Responses of ventral respiratory neurones in the rat to vagus stimulation and the functional division of expiration.

In anaesthetized rats, extracellular and intracellular recordings were taken from 106 respiratory neurones in the intermediate region of the nucleus ambiguus. We observed unprovoked shortening of expiratory time accompanied, in all classes of respiratory neurone, by the elimination of the changes in membrane potential that were characteristic of stage II expiration. The demonstration of the elimination of stage II expiration in both the rat and cat strongly supports the functional division of expiration into stage I expiration (post-inspiration) and stage II expiration. In order to identify the neurones in the rat that receive inputs from vagal afferents and modulate the central respiratory rhythm, we examined whether any respiratory neurones responded to stimulation of the vagus nerve. Some post-inspiratory and stage II expiratory neurones responded. The short latency (< 2 ms) of four of the responses indicates that some vagal afferents act on post-inspiratory neurones via a disynaptic pathway. While repetitive stimulation of the vagus nerve could inhibit the phrenic rhythm, it appears that most inspiratory neurones in the intermediate region of the nucleus ambiguous complex are not directly involved in integrating the information from vagal afferents with the central respiratory rhythm.     

A combined blockade of glycine and calcium-dependent potassium channels abolishes the respiratory rhythm

In order to test whether glycinergic inhibition is essential for the in vivo respiratory rhythm, we analysed the discharge properties of neurones in the medullary respiratory network after blockade of glycine receptors in the in situ perfused brainstem preparation of mature wild type and oscillator mice with a deficient glycine receptor. In wild type mice, selective blockade of glycine receptors with low concentrations of strychnine (0.03-0.3 microM) provoked considerable changes in neuronal discharge characteristics: The cycle phase relationship of inspiratory, post-inspiratory and expiratory specific patterns of membrane potential changes was altered profoundly. Inspiratory, post-inspiratory and expiratory neurones developed a propensity for fast voltage oscillations that were accompanied by multiple burst discharges. These burst discharges were followed by "after-burst" hyperpolarisations that were capable of triggering secondary burst discharges. Blockade of glycine receptors and the "big" Ca2+-dependent K+-conductance by charybdotoxin (3.3 nM) resulted in loss of the respiratory rhythm, whilst only tonic discharge activity remained. In contrast, rhythmic activity was only weakened, but preserved after the "small" Ca2+-dependent activated K+ conductance was blocked with apamin (8 nM). Also low concentrations of pentobarbital sodium (6 mg/kg) abolished rhythmic respiratory activity after blockade of glycine receptors in the wild type mice and in glycine receptor deficient oscillator mice. The data imply that failure of glycine receptors provokes enhanced bursting behaviour of respiratory neurones, whilst the additional blockade of BKCa channels by charybdotoxin or with pentobarbital abolishes the respiratory rhythm.     

Inhibitory synaptic mechanisms regulating upper airway patency

The breathing cycle of vertebrates comprises three phases (inspiration, postinspiration and expiration) that are apparent in the activities generated in the ponto-medullary respiratory network. A large body of evidence now indicates that in adult mammals generation of this three-phase pattern is based on reciprocal synaptic inhibition between distinct subsets of respiratory neurones. This review summarises our recent experiments focused on the role of glycinergic inhibition in respiratory pattern formation: e.g. in co-ordinating the activity of spinal and cranial motor outputs that drive the ventilatory pump (thoracic and abdominal muscles) and adjust airflow by regulating laryngeal resistance (laryngeal abductors and adductors). We used arterially perfused in situ preparations of neonatal and mature rat and show that specific blockade of glycine receptors within the ponto-medullary network caused a severe disruption of the co-ordination of spinal and cranial motor outputs: postinspiratory neurones lose their characteristic inspiratory inhibition revealing excitatory synaptic drive coincident with inspiratory phrenic nerve activity. The resulting simultaneous discharge of inspiratory and postinspiratory neurones caused co-activation of both glottal abductors and adductors during neural inspiration. The latter resulted in a paradoxical inspiratory adduction of the vocal fold and severe disruption of the eupneic breathing pattern.The effect of blocking glycine receptors was the same in both mature and newborn rats suggesting that glycinergic inhibition is essential for co-ordinating cranial and spinal motor outputs from birth.     

