Vagal-dependent nonlinear variability in the respiratory pattern of anesthetized, spontaneously breathing rats

Physiological rhythms, including respiration, exhibit endogenous variability associated with health, and deviations from this are associated with disease. Specific changes in the linear and nonlinear sources of breathing variability have not been investigated. In this study, we used information theory-based techniques, combined with surrogate data testing, to quantify and characterize the vagal-dependent nonlinear pattern variability in urethane-anesthetized, spontaneously breathing adult rats. Surrogate data sets preserved the amplitude distribution and linear correlations of the original data set, but nonlinear correlation structure in the data was removed. Differences in mutual information and sample entropy between original and surrogate data sets indicated the presence of deterministic nonlinear or stochastic non-Gaussian variability. With vagi intact (n = 11), the respiratory cycle exhibited significant nonlinear behavior in templates of points separated by time delays ranging from one sample to one cycle length. After vagotomy (n = 6), even though nonlinear variability was reduced significantly, nonlinear properties were still evident at various time delays. Nonlinear deterministic variability did not change further after subsequent bilateral microinjection of MK-801, an N-methyl-D-aspartate receptor antagonist, in the Kölliker-Fuse nuclei. Reversing the sequence (n = 5), blocking N-methyl-D-aspartate receptors bilaterally in the dorsolateral pons significantly decreased nonlinear variability in the respiratory pattern, even with the vagi intact, and subsequent vagotomy did not change nonlinear variability. Thus both vagal and dorsolateral pontine influences contribute to nonlinear respiratory pattern variability. Furthermore, breathing dynamics of the intact system are mutually dependent on vagal and pontine sources of nonlinear complexity. Understanding the structure and modulation of variability provides insight into disease effects on respiratory patterning.

Respiratory and Mayer wave-related discharge patterns of raphé and pontine neurons change with vagotomy

Previous models have attributed changes in respiratory modulation of pontine neurons after vagotomy to a loss of pulmonary stretch receptor "gating" of an efference copy of inspiratory drive. Recently, our group confirmed that pontine neurons change firing patterns and become more respiratory modulated after vagotomy, although average peak and mean firing rates of the sample did not increase (Dick et al., J Physiol 586: 4265-4282, 2008). Because raphé neurons are also elements of the brain stem respiratory network, we tested the hypotheses that after vagotomy raphé neurons have increased respiratory modulation and that alterations in their firing patterns are similar to those seen for pontine neurons during withheld lung inflation. Raphé and pontine neurons were recorded simultaneously before and after vagotomy in decerebrated cats. Before vagotomy, 14% of 95 raphé neurons had increased activity during single respiratory cycles prolonged by withholding lung inflation; 13% exhibited decreased activity. After vagotomy, the average index of respiratory modulation (eta(2)) increased (0.05 +/- 0.10 to 0.12 +/- 0.18 SD; Student's paired t-test, P < 0.01). Time series and frequency domain analyses identified pontine and raphé neuron firing rate modulations with a 0.1-Hz rhythm coherent with blood pressure Mayer waves. These "Mayer wave-related oscillations" (MWROs) were coupled with central respiratory drive and became synchronized with the central respiratory rhythm after vagotomy (7 of 10 animals). Cross-correlation analysis identified functional connectivity in 52 of 360 pairs of neurons with MWROs. Collectively, the results suggest that a distributed network participates in the generation of MWROs and in the coordination of respiratory and vasomotor rhythms.

Reconfiguration of the pontomedullary respiratory network: a computational modeling study with coordinated in vivo experiments

A large body of data suggests that the pontine respiratory group (PRG) is involved in respiratory phase-switching and the reconfiguration of the brain stem respiratory network. However, connectivity between the PRG and ventral respiratory column (VRC) in computational models has been largely ad hoc. We developed a network model with PRG-VRC connectivity inferred from coordinated in vivo experiments. Neurons were modeled in the "integrate-and-fire" style; some neurons had pacemaker properties derived from the model of Breen et al. We recapitulated earlier modeling results, including reproduction of activity profiles of different respiratory neurons and motor outputs, and their changes under different conditions (vagotomy, pontine lesions, etc.). The model also reproduced characteristic changes in neuronal and motor patterns observed in vivo during fictive cough and during hypoxia in non-rapid eye movement sleep. Our simulations suggested possible mechanisms for respiratory pattern reorganization during these behaviors. The model predicted that network- and pacemaker-generated rhythms could be co-expressed during the transition from gasping to eupnea, producing a combined "burst-ramp" pattern of phrenic discharges. To test this prediction, phrenic activity and multiple single neuron spike trains were monitored in vagotomized, decerebrate, immobilized, thoracotomized, and artificially ventilated cats during hypoxia and recovery. In most experiments, phrenic discharge patterns during recovery from hypoxia were similar to those predicted by the model. We conclude that under certain conditions, e.g., during recovery from severe brain hypoxia, components of a distributed network activity present during eupnea can be co-expressed with gasp patterns generated by a distinct, functionally "simplified" mechanism.

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.

Invited review: Neural network plasticity in respiratory control

Respiratory network plasticity is a modification in respiratory control that persists longer than the stimuli that evoke it or that changes the behavior produced by the network. Different durations and patterns of hypoxia can induce different types of respiratory memories. Lateral pontine neurons are required for decreases in respiratory frequency that follow brief hypoxia. Changes in synchrony and firing rates of ventrolateral and midline medullary neurons may contribute to the long-term facilitation of breathing after brief intermittent hypoxia. Long-term changes in central respiratory motor control may occur after spinal cord injury, and the brain stem network implicated in the production of the respiratory rhythm could be reconfigured to produce the cough motor pattern. Preliminary analysis suggests that elements of brain stem respiratory neural networks respond differently to hypoxia and hypercapnia and interact with areas involved in cardiovascular control. Plasticity or alterations in these networks may contribute to the chronic upregulation of sympathetic nerve activity and hypertension in sleep apnea syndrome and may also be involved in sudden infant death syndrome.

Incorporating inheritance into models for understanding ventilatory behavior

Ventilation and its components (frequency and tidal volume) appear to be determined to a significant extent by inheritance. Gene manipulation, gene identification, and functional genomics now offer powerful tools to identify the strength and mode of inheritance for ventilatory behavior under steady-state and non-steady-state conditions, in health and in disease. Conscious integration of genetic principles into existing explanatory models may increase the likelihood of detecting traits that correlate with protein systems responsible for the structures and the functional components of respiration.

Ventilatory behavior after hypoxia in C57BL/6J and A/J mice

Given the environmental forcing by extremes in hypoxia-reoxygenation, there might be no genetic effect on posthypoxic short-term potentiation of ventilation. Minute ventilation (VE), respiratory frequency (f), tidal volume (VT), and the airway resistance during chemical loading were assessed in unanesthetized unrestrained C57BL/6J (B6) and A/J mice using whole body plethysmography. Static pressure-volume curves were also performed. In 12 males for each strain, after 5 min of 8% O2 exposure, B6 mice had a prominent decrease in VE on reoxygenation with either air (-11%) or 100% O2 (-20%), due to the decline of f. In contrast, A/J animals had no ventilatory undershoot or f decline. After 5 min of 3% CO2-10% O2 exposure, B6 exhibited significant decrease in VE (-28.4 vs. -38.7%, air vs. 100% O2) and f (-13.8 vs. -22.3%, air vs. 100% O2) during reoxygenation with both air and 100% O2; however, A/J mice showed significant increase in VE (+116%) and f (+62.2%) during air reoxygenation and significant increase in VE (+68.2%) during 100% O2 reoxygenation. There were no strain differences in dynamic airway resistance during gas challenges or in steady-state total respiratory compliance measured postmortem. Strain differences in ventilatory responses to reoxygenation indicate that genetic mechanisms strongly influence posthypoxic ventilatory behavior.

