Alpha2 receptor binding in the medulla oblongata in the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is the leading cause of postnatal infant mortality in the United States. Its etiology remains unknown. We propose that SIDS, or a subset of SIDS, is due to a failure of autoresuscitation, a protective brainstem response to asphyxia or hypoxia, in a vulnerable infant during a critical developmental period. Gasping is an important component of autoresuscitation that is thought to be mediated by the "gasping center" in the lateral tegmentum of the medulla, a region homologous in its cytoarchitecture and chemical anatomy to the intermediate reticular zone (IRZ) in the human. Since we found that [3H]para-aminoclonidine ([3H]PAC) binding to alpha2-adrenergic receptors localizes to this region in human infants and, thereby provides a neurochemical marker for it, we tested the hypothesis that [3H]PAC binding to alpha2-adrenergic receptors is decreased in the IRZ in SIDS victims. Using quantitative tissue autoradiography with [3H]PAC as the radioligand and phentolamine as the displacer, we analyzed alpha2-receptor binding density in the IRZ, as well as in 7 additional sites for comparison, in 10 SIDS and 10 control medullae. There were no significant differences in alpha2 receptor binding in the IRZ, vagal nuclei, or other medullary sites examined between SIDS and control cases. These results suggest that the putative gasping defect in the IRZ in SIDS victims is not related to [3H]PAC binding to alpha2-adrenergic receptors.

Rostral medullary respiratory neuronal activities of decerebrate cats in eupnea, apneusis and gasping

Eupnea is generated by mechanisms within the pons and medulla. Following removal of pons or exposure to anoxia, gasping is elicited. Eupnea and gasping are markedly different ventilatory patterns. The genesis of gasping is dependent upon rostral medullary neuronal activities. To generate the gasp, these activities should commence before the phrenic burst. In decerebrate, vagotomized, paralyzed and ventilated cats, eupnea was altered to gasping in anoxia. Rostral medullary neuronal activities had inspiratory, expiratory and phase-spanning patterns in eupnea. During gasping, some inspiratory neuronal activities commenced before the phrenic gasp; these same neurons had commenced activities after the onset of the eupneic phrenic burst. Expiratory and phase-spanning neurons did not discharge. Neuronal activities which are consonant with a role in the neurogenesis of gasping had very different discharge patterns in eupnea. Results support the concept that medullary mechanisms for gasping are incorporated in the ponto-medullary circuit responsible for the neurogenesis and expression of eupnea.

Pontine cholinergic respiratory depression in neonatal and young rats

We postulated that activation of pontine cholinergic mechanisms would cause respiratory depression in neonatal and young rats. Phrenic activity was recorded in decerebrate, paralyzed, ventilated and vagotomized rats of 4 to 22 days after birth. Small volumes (10-60 nl) of carbachol (44-88 mM) were injected into the medial portion of the rostral pons. The injection of carbachol, but not saline, decreased phrenic peak activity (83 +/- 6% of control) and respiratory frequency (64 +/- 9.5% of control) within 2 min following the injection in neonates and the depression lasted for less than 10 min. The site of injection in the pontine reticular formation was confirmed by histology. Results suggest that cholinergic mechanisms in the medial pons depress respiratory activity in the neonate.

Lesions of regions for in vitro ventilatory genesis eliminate gasping but not eupnea

Medullary regions, termed 'pre-inspiratory' and 'pre-Bötzinger', are considered critical for the neurogenesis of rhythmic ventilatory activity of in vitro preparations of the neonatal rat. We examined the influence of destruction of neurons in these regions, by microinjections of kainic acid, upon eupnea and gasping in vivo. Decerebrate, vagotomized, paralyzed and ventilated rats of age 8-15 days were used; the phrenic nerve activity was recorded. Eupnea was not consistently altered following destruction of neurons in any region. However, in the majority of animals, anoxia-induced gasping was not observed following injections of kainic acid into the 'pre-inspiratory' region, 'pre-Bötzinger' complex or lateral tegmental field; the latter region is important for the neurogenesis of gasping in adults. Injections into other regions did not prevent the elicitation of gasping. These results do not support the possibility that neuronal activities which are responsible for respiratory rhythm generation in vitro underlie the neurogenesis of eupnea in vivo.

The morphology and connections of neurons in the gasping centre of adult rats

Neuronal activities in the intermediate reticular nucleus and adjacent lateral tegmental field are critical for the neurogenesis of the ventilatory pattern of gasping. We report herein the anatomical features of these neurons, their axonal projections and the location of neurons providing afferent inputs. These neuroanatomical evaluations were performed by iontophoretic injection of the tracer Neurobiotin into the region of the intermediate reticular nucleus of the rat. At the site of injection, neurons having soma of 30-50 microns were filled. Labelled axons and terminals were observed in ipsilateral regions which contain neurons having established functions in the control of ventilatory activity. These regions include the nucleus ambiguous and motor nuclei of the hypoglossal and facial nerves. In addition, axonal projections extended to the contralateral region of the intermediate reticular nucleus. From this contralateral region, retrograde tracing revealed projections to the site of injection. Similarly, many ipsilateral regions which received axonal terminals from the region of the intermediate reticular nucleus had reciprocal projections to this region. These anatomical results support the physiological observation that the neurogenesis of gasping involves a synchronized activation of diverse components of the brainstem ventilatory control system.