The differential organization of medullary post-inspiratory activities

Membrane potential trajectories of 68 bulbar respiratory neurones from the peri-solitary and peri-ambigual areas of the brain-stem were recorded in anaesthetized cats to explore the synaptic influences of post-inspiratory neurones upon the medullary inspiratory network.A declining wave of inhibitory postsynaptic potentials resembling the discharge of postpinspiratory neurones was seen in both bulbospinal and non-bulbospinal inspiratory neurones, including alpha- and beta-inspiratory, early-inspiratory, late-inspiratory and ramp-inspiratory neurones.Activation of laryngeal and high-threshold pulmonary receptor afferents excited bulbar post-inspiratory neurones, whilst in the case of inspiratory neurones such stimulation produced enhanced postsynaptic inhibition during the same period of the cycle. Activation of post-inspiratory neurones and enhanced post-inspiratory inhibition of inspiratory bulbospinal neurones was accompanied by supression of the after-discharge of phrenic motoneurones.These results suggest that a population of post-inspiratory neurones exerts a widespread inhibitory function at the lower brain-stem level. Implications of such an inhibitory function for the organization of the respiratory network are discussed in relation to the generation of the respiratory rhythm.     

Electrical stimulation of arterial and central chemosensory afferents at different times in the respiratory cycle of the cat: II. Responses of respiratory muscles and their motor nerves

The response patterns of the electrical activity of the respiratory motor nerves and muscles to brief electrical stimulation of the arterial and the intracranial chemosensory afferents were studied in anesthetized cats. Stimulation during inspiration increased the activity of phrenic nerve and the inspiratory muscles (intercostal, diaphragm) with a latency of 15–25 ms, whereas expiratory muscle activity in the following expiration remained almost unaltered. Stimulation during expiration increased the activity of expiratory nerves and muscles (intercostal, abdominal) after a delay of 80–120 ms. The later the stimulation occurred in the insor expiratory period the larger the increase in amplitude and in steepness of rise of the respective integrated activity in respiratory nerves and muscles. Stimulation in early inspiration shortened the discharge period of inspiratory muscles, whereas excitation in early expiration caused an earlier onset and prolonged the activity in the expiratory muscles. Stimulation in the late phase of ins- or expiration prolonged the discharge of the respective nerves and muscles. Both the arterial (carotid sinus nerve, CSN, and aortic nerve, AN) and intracranial chemosensory (VM) afferents stimuli were able to affect both the inspiratory and the expiratory mechanisms. The restriction of the effects to the phase of the stimulus suggests a mechanism by which these afferents, when activated during inspiration, effectively project only to inspiratory neurones, and vice versa for expiration.Supported by the Deutsche Forschungsgemeinschaft, SFB 114 Bionach     

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

Summary Our aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the presynaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and offswitch neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of off-switch, postinspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration. The variability of inspiratory duration increased five-fold whereas the variability of expiration was unchanged. We conclude that in the apneustic state, (1) inspiratory on-switching and the successive activation of the different inspiratory neuronal types are preserved; (2) near the end of the inspiratory ramp, the reversible phase of inspiratory off-switching is prolonged, producing the apneustic plateau, and (3) the irreversible phase of offswitching is impaired by a reduced activity of postinspiratory neurons. These results support the 3-phase model of respiratory rhythm generation, in which key roles are played by early-inspiratory and post-inspiratory neurons.     