Heterogeneity within geniohyoid motor unit subpopulations in firing patterns during breathing

Respiratory motor units (MU) segregate into subpopulations, which differ in firing patterns during resting and stimulated breathing. For phrenic/diaphragm MUs, diversity also exists within subpopulations, and is greater for late than early-onset MUs. The present study characterized the extent of diversity within upper airway respiratory MU subpopulations by recording geniohyoid MUs in anesthetized cats. Inspiratory MUs (I-MU, n=21) had a wide range of firing durations (coefficient of variation (CV)=42%). In contrast, inspiratory-expiratory MUs (I/E-MU, n=19) had a narrow range of firing durations during inspiration (CV=13%), but a wide range of firing durations during expiration (CV=36%). Mean firing frequency had similar degrees of diversity among units for I-MU and I/E-MU (CV=31-40%). For I-MU firing duration correlated with mean firing frequency, whereas no such relationship was apparent for I/E-MU. Single-breath end-expiratory airway occlusion decreased heterogeneity in firing duration during inspiration and increased it during expiration, whereas end-inspiratory airway occlusion decreased heterogeneity during expiration. In conclusion, (a) there is considerable diversity within geniohyoid MU subpopulations receiving respiratory drive; (b) the degree of diversity within subpopulations differs for I-MU and I/E-MU; and (c) diversity within subpopulations in timing of activity is modulated by single-breath airway occlusion.

Modulation of diaphragm action potentials by K(+) channel blockers

K(+) channels regulate diaphragm contractility. The present study examined the electrophysiological mechanisms accounting for diversity among K(+) channel blockers in their inotropic actions on the diaphragm. Rat diaphragmatic muscle fibers were recorded intracellularly in vitro at 37 degrees C. Apamin and charybdotoxin (Ca2+)-activated K(+) channel blockers) did not alter resting membrane potential or action potentials. Glibenclamide (ATP-sensitive K(+) channel blocker) slowed action potential repolarization by 12% (P<0.05) and increased action potential area by 25% (P<0.005). Tetraethylammonium (which blocks several types of K(+) channels) increased action potential overshoot by 20% (P<0.01) and prolonged action potential rise time by 17% (P<0.02). 4-Aminopyridine and 3,4-diaminopyridine (which also block several types of K(+) channels) slowed action potential repolarization by 163% (P<0.0001) and 253% (P<0.0001), and increased action potential area by 183% (P<0.0001) and 298% (P<0.0001), respectively. Slowing of repolarization for the aminopyridines was especially marked at voltages approaching resting membrane potential, thereby changing action potential repolarization from a first to a second order decay. Previously reported variability in inotropic effects among K(+) channel blockers correlated significantly with the extent to which they slowed action potential repolarization and increased action potential area, but not with changes in other action potential properties.

Ventrolateral pons mediates short-term depression of respiratory frequency after brief hypoxia

The respiratory response to hypoxia is dynamic in the adult anesthetized Sprague-Dawley rat. Hypoxia elicits acute increases in both tidal volume (VT) and respiratory frequency (fR) followed by short-term increases in VT and short-term decreases in fR. After brief hypoxia (<1 min), recovery of the breathing pattern is again dynamic, where both VT and fR decrease immediately, but where VT remains above, and fR drops below, baseline. These acute changes are followed by a short-term progressive decrease in VT and increase in fR to baseline. We have identified a potential neural mechanism that depends on the integrity of the ventrolateral (vl) pons. Our studies show that: (a) blockade of activity in the vl pons prevents the short-term decrease in fR after hypoxia (b) stimulation of the vl pons decreases fR, and (c) vl pontine expiratory neurons are activated after hypoxia. These neurons may not be acting through alpha(2) -adrenergic receptors, but their effect does depend on NMDA-type receptor function. We conclude that the vl pons is a critical element in the pontomedullary network that generates and modulates the fR response to acute hypoxia.

A role for NMDA receptors in posthypoxic frequency decline in the rat

Posthypoxic frequency decline (PHFD) refers to the undershoot in respiratory frequency that follows brief hypoxic exposures. Lateral pontine neurons are required for PHFD. The neurotransmitters involved in the circuit that activate and/or are released by these pontine neurons regulating PHFD are unknown. We hypothesized that N-methyl-D-aspartate (NMDA) receptors are required for PHFD, because of the similarity in respiratory pattern after blocking lateral pontine activity or NMDA receptors. Furthermore, we hypothesized that the location of these NMDA receptors could be visualized by optimizing binding affinity with spermidine. In vagotomized, anesthetized rats (n = 16), cardiorespiratory responses to hypoxia (8% O2, 30-90 s) were recorded before and after dizocilpine (10 microg-1 mg/kg iv), and NMDA receptors were mapped with [3H]dizocilpine (n = 6). Dizocilpine elicited a dose-related effect on PHFD, blocking PHFD at high doses. Resting arterial blood pressure and breathing frequency decreased with high doses of dizocilpine, but the respiratory response to hypoxia remained intact. Our novel anatomical data indicate that NMDA receptors were widespread but distributed differentially in the brain stem. We conclude that NMDA receptors are located in pontine and medullary respiratory-related regions and that PHFD requires NMDA-receptor activation.

Post-hypoxic frequency decline does not depend on alpha2-adrenergic receptors in the adult rat

The aim of this study was to determine whether post-hypoxic frequency decline (PHFD) requires central activation of alpha2-adrenergic receptors. PHFD is defined as the undershoot in respiratory frequency that occurs immediately following brief hypoxic periods. Adult anesthetized, vagotomized rats were exposed to hypoxia (8% O2, mean=45 s) before and after intracerebroventricular (i.c.v.) infusion of vehicle or alpha2-antagonist. The efficacy of the i.c.v. antagonist was assessed by recording the response to intravenous injection of alpha2-agonist before and after the infusion. We compared breathing frequencies before, during, and after hypoxia, both before and after treatments. The decline in breathing frequency after hypoxia was not prevented by the alpha2-antagonists, RX 821002 or SK&F-86466. Guanabenz, an alpha2-agonist, prolonged baseline expiration and potentiated PHFD. Prior treatment with SK&F-86466 blocked the agonist-evoked response which was also reversed by subsequent administration of SK&F-86466. We conclude that PHFD does not require the activation of alpha2-adrenergic receptors, but that alpha2-adrenergic receptors can modulate resting and post-hypoxic respiratory frequency.

Persistence of the biphasic ventilatory response to hypoxia in preterm infants

OBJECTIVE

To characterize postnatal maturation of the biphasic ventilatory response to hypoxia in order to determine whether it persists beyond the first weeks of life in preterm infants, and the contributions of respiratory frequency and tidal volume to this response.

METHODS

Stable preterm infants were studied at two postnatal ages, 2 to 3 weeks (n = 12) and 4 to 8 weeks (n = 12), before hospital discharge at 35 weeks (range, 33 to 38 weeks) of postconceptional age. Infants were exposed to 5 minutes of 15% (or 13%) inspired oxygen; ventilation, oxygen saturation, end-tidal partial pressure of carbon dioxide, and heart rate were simultaneously recorded.