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

Gasping is a critical mechanism for survival in that it serves as a mechanism for autoresuscitation when eupnea fails. Eupnea and gasping are separable patterns of automatic ventilatory activity in all mammalian species from the day of birth. The neurogenesis of the gasp is dependent on the discharge of neurons in the rostroventral medulla. This gasping center overlaps a region termed "the pre-Bötzinger complex." Neuronal activities of this complex, characterized in an in vitro brain stem spinal cord preparation of the neonatal rat, have been hypothesized to underlie respiratory rhythm generation. Yet, the rhythmic activity of this in vitro preparation is markedly different from eupnea but identical with gasping in vivo. In eupnea, medullary neuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of the pontomedullary circuit defining eupneic ventilatory activity. However, these medullary neuronal activities do not appear critical for the neurogenesis of eupnea, per se.

Phrenic response to hypercapnia in the unanesthetized, decerbrate, 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.

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.

Characterization of ventilatory responses to hypoxia in neonatal rats

Newborn animals exhibit a biphasic response to hypoxia, with ventilation increasing and then declining. Our purpose was to define if this response could be supported by the pontile and medullary respiratory centers. Spontaneously breathing and paralyzed and ventilated decerebrate or anesthetized, vagotomized rats were studied from birth to 13 days thereafter. Peak integrated phrenic activity, or tidal volume, and frequency initially increased and then declined after inspired oxygen was reduced from hyperoxic to hypoxic levels; most animals became apneic in hypoxia. Apnea occurred in a greater proportion of animals and more quickly with more severe hypoxia. Following sectioning of the carotid sinus nerves, ventilatory activity declined with a change from hyperoxia to normoxia. We conclude that the biphasic ventilatory response to hypoxia represents a balance between synaptically-induced augmentations and reductions of brainstem neuronal activities. The carotid chemoreceptors play a fundamental role in the augmentations, and reductions appear dependent upon actions of hypoxia upon brainstem mechanisms.

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.

Medullary neuronal activities in gasping induced by pharyngeal stimulation and hypoxia

We examined the hypothesis that medullary respiratory-related and non-respiratory-related neuronal activities are similarly altered with the "aspiration reflex", induced by mechanical stimulation of the epipharyngeal mucosa, and gasping, induced by severe hypoxia. Extracellular neuronal activities were recorded in decerebrate, paralyzed and ventilated cats. Phrenic activity and neuronal activities were monitored in eupnea and gasping. Seventy-one unit activities were recorded in the lateral medulla including the nucleus tractus solitorii (NTS), lateral tegmental field (LTF) and the nucleus ambiguus (NA). The respiratory modulation of a neuronal activity was quantified by a eta 2 statistic (Orem, J. and Dick, T., 1983, J. Neurophysiol. 50: 1098-1107). The eta 2 values of the units ranged from 0.02 to 0.93. Inspiratory-related activities with relative high eta 2 values (n = 16) were recorded in the region closed to the NTS. Phase-spanning (n = 7) and expiratory-related activities (n = 10) were recorded in the ventral medullary region. Units with low eta 2 values (n = 29) and with no spontaneous activity (n = 9) in eupnea were recorded in the region of the LTF. In both "aspiration reflex" and gasping, inspiratory-related activities were augmented and expiratory-related activities were suppressed. Tonic units were activated and additional activities were recruited. The modulation of the neuronal activities to gasping induced by anoxia was identical to that induced by pharyngeal stimulation in either hyperoxia or severe hypoxia. We concluded that medullary gasping mechanism is recruited by pharyngeal stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Expiratory neural activities in gasping induced by pharyngeal stimulation and hypoxia

The purpose was to characterize expiratory neural activities in gasping elicited during the aspiration reflex (AR) in hyperoxia and during hypoxia-induced gasping. In decerebrate, vagotomized and paralyzed cats, we recorded activities of inspiratory and expiratory cranial and spinal nerves. The AR was elicited by touching the epipharyngeal mucosa. In eupnea, spinal expiratory activities were greatly decreased during AR whereas laryngeal expiratory activities were increased. In hypoxia-induced gasping, both the laryngeal and spinal expiratory activities were reduced. All of the inspiratory activities were increased during both gasping and the AR. In addition, neural activities were below control levels following AR; activities gradually recovered to control levels. We conclude that spinal expiratory activities are inhibited during the AR and gasping. Results are consistent with the concept that medullary mechanisms for gasping are recruited by mechanical stimulation of the epipharynx. In hypoxia-induced gasping, the hypoxia, per se, causes a separate suppression of laryngeal expiratory activities.

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.

Neuronal activities underlying inspiratory termination by pneumotaxic mechanisms

The purpose was to identify and characterize the discharge patterns of pontile neurons which are responsible for the termination of inspiratory activity. Phrenic discharge is prolonged following destruction of neurons at the junction of mesencephalon and pons by neurotoxins. Neuronal activities were recorded in this region in decerebrate, vagotomized, paralyzed and ventilated cats. At normocapnia, neurons had tonic discharge patterns, most of which were linked to phasic periods of phrenic activity. Peak activities occurred in late neural inspiration or early expiration. In hypercapnia, neuronal discharge frequencies did not increase, rather activity became more concentrated during one portion of the respiratory cycle. In severe hypoxia, neuronal activities diminished in parallel with the prolongation of phrenic discharge and establishment of apneusis. During recovery, some neurons transiently acquired phasic, respiratory-modulated discharge patterns. Neuronal activities from neighboring regions did not exhibit comparable changes in hypercapnia or hypoxia. We conclude that rostral pontile neuronal activities are a primary determinant of the reversible and irreversible terminations of eupneic inspiratory activity.