Inhibitory synaptic transmission governs inspiratory motoneuron synchronization

Neurons within the intact respiratory network produce bursts of action potentials that cause inspiration or expiration. Within inspiratory bursts, activity is synchronized on a shorter timescale to generate clusters of action potentials that occur in a set frequency range and are called synchronous oscillations. We investigated how GABA and glycine modulate synchronous oscillations and respiratory rhythm during postnatal development. We recorded inspiratory activity from hypoglossal nerves using the in vitro rhythmically active mouse medullary slice preparation from P0-P11 mice. Average oscillation frequency increased with postnatal development, from 17 +/- 12 Hz in P0-P6 mice (n = 15) to 38 +/- 7 Hz in P7-P11 mice (n = 37) (P < 0.0001). Bath application of GABAA and GlyR antagonists significantly reduced oscillation power in neonates (P0-P6) and juveniles (P7-P10) and increased peak integrated activity in both age groups. To test whether elevating slice excitability is sufficient to reduce oscillation power, Substance P was bath applied alone. Substance P, although increasing peak integrated activity, had no significant effect on oscillation power. Prolonging the time course of GABAergic synaptic currents with zolpidem decreased the median oscillation frequency in P9-P10 mouse slices. These data demonstrate that oscillation frequency increases with postnatal development and that both GABAergic and glycinergic transmission contribute to synchronization of activity. Further, the time course of synaptic GABAergic currents is a determinant of oscillation frequency.     

Reconfiguration of respiratory-related population activity in a rostrally tilted transversal slice preparation following blockade of inhibitory neurotransmission in neonatal rats

Recent studies showed that respiratory rhythm generation depends on oscillators located in the pre-Bötzinger complex (pre-BötC) and the parafacial respiratory group (pFRG). To study inhibitory synaptic interactions between these two oscillators, we developed a rostrally tilted transversal slice preparation, which preserves these regions. The onset of rhythmic mass activity in the retrotrapezoid nucleus (RTN)/pFRG preceded that of the pre-BötC. Blockade of glycinergic and gamma-aminobutyric acidic inhibition synchronized pre-BötC and RTN/pFRG activity and significantly increased pre-BötC burst frequency, amplitude, and duration. Population imaging revealed recruitment of inspiratory-like neurones, while expiratory-like neurones lost their phasic activity. The reconfiguration after disinhibition reveals: (1) synaptic inhibition of the pre-BötC arising from the RTN/pFRG, (2) excitatory drive from the RTN/pFRG that triggers the pre-BötC burst. Our findings support the view that these synaptic interactions in vitro relate to the initiation of the inspiratory phase or to the steering of the expiratory–inspiratory phase transition in vivo.     

Activity of Brainstem Respiratory Neurones just before the Expiration-Inspiration Transition in the Rat

Inspiratory activity of the hypoglossal nerve (XIIn) often precedes that of the phrenic nerve (PHRn). By manipulating artificial respiration, this preceding activity (pre-I XIIn activity) can be lengthened or isolated prematurely (decoupled XIIn activity) without developing into overt PHRn-associated inspiratory bursts. We hypothesized that these pre-I and decoupled XIIn activities, collectively termed 'XIIn-w/o-PHRn activity', reflect certain internal states of the respiratory centre at the period just prior to the transition from the expiratory phase to the inspiratory phase. In decerebrate, neuromuscularly blocked and artificially ventilated rats, the firing properties of medullary respiratory neurones were examined during the period of the XIIn-w/o-PHRn activity. The majority of the inspiratory neurones examined could be classified into two types: one was active (XIIn-type) and the other was inactive (PHRn-type) during the XIIn-w/o-PHRn period. On the other hand, augmenting expiratory (E-AUG) neurones of the Bötzinger complex (BOT) and the caudal ventral respiratory group (VRG) fired intensively during this period. Their firing stopped at the onset of the overt inspiratory bursts in the XIIn and PHRn, suggesting that BOT E-AUG neurones inhibit PHRn-type, but not XIIn-type, inspiratory neurones. We hypothesize that XIIn-type inspiratory activity facilitates the phase change from expiration to inspiration, through activation of certain inspiratory neurones that inhibit the firing of BOT E-AUG neurones and generation of the overt inspiratory bursts in XIIn-type and PHRn-type inspiratory neurones.     