RESULTS

Minute ventilation exhibited a characteristic biphasic response to hypoxia at both postnatal ages, regardless of the development of periodic breathing. At both ages there was a transient increase in tidal volume, which peaked at 1 minute, accompanied by a sustained decrease in respiratory frequency as a result of significant prolongation of expiratory time.

CONCLUSION

The characteristic biphasic ventilatory response to hypoxia persists into the second month of postnatal life in preterm infants. We speculate that this finding is consistent with the prolonged vulnerability of such infants to neonatal apnea.

Prolongation in expiration evoked from ventrolateral pons of adult rats

Activation of neurons in the ventrolateral (vl) pons was hypothesized to alter the breathing pattern because previous studies demonstrated apneusis after inhibiting neuronal activity with bilateral muscimol (10 mM) microinjections into the vl pons (17). The excitatory amino acid L-glutamate (10 mM) was microinjected (10-100 nl) into the vl pons in anesthetized, vagotomized, paralyzed, and ventilated adult rats (n = 8). In four of these animals, the target site was approached from the ventral surface of the pons to avoid penetrating the dorsolateral (dl) pons. The expiratory phase was prolonged transiently and concurrently with the microinjection. The location of the injection sites included the A5 area, was independent of the approach, and was distinct from the dl pons. These results complement our previous data and indicate that neurons located in the vl pons influence respiration specifically by prolonging expiration when activated and by delaying the inspiratory-to-expiratory phase transition when inhibited.

The role of pulmonary stretch receptor activation during cough in dogs

The role of pulmonary stretch receptors in the modulation of expiratory muscle activity during cough is controversial. To evaluate their potential influence on expiratory effort during cough, we compared expiratory muscle activity during unobstructed cough to that during obstructed cough in which the trachea was occluded at the end-inspiration and maintained throughout the subsequent expiration. Cough was evoked by mechanical stimulation of the intrathoracic trachea in 9 anesthetized, tracheotomized dogs. Peak triangularis sterni (TS), internal intercostal (IIC) and transversus abdominis (TA) muscle EMG were monitored to assess both rib cage and abdominal muscle activation during expiration. During cough, expiratory activity increased and peak activity shifted from Stage II to Stage I expiration. Peak expiratory muscle activation during unobstructed and occluded coughs were not significantly different: during unobstructed coughs, peak EMG's (mean +/- SE as percent of resting breathing) were TS, 212 +/- 18; IIC, 425 +/- 72; TA, 406 +/- 66; and during obstructed cough: TS, 188 +/- 24; IIC, 365 +/- 44; TA, 387 +/- 77 (n = 9). These data indicate that enhanced vagal stimulation resulting from airway occlusion does not affect expiratory activity during cough. We suggest that during cough, the expiratory muscles are activated in a stereotypical pattern by the neural network generating the cough and this pattern of activation is not affected by phasic vagal input.

Neurones in the ventrolateral pons are required for post-hypoxic frequency decline in rats

1. The breathing pattern following acute hypoxia (arterial O2 pressure (Pa,O2), 27.4 +/- 7.7 mmHg) was measured in intact, anaesthetized and spontaneously breathing adult rats (n = 4) and in anaesthetized, vagotomized, paralysed and ventilated animals (n = 14). Measurements were made both before and after bilateral lesions or chemical inactivation of neurones in the lateral pons. Respiratory motor activity was recorded as an index of the respiratory cycle. We tested the hypothesis that the ventrolateral pons is required for expression of post-hypoxic frequency decline, defined as a decrease in respiratory frequency below steady-state baseline levels following brief exposures to hypoxia. 2. We identified an area in the ventrolateral pons where brief (1 ms) low current (< or = 20 microA) pulses evoked a short-latency inhibitor of phrenic nerve activity. At this site, bilateral electrical or chemical lesions (n = 3) were performed, or neural activity was inhibited by focal injections of 10 mM muscimol (n = 9). In six control animals, neural activity was inhibited by muscimol injections into the lateral pons, dorsal to the target site. 3. Prior to pontine intervention, respiratory frequency decreased below baseline levels following 20-110 s of 8% O2. The decrease in frequency resulted from a prolongation of expiration (up to 276%), which gradually returned to baseline levels (tau = 45 s). 4. Following lesions or inhibition of neural activity in the ventrolateral pons, baseline inspiratory (TI) and expiratory (TE) durations were altered, albeit minimally, in the animals with intact vagus nerves. Expiratory duration following hypoxia was not different from baseline levels either in vagotomized (P = 0.18) or intact (P > 0.05) animals. In contrast, injections of muscimol at more dorsal sites did not alter the decrease in frequency normally seen following hypoxia. 5. Histological examination revealed that effective lesion or injection sites were within the lateral pontine tegmental field and included portions of the noradrenergic A5 cell group. 6. We conclude that the mechanism responsible for post-hypoxic frequency decline involves an active neural process that depends on the integrity of the ventrolateral pons.

Respiratory responses to tracheobronchial stimulation during sleep and wakefulness in the adult cat

The response to tracheal stimulation (50 microliters of tap water) during wakefulness, non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep was investigated in adult cats. In wakefulness, repetitive coughing occurred on 80% of the trials. In NREM and REM sleep, the most frequent response (approximately 69% and 58% of the trials, respectively) was arousal, followed by coughing. Apneas occurred following the stimulus and before arousal in 11% and 24% of the trials in NREM and REM sleep, respectively. In NREM sleep, the tracheal stimulus sometimes evoked expiratory efforts following a normal inspiratory effort (11% of the trials). These were much weaker than the expiratory efforts during coughing in wakefulness. In REM sleep, stimulation in 11% of the trials elicited increased inspiratory efforts. Although these may have been diminutive preparatory inspirations for coughing, they were much smaller than preparatory inspirations associated with coughing in wakefulness, and they were never followed by active expiratory efforts. Arousal from either NREM or REM sleep in response to tracheal stimulation was sometimes associated with an augmented breath. This response, which is common upon spontaneous arousal, may lead to deeper aspiration of the tracheal fluid. We conclude that in cats coughing requires wakefulness and that airway stimuli in sleep cause a variety of respiratory responses, some of which may be maladaptive.

Swallowing in sleep and wakefulness in adult cats

Clinical evidence indicates that swallowing, a vital function, may be impaired in sleep. To address this issue, we elicited swallows in awake and sleeping adult cats by injecting water through a nasopharyngeal tube. Our results indicate that swallowing occurs not only in non-rapid eye movement (NREM) sleep, but also in rapid eye movement (REM) sleep. In NREM sleep, the injections often caused arousal followed by swallowing, but, in the majority of cases, swallowing occurred in NREM sleep before arousal. These swallows in NREM sleep were entirely comparable to swallows in wakefulness. In contrast, the injections in REM sleep were less likely to cause arousal, and the swallows occurred as hypotonic events. Furthermore, apneas were sometimes elicited by the injections in REM sleep, and there was repetitive swallowing upon arousal. These results suggest that the hypotonic swallows of REM sleep were ineffective.

Paradoxical phase response at late expiration by superior laryngeal nerve stimulation

Superior laryngeal nerve (SLN) stimulation during expiration prolongs the respiratory cycle in decerebrate, vagotomized and paralysed cats. In a few animals, however, the cycle can be terminated prematurely by the same stimulus. We developed a mathematical model of the respiratory neural network to stimulate these responses. The model contained inspiratory decrementing (I-DEC), and augmenting (I-AUG) and expiratory decrementing (E-DEC), and augmenting (E-AUG) neurones. Connections were based on published findings. SLN stimulation during late expiration prolonged the cycle when it was assumed to excite principally E-DEC neurones, whereas it terminated the cycle prematurely when it was assumed to excite both I-DEC and E-DEC neurones. Therefore, phase-resetting depends on the differential strength of afferent connections on the network's elements.

