increases in end-expiratory lung volume were induced by application of 3-4 cm H2O of positive end-expiratory pressure (PEEP). Steady-state PEEP did not influence nerve activities or the breathing pattern. Bilateral vagotomy increased TI, TE, and the activities of all three nerves. No response to withoholding lung inflation could be discerned after vagal section. The results provide further definition of the influence of vagally mediated, lung volume dependent reflexes on the control of upper airway muscles. These reflexes are well suited to relieve or prevent upper airway obstruction.

Phrenic response to hypercapnia in the unanesthetized, decerebrate, newborn rat

We developed a decerebrate, vagotomized, newborn rat preparation to investigate brainstem respiratory control mechanisms without the influence of anesthesia, supra-pontine structures, or vagally mediated feedback mechanisms. We measured the changes in phrenic nerve electrical activity in response to breathing 3% and 5% CO2 in unanesthetized, vagotomized, decerebrate newborn rats from 0 to 10 days of age and compared them with the changes in anesthetized, vagotomized, newborn rats and adult, vagotomized, decerebrate or anesthetized, animals. Phrenic nerve activity was irregular in the young newborn rats and became more regular between 7 and 10 days of age. T1 and T1/Ttot increased with age but increasing age had no influence on the response to CO2. The response to CO2 was dominated by increases in phrenic amplitude, minute activity, and inspiratory slope with no change in timing variables. These responses are similar to those that have been reported previously in vagally intact animals, suggesting that vagal feedback contributes little to the response to hypercapnia in the newborn rat. In summary, decerebrate newborn rats consistently respond to hypercapnia by increasing inspiratory drive similar to conscious animals.

The functional expression of a pontine pneumotaxic centre in neonatal rats.

1. Our purpose was to determine whether a pneumotaxic centre could be localized to the rostral pons in newborn rats. We recorded efferent activity of the phrenic nerve in decerebrate, paralysed, vagotomized and ventilated rats, whose age varied from the day of birth to 22 days. 2. The rostral pontine tegmentum was ablated by aspiration and electrolytic lesions. Neuronal activities were blocked by microinjections of the glutamate antagonist MK‐801 and were destroyed by the neurotoxins kainic acid and domoic acid. 3. Unilateral ablation or lesions of the pontine tegmentum caused a significant prolongation of the duration of the phrenic burst in animals of all ages. This duration increased further following contralateral destruction and apneusis was established. The period between phrenic bursts increased in most rats whereas peak phrenic height was not consistently altered. 4. Similar changes to those following physical ablations or lesions were recorded after microinjections of MK‐801 or neurotoxins. 5. A common region of ablation, lesion and microinjection was the parabrachialis and Köllicker‐Fuse nucleus. 6. Exposure to anoxia resulted in an alteration from apnoeusis to gasping. 7. We conclude that from the day of birth, rostral pontine pneumotaxic mechanisms play a significant role in the definition of eupnoea. Moreover, from the day of birth, rats can exhibit the classical ventilatory patterns of eupnoea, apneusis and gasping.

Generation of the respiratory rhythm: state-dependency and switching

Abstract Two alternative concepts have been offered to explain the neural mechanisms responsible for the generation of the respiratory motor pattern in the brainstem: a network paradigm and a hybrid pacemaker-network paradigm. Our computational and experimental studies were aimed at “building a bridge” between these concepts and considering the conditions that may define switching from one mechanism of rhythmogenesis to the other.

Power spectral analysis of respiratory responses to pharyngeal stimulation in cats: comparisons with eupnoea and gasping.

1. Based on similarities between properties of gasping and the aspiration reflex, we hypothesized that this reflex activates the central pattern generator for gasping. To evaluate this hypothesis, we have analysed high‐frequency oscillations in phrenic and hypoglossal neural activities. These oscillations, analysed by power and coherence spectra, are considered as signatures of the central pattern generators for automatic ventilatory activity. 2. In decerebrate, vagotomized, paralysed and ventilated cats, the aspiration reflex was elicited in eupnoea and gasping by mechanical stimulation of the pharynx and electrical stimulation of the glossopharyngeal nerve. 3. Compared with eupnoeic values, the peaks in the power spectra occurred at higher frequencies in spontaneous gasping. Peaks in the coherence spectra showed identical changes. 4. Power and coherence spectra of inspiratory neural activities during the aspiration reflex differed markedly from those of eupnoea, but were similar to those in gasping. 5. We conclude that mechanical stimulation of the pharynx or electrical stimulation of the glossopharyngeal nerve activates a reflex by which the central pattern generator for eupnoea is depressed, and that for gasping is activated. Our results also support the concept that separate brainstem mechanisms generate ventilatory activity in eupnoea and gasping.

Adaptation of pulmonary stretch receptor in different mammalian species

A consequence of the adaptation of pulmonary stretch receptors is that their pattern of discharge depends on both lung volume and its rate of change. For adaptation dependent, flow related information to modify the breathing pattern by a feedback process, appreciable receptor adaptation must take place during a single respiratory cycle. Moreover, for flow related information to have the same significance in all animals, the time course of stretch receptor adaptation would have to vary among species, being more rapid in small animals, with high respiratory frequencies. We recorded single fiber action potentials from 84 pulmonary stretch receptors in six species of mammals, ranging from hamster to dog. The response of the receptors to maintained transpulmonary pressures was similar in the different species,as was the time course of receptor adptation following sudden lung inflations to 5 and 10 cm H2O. These findings show that the behavior of pulmonary stretch receptors is similar in animals with widely differing respiratory frequencies, and that the significance of receptor adaptation for regulation of the breathing pattern must vary interspecifically.