Inspiratory on-switch evoked by mesencephalic stimulation: activity of medullary respiratory neurones

Summary The activity of medullary inspiratory and expiratory neurones was studied in urethan-chloralose anaesthetized cats during stimulus — evoked inspiratory phase (inspiratory on-switch). All neurones were characterized according to their axonal destination (i.e. bulbospinal neurones or vagal motoneurones) or the absence of such axonal projections (i.e. propriobulbar neurones), and to their location in the dorsal or ventral respiratory nuclei. 1. The inspiratory on-switch effects were elicited during expiration (E phase) by brief tetanic electrical stimulation (50 to 100 ms duration; 0.5 mA; 300 Hz) delivered to the mesencephalic periaqueductal central gray and the adjacent reticular formation. The evoked inspiratory effects observed on the phrenic nerve discharge consisted of: (i) an immediate response (latency 20 ± 5 ms) of stable duration related to the stimulus (primary response: Prim.R.), (ii) a delayed response (patterned response: Patt.R.) appearing after a latent period (silent phase: Sil.P.) of 100 ms maximal duration. The later the stimulus in the E phase, the longer was the duration of the Patt.R. (300 to 1000 ms). 2. The stimulation evoked an earlier activation of the inspiratory bulbospinal neurones (latency 12 ± 6 ms) than that obtained in the phrenic nerve (Prim.R.). Hence, the Prim.R. originated from the bulbospinal pathway and not from a pathway directly impinging on the motoneurones. Conversely during stimulation very few inspiratory propriobulbar neurones were activated and no expiratory neurone discharged. 3. During the phrenic Sil.P., 46% of the inspiratory bulbospinal neurones continued to discharge with a firing rate lower than that during the stimulus train, while most of the inspiratory propriobulbar and expiratory neurones were not active. 4. During the Patt.R. all inspiratory bulbospinal neurones discharged early and were strongly activated whatever the Patt.R. duration whereas the expiratory neurones were not active. Inspiratory propriobulbar neurones were either not recruited or recruited later, and the number of active neurones increased as the duration of the Patt.R. lengthened. 5. Our results suggest that the eliciting of the stimulus-evoked inspiration (Patt.R.) primarily depends on the activation of the inspiratory bulbospinal neurones. These neurones therefore would not only be the output neurones of the medullary respiratory centres, but they would serve other roles such as building up of the excitation in other respiratory neurones, thus acting as a component of the inspiratory ramp generator.Abbreviations Prim.R Primary response - Patt.R Patterned response - Sil.P Silent phase - I phase Inspiratory phase - E phase Expiratory phase - IBSN Inspiratory bulbospinal neurones - IPBN Inspiratory propriobulbar neurones - EBSN Expiratory bulbospinal neurones - EPBN Expiratory propriobulbar neurones - DRN Dorsal respiratory nucleus - VRN Ventral respiratory nucleus Supported by CNRS (LA 205 and ATP no 4188) and Fondation pour Ia recherche médicale     

Studies on the synaptic interconnection between bulbar respiratory neurones of cats