A 'pneumotaxic centre' in rats

Electrical and chemical lesions in the ventrolateral pons produced apneustic breathing in anesthetized, vagotomized, paralyzed, ventilated adult rats (n = 13). Apneustic breathing did not develop if the vagi remained intact and was reversed partially with vagal (proximal end) stimulation. Physiologically, these data are similar to those obtained following dorsolateral pontine lesion in rat and other mammalian species and support the hypothesis that pontine neurons influence breathing similarly across mammalian species.

Pontine respiratory neurons in anesthetized cats

The pontine respiratory neurons (PRG) in the 'pneumotaxic centre' have been hypothesized to contribute to phase-switching of neural respiratory activity, especially in terminating inspiration. To define the neural elements involved in phase-switching, we recorded respiratory neurons extra- and intracellularly in anesthetized cats with an intact central nervous system. In total, 54 neurons were recorded: 49 neurons with activity modulated by central respiratory rhythm (20 inspiratory, 17 postinspiratory and 12 expiratory) and 5 neurons with activity correlated to tracheal pressure. The recorded neurons were clustered in dorsolateral pontine tegmentum within the Kölliker-Fuse (KF) subnucleus of the parabrachial nuclei. Stable intracellular membrane potential was recorded in 11 of the 49 respiratory neurons (8 postinspiratory, 1 early inspiratory and 2 inspiratory). During continuous injection of chloride ions (n = 6), synaptic noise increased and IPSPs reversed, including a wave of IPSPs during stage-2 expiration in postinspiratory neurons. Further, relative input resistance varied through the respiratory cycle such that the least input resistance occurred during the neuron's (n = 5) quiescent period. No IPSPs nor EPSPs were evoked in pontine respiratory neurons by vagal stimulation. In conclusion, various types of respiratory neurons were recorded in the KF nucleus. Prominent excitatory and inhibitory postsynaptic activities were similar to those described for medullary neurons. These pontine respiratory neurons do not appear to receive a strong afferent input from the vagus. Rather, vagal afferent inputs seem to be directed towards non-respiratory neurons that are located more medially in the dorsal pons.

Interaction between central pattern generators for breathing and swallowing in the cat

1. We examined the interaction between central pattern generators for respiration and deglutition in decerebrate, vagotomized, paralysed and ventilated cats (n = 10), by recording activity from the following nerves: hypoglossal, phrenic, thyroarytenoid and triangularis sterni. Fictive breathing was spontaneous with carbon dioxide above the apnoeic threshold (end-tidal PCO2, 32 +/- 4 mmHg) and fictive swallowing was induced by stimulating the internal branch of the left superior laryngeal nerve (SLN) continuously (0.2 ms pulse duration, 10 Hz). 2. In all ten animals, SLN stimulation evoked short bursts of thyroarytenoid and hypoglossal nerve activity indicative of fictive swallowing. In two of ten animals, respiration was inhibited completely during deglutition. In the other eight animals, fictive breathing and swallowing occurred simultaneously. 3. With SLN stimulation below threshold for eliciting swallowing, the respiratory rhythm decreased, the duration of inspiration did not change but the duration of expiration, especially stage II, increased. Integrated nerve activities indicated that the rate of rise and peak of phrenic nerve activity decreased, stage I expiratory activity of the thyroarytenoid and especially that of the hypoglossal nerve increased and stage II expiratory activity of the triangularis sterni nerve was suppressed completely. However, if inspired carbon dioxide was increased, i.e. hypercapnic ventilation, stage II expiratory activity remained partially during continuous SLN stimulation. 4. Fictive-swallowing bursts occurred only at respiratory phase transitions. At the minimal stimulus intensity that evoked repetitive swallowing bursts, the pattern of interaction between breathing and swallowing central pattern generators was consistent for each animal (n = 7) but was different across animals. In four animals, fictive swallows occurred at the phase transition between stage II expiration and inspiration, at the transition between inspiration and stage I expiration in one animal; and in two other animals, at the transition between stage I and II of expiration. 5. The response to SLN stimulation accommodated during the stimulus train. Accommodation was evident in both the interswallow interval (ISI) which lengthened, and the interaction pattern which had fewer swallows per breath as the stimulus period progressed. In contrast to the ISI, characteristics of the fictive swallow did not accommodate. For example, duration of the swallow was constant, distributed over a narrow range throughout the stimulus train. 6. We conclude that the central pattern generators for swallowing and breathing interact. The pattern of interaction supports the three-phase theory of respiratory pattern generation.(ABSTRACT TRUNCATED AT 400 WORDS)

Phase-dependent dynamic responses of respiratory motor activities following perturbation of the cycle in the cat

1. Electroneurographical (ENG) activities of a phrenic nerve, a thyroarytenoid (TA) branch of a recurrent laryngeal nerve, and a triangularis sterni (TS) branch of an internal intercostal nerve were recorded in decerebrate, vagotomized and paralysed cats. A superior laryngeal nerve (SLN) was stimulated electrically. Our objective was to evaluate transient changes in motor activity following a brief perturbation of the respiratory cycle by SLN stimulation. 2. Each motor nerve recorded represents a separate phase of the respiratory cycle. We measured the duration of phrenic ENG activity for inspiratory phase duration (TI) and similarly the duration of TA and TS ENG activity for the duration of stages I and II of expiration, respectively. Changes in the duration of the total respiratory cycle (TTOT) were also measured. Therefore, the changes in TTOT were accounted for directly by changes in each phase of the respiratory cycle. 3. Perturbation during the inspiratory phase inhibited phrenic activity either reversibly or irreversibly (premature termination of inspiration) depending on the strength and timing of the stimulus. Reversible inhibition of inspiration was associated with a transient activation (< 100 ms) of the TA nerve followed by a reactivation of the phrenic nerve, but the duration of the subsequent stages I and II of expiration remained the same. Thus, the prolongation of TTOT was completely accounted for by the lengthening of TI. 4. Premature termination of inspiration was followed by either a shortening (the first half of inspiration) or a lengthening (the second half of inspiration) of the duration of stage I expiration and consistently by a shortening of the duration of stage II expiration. The magnitude of these changes in the durations of stages I and II of expiration was phase dependent. Changes in the duration of all three phases of motor activity contributed to the changes in TTOT. 5. Perturbation during stage I expiration prolonged this stage but did not affect the duration of the succeeding stage (stage II expiration). The increase in the duration of stage I expiration appeared constant and not dependent on the time when the perturbation was delivered in stage I expiration. Thus, the change in TTOT was less phase dependent during stage I expiration than during inspiration and stage II expiration and was accounted for by changes in the duration of TA activity alone. 6. Perturbation during stage II expiration inhibited TS activity and evoked TA activity transiently.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsic properties of pharyngeal and diaphragmatic respiratory motoneurons and muscles

Breathing is a complex act requiring the coordinated activity of multiple groups of muscles. Thoracic and abdominal respiratory muscles expand and contract the lungs, whereas pharyngeal and laryngeal respiratory muscles maintain upper airway patency and regulate upper airway resistance. An appreciation of the importance of the latter muscle group in maintaining ventilatory homeostasis and in the pathophysiology of sleep apnea has led to extensive studies examining the neural regulation of pharyngeal dilator muscles. The present review examines the role of heterogeneity in motoneuron and muscle properties in determining the diversity in the electrical and mechanical behaviors of thoracic compared with pharyngeal muscle groups. Specifically, phrenic and hypoglossal motoneuron electrophysiological properties influence whether and the extent to which these neurons will fire in response to a given synaptic input arising from chemo- and mechanoreceptors and from respiratory and nonrespiratory pattern generators. Furthermore, thoracic and pharyngeal muscle properties determine the mechanical response to motoneuronal activity, including the speed of contraction, relationships between motoneuron firing frequency and force production, and whether force is maintained during repetitive activation. Heterogeneity in the functional capabilities of these motoneurons and muscles is in turn determined by diversity of their structural and biochemical properties. Thus, intrinsic properties of respiratory motoneurons and muscles act in concert with neuronal drives in defining the complex electrical and mechanical behavior of pharyngeal and thoracic respiratory motor systems.

