Hypoglossal motoneuron responses to pulmonary and superior laryngeal afferent inputs

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

Activation of upper airway muscles during breathing and swallowing

The upper airway is a complex muscular tube that is used by the respiratory and digestive systems. The upper airway is invested with several small and anatomically peculiar muscles. The muscle fiber orientations and their nervous innervation are both extremely complex, and how the activity of the muscles is initiated and adjusted during complex behaviors is poorly understood. The bulk of the evidence suggests that the entire assembly of tongue and laryngeal muscles operate together but differently during breathing and swallowing, like a ballet rather than a solo performance. Here we review the functional anatomy of the tongue and laryngeal muscles, and their neural innervation. We also consider how muscular activity is altered as respiratory drive changes, and briefly address upper airway muscle control during swallowing.

Human hypoglossal motor unit activities in exercise

The genioglossus (GG) muscle is considered the principal protruder muscle of the tongue that dilates and stiffens the pharyngeal airway. We recorded whole muscle and single motor unit (MU) activities in healthy adults performing progressive intensity exercise on a cycle ergometer. Tungsten microelectrodes were inserted percutaneously into the GG of 11 subjects (20-40 years) to record electromyographic (EMG) activities and pulmonary ventilation (VI) at rest and at workload increments up to 300 W. Increases in respiratory drive were associated with increases in VI, mean inspiratory flow (Vt/Ti) and tonic and phasic components of the GG EMG activity. In contrast, individual MUs typically showed expiration-related decreases in firing as exercise intensity increased. We suggest the decrease in MU activity may occur secondary to afferent feedback from lungs/chest wall and that compensation for more negative inspiratory airway pressures generated during heavy exercise occurs primarily via recruitment of previously silent MUs.

Role of inhibitory neurotransmission in the control of canine hypoglossal motoneuron activity in vivo

Hypoglossal motoneurons (HMNs) innervate all tongue muscles and are vital for maintenance of upper airway patency during inspiration. The relative contributions of the various synaptic inputs to the spontaneous discharge of HMNs in vivo are incompletely understood, especially at the cellular level. The purpose of this study was to determine the role of endogenously activated GABA(A) and glycine receptors in the control of the inspiratory HMN (IHMN) activity in a decerebrate dog model. Multibarrel micropipettes were used to record extracellular unit activity of individual IHMNs during local antagonism of GABA(A) receptors with bicuculline and picrotoxin or glycine receptors with strychnine. Only bicuculline had a significant effect on peak and average discharge frequency and on the slope of the augmenting neuronal discharge pattern. These parameters were increased by 30 +/- 7% (P < 0.001), 30 +/- 8% (P < 0.001), and 25 +/- 7% (P < 0.001), respectively. The effects of picrotoxin and strychnine on the spontaneous neuronal discharge and its pattern were negligible. Our data suggest that bicuculline-sensitive GABAergic, but not picrotoxin-sensitive GABAergic or glycinergic, inhibitory mechanisms actively attenuate the activity of IHMNs in vagotomized decerebrate dogs during hyperoxic hypercapnia. The pattern of GABAergic attenuation of IHMN discharge is characteristic of gain modulation similar to that in respiratory bulbospinal premotor neurons, but the degree of attenuation ( approximately 25%) is less than that seen in bulbospinal premotor neurons ( approximately 60%). The current studies only assess effects on active neuron discharge and do not resolve whether the lack of effect of picrotoxin and strychnine on IHMNs also extends to the inactive expiratory phase.

Modulation of upper airway muscle activities by bronchopulmonary afferents

Here we review the influence of bronchopulmonary receptors (slowly and rapidly adapting pulmonary stretch receptors, and pulmonary/bronchial C-fiber receptors) on respiratory-related motor output to upper airway muscles acting on the larynx, tongue, and hyoid arch. Review of the literature shows that all muscles in all three regions are profoundly inhibited by lung inflation, which excites slowly adapting pulmonary stretch receptors. This widespread coactivation includes the recruitment of muscles that have opposing mechanical actions, suggesting that the stiffness of upper airway muscles is highly regulated. A profound lack of information on the modulation of upper airway muscles by rapidly adapting receptors and bronchopulmonary C-fiber receptors prohibits formulation of a conclusive opinion as to their actions and underscores an urgent need for new studies in this area. The preponderance of the data support the view that discharge arising in slowly adapting pulmonary stretch receptors plays an important role in the initiation of the widespread and highly coordinated recruitment of laryngeal, tongue, and hyoid muscles during airway obstruction.