Involvement of pontile NMDA receptors in inspiratory termination in rat

We evaluated the hypothesis that N-methyl-D-aspartate (NMDA) receptors in the rostral pons mediate the off-switch of inspiration in the adult rat. Experiments were performed on decerebrate, vagotomized, paralyzed and ventilated animals. Activity of phrenic nerve was recorded. Small volumes (10 nl) of NMDA antagonists, MK-801 and AP-5, or non-NMDA antagonists, CNQX and DNQX, were injected into the rostral pons. We found that injections of MK-801 reversibly increased the duration of neural inspiration (TI), and the increase was dose-dependent. Injections of AP-5 also increased TI. Injections of the DNQX and CNQX in these same loci resulted in no significant changes in the duration of neural inspiration, expiration or peak phrenic activity (PNA). However, injections of kainic acid (KA, 4.7 mM) in the loci increased TI and decreased PNA. We conclude that neurons regulating the off-switch mechanism are located in the rostral pons. Further, the binding of NMDA receptors in the rostral pons is involved in this off-switch mechanism.

Separation of multiple functions in ventilatory control of pneumotaxic mechanisms

Multiple functions have been ascribed to the pontile pneumotaxic center. We hypothesized that these functions might be separable among neurons in different regions. In decerebrate, vagotomized, paralyzed and ventilated cats, activities of the phrenic and triangularis sterni nerves were recorded. Microinjections of kainic acid were used to destroy neurons. Neurons in the rostrolateral tegmentum at the ponto-mesencephalic border controlled the duration of neural inspiration. Expiratory duration was controlled by neurons in the more caudal nucleus parabrachialis medialis and Kolliker-Fuse nucleus. Ventilatory responses to hypercapnia were depressed following injections of kainic acid into regions controlling either inspiratory or expiratory durations. The phases of expiration were regulated by two groups of neurons, located medial and lateral in the rostral pons. We conclude that rostral pontile and mesencephalic mechanisms control multiple aspects of the eupneic ventilatory cycle. There mechanisms are served by neurons in separable anatomical regions.

Electrical stimulation of pneumotaxic center: activation of fibers and neurons

Electrical stimulations of the pneumotaxic center can result in a phase-switch from neural inspiration to expiration or the reverse. Terminations of inspiration are also obtained from many loci ventral to the pneumotaxic center. We hypothesized that responses to some stimulations reflect an activation of fibers rather than neurons. Studies were conducted in decerebrate, vagotomized, paralyzed and ventilated cats. Activities of the phrenic and triangularis sterni nerves were recorded. A multibarreled pipette was inserted into the rostral pons. Electrical stimulation was delivered through two barrels; another contained kainic acid to destroy neurons. The threshold current for producing inspiratory termination was not altered in most regions following the injections of kainic acid. However, stimulation of neurons did appear to underlie the premature onset of phrenic activity. In addition, neurons in medial pontile regions regulate triangularis sterni activity. We conclude that some functions ascribed to the pneumotaxic center are, in fact, subserved by neurons in other regions of the central nervous system.

Reflex recruitment of medullary gasping mechanisms in eupnoea by pharyngeal stimulation in cats

1. Mechanical stimulation of the naso- and oropharynx causes the replacement of the eupnoeic ventilatory pattern by a brief, but large, burst of activity of the phrenic nerve. Our purpose was to define whether these changes in phrenic activity represent a switch to gasping. 2. In decerebrate, vagotomized, paralysed and ventilated cats, mechanical stimulation of the pharynx was performed during eupnoea, apneusis and gasping. The latter two ventilatory patterns were produced by ventilating the experimental animal with 1.0% carbon monoxide in air or with 100% nitrogen. Eupnoea could be re-established by a recommencement of ventilation with oxygen. 3. The rate of rise of phrenic activity and its peak height were much greater following mechanical stimulation of the pharynx than the phrenic bursts of eupnoea or apneusis. The durations of phrenic burst and the period between these were much less following pharyngeal stimulation. In contrast, these variables of phrenic activity were the same during pharyngeal stimulation and in gasping. 4. Previous studies had established that activity within a region of the lateral tegmental field of medulla is critical for the manifestation of gasping. Hence, electrical stimulation of this region during gasping elicits premature gasps whereas its ablation irreversibly eliminates gasping. 5. We positioned a multibarrelled pipette in the critical medullary region for gasping. Its location was verified, once gasping was established in hypoxia or anoxia, by the elicitation of premature gasps following electrical stimulation. Neurons in this region were destroyed by microinjections of the neurotoxin kainic acid; in a few experiments the region was destroyed by electrolytic lesions. 6. Following destruction of the region of the lateral tegmental field, gasping could no longer be provoked in anoxia. In contrast, the eupnoeic pattern of phrenic activity continued. However, mechanical stimulation of the pharynx no longer caused any changes in the on-going pattern of phrenic activity. 7. We conclude that mechanical stimulation of the pharynx elicits a powerful reflex by which eupnoea is suppressed and gasping is elicited. Stated differently, the changes in phrenic activity during this pharyngeal stimulation in fact represent gasps. 8. Gasps are dependent upon activity within a region of the lateral tegmental field of the medulla. This region plays no role in the neurogenesis of eupnoea. Hence, our results provide additional support for the concept that there are multiple sites for ventilatory neurogenesis in the mammalian brainstem.

Responses of respiratory modulated and tonic units in the retrotrapezoid nucleus to CO2