In cats anaesthetized with pentobarbital, medullary respiratory neurones of both dorsal and ventral populations were recorded intracellularly with 1 mol·l^–1 KCl-electrodes. The neurones were classified according to the projection of their axons to the spinal cord (bulbospinal neurones) or to the vagal nerves (vagal neurones). Those neurones which could not be activated antidromically (NAA-neurones) by either procedure were subdivided into (inspiratory) R-neurones, which were monosynaptically excited by lung stretch receptor afferents, and into inspiratory and expiratory NAA-neurones, which did not receive a direct synaptic input, from these afferents.All types of neurone investigated revealed postsynaptic activity during both inspiration and expiration. The periods when synaptic activity was minimal were the periods of transition between respiratory phases.The input resistance of most respiratory neurones varied in parallel with the respiratory cycle. A drastic fall of the input resistance during expiration was observed in R-neurones and in some inspiratory vagal neurones. This was not seen in inspiratory bulbospinal neurones.In stable intracellular recordings, periodic postsynaptic inhibition was demonstrated in 52 of 53 respiratory neurones by IPSP reversal following chloride injection. Maximal membrane potential then was generally reached during one of the periods of respiratory phase transition. Reasons for the failure of others to demonstrate these IPSPs are presented and discrepancies between other findings and these are discussed. It is concluded that reciprocal inhibition between bulbar respiratory neurones does exist and is a general phenomenon.It is argued that reciprocal inhibition is the fundamental mechanism underlying respiratory gating of afferent inputs.The probable existence of recurrent inhibition is inferred from the changes in the pattern of membrane depolarization during the active period of neurones.Supported by the Deutsche Forschungsgemeinschaft     

Studies of the pulmonary vagal control of central respiratory rhythm in the absence of breathing movements

1. Breuer's hypothesis that the vagus nerves exert a tonic control of respiratory rhythm, in addition to the phasic control, was examined.2. Closed-chest cardiopulmonary bypass was instituted in dogs weighing 20-30 kg anaesthetized with chloralose. Respiratory rhythm was recorded from a phrenic electroneurogram.3. Complete muscular paralysis induced with gallamine triethiodide produced an increase in the duration of inspiration and an increase in the amplitude of the integrated phrenic electroneurogram. There was no consistent effect on expiratory duration. Gallamine produced no effect when given after vagotomy.4. In the paralysed state, an increase in lung volume of 25-100 ml. for 30-60 sec produced a sustained increase in the duration of expiration and a decrease of respiratory rate: there was little effect on inspiratory duration, or the amplitude of the integrated phrenic electroneurogram.5. A decrease in lung volume of the same order of magnitude for the same period produced a sustained decrease in the duration of expiration and an increase of respiratory rate: there was little effect on inspiratory duration or the amplitude of the integrated phrenic electroneurogram.6. The phenomena described in (5) and (6) constitute a high gain respiratory frequency controller. They did not occur after bilateral cervical vagotomy.7. Bilateral cervical vagotomy during complete muscular paralysis produced a further increase in the duration of inspiration and in the amplitude of the integrated phrenic electroneurogram; there was no consistent effect on expiratory duration.8. The results confirmed Breuer's hypothesis and showed that inspiratory duration and expiratory duration are controlled independently.     

The importance of timing on the respiratory effects of intermittent carotid sinus nerve stimulation

1. The respiratory response, measured directly as tidal volume or indirectly by using integrated peak phrenic activity, to intermittent electrical stimulation of the carotid sinus nerve was determined in anaesthetized cats.2. Stimulation at rates of 20-25 Hz for 0.5 sec had a rapid effect, increasing inspiratory airflow and phrenic discharge, but only if applied during inspiration. An increase in tidal volume or peak level of integrated phrenic discharge occurred only if the stimulus was exhibited during the second half of inspiration. Continuous stimulation had no greater effect on size or frequency of breathing than did intermittent inspiratory stimuli alone. Stimulation during expiration had no effect on the form or magnitude of subsequent breaths.3. Stimuli in expiration led to a prolongation of expiration. Stimuli in late inspiration caused a prolongation of both inspiration and expiration. Because of these effects, the respiratory rate could be changed by stimulation; in some instances entrainment of respiration by the intermittent carotid sinus nerve stimuli occurred.4. The findings are attributable to modulation of incoming carotid sinus nerve information by the central respiratory neurones, which use primarily that which arrives during inspiration. They show a possible mechanism by which oscillating signals may have a different effect than their mean level would indicate.     