Breath-to-breath variability in hypoglossal motor unit firing

Instability in the magnitude and timing of motor output to pharyngeal dilator muscles occurs during breathing. This contributes to alterations in upper airway resistance, and is one of several factors that play a role in the pathophysiology of obstructive apneas. To define the motor unit mechanisms accounting for such variability, geniohyoid motor unit activity was recorded simultaneously with diaphragm EMG in anesthetized cats spontaneously breathing 7% CO2 in O2. Variability was quantified with the coefficient of variation [CV = (SD/mean) x 100%]. In this preparation, we confirmed greater breath-to-breath variability of geniohyoid compared to diaphragm peak moving average EMGs. During recordings of geniohyoid motor unit activity, average CV of other respiratory parameters were as follows: peak diaphragm EMG 5.8%, inspiratory time 3.5%, expiratory time 3.8%. The average CV for geniohyoid motor unit activity patterns were substantially higher: spikes per breath 15.6%, mean firing frequency 13.3%, peak firing frequency 19.0%, minimal firing frequency 26.3%, onset time 40.9%, offset time 10.0% and duration of firing 12.8%. Values differed considerably among motor units, even when activity was recorded simultaneously. These findings suggest that variability is present in both intensity and timing of geniohyoid motor unit firing during breathing, and that different geniohyoid motor units appear to have varying degrees of stability during breathing.

Salivary secretion elicited by activation of the parabrachial nuclei in the cat

The lateral pontine tegmentum contains the parabrachial nuclei (NPB) which have been identified as a relay nucleus for cardiovascular, respiratory and gustatory systems, but their role in the regulation of these systems is not well understood. We examined the effects of electrical and chemical stimulation of the NPB on blood pressure, phrenic and hypoglossal nerve activity and salivary secretion. These variables were measured in eight anesthetized (alpha-chloralose/urethane, 30/150 mg/kg, n = 5) or decerebrate (n = 3) cats before, during, and after trains of electrical stimulation (1 ms pulse duration, 10 Hz 5 min train duration, currents as low as 10 microA) delivered unilaterally to NPB. Stimulation of the NPB elicited copious salivary secretion (1100 +/- 270 mg, mean +/- S.D.; P less than 0.001). Secretion was blocked completely by prior administration of atropine. The effects of the stimulus train on the respiratory and cardiovascular systems were variable and inconsistent even though short-latency responses of phrenic and hypoglossal nerve activities to single pulses were consistent. The short-latency response of phrenic nerve activity was biphasic, a decrease followed by an increase in activity; the response of hypoglossal nerve activity was monophasic, a transient increase in activity. Effects of electrical stimulation were replicated by the injection of an excitatory amino acid agonist (kainic acid) into the dorsolateral pons. Injection of 50 nl of 10 mM kainic acid into the NPB evoked salivary secretion, indicating that this response was elicited by stimulation of cell bodies in the region. In addition, chemical excitation increased breathing frequency, peak phrenic nerve activity, and blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Phase resetting of the respiratory cycle before and after unilateral pontine lesion in cat

The pontine respiratory group (PRG) facilitates the mechanism for terminating the inspiratory phase but may influence other phases in the respiratory cycle as well. We determined the effects of PRG lesions on the response of the respiratory cycle to superior laryngeal nerve stimulation delivered in each phase of the cycle in decerebrate, vagotomized, paralyzed, and ventilated cats (n = 6). We measured the duration of inspiration (TI) and expiration (TE) for three breaths before and in the perturbed breath and TI for three breaths after the perturbation. The delay to next inspiration was plotted against the phase at which the stimulus was delivered. Before lesioning, premature inspiratory termination was followed by phase-dependent shortening of TE. After lesioning, premature inspiratory termination did not systematically change the following TE. Breath-by-breath variability (measured 50 breaths) increased and stimulus after-effects (prolonged TI in the subsequent cycle) were augmented following lesions. These data indicate that the PRG plays an important role in the control of TE after perturbation and in the stability of the respiratory central pattern generator.

Contractile and endurance properties of geniohyoid and diaphragm muscles

Despite the wealth of information about the neural control of pharyngeal dilator muscles, little is known about their intrinsic physiological properties. In the present study the in situ isometric contractility and endurance of a pharyngeal dilator, the geniohyoid muscle, were compared with properties of the diaphragm in 12 anesthetized artificially ventilated cats. The contraction time (means +/- SE) of the geniohyoid (27 +/- 2 ms) was shorter than that of the diaphragm (36 +/- 3 ms; P less than 0.0005), as was the half-relaxation time (29 +/- 2 vs. 45 +/- 4 ms; P less than 0.002). The faster contraction and relaxation of the geniohyoid compared with the diaphragm were appropriately reflected in the shape of the force-frequency curves for the two muscles, with that of the geniohyoid located to the right of the diaphragm force-frequency curve. The endurance properties of the two muscles were assessed using repetitive stimulation at 40 Hz in trains lasting 0.33 s, with one train repeated every second. The ratio of force at the end of 2 min of repetitive stimulation to initial force was 0.67 +/- 0.06 for the geniohyoid and 0.15 +/- 0.03 for the diaphragm (P less than 0.00001). After the repetitive stimulation, the muscle force generated in response to a range of stimulus frequencies was reduced to a greater extent for the diaphragm than for the geniohyoid muscle. These results indicate that the geniohyoid muscle has a faster physiological profile than does the diaphragm yet is relatively resistant to fatigue when driven at high rates.

Artifactual averaged 'twitch tension' waveforms resulting from synchronized activity: recording from feline diaphragmatic motor units

Averaging techniques have been used to measure contractile properties of spontaneously active motor units (MUs). This study examined the potential for artifactual results due to synchronization between the triggering, single-MU action potentials, and activity of other MUs within the muscle. A muscle strip was formed in situ from feline diaphragm. Single MUs were recorded from the strip and from the contralateral diaphragm. The diaphragm including the muscle strip continue to contract rhythmically in this preparation and a high-gain, AC-coupled recording of force was averaged using MUs recorded in either hemidiaphragm to trigger the averager. Twitch-tension waveforms occurred in 42 of 49 cases triggering from spikes of MUs contained within the strip and in 13 of 19 averages triggered from contralateral MUs. The waveforms generated using contralateral MUs as triggers could only arise from synchronization with MUs contained within the diaphragmatic strip. Although twitch waveforms that were generated from external and internal triggers could appear similar qualitatively, contraction times were significantly (P less than 0.05) longer for averages using contralateral MUs. This study demonstrates that synchronization of triggering events is a major source for error in determining mechanical properties of MUs.