We hypothesized that the retrotrapezoid nucleus (RTN) contains both respiratory modulated (RM) and non-respiratory modulated (NRM) neurons which participate in the ventilatory response to increased CO2. We made extracellular recordings of the activity of 46 single units in the RTN of 9 decerebrate, paralyzed, ventilated cats (5 intact; 4 with carotid body and sinus ablation) under eucapnic (PCO2 = 34.2 +/- 3.5 mmHg; mean +/- SD) and hypercapnic (PCO2 = 47.4 +/- 3.4 conditions. To define a RM unit, we used the eta 2 statistic which is the ratio of the variance of the unit firing rate within respiratory cycles to that across respiratory cycles. We classified the units as RM (N = 17) if the eta 2 values in eucapnia or hypercapnia were > or = 0.25 and as NRM (N = 29) if the values were < 0.25. Overall, 19/46 units (41%) increased their firing rate with increased CO2, 5 decreased their firing rate, and 22 had no significant change in firing rate. Of 17 RM units, 8 (47%) increased their mean firing rate with hypercapnia from 7.6 +/- 3.9 to 23.2 +/- 6.8 spikes/sec. These included 5 inspiratory units, 2 inspiratory units that had an onset of firing in late expiration (Pre-I/I), and 1 expiratory unit. Seven of these also changed their discharge pattern (eucapnic eta 2 = 0.02 to 0.12; hypercapnic eta 2 = 0.34 to 0.79) Of 29 NRM units, 11 (38%) showed a significant increase in mean firing rate with CO2 stimulation from 19.8 +/- 7.2 to 31.3 +/- 8.2 spikes/sec. The RTN has RM units which change their discharge pattern and firing rate in response to increased CO2, as do units within the medulla and pons, and it has NRM units which are also responsive to increased CO2. These data indicate that some neurons of the RTN are involved in the central chemoreceptor response but they provide no direct evidence that chemoreception resides within the RTN.

Pontile regulation of ventilatory activity in the adult rat

Our purpose was to characterize the pontile components of the brain stem ventilatory control system in rats. This study was precipitated by reports that this pontile component might differ fundamentally from that of other species. Efferent activity of the phrenic nerve was recorded in anesthetized, vagotomized, paralyzed, and ventilated adult rats. As in other species, electrical stimulations of the rostral pons caused premature terminations and/or onsets of phrenic activity in eupnea. Electrolytic lesions of rostrolateral pons resulted in apneusis, characterized by significant prolongations of the phrenic burst. Some effective lesions were in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus, the site of the pneumotaxic center. Other lesions resulting in apneusis were ventral to the pneumotaxic center. As in cats, lesions in the caudal pontile reticular formation caused the duration of the apneustic neural inspiration to return toward that of eupnea. Again, as in other species, gradual alterations from eupnea to gasping in the rat were recorded during hypoxia, which was induced by ventilation with carbon monoxide. We conclude that the brain stem respiratory control system is similarly organized in rats and other mammalian species. These results have implications for contemporary hypotheses concerning the neurogenesis of ventilatory activity.

Facilitation and inhibition of phrenic motoneuronal activities by lung inflation

The purpose was to evaluate the facilitatory influence of pulmonary inflations on phrenic activity. In decerebrate cats, activities of phrenic motoneurons and nerve were recorded during ventilatory cycles in which the lungs were inflated to different levels and inflations were withheld. Motoneuronal activities were divided into "early" and "late" populations depending on their onset of activity. In normocapnia, facilitation was manifested by an increase in the rate of rise of phrenic activity. Facilitation increased with an increased level of inflations and fell when inflations were withheld. This facilitation was largely due to an increased rate of change and earlier onset of late motoneuronal activities. These variables for early motoneuronal activities were little altered by changes in inflations. Peak discharge frequencies of both early and late motoneurons increased during noninflation cycles. Facilitation was still evident during hypercapnia and in anesthetized animals; however, under these conditions, the earlier onset of late motoneuronal activities was no longer observed. We conclude that facilitation by pulmonary stretch receptor discharge is a constant determinant of phrenic neuronal and neural activities.

Cerebellar control of expiratory activities of medullary neurons and spinal nerves

Expiratory-related activities of spinal nerves are augmented after stimulation of the infracerebellar nucleus of the cerebellum. These stimulations alter neither inspiratory spinal neural activities nor inspiratory and expiratory laryngeal neural activities. It was hypothesized that efferents from the infracerebellar nucleus impinge on spinal motoneurons by a pathway that bypasses the medulla. In decerebrate, paralyzed, and vagotomized cats, phrenic and expiratory triangularis sterni activities and activities of medullary respiratory neurons were recorded. During infracerebellar stimulation, activities increased for expiratory bulbospinal neurons and neurons with tonic discharge patterns. After unilateral ablation of the infracerebellar nucleus by kainic acid, triangularis sterni discharge was eliminated and activities of expiratory bulbospinal neurons continued at reduced frequencies. Stimulations of the anterior interposed nucleus caused both inspiratory and expiratory activities to increase, whereas no systematic changes followed stimulations of the vermis. Results establish that at least a portion of the changes in expiratory activities of spinal motoneurons after perturbations of the infracerebellar nucleus occurs secondarily to changes in activities of medullary neurons. The possibility of separate cerebellar projections to medullary and spinal neurons is discussed.

Reductions of neural activities to upper airway muscles after elevations in static lung volume

We evaluated the hypothesis that the tonic discharge of pulmonary stretch receptors significantly influences the respiratory-modulated activities of cranial nerves. Decerebrate and paralyzed cats were ventilated with a servo-respirator, which produced changes in lung volume in parallel with integrated phrenic activity. Activities of the facial, hypoglossal, and recurrent laryngeal nerves and nerves to the thyroarytenoid muscle and triangularis sterni were recorded. After a stereotyped pattern of lung inflation, tracheal pressure was held at 1, 2, 4, or 6 cmH2O during the subsequent ventilatory cycle. Increases in tracheal pressure caused progressive reductions in both inspiratory and expiratory cranial nerve activities and progressive elevations in triangularis sterni discharge; peak levels of phrenic activity declined modestly. Similar changes were observed in normocapnia and hypercapnia. We conclude that the tonic discharge of pulmonary stretch receptors is an important determinant of the presence and magnitude of respiratory-modulated cranial nerve activity. This reflex mechanism may maintain upper airway patency and also regulate expiratory airflow.