Respiratory activation of the genioglossus muscle involves both non-NMDA and NMDA glutamate receptors at the hypoglossal motor nucleus in vivo

Brainstem respiratory neurons innervate the hypoglossal motor nucleus which in turn transmits this respiratory drive signal to the genioglossus muscle of the tongue. The mechanism of this transmission is important to help maintain an open airspace for effective breathing, and is thought to rely almost exclusively on non-N-methyl-d-aspartate (non-NMDA) glutamate receptor activation during respiration. However those studies were performed in slices of medulla from neonatal animals in vitro which may have led to an underestimation of the contribution of NMDA glutamate receptors that may normally operate in intact preparations. The current study tests the hypothesis that both NMDA and non-NMDA receptors contribute to respiratory drive transmission at the hypoglossal motor nucleus in vivo. Experiments were performed in urethane-anesthetized and tracheotomized adult Wistar rats in which vagus nerves were either intact or sectioned. In the presence of augmented genioglossus activity produced by vagotomy, microdialysis perfusion of either an NMDA receptor antagonist (D-2-amino-5-phosphonovaleric acid, 0.001-10 mM) or a non-NMDA receptor antagonist (6-cyano-7-nitroquinoxaline-2, 3-dione disodium salt, 0.001-1 mM) to the hypoglossal motor nucleus reduced respiratory-related genioglossus activity in a dose-dependent manner (P < 0.001) indicating that both NMDA and non-NMDA glutamate receptors are necessary for transmission of the respiratory drive signal to genioglossus muscle in vivo. Similar effects were observed in the vagus nerve intact rats. Further experiments demonstrated that each delivered antagonist had effects that were specific to its respective receptor. Regression analysis also revealed that the activity of both NMDA and non-NMDA receptors at the hypoglossal motor nucleus is related to levels of the prevailing respiratory drive. These results show that both NMDA and non-NMDA glutamate receptors at the hypoglossal motor nucleus are involved in transmission of the respiratory drive signal to genioglossus muscle in vivo.

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.

Suppression of genioglossus muscle tone and activity during reflex hypercapnic stimulation by GABA(A) mechanisms at the hypoglossal motor nucleus in vivo

The genioglossus muscle is involved in the maintenance of an open airway for effective breathing. Inhibitory neurotransmitters may be responsible for the major suppression of hypoglossal motor output to genioglossus muscle that occurs in certain behaviours such as rapid-eye-movement sleep. There is evidence for GABA(A) receptor-mediated inhibition of hypoglossal motoneurons in vitro. However, comparable studies have not been performed in vivo and the interactions of such mechanisms with integrative reflex respiratory control have also not been determined. Urethane-anaesthetised, tracheotomized and vagotomized rats were studied whilst diaphragm and genioglossus muscle activities, blood pressure and the electroencephalogram were recorded. Microdialysis probes were implanted into the hypoglossal motor nucleus, with sites verified by histology. Genioglossus responses to microdialysis perfusion of muscimol (GABA(A) agonist: 0, 0.1, 1 and 10 microM in artificial cerebrospinal fluid) were recorded at inspired CO(2)s of 0, 5 and 7.5% in six rats. Responses to bicuculline (GABA(A) antagonist, 0, 1, 10, 100 and 1000 microM) were also studied in six rats with and without CO(2) stimulation. Genioglossus activity decreased with muscimol (P<0.0001), with major suppression at 1 and 10 microM during air breathing (decreases=70.2% and 92.8%, P<0.005). Genioglossus activity increased with CO(2) (P=0.003), but genioglossus activation with 5 and 7.5% CO(2) were almost abolished with 10-microM muscimol. Responses were specific to genioglossus muscle as there were no changes in diaphragm, respiratory rate or blood pressure with muscimol (P>0.144). Antagonism of GABA(A) receptors increased genioglossus activity (P<0.001). These results show that GABA(A) receptor stimulation at the hypoglossal motor nucleus suppresses both genioglossus muscle tone and activity in the presence of reflex stimulation produced by hypercapnia. Recruitment of such mechanisms may contribute to the major suppression of genioglossus activity observed with and without CO(2) stimulation in behaviours such as rapid-eye-movement sleep.