Tonic and phasic respiratory drives to human genioglossus motoneurons during breathing

A tongue muscle, the genioglossus (GG), is important in maintaining pharyngeal airway patency. Previous recordings of multiunit electromyogram (EMG) suggest it is activated during inspiration in humans with some tonic activity in expiration. We recorded from populations of single motor units in GG in seven subjects during quiet breathing when awake. Ultrasonography assisted electrode placement. The activity of single units was separated into six classes based on a step-wise analysis of the discharge pattern. Phasic and tonic activities were analyzed statistically with the coefficient of determination (r2) between discharge frequency and lung volume. Of the 110 motor units, 29% discharged tonically without phasic respiratory modulation (firing rate approximately 19 Hz). Further, 16% of units increased their discharge during expiration (expiratory phasic and expiratory tonic units). Only half the units increased their discharge during inspiration (inspiratory phasic and inspiratory tonic units). Units firing tonically with an inspiratory increase had significantly higher discharge rates than those units that only fired phasically (peak rates 25 vs. 16 Hz, respectively). Simultaneous recordings of two or three motor units showed neighboring units with differing respiratory and tonic drives. Our results provide a classification and the first quantitative measures of human GG motor-unit behavior and suggest this activity results from a complex interaction of inspiratory, expiratory, and tonic drives at the hypoglossal motor nucleus. The presence of different drives to GG implies that complex premotor networks can differentially engage human hypoglossal motoneurons during respiration. This is unlike the ordered recruitment of motor units in limb and axial muscles.     

Bötzinger-complex, bulbospinal expiratory neurones monosynaptically inhibit ventral-group respiratory neurones in the decerebrate rat

Extracellularly recorded action potentials from 49 Bötzinger-complex, bulbospinal expiratory neurones were used as triggers to compute 162 spike-triggered averages (STAs) of intracellular potentials recorded from 167 respiratory neurones in the ventral respiratory group (VRG) near the obex in 15 vagotomized, paralysed, ventilated and decerebrated rats. All of the Bötzinger-complex expiratory neurones were antidromically activated from the ipsilateral border between the C2/C3 segments of the spinal cord and discharged only during the late part of expiration with an augmenting pattern. We found evidence for monosynaptic inhibitory post-synaptic potentials (IPSPs) in 74 (～44%) of the STAs computed using 34 (～69%) of the trigger neurones. For vagal motoneurones, IPSPs were found in 24 of the 53 STAs of expiratory motoneurones, but in none of the 12 STAs of inspiratory motoneurones. For inspiratory neurones, IPSPs were found in 23 of the 33 STAs of bulbospinal neurones and in 6 of the 26 STAs of not antidromically activated (NAA) neurones. For expiratory neurones, IPSPs were found in one of the two STAs of bulbospinal neurones and in 20 of the 36 STAs of NAA neurones. We conclude that Bötzinger-complex, bulbospinal expiratory neurones monosynaptically inhibit bulbospinal inspiratory neurones, expiratory vagal motoneurones and other unidentified inspiratory and expiratory neurones in the VRG of rats during the late part of expiration.     

Expiratory changes in pressure: flow ratio during sleep in patients with sleep-disordered breathing