Fiber subtype distribution of pharyngeal dilator muscles and diaphragm in the cat

In previous studies differences were frequently found between the pharyngeal dilator muscles and the thoracic respiratory muscles in their patterns of electrical and mechanical activity during the respiratory cycle, with both resting and stimulated breathing. However, little is known about the intrinsic properties of the pharyngeal muscles and how they relate to the intrinsic properties of the diaphragm. In the present study, the fiber subtype distributions of two pharyngeal dilator muscles, the geniohyoid and the sternohyoid, were ascertained histochemically in the cat. The geniohyoid and the sternohyoid muscles had a preponderance of fast glycolytic (FG) fibers (mean 48 and 55%, respectively), a smaller number of fast oxidative-glycolytic (FOG) fibers (mean 36 and 31%, respectively), and few slow oxidative (SO) fibers (mean 16 and 14%, respectively). The percentages of SO fibers of both hyoid muscles were significantly (P less than 0.01) lower than that of the costal diaphragm, and the percentages of FOG and FG fibers were significantly higher than that of the diaphragm. In conclusion, the geniohyoid and sternohyoid muscles have histochemical characteristics usually associated with fast contraction and intermediate endurance properties.

Motor unit regulation of mammalian pharyngeal dilator muscle activity

The present study examined the cellular regulation of one of the pharyngeal dilator muscles, the geniohyoid, by assessing its motor unit (MU) behavior in anesthetized cats. During spontaneous breathing, MU that (a) were active during inspiration only (I-MU) and (b) were active during both inspiration and expiration (I/E-MU) were identified. I-MU had a later inspiratory onset time and a shorter duration of inspiratory firing than did I/E-MU (P less than 0.002 and P less than 0.0001, respectively). I-MU were usually quiescent whereas I/E-MU were usually active during the last 20% of inspiration. I/E-MU fired more rapidly (P less than 0.00001) and for relatively longer periods of time (P less than 0.00001) during inspiration than during expiration. End-expiratory airway occlusion (preventing lung expansion during inspiration) augmented the inspiratory activity of both I-MU and I/E-MU. Conversely, end-expiratory airway occlusion reduced the absolute and relative firing durations (P less than 0.002 and P less than 0.00002, respectively) and the firing frequency (P less than 0.001) of I/E-MU activity during expiration. These results indicate that (a) the complex pattern of pharyngeal dilator muscle activity is due to the integrated activity of a heterogeneous group of MU, (b) changes in the degree to which pharyngeal dilator muscles are active result from combinations of MU recruitment/decruitment and modulations of the frequency and duration of MU firing, and (c) gating of lung-volume afferent information occurs during the respiratory cycle.

Relationship between diaphragmatic activation and twitch tension to superimposed electrical stimulation in the cat

The purpose of these experiments was to evaluate the validity of the 'twitch-occlusion' method as an index of the extent of diaphragmatic activation and to assess the extent of diaphragmatic activation during inspiration. Studies were performed in situ on innervated, perfused muscle strips from the costal region of the diaphragm in ten anesthetized cats. We measured isometric tension generated by the diaphragm during inspiration and following an interpolated electrical stimulation (2 Hz, 100 microseconds, 3.0 x threshold) of the nerve. The extent of MU activation was assessed by comparing twitch amplitudes during electrical stimulation applied in expiration and in inspiration. Spontaneous inspiratory activity was induced by adding CO2 to the inspired oxygen. Within an animal, the relationship between diaphragmatic activation and twitch occlusion was linear (range of r values was from -0.88 to -0.94). The extent of spontaneous diaphragmatic activation was normalized by dividing tension at end inspiration by the average twitch tension caused by stimuli applied during expiration. Across animals, twitch amplitude was inversely related to diaphragmatic activation (y = -0.36x + 1.13, r = -0.94). At a respiratory drive with end-tidal PCO2 approximately 1% above apneic threshold (end-tidal PCO2 between 5 and 6%), twitch occlusion was less than 5.0%. Increasing end-tidal PCO2 to at least 5% above apneic threshold (end-tidal PCO2 between 9 and 11%), twitch occlusion was still less than 50%. These results from a preparation that allows direct measurement of isometric tension of the diaphragm show that the interpolated twitch is linearly related to the extent of muscle activation through a broad range of muscle activity. In addition, these data indicate that a higher respiratory drives there exists a large reserve in the phrenic motor pool.

Repetitive firing properties of phrenic motoneurons in the cat

1. Using both rectangular- and ramp-shaped intracellularly injected currents, the repetitive firing properties of 23 antidromically identified phrenic motoneurons were determined in anesthetized, paralyzed, and artificially ventilated cats during hypocapnic apnea. In response to rectangular depolarizing current injections, regular repetitive firing was observed in all cells. 2. At the beginning of a rectangular current pulse, the firing pattern was characterized by high frequency of firing that rapidly adapted to a much lower steady-state value. The relationship between the reciprocal of the first interspike interval (F1-2) and injected current was described by an initial linear portion of shallow slope, followed by a much steeper segment that smoothly reached a plateau value. The plateau value of F1-2 did not change with further increase in injected current. 3. The steady-state firing frequency versus injected current relationship was represented by a line of shallow slope over the entire range of injected currents. The slope of this line ranged between 1.1 and 4.5 Hz/nA. 4. A weaker correlation between minimal firing frequency for continuous activity and the reciprocal of the after hyperpolarization duration (1/AHPdur) was found for phrenic motoneurons than exists for lumbosacral motoneurons (26). Comparison of AHP shape at different levels of repetitive firing revealed that the slopes of the ascending portions of the AHP were similar except at the higher injected currents. Further, in the same cells during natural inspiratory activity the ascending part of the AHP was similar to that observed during current injection. 5. Depolarizing current ramps (approximately 1-s duration) were injected into 11 phrenic motoneurons. Instantaneous firing frequency was directly proportional to the intensity of the instantaneous injected current and independent of the rate of change of current for the range of ramp slopes tested (5-80 nA/s). Ramp-and-hold current injections were done in three motoneurons, and the peak instantaneous firing frequency showed little adaptation during the hold maneuver. 6. During hypocapnic apnea, we mimicked the expiratory-phase inhibition and inspiratory-phase excitation of phrenic motoneurons by injecting a 1-s depolarizing current ramp that was immediately preceded by a 1-s hyperpolarizing current ramp of the same absolute peak current intensity. Compared with the effects of positive current ramps alone the spike onsets during the negative-positive current ramp paradigm were either facilitated or retarded. Various ionic mechanisms are suggested for these effects as well as their function in determining the onset of firing during natur

Upper airway pressure receptors alter expiratory muscle EMG and motor unit firing

To examine the effects of upper airway negative pressure (UAW NP) afferents on respiratory muscle activity during expiration (TE), diaphragm electromyograms (EMG) and triangularis sterni EMG and single motor unit activity were recorded from supine anesthetized tracheotomized cats while they breathed 100% O2. The period of TE during which the diaphragm was electrically active (TE-1) and the period of TE during which the diaphragm was quiescent (TE-2) were both increased with continuous UAW NP (P less than 0.001 and P less than 0.05, respectively), as was TE-1 as a percent of TE (P less than 0.001). Continuous UAW NP reduced peak triangularis sterni EMG (P less than 0.001) and delayed its expiratory onset (P less than 0.005) but did not alter its duration of firing. Changes in triangularis sterni EMG were due to a combination of complete cessation of motor unit activity (2 of 17 motor units), a reduction in mean motor unit firing frequency (P less than 0.02), and a delay in the expiratory onset of motor unit activity (P less than 0.001). Qualitatively similar results were obtained when UAW NP was applied during inspiration only. We conclude that 1) UAW NP has reciprocal stimulatory and inhibitory influences on diaphragm and triangularis sterni muscle electrical activity, respectively, during expiration, and 2) the reductions in triangularis sterni EMG are due to both motor unit derecruitment and a slowing of motor unit firing frequency.