Intralaryngeal neuroanatomy of the recurrent laryngeal nerve of the rabbit

We undertook this study to determine the detailed neuroanatomy of the terminal branches of the recurrent laryngeal nerve (RLN) in the rabbit to facilitate future neurophysiological recordings from identified branches of this nerve. The whole larynx was isolated post mortem in 17 adult New Zealand White rabbits and prepared using a modified Sihler's technique, which stains axons and renders other tissues transparent so that nerve branches can be seen in whole mount preparations. Of the 34 hemi-laryngeal preparations processed, 28 stained well and these were dissected and used to characterize the neuroanatomy of the RLN. In most cases (23/28) the posterior cricoarytenoid muscle (PCA) was supplied by a single branch arising from the RLN, though in five PCA specimens there were two or three separate branches to the PCA. The interarytenoid muscle (IA) was supplied by two parallel filaments arising from the main trunk of the RLN rostral to the branch(es) to the PCA. The lateral cricoarytenoid muscle (LCA) commonly received innervation from two fine twigs branching from the RLN main trunk and travelling laterally towards the LCA. The remaining fibres of the RLN innervated the thyroarytenoid muscle (TA) and comprised two distinct branches, one supplying the pars vocalis and the other branching extensively to supply the remainder of the TA. No communicating anastomosis between the RLN and superior laryngeal nerve within the larynx was found. Our results suggest it is feasible to make electrophysiological recordings from identified terminal branches of the RLN supplying laryngeal adductor muscles separate from the branch or branches to the PCA. However, the very small size of the motor nerves to the IA and LCA suggests that it would be very difficult to record selectively from the nerve supply to individual laryngeal adductor muscles.

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.

Glycine at hypoglossal motor nucleus: genioglossus activity, CO(2) responses, and the additive effects of GABA

There is evidence for glycine and GABA(A)-receptor-mediated inhibition of hypoglossal motoneurons in vitro. However, comparable studies have not been performed in vivo, and the interactions of such mechanisms with integrative reflex respiratory control have also not been determined. This study tests the hypotheses that glycine at the hypoglossal motor nucleus (HMN) will suppress genioglossus (GG) muscle activity, even in the presence of hypercapnic respiratory stimulation, and the effects of glycine will be blocked by strychnine. We also determined whether coapplication of glycine and muscimol (GABA(A)- receptor agonist) to the HMN is additive in suppressing GG activity. Twenty-four urethane-anesthetized, tracheotomized, and vagotomized rats were studied. Diaphragm and GG activities, the electroencephalogram, and blood pressure were recorded. Microdialysis probes were implanted into the HMN for delivery of artificial cerebrospinal fluid (control), glycine (0.0001-10 mM), or muscimol (0.1 microM). Increasing glycine at the HMN produced graded suppression of GG activity (P < 0.001), although the GG still responded to stimulation with 7% inspired CO(2) (P = 0.002). Strychnine (0.1 mM) reversed the glycine-mediated suppression of GG activity, whereas combined glycine and muscimol were additive in GG muscle suppression. It remains to be determined whether the recruitment of such glycine and GABA mechanisms explains the periods of major GG suppression in behaviors such as rapid eye movement sleep.

Difference between hypoglossal and phrenic activities during lung inflation and swallowing in the rat