Activity of abdominal muscle motoneurons during hypercapnia

Our purpose was to examine the influence of hypercapnia on the activity of motoneurons innervating the transversus abdominis and internal oblique abdominal muscles, and of integrated phrenic and abdominal motor nerve activities. Studies were done in nine adult cats that were decerebrated, vagotomized, thoracotomized, paralyzed and ventilated mechanically. Of 42 motoneurons examined, 24 showed strong respiratory modulation (RM neurons), with the discharge confined primarily to the central expiratory period. The remaining 18 motoneurons discharged tonically, and failed to show respiratory modulation even at increased levels of central respiratory drive. Hyperoxic hypercapnia augmented the activities of the phrenic and abdominal nerves and increased the early expiratory discharge frequency of the RM neurons. The hypercapnia-induced increase in firing frequency during early expiration was accompanied by a corresponding decline in late expiration, and a virtual abolition of the inspiratory activity in the few neurons that discharged in this phase under normocapnic conditions. Finally, hypercapnia induced an increase in the number of spikes generated during each expiratory period in about half of the RM neurons, whereas the remaining cells showed a decrease. Thus, the increased peak activity of the integrated whole abdominal nerve burst with hypercapnia was brought about by a shift in the temporal pattern of motoneuron firing, or by an increase in the number of spikes generated during the expiratory period. The steep rate of rise and the pronounced early expiratory peak observed in the integrated abdominal nerve burst during hypercapnia in this preparation are consistent with the increase in motoneuron firing frequency during the early stages of the expiratory phase.

Rostral pontile mechanisms regulate durations of expiratory phases

Neural expiration can be divided into two phases. Phase I corresponds to the period of laryngeal adduction, whereas many spinal nerves reach peak discharge in phase II. The present studies evaluated the hypothesis that rostral pontile mechanisms contribute to determining the time of onset of spinal motoneuronal activities in phase II. In decerebrate and paralyzed cats, efferent activities were recorded from the phrenic nerve and from single fibers of the branch of the intercostal nerve innervating the triangularis sterni muscle. These activities were recorded in eupnea and apneusis; the latter was produced by cooling the rostral pons by a fork thermode. In eupnea, there was a delay between the rapid decline of phrenic discharge from peak levels and the commencement of activities of motoneurons of the triangularis sterni. This delay was significantly reduced in apneusis. Peak discharge frequencies of triangularis sterni motoneurons were the same in eupnea and apneusis. We conclude that rostral pontile mechanisms contribute significantly to defining the phases of neural expiration.

Lesions in retrotrapezoid nucleus decrease ventilatory output in anesthetized or decerebrate cats

Kainic acid (KA) injections into the retrotrapezoid nucleus (RTN) of anesthetized deafferented cats profoundly decreased phrenic activity (PA) and CO2 sensitivity (J. Appl. Physiol. 68: 1157-1166, 1990). In this study small electrolytic lesions of the RTN produced the same results, indicating that the KA destroyed cells. We then asked whether anesthetic depression or the absence of peripheral chemoreceptors could explain the degree of respiratory depression observed. In decerebrate cats electrolytic lesions of the RTN resulted in a decrease in PA similar to that seen under anesthesia. CO2 sensitivity was decreased by RTN lesions that extended into the caudal RTN but less so than under anesthesia. KA injections resulted in an initial increase in PA followed by a continuous decrease, a pattern similar to that seen under anesthesia but with a slower time course. CO2 sensitivity was essentially absent. Peripheral chemodenervation produced a small further decrease in PA and a downward shift of the CO2 response without change in slope. Blood pressure was unaffected by RTN lesions but was decreased by more-caudal lesions without respiratory effects. The RTN appears to be necessary for the maintenance of eupneic phrenic activity and CO2 sensitivity even in decerebrate cats with intact peripheral chemoreceptors.

Vestibular and cerebellar modulation of expiratory motor activities in the cat

1. The purpose of our investigation was to evaluate the hypothesis that components of the vestibular and cerebellar systems regulate efferent respiratory-modulated activities of cranial and spinal nerves. The hypothesis was based upon the observation that spinal neural activities during expiration are greatly altered subsequent to a change in posture. 2. In decerebrate and paralysed cats, efferent activities were recorded from the central cut ends of the phrenic nerve, intercostal nerve, branch of the intercostal nerve innervating the triangularis sterni, cranial iliohypogastric (abdominal) nerve and recurrent laryngeal nerve. 3. Animals were artificially ventilated. Those with intact vagi were ventilated by a servo-respirator which produced changes in lung volume in parallel with alterations in integrated activity of the phrenic nerve. Animals with bilateral vagotomy were ventilated with a standard respirator. 4. Aspiration of the entire cerebellar cortex did not produce alterations in levels of neural activities; the respiratory frequency was increased modestly. Following ablation of the ventrolateral portion of corpus medullare and cerebellar peduncles, expiratory activities of spinal nerves were completely eliminated whereas inspiratory activities were not greatly altered. Results were similar in animals having either intact or sectioned vagi. 5. Electrical stimulation or chemical stimulation by glutamate of regions of the ventrolateral cerebellum produced little change in respiratory neural activities except when these stimulations were within the infracerebellar nucleus. Stimulations in this nucleus caused pronounced increases in expiratory activities of spinal nerves. Neither inspiratory activities of spinal nerves nor inspiratory or expiratory activities of the recurrent laryngeal nerve were altered. Studies in animals having intact or sectioned vagi yielded similar results. 6. Bilateral lesions of neurons in the infracerebellar nucleus by injections of kainic acid in animals having intact or sectioned vagi caused an irreversible loss of expiratory activities of spinal nerves with neither inspiratory spinal activities nor inspiratory and expiratory laryngeal activities being altered. Similar findings were obtained following unilateral ablation of the infracerebellar nucleus in vagotomized cats. However, in cats with intact vagi, unilateral ablation of the infracerebellar nucleus produced only transient changes in either inspiratory or expiratory neural activities.(ABSTRACT TRUNCATED AT 400 WORDS)