Electrophysiological determination of the axonal projections of single dorsal respiratory group neurons to the cervical spinal cord of cat

Antidromic microstimulation and orthodromic extracellular spike-triggered averaging were used to determine the axonal positions, divergence and terminations of 16 axons arising from bulbospinal, inspiratory (I) neurons. Activity from these neurons was recorded in the dorsal respiratory groups (DRG) of 12 cats. Systematic tracking was done both transversely and longitudinally in the contralateral fifth and sixth cervical segments of the spinal cord. Axonal position was determined by antidromically activating axons and by recording axonal field potentials. Thirteen axons were located in the lateral funiculus, two in the ventrolateral funiculus and one in the ventral funiculus. Axonal conduction velocity (CV) was calculated from conduction distance and conduction time, the latter defined as the interval of time from the recorded action potential in the medulla to the recorded averaged axonal potential in the spinal cord. Average (+/- S.D.) axonal CV was 52 +/- 15 m/s. Terminal potentials were recorded for 13 of these axons and were coincident with the location of evoked field potentials resulting from antidromic stimulation of phrenic motoneurons. In addition, terminal potentials from single I cells were recorded at multiple sites along the longitudinal axis of the phrenic motor column. These data indicate that axons of spontaneously active, DRG bulbospinal I cells descend predominantly in the lateral columns and diverge and terminate extensively within the phrenic motor column.

Inhibition of expiratory muscle EMG and motor unit activity during augmented breaths in cats

To test the hypothesis that expiratory muscle activity is reduced during augmented breaths, electromyographic activity (EMG) of the triangularis sterni (TS) was recorded from eight pentobarbital anesthetized cats. Augmented breaths significantly increased tidal volume and peak diaphragm EMG, and prolonged inspiratory time and the first phase of expiration. However, the duration of the second phase of expiration was unchanged. Peak TS EMG was reduced during sighs in all animals, from 25 +/- 5 to 12 +/- 2 arbitrary units (P less than 0.005). Furthermore, the onset of TS activity during expiration was significantly delayed during augmented breaths (P less than 0.002), whereas the duration of expiratory firing tended to decrease but not significantly. Electrical activity was recorded from eight motor units of the TS in five cats. During resting breathing the motor units had a mean relative expiratory onset time of 46 +/- 4% of expiration, and a mean firing frequency of 19 +/- 2 impulses/sec. Two motor units became quiescent during augmented breaths. Of the remaining six motor units, three minimally shortened their duration of activity (by less than 15%) while three substantially abbreviated their period of firing (by 50% or more). In addition, all TS motor units reduced their mean firing frequency (P less than 0.05) and number of impulses per breath (P less than 0.002) during sighs. We conclude that expiratory activity of the triangularis sterni muscle is reduced during augmented breaths, due to a combination of motor unit derecruitment and a slowing of motor unit firing frequency.

Projections and terminations of single respiratory axons in the cervical spinal cord of cat

Position, divergence, branching, and termination patterns of single, respiratory axons were studied in cat cervical spinal cord by injecting horseradish peroxidase (HRP) intra-axonally. We stained 12 axons which were characterized by their firing patterns and by electrical stimulation. Five axons discharged during inspiration (I); the remaining 7 discharged during expiration (E). No injected axon was evoked by stimulating ipsilateral phrenic nerve roots while 7 (4 I, 3 E) of 12 were excited at a short latency from stimulating at a medullary site (on the midline, 1-2 mm rostral to the obex, approximately 3 mm below the dorsal medullary surface) where many bulbospinal respiratory axons decussate. All injected stem axons were located in the ventral and ventrolateral funiculi, traversed in a rostrocaudal direction, and were stained for lengths ranging from 3.6 to 12.4 mm. Mean axonal diameter was 2.9 microns. In 6 axons (4 I, 2 E), 14 collaterals were stained: 1 on each E axon, 2 on one I axon, 3 each on 2 others and 4 on another I axon. Collaterals emerged perpendicularly from the descending stem axon and projected directly to the ventral horn. The average distance between neighboring collaterals was 1.0 mm (n = 7). Collaterals did not arborize until they were near or within the ventral horn. Both en passant and terminaux types of presynaptic boutons were found primarily within the rostrocaudal cylinder that defined the phrenic motor column. In addition, some boutons were located dorsomedial to the phrenic motor column. We conclude that I axons, presumably of medullary origin, have multiple collaterals which terminate primarily in the phrenic motor column. However, the same axon can have terminals in different regions of the ventral horn, which are known to contain dendrites of phrenic motoneurons.

Connectivity of slowly adapting pulmonary stretch receptors with dorsal medullary respiratory neurons

1. Intracellular recordings were made from 50 dorsal respiratory group (DRG) neurons in the region of the ventrolateral nucleus of the solitary tract in anesthetized, paralyzed cats ventilated with a cycle-triggered pump whose inflation stroke was triggered by the onset of phrenic nerve inspiratory (I) discharge. Activity was recorded simultaneously in the ipsilateral nodose ganglion from sensory cell bodies of slowly adapting pulmonary stretch receptors (PSRs). 2. Respiratory cycle-related membrane potential changes of DRG neurons were recorded. Twenty-six neurons that did not exhibit spikes were classified as I alpha, I beta or pump (P)-cells by comparing their membrane potential trajectories during I in the presence of lung inflation with that observed during I, but with lung inflation withheld. The remaining 24 neurons were classified similarly, but the classification was based upon a comparison of their I-phase spike activity responses with and without lung inflation. I phase-related histograms of either membrane potential or spike activity were constructed to facilitate DRG neuronal classification. Additionally, steady lung inflation of varying magnitudes was applied during the expiratory phase. This prolonged expiration and produced different responses in the neurons. Generally, I beta and P-cells were depolarized, whereas I alpha cells were hyperpolarized. 3. Low-intensity electrical stimulation of the ipsilateral vagus nerve evoked excitatory postsynaptic potentials (EPSPs) in all three DRG neuronal types. P-cells and I beta cells exhibited EPSPs in response to the lowest intensity; generally this intensity was below threshold for the simultaneously recorded PSR. Overall, EPSPs in I alpha cells had the highest thresholds, but some EPSPs could be evoked at thresholds similar to those of the I beta cells. The distributions of the average onset latency of the evoked EPSP overlapped considerably. Thus vagal electrical stimulation cannot be used for unequivocal classification of DRG neurons into I alpha, I beta, and P-cell subpopulations. 4. Using intracellular spike-triggered averaging, single PSRs were shown to generate monosynaptic EPSPs in I beta neurons and P-cells but not I alpha cells. Divergence of single PSR afferents also was observed. Relationships between EPSP shape factors, amplitudes, and PSR afferent conduction velocity are similar to those previously observed for monosynaptic EPSPs in hindlimb motoneurons generated by spinal afferents.