We aimed in this study to elucidate the discharge properties and neuronal mechanisms of the dissociation between hypoglossal and phrenic inspiratory activities in decerebrate rats, which had been subjected to neuromuscular blockade and artificially ventilated. The discharge of the hypoglossal nerve and the intracellular activity of hypoglossal motoneurones were monitored during respiration and fictive-swallowing evoked by electrical stimulation of the superior laryngeal nerve, and were compared with the activity of the phrenic nerve. The hypoglossal nerve activity was characterized by its onset preceding the phrenic nerve activity ('pre-I' activity). By manipulating artificial respiration, we could augment the 'pre-I' activity, and could elicit another type of hypoglossal activity decoupled from the phrenic-associated inspiratory bursts ('decoupled' activity). We further scrutinized the correlatives of 'pre-I' and 'decoupled' activities in individual hypoglossal motoneurones. Hypoglossal motoneurones consisted of inspiratory (n = 42), expiratory (n = 18) and non-respiratory (n = 1) neurones and were classified by their swallowing activity into depolarized, hyperpolarized, hyperpolarized-depolarized and unresponsive groups. All of the inspiratory neurones were depolarized in accordance with the 'pre-I' and 'decoupled' activities, and all of the expiratory neurones were hyperpolarized during these activities. Fictive swallowing, which was characterized by its frequent emergence just after the phrenic inspiratory activity, was also evoked just after the 'decoupled' hypoglossal activity, suggesting that this activity may have similar effects on swallowing as the 'overt' inspiratory activity. Such a coupling between 'decoupled' and swallowing activities was also revealed in each motoneurone. These findings suggest that the 'pre-I' and 'decoupled' activities may reflect some internal inspiratory activity of the respiratory centre and that hypoglossal motoneurones may be driven by a distinct group of premotor neurones that possibly play a role in the coordination of respiration and swallowing.

High frequency oscillations in respiratory networks: functionally significant or phenomenological?

Inspiratory activities, whether recorded from medullary neurons, motoneurons or motor nerves, feature prominent oscillations in high (50-120 Hz) and medium (15-50 Hz) frequency ranges. These oscillations have been extensively characterized and are considered signatures of respiratory network activity. Their functional significance, however, if any, remains unknown. Here we review the literature describing the nature and origin of these oscillations as well as their modulation during development and by mechanoreceptive and chemoreceptive feedback, respiratory- and non-respiratory-related behaviors, temperature and anesthesia. We then consider the potential significance of these oscillations for respiratory network function by drawing on analyses of distributed motor and sensory networks of the cortex where current interest in oscillatory activity, and the synchronization of neural discharge that can result, is based on the increased efficacy with which synchronous inputs influence neuronal output, and the role that synchronous activity may play in information coding. We speculate that synchronized oscillations at the network level help coordinate activity in distributed rhythm and pattern generating systems and at the muscle level enhance force development. Data most strongly support that oscillatory synaptic inputs play an important role in controlling timing and pattern of action potential output.

Effects of sleep-wake state on the genioglossus vs.diaphragm muscle response to CO(2) in rats

The effects of sleep on the ventilatory responses to hypercapnia have been well described in animals and in humans. In contrast, there is little information for genioglossus (GG) responses to a range of CO(2) stimuli across all sleep-wake states. Given the notion that sleep, especially rapid eye movement (REM) sleep, may cause greater suppression of muscles with both respiratory and nonrespiratory functions, this study tests the hypothesis that GG activity will be differentially affected by sleep-wake states with major suppression in REM sleep despite excitation by CO(2). Seven rats were chronically implanted with electroencephalogram, neck, GG, and diaphragm electrodes, and responses to 0, 1, 3, 5, 7, and 9% CO(2) were recorded. Diaphragm activity and respiratory rate increased with CO(2) (P < 0.001) across sleep-wake states with significant increases at 3-5% CO(2) compared with 0% CO(2) controls (P < 0.05). Phasic GG activity also increased in hypercapnia but required higher CO(2) (7-9%) for significant activation (P < 0.05). Further studies in 15 urethane-anesthetized rats with the vagi intact (n = 6) and cut (n = 9) showed that intact vagi delayed GG recruitment with hypercapnia but did not affect diaphragm responses. In the naturally sleeping rats, we also showed that GG activity was significantly reduced in non-REM and REM sleep (P < 0.04) and was almost abolished in REM even with stimulation by 9% CO(2) (decrease = 80.4% vs. wakefulness). Such major suppression of GG activity in REM, even with significant respiratory stimulation, may explain why obstructive apneas are more common in REM sleep.