Differing activities of medullary respiratory neurons in eupnea and gasping

Our purpose was to compare further eupneic ventilatory activity with that of gasping. Decerebrate, paralyzed, and ventilated cats were used; the vagi were sectioned within the thorax caudal to the laryngeal branches. Activities of the phrenic nerve and medullary respiratory neurons were recorded. Antidromic invasion was used to define bulbospinal, laryngeal, or not antidromically activated units. The ventilatory pattern was reversibly altered to gasping by exposure to 1% carbon monoxide in air. In eupnea, activities of inspiratory neurons commenced at various times during inspiration, and for most the discharge frequency gradually increased. In gasping, the peak discharge frequency of inspiratory neurons was unaltered. However, all commenced activities at the start of the phrenic burst and reached peak discharge almost immediately. The discharge frequencies of all groups of expiratory neurons fell in gasping, with many neurons ceasing activity entirely. These data are consistent with the hypothesis that brain stem mechanisms controlling eupnea and gasping differ fundamentally.

Influence of phasic volume feedback on abdominal expiratory nerve activity

Our purpose was to examine the influence of phasic lung volume feedback on the activities of motor nerves innervating the diaphragm and transversus abdominis muscles during hypercapnia and hypoxia. We studied seventeen decerebrate cats that were paralyzed and ventilated with a servo-respirator controlled by the integrated phrenic neurogram. The effects of phasic lung volume feedback were assessed by withholding pulmonary inflation during the central inspiratory period. Withholding lung inflation for a single respiratory cycle under hyperoxic, normocapnic conditions consistently prolonged the durations of the inspiratory and expiratory periods, and caused marked increases in the peak electrical activities of both phrenic and abdominal nerves. Hyperoxic hypercapnia (PaCO2 50-80 mmHg) and isocapnic hypoxia (PaO2 60-35 mmHg) increased peak phrenic and abdominal neural activities, and withholding pulmonary inflation under these conditions caused even greater augmentations of inspiratory and expiratory motor output. The augmentation of expiratory activity by withholding lung inflation was proportionately greater than the concomitant prolongation of the central expiratory period. All responses to non-inflation maneuvers were abolished following bilateral cervical vagotomy. The results indicate that vagally mediated volume feedback during inspiration can attenuate the output of abdominal motoneurons in the subsequent expiratory period. Moreover, hypoxia, which attenuates abdominal motor activity in vagotomized animals, enhances this activity when the vagi are intact.

Discharge of vagal pulmonary receptors differentially alters neural activities during various stages of expiration in the cat

1. The purpose was to evaluate the hypothesis that neural expiration is composed of two phases: I, a post inspiratory period; and II, the period at which expiratory activities of spinal nerves reach peak values. We hypothesized that the discharge of pulmonary stretch receptors might differentially alter neural activities during these two phases. 2. Activities of the phrenic nerve, intercostal nerve and nerves innervating the thyroarytenoid muscle of the larynx and triangularis sterni muscle of the chest wall were recorded in decerebrate and paralysed cats. 3. The experimental animals were ventilated with a servo-respirator which produced changes in tracheal pressure, and lung volume, in parallel with alterations in integrated activity of the phrenic nerve. 4. In order to assess the influence of the discharge of slowly adapting pulmonary stretch receptors upon neural activities during expiration, lung volume was held at end-expiratory or end-inspiratory levels for individual respiratory cycles. 5. When pulmonary inflation was prevented, phrenic activity increased, as did activity of the thyroarytenoid nerve during early expiration. In contrast, activities of the triangularis sterni and intercostal nerves during mid- to late expiration declined. 6. Holding the lungs at end-inspiratory levels caused a reduction of thyroarytenoid activity and increases in peak triangularis sterni and intercostal activities. Neural expiration typically continued as long as the lungs were maintained at the end-inspiratory level. 7. Responses were qualitatively similar in hypocapnia, normocapnia and hypercapnia, but the magnitude of changes in neural activities was typically augmented with elevations in end-tidal fractional concentrations of CO2. 8. We conclude that the discharge of slowly adapting pulmonary stretch receptors inhibits neural activities during early expiration and augments activities during mid-to late expiration. Hence, our data support the concept that neural expiration is composed of two stages in which neural activities may be differentially controlled.

Neurogenesis, control, and functional significance of gasping

Gasps are frequently the first and last breaths of life. Gasping, which is generated by intrinsic medullary mechanisms, differs fundamentally from other automatic ventilatory patterns. A region of the lateral tegmental field of the medulla is critical for the neurogenesis of the gasp but has no role in eupnea. Neuronal mechanisms in separate brain stem regions may be responsible for the neurogenesis of different ventilatory patterns. This hypothesis is supported by the recording of independent respiratory rhythms simultaneously from isolated brain stem segments. Data from fetal and neonatal animals also support gasping and eupnea being generated by separate mechanisms. Gasping may represent the output of a simple but rugged pattern generator that functions as a backup system until the control system for eupnea is developed. Pacemaker elements are hypothesized as underlying the onset of inspiratory activity in gasping. Similar elements, in a different brain stem region, may be responsible for the onset of the eupneic inspiration with neural circuits involving the pons, the medulla, and the spinal cord serving to shape efferent respiratory-modulated neural discharges.