OBJECTIVES: The size of the upper airway is smallest during sleep, at the end of expiration. This may favor upper-airway collapse in patients with obstructive sleep apnea. In the respiratory cycles preceding obstructive events during sleep, our hypothesis is that upper-airway resistance (UAR) increased earlier during expiration prior to changes occurring during inspiration. DESIGN: We analyzed the pharyngeal pressure-to-flow ratios in order to determine variations in UAR for both inspiration and expiration during stable respiration and the 4 consecutive breaths preceding upper-airway obstructive events in stage 2 sleep. To assess the variation of resistance throughout the within-breath period during stable respiration and the 4 breaths preceding obstructive events, results were expressed as the instantaneous resistance at fixed points 10%, 30%, 50%, 70%, and 90% of time values of inspiration and expiration. Global inspiratory and expiratory UARs during wakefulness and sleep in stable respiration were expressed by the median of instantaneous UAR values. SETTING: Tertiary-care academic medical center. PATIENTS: Eleven patients with moderate to severe sleep-disordered breathing. INTERVENTION: None. MEASUREMENTS AND RESULTS: During stable respiration, both inspiratory and expiratory resistance increased during sleep, compared to values while awake. The difference between inspiratory and expiratory UAR increased when sleep deepened. During the respiratory cycle, the increase in the end-expiratory UAR occurred earlier than during inspiration; during stable respiration, UAR was much aggravated during the last three breaths preceding an obstructive event. CONCLUSION: Increases in the expiratory UAR occurred earlier than during inspiration in the cycles preceding upper-airway collapse in patients with sleep apnea. This finding suggested an important role of the expiratory phase in promoting upper-airway collapse and is in accordance with the inspiratory pharyngeal instability occurring when lowering the expiratory pressure in patients with obstructive sleep apnea.     

Role of synaptic inhibition in turtle respiratory rhythm generation

In vitro brainstem and brainstem-spinal cord preparations were used to determine the role of synaptic inhibition in respiratory rhythm generation in adult turtles. Bath application of bicuculline (a GABA_A receptor antagonist) to brainstems increased hypoglossal burst frequency and amplitude, with peak discharge shifted towards the burst onset. Strychnine (a glycine receptor antagonist) increased amplitude and frequency, and decreased burst duration, but only at relatively high concentrations (10-100 μM). Rhythmic activity persisted during combined bicuculline and strychnine application (50 μM each) with increased amplitude and frequency, decreased burst duration, and a rapid onset-decrementing burst pattern. The bicuculline-strychnine rhythm frequency decreased during μ-opioid receptor activation or decreased bath P _CO2. Synaptic inhibition blockade in the brainstem of brainstem-spinal cord preparations increased burst amplitude in spinal expiratory (pectoralis) nerves and nearly abolished spinal inspiratory activity (serratus nerves), suggesting that medullary expiratory motoneurons were mainly active. Under conditions of synaptic inhibition blockade in vitro , the turtle respiratory network was able to produce a rhythm that was sensitive to characteristic respiratory stimuli, perhaps via an expiratory (rather than inspiratory) pacemaker-driven mechanism. Thus, these data indicate that the adult turtle respiratory rhythm generator has the potential to operate in a pacemaker-driven manner.     

Synaptic effects of intercostal tendon organs on membrane potentials of medullary respiratory neurons

1. Stimulation of intercostal muscle tendon organs or their afferent fibers reduces medullary inspiratory neuron activity, decreases motor output to inspiratory muscles, and increases the activity of expiratory laryngeal motoneurons. The present study examines the synaptic mechanisms underlying these changes to obtain information about medullary neurons that participate in the afferent limb of this reflex pathway. 2. Membrane potentials of medullary respiratory neurons were recorded in decerebrate paralyzed cats. Postsynaptic potentials (PSPs) elicited in these neurons by intercostal nerve stimulation (INS) were compared before and after intracellular iontophoresis of chloride ions. After chloride injection, the normal hyperpolarization caused by inhibitory (I) PSPs is "reversed" to depolarization. 3. In inspiratory neurons, reversal of IPSPs by chloride injection also reversed hyperpolarization produced by INS when applied during any portion of the respiratory cycle. This observation suggests that increased chloride conductance of the postsynaptic membrane mediated the inhibition. Further, it is very likely that the last-order interneuron in the afferent pathway must be excited by INS and alter inspiratory neuron activity via an inhibitory synapse. The linear relationship between the amplitude of the INS induced PSP and membrane potential of inspiratory neurons provided evidence that neurons in the afferent pathway are not respiratory modulated. 4. The membranes of expiratory vagal motoneurons and post-inspiratory neurons were depolarized by INS during all portions of the respiratory cycle before IPSP reversal. Reversal of IPSPs affected neither this depolarization of expiratory vagal motoneurons during stage I and II expiration nor that of post-inspiratory neurons during stage I expiration. Thus this depolarization probably resulted from synaptic excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory connections among rostral medullary expiratory neurones detected with cross-correlation in the decerebrate rat