Reflex respiratory response to changes in upper airway pressure in the anaesthetized rat

1. We examined the upper airway (UA) motor response to upper airway negative pressure (UANP) in the rat. We hypothesized that this response is mediated by superior laryngeal nerve (SLN) afferents and is not confined to airway dilator muscles but also involves an increase in motor drive to tongue retractor and pharyngeal constrictor muscles, reflecting a role for these muscles in stabilizing the UA. 2. Experiments were performed in 49 chloralose-anaesthetized, tracheostomized rats. Subatmospheric pressure in the range 0 to -30 cmH(2)O was applied to the isolated UA. Motor activity was recorded in separate experiments from the main trunk of the hypoglossal nerve (XII, n = 8), the pharyngeal branch of the glossopharyngeal nerve (n = 8), the medial and lateral branches of the XII (n = 8) and the pharyngeal branch of the vagus (n = 8). Afferent activity was recorded from the whole SLN in six experiments. 3. All UA motor outflows exhibited phasic inspiratory activity and this was significantly (P < 0.05) increased by UANP. Tonic end-expiratory activity increased significantly in response to pressures more negative than -20 cmH(2)O. Bilateral section of the SLN also increased (P < 0.05) motor activity and abolished the responses to UANP. Electrical stimulation of the SLN inhibited inspiratory XII activity. SLN afferents were tonically active and inhibited by UANP. 4. We conclude that UANP causes a reflex increase in motor drive to pharyngeal dilator, tongue retractor and pharyngeal constrictor muscles via afferent fibres in the SLN. Tonic activity in SLN afferent fibres at zero transmural pressure exerts a marked inhibitory effect on UA motor outflow.

Neural control of tongue movement with respect to respiration and swallowing

The tongue must move with remarkable speed and precision between multiple orofacial motor behaviors that are executed virtually simultaneously. Our present understanding of these highly integrated relationships has been limited by their complexity. Recent research indicates that the tongue s contribution to complex orofacial movements is much greater than previously thought. The purpose of this paper is to review the neural control of tongue movement and relate it to complex orofacial behaviors. Particular attention will be given to the interaction of tongue movement with respiration and swallowing, because the morbidity and mortality associated with these relationships make this a primary focus of many current investigations. This review will begin with a discussion of peripheral tongue muscle and nerve physiology that will include new data on tongue contractile properties. Other relevant peripheral oral cavity and oropharyngeal neurophysiology will also be discussed. Much of the review will focus on brainstem control of tongue movement and modulation by neurons that control swallowing and respiration, because it is in the brainstem that orofacial motor behaviors sort themselves out from their common peripheral structures. There is abundant evidence indicating that the neural control of protrusive tongue movement by motoneurons in the ventral hypoglossal nucleus is modulated by respiratory neurons that control inspiratory drive. Yet, little is known of hypoglossal motoneuron modulation by neurons controlling swallowing or other complex movements. There is evidence, however, suggesting that functional segregation of respiration and swallowing within the brainstem is reflected in somatotopy within the hypoglossal nucleus. Also, subtle changes in the neural control of tongue movement may signal the transition between respiration and swallowing. The final section of this review will focus on the cortical integration of tongue movement with complex orofacial movements. This section will conclude with a discussion of the functional and clinical significance of cortical control with respect to recent advances in our understanding of the peripheral and brainstem physiology of tongue movement.

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.

Effect of pulmonary stretch receptor feedback and CO(2) on upper airway and respiratory pump muscle activity in the rat

1. Our purpose was to examine the effects of chemoreceptor stimulation and lung inflation on neural drive to tongue protrudor and retractor muscles in the rat. 2. Inspiratory flow, tidal volume, transpulmonary pressure, compliance and electromyographic (EMG) activity of genioglossus (GG), hyoglossus (HG) and inspiratory intercostal (IIC) muscles were studied in 11 anaesthetized, tracheotomized and spontaneously breathing rats. Mean EMG activity during inspiration was compared with mean EMG activity during an occluded inspiration, at each of five levels of inspired CO(2) (0, 3, 6, 9 and 12 %). 3. Lung inflation suppressed EMG activity in all muscles, with the effect on both tongue muscles exceeding that of the intercostal muscles. Static elevations of end-expiratory lung volume evoked by 2 cmH(2)O positive end-expiratory pressure (PEEP) had no effect on tongue muscle activity. 4. Despite increasing inspiratory flow, tidal volume and transpulmonary pressure, the inhibition of tongue muscle activity by lung inflation diminished as arterial PCO2 (P(a),CO(2)) increased. 5. The onset of tongue muscle