Differing control of neural activities during various portions of expiration in the cat

1. Activities of the phrenic nerve, intercostal nerve and nerves innervating the thyroarytenoid (TA) muscle of the larynx and triangularis sterni (TS) muscle of the chest wall were recorded in decerebrate, vagotomized, paralysed and ventilated cats. 2. Neural inspiration was defined by the phase of phrenic activity. Neural expiration was divided into two phases with phase I corresponding to the duration of TA activity and phase II to TS activity: intercostal nerves discharged across both phases. 3. Phrenic activity was terminated prematurely by electrical stimulation of the superior laryngeal nerve or of the dorsolateral region of the rostral pons. Following stimulation, neural activities during phase I of expiration rose and those during phase II fell in most animals. 4. Stimulation of the superior laryngeal nerve during phase I caused augmentations of both TA and TS activity. At the termination of stimulation, a phase of TA discharge was recorded followed by a phase of TS activity. The durations of these post-stimulation phases of TA and TS activities approximated those of cycles without stimulation. 5. Stimulation of the superior laryngeal nerve during phase II caused a resetting of neural expiration. Following stimulation, phases of TA and TS activity were recorded which had durations approximating those of cycles without stimulation. 6. The current required to induce a premature onset of phrenic activity by stimulation of the dorsolateral region of the rostral pons fell dramatically with the change from phase I to phase II of expiration. 7. We conclude that the control of neural activities differs markedly between phase I and phase II of expiration. The data support the hypothesis that post-inspiratory medullary respiratory neurones play a fundamental role in the definition of the ventilatory cycle.

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.

Respiratory activities of intralaryngeal branches of the recurrent laryngeal nerve

To distinguish experimentally between motor nerve activity destined for vocal cord abductor muscles and that bound for muscles that adduct the cords, we recorded efferent activities of intralaryngeal branches of the recurrent laryngeal nerve (RLN) in decerebrate, vagotomized, paralyzed, ventilated cats. Activities of the whole RLN and phrenic nerve were also recorded. Nerve activities were assessed at several steady-state end-tidal O2 and CO2 concentrations. The nerve to the thyroarytenoid (TA) muscle, a vocal cord adductor, was only slightly active under base-line (normocapnic, hyperoxic) conditions but in most cats developed strong activity during expiration in hypocapnia or hypoxia. In severe hypocapnia, phasic expiratory TA activity persisted even during phrenic apnea, indicating continuing activity of the respiratory rhythm generator. The nerve to the posterior cricoarytenoid (PCA) muscle, the vocal cord abductor, was always active in inspiration but often showed expiratory activity as well. This expiratory activity was usually enhanced by hypercapnia and often inhibited by hypoxia. The results are consistent with previous electromyographic findings and emphasize the importance of distinguishing abductor from adductor activity in studies of laryngeal control.

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.

Activities of pulmonary stretch receptors during ventilatory cycles without lung inflation

When lung inflation is temporarily withheld in paralyzed, ventilated cats with intact vagi, the activities of inspiratory motor nerves are greater during the second cycle without inflation than during the first. This response is not easily attributable to increasing drive from chemoreceptors as it is abolished by vagotomy. We examined the hypothesis that the increasing inspiratory activity is the result of decreasing inhibitory feedback from pulmonary stretch receptors (PSRs). Decerebrate, paralyzed cats were ventilated by a servo-respirator in accordance with their own phrenic nerve activity. Afferent activities from individual PSRs were recorded from a few cut fibers of one vagus nerve; the vagi were otherwise intact. When lung inflation was withheld, phrenic and hypoglossal nerve activities and the durations of inspiration and expiration all increased and were significantly greater during the second cycle without inflation than during the first. The frequency of PSR discharge was also greater during the second cycle and thus did not account for the responses recorded from the motor nerves. We conclude that the latter responses probably reflect neural processes within the brain stem, involving a persistent inhibitory influence from lung inflation, which outlasts the inflation itself.

Functions of the retrofacial nucleus in chemosensitivity and ventilatory neurogenesis

The hypothesis was evaluated that neurons within the retrofacial nucleus of medulla integrate afferent stimuli from the central chemoreceptors. In decerebrate, vagotomized, paralyzed and ventilated cats, activity of the phrenic nerve was monitored. Peak integrated phrenic activity increased in hypercapnia; the frequency of phrenic bursts typically declined slightly. The retrofacial nucleus was ablated by radio-frequency lesions or neurons within this nucleus were destroyed by microinjections of kainic acid. Results were similar following lesions or injections. Following unilateral ablations, peak phrenic activity was greatly reduced at normocapnia and hypercapnia; the frequency of phrenic bursts typically rose. Both frequency and peak phrenic activity fell further after the contralateral destruction with a cessation of all phasic phrenic discharge being observed in most animals. Injections of kainic acid in regions rostral, caudal or medial to the retrofacial nucleus produced no consistent changes in phrenic activity. We conclude that neuronal activities in the region of the retrofacial nucleus are important both in the integration of stimuli from the central chemoreceptors and in defining the discharge patterns of respiratory neurons.

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.

Kainic acid on the rostral ventrolateral medulla inhibits phrenic output and CO2 sensitivity

We used the neurotoxin, kainic acid, which is known to stimulate neuronal cell bodies as opposed to axons of passage by binding to specific amino acid receptors to determine whether cells with such receptors have access to the ventrolateral medullary surface and are involved in central ventilatory chemosensitivity. Pledgets with 4.7 mM kainic acid were placed bilaterally on the rostral, intermediate, or caudal ventilatory chemosensitive areas for 1-2 min in chloralose-urethan-anesthetized, paralyzed, vagotomized, glomectomized, and servo-ventilated cats. Application of kainic acid on the caudal or intermediate areas produced no consistent significant effects on eucapnic phrenic output or on the slope or maximum value of the phrenic nerve response to increased end-tidal PCO2. Rostral area kainic acid produced immediate augmentation and then diminution of blood pressure and phrenic output. Apnea developed in six of nine cats by 40 min. In all five cats in which it could be tested, the slope of the CO2 response was clearly decreased. Of [3H]kainic acid applied to the rostral area, 88.4% was shown to be within 2 mm of the ventral surface. Comparison of surface application sites of this and other studies suggests that an area overlapping the border of the original rostral and intermediate areas allows access to neurons involved in the chemoreception process, which may also provide tonic facilitatory input to cardiorespiratory systems.