Serotonin-mediated excitation of recurrent laryngeal and phrenic motoneurons evoked by stimulation of the raphe obscurus

Short-latency averaged responses in the recurrent laryngeal nerve (RLN) and C5 phrenic nerve to electrical stimulation (2.5-80 microA; 2.5-160 Hz; 150 microseconds pulse duration) of the medullary nucleus raphe obscurus (RO) were investigated in anesthetized, paralyzed and artificially ventilated cats. The response evoked in RLN by stimulation within RO was excitatory and consisted of a single peak. Characteristics of this response in RLN were compared with those of the delayed excitatory response in C5 phrenic nerve, which we previously showed to be elicited by stimulation within RO. Mean latency to onset for the excitatory response in RLN was 5.7 +/- 0.3 ms, while the delayed excitatory response in C5 phrenic nerve occurred at 7.0 +/- 0.3 ms. The excitatory response in both nerves could be evoked when stimulation was applied during inspiration as well as during expiration. The stimulus threshold varied between 2.5 and 5 microA for evoking the inspiratory-phase response in each nerve. The magnitude of this response in RLN and in C5 phrenic nerve was directly related to current intensity and was dependent upon stimulus frequency. Intravenous administration of the serotonin receptor antagonist, methysergide (0.1-2.4 mg/kg) caused significant dose-related reductions in the response in each nerve. In summary, characteristics of the evoked response in RLN and phrenic nerve are similar in several ways. Both responses are: (1) excitatory in nature, (2) elicited at small stimulus currents, (3) affected similarly by increasing stimulus current and frequency, (4) elicited by stimulation during inspiration and expiration, and (5) mediated at least in part by activation of pathways using serotonin as a neurotransmitter.

Electrical properties of phrenic motoneurons in the cat: correlation with inspiratory drive

1. Resting membrane potential (Vmp), input resistance (Rn), rheobase (Irh), and after hyperpolarization duration (AHPdur) and amplitude (AHPamp) were measured in 38 phrenic motoneurons of anesthetized, paralyzed, and artificially ventilated cats during hypocapnic apnea. The mean +/- SD and range of values for these variables were as follows: Vmp, -68 +/- 5mV (range: -60 to -82); Rn, 1.3 +/- 0.6 M omega (0.6-2.4); Irh, 9.7 +/- 5 nA (2-20); AHPdur, 68 +/- 19 ms (37-134); AHPamp, 3.3 +/- 1.8 mV (1.0-8.5). In 31 motoneurons, the membrane potential level at which firing occurred (Vthr) during intracellular current injection was measured. The mean value of Vthr was -58 +/- 3 mV (range: -52 to -64). 2. A histogram of Rn revealed a bimodal distribution. Also a plot of Irh against Rn showed a grouping of the motoneurons into two subpopulations: 1) low-Rn and high-Irh cells, called type L neurons, and 2) high-Rn, low-Irh cells, called type H neurons. The overall negative linear correlation between Irh and Rn (r = -0.85; P less than 0.0001) resulted from this grouping rather than from a strictly linear relation between these two variables. 3. Electrical properties were compared for type L (n = 20) and type H (n = 18) phrenic motoneurons. The following mean values were found for each group, respectively: Rn, 0.8 and 1.8 M omega; Irh, 13.7 and 5.3 nA; AHPdur, 58 and 79 ms; AHPamp, 2.4 and 4.4 mV. All differences were significant (t test, P less than 0.001). Mean Vthr was the same for the two groups. 4. Comparison of these data with those available for lumbosacral motoneurons revealed that almost all investigated electrical properties of type L and type H phrenic motoneurons are similar to the analogous properties of type F (fast twitch) and type S (slow twitch) lumbosacral motoneurons, respectively. The apparent exception is the lower mean value of Irh for type L phrenic motoneurons compared with type F lumbosacral motoneurons. 5. For 13 cells, membrane potential was continuously monitored while spontaneous respiratory activity was restored by increasing CO2. It was found that at approximately the same end-tidal CO2 (about 7%) and a similar end-expiratory mean membrane potential level (approximately -70 mV), mean amplitude of peak inspiratory synaptic depolarization was higher in type H motoneurons (8.8 mV, n = 5) than in type L (2.9 mV, n = 8; P less than 0.001).(ABSTRACT TRUNCATED AT 400 WORDS)

Correlation of recruitment order with axonal conduction velocity for supraspinally driven diaphragmatic motor units

Spontaneous activities of pairs of single diaphragmatic motor units (MUs) were recorded via two electrodes in anesthetized cats, ventilated with CO2 added to the inspired gas, which slightly enhanced respiratory drive (endtidal CO2 less than 6%). These MUs were characterized by their axonal conduction velocities (CVs) and relative onset times (defined as the time after onset of phrenic nerve activity until the MU began discharging divided by the duration of inspiration). Motor unit axonal CV was estimated by the conduction time and the distance between two points on the phrenic nerve. Results were compared from two experimental preparations: one with dorsal roots intact and the other with dorsal roots transected bilaterally between fourth (C4) and seventh (C7) cervical segments. Estimated mean CV for phrenic MUs was 46.2 m/s(n = 180 MU). Motor units were classified as early and late recruited MUs depending on their relative onset times. We correlated MU axonal CV with its relative recruitment time. A highly significant (P less than 0.0001), positive correlation between axonal CV and relative recruitment time was established for those diaphragmatic MUs recruited with this respiratory drive. Correlation coefficients were r = 0.70 for intact animals, r = 0.72 for dorsal rhizotomized animals, and r = 0.72 overall population. For pairs of MUs, the CV of the earlier recruited unit was compared with the CV of the later recruited unit. In 96% of pairs from intact animals and 92% of pairs from dorsal rhizotomized animals, the first MU had a lower CV than the MU recruited later. Difference in relative times of recruitment was directly related to difference in axonal CVs. However, a portion of the motor pool with high-axonal CVs was not sampled. Under conditions of these experiments, afferent input in cervical dorsal roots, including that from diaphragmatic receptors, did not influence the distribution of MU relative onset times. Further, a similar proportion of MU pairs wherein the earlier recruited MU had a CV lower than the later recruited unit was observed in intact and dorsal rhizotomized animals. We also cross-correlated 31 pairs of simultaneously recorded MUs to assess common input onto phrenic motoneurons. Common input was characterized by the presence of peaks having widths of greater than or equal to 3 ms in the cross-correlation histograms (CCHs) and occurring within 20 ms of the trigger event. Peaks were judged significant if the bin with the largest number of occurrences was significantly greater than base line and if neighboring bins were above base line.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of serotonin in the excitation of phrenic motoneurons evoked by stimulation of the raphe obscurus

Short-latency averaged responses in the C5 phrenic nerves to electrical stimulation (2.5-80 microA; 5-80 Hz; 150 microseconds pulse duration) of raphe pallidus (RP) and raphe obscurus (RO) were investigated in anesthetized, paralyzed, and artificially ventilated cats. The responses to stimulation of RO were excitatory, whereas a mixture of inhibitory and excitatory responses of lesser magnitude were observed after stimulating in RP. The maximal response was obtained from the ventral part of RO and consisted of early and delayed excitatory responses that were of equal magnitude in both left and right C5 phrenic nerve roots. The mean latency for the early response was 2.5 +/- 0.1 msec and for the delayed response was 7.0 +/- 0.2 msec. Both responses were elicited during inspiratory phase stimulation, but only the delayed response was present during expiratory phase stimulation. The stimulus threshold of the early response was 5 microA; the delayed response was elicited at currents as small as 2.5 microA. Early and delayed responses were affected in different ways by increasing stimulus current and by increasing stimulus frequency. Intravenous administration of serotonin receptor antagonists methysergide (0.1-0.7 mg/kg), metergoline (33-244 micrograms/kg), and cinanserin (1.5-9.0 micrograms/kg) caused significant dose-related reductions in the magnitude of the delayed response, but did not significantly affect the early response. These data suggest that the early and delayed excitatory responses are mediated by different neuronal pathways. The early response does not involve serotonin release, while the later response is mediated at least in part by activation of a serotonergic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)