Expiratory neurones, with a decrementing firing pattern during the first phase of expiration (E-DEC) and located in the rostral ventrolateral medulla, are thought to be involved in the network generating respiratory rhythm, which also includes expiratory neurones with augmenting firing patterns (E-AUG). We used cross-correlation to detect their synaptic interconnections and connections to phrenic motoneurones in 32 vagotomised, decerebrate, paralysed and ventilated male rats. Pairs of neurones were recorded extracellularly with glass-insulated tungsten microelectrodes and the whole phrenic nerve with bipolar silver wire electrodes. Of the 79 cross-correlograms computed between pairs of E-DEC neurones, 8 (approximately 10%) showed evidence for inhibitory connections. Of the 67 cross-correlograms computed between E-DEC and E-AUG neurones, 5 (7.5%) showed evidence for a monosynaptic inhibition of the E-AUG neurone by the E-DEC neurone, while 3 (4.5%) showed evidence for a monosynaptic inhibition of the E-DEC neurone by the E-AUG neurone. An inhibitory connection from E-DEC neurones to phrenic motoneurones was detected in 5 (approximately 2%) of the cross-correlograms, and from E-AUG neurones to phrenic motoneurones in 4 (approximately 3.7%). These results are the first demonstration that network models of rhythm generation in the rat involving reciprocal inhibition between E-DEC and E-AUG neurones could have a neurophysiological basis, and the first to demonstrate that phrenic motoneurones are inhibited during the early phase of expiration by E-DEC neurones.     

Biphasic effects of substance P on respiratory activity and respiration-related neurones in ventrolateral medulla in the neonatal rat brainstem in vitro

The effects of substance P (SP) on respiratory activity in the brainstem-spinal cord preparation from neonatal rats (0-4 days old) were investigated. The respiratory activity was recorded from C4 ventral roots and intracellularly from three types of respiration-related neurones, i.e. pre-inspiratory (or biphasic E), three subtypes of inspiratory; expiratory and tonic neurones in the ventrolateral medulla (VLM). After the onset of SP bath application (10 nM-1 microM) a dose-dependent decline of burst rate (by 48%) occurred, followed by a weaker dose-dependent increase (by 17.5%) in burst rate. The biphasic effect of SP on inspiratory burst rate was associated with sustained membrane depolarization (in a range of 0.5-13 mV) of respiration-related and tonic neurones. There were no significant changes in membrane resistance in any type of neurones when SP was applied alone or when synaptic transmission was blocked with tetrodotoxin (TTX). The initial depolarization was associated with an increase in inspiratory drive potential (by 25%) as well as in bursting time (by 65%) and membrane excitability in inspiratory and pre-inspiratory neurones, which corresponded to the decrease in burst rate (C4 activity). The spiking frequency of expiratory and tonic neurones was also increased (by 36 and 48%). This activation was followed by restoration of the synaptic drive potential and bursting time in inspiratory and to a less extent in pre-inspiratory neurones, which corresponded to the increase in burst rate. The discharge frequency of expiratory and tonic neurones also decreased to control values. This phase followed the peak membrane depolarization. At the peak depolarization, SP reduced the amplitude of the action potential by 4-8% in all types of neurones. Our results suggest that SP exerts a general excitatory effect on respiration-related neurones and synaptic coupling within the respiratory network in the VLM. The transient changes in neuronal activity in the VLM may underlie the biphasic effect of SP in the brainstem respiration activity recorded in C4 roots. However, the biphasic effect of SP on inspiratory burst rate seems to be also defined by the balance in activity of other SP-sensitive systems and neurones in the respiratory network in the brainstem and spinal cord, which can modify the activity of medullary respiratory rhythm generator.