Characterization of respiratory-related activity of the facial nerve

Activities of the facial, hypoglossal and phrenic nerves were recorded in decerebrate and paralyzed cats. These animals were ventilated with a servo-respirator which produced lung inflations in parallel with phrenic activity. Peak inspiratory phrenic, hypoglossal and facial activities increased in hypercapnia or hypoxia. When pulmonary inflation was prevented, hypoglossal and facial activities increased more than phrenic. Responses to withholding lung inflation differed from those following vagotomy. These differences were observed in expiratory facial and hypoglossal activities and in hypercapnia- and hypoxia-induced changes in facial activity. Administration of pentobarbital or hyperventilation to hypocapnia caused greater suppressions of hypoglossal than facial activity; the latter declined more than phrenic activity. The results support the hypothesis that influences from the brainstem reticular formation and from pulmonary stretch receptors are differentially distributed to motoneurons innervating upper airway muscles compared to those of the bulbospinal-phrenic system. The concept that ventilatory activity is influenced by tonic, as well as phasic discharge of pulmonary receptors is discussed.

Respiratory-modulated activities of motor units of the facial nerve

The purpose of this work was to characterize the influence of activity of vagal pulmonary receptors upon the discharge pattern of motor units of the facial nerve. Decerebrate and paralyzed cats were ventilated with a servo-respirator which produced pulmonary inflations in parallel with activity of the phrenic nerve. At normocapnia, facial units discharged phasically during neural inspiration, expiration or across both phases or discharged tonically throughout the respiratory cycle. When pulmonary inflation was withheld, the tonic discharge of some units became phasic; others changed the pattern of phasic discharge. In hypercapnia, the number of tonic fiber activities increased and, again, some phasic discharge patterns were altered. Withholding inflation caused similar alterations as in normocapnia. Activities of facial fibers in vagotomized animals differed in that no tonic activities were recorded, and no change in phasic discharge patterns was induced by hypercapnia. We conclude that afferents from pulmonary stretch receptors influence ventilatory activity throughout the entire respiratory cycle. The concept is discussed that the tonic, as well as phasic discharge of these receptors, is important for the regulation of activity of motoneurons to upper airway muscles.

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.

An inexpensive servo-respirator based upon regulation of a shunt resistance

We have designed and built an inexpensive servo-respirator for use in investigations of respiratory control in small animals. The device uses a butterfly valve to alter the resistance of an outflow shunt from a manifold that connects the animal's tracheal cannula to a pressure source. Tracheal pressure is regulated in response to a command provided by a suitably processed neural signal, often the integrated phrenic neurogram. As the valve opens, tracheal pressure approaches atmospheric; as it closes, tracheal pressure approaches the source pressure. An electronic controller circuit was developed to permit experimental procedures that include withholding volume delivery while maintaining a desired level of positive end-expiratory pressure. The device is able to track the neural command signal satisfactorily, and its performance appears to be limited primarily by the constraints applied by the respiratory system mechanics.

Respiratory neural activities after caudal-to-rostral ablation of medullary regions

The purpose is to assess the importance of medullary mechanisms for the neurogenesis of eupnea. Cats that were used were decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated. Activities of the phrenic, facial, and mylohyoid nerves were monitored. Progressive caudal-to-rostral transections of the spinal cord and medulla were performed. Phrenic activity was eliminated by C1 spinal transections. Only modest changes in facial and mylohyoid activities resulted from transections as far rostral as the level of the dorsal respiratory nucleus. Rhythmic discharges ceased on transections at the pontomedullary junction. However, rhythmic mylohyoid discharges were maintained if protriptyline and strychnine were administered before and during the transection. In other studies rhythmic phrenic, facial, and mylohyoid discharges continued, albeit with an altered rhythm, after destruction of neurons in the dorsal respiratory nucleus by kainic acid. We conclude that caudal medullary mechanisms do not play an essential role in the neurogenesis of breathing movements. Rather, structures in rostral medulla and pons appear necessary for sustaining eupneic neural activities. The concept of multiple brain stem sites for ventilatory neurogenesis is discussed.

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.

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.

Alterations of hypoglossal motoneuronal activities during pulmonary inflations

Preventing pulmonary inflation during inspiration results in greater augmentations in activity of the hypoglossal nerve than in the phrenic nerve. Our purpose was to characterize the hypoglossal motoneuronal activities which underlie these augmentations. Activities of the phrenic and hypoglossal nerves and single hypoglossal fibers were recorded in decerebrate and paralyzed cats. Ventilation was by a servo-respirator which produced changes in lung volume in parallel with phrenic activity. The number of motoneurons that discharged during cycles in which the lungs were inflated increased with elevations of end-tidal fractional concentrations of CO2 (FETCO2) from 0.05 to 0.06 and 0.09. At each FETCO2, the discharge frequency increased when pulmonary inflation was withheld. In addition, withholding inflation resulted in the recruitment of other motoneuronal activities. Most motoneurons discharged during the period of the phrenic burst (inspiratory neurons). Lesser numbers of inspiratory-expiratory, expiratory-inspiratory, and tonic motoneuronal activities were also recorded. Results are considered in the context of the inhibition of respiratory motoneuronal activity by vagal pulmonary afferent fibers. The possible role of such inhibition, and release from this inhibition, in maintenance of patency of the upper airways is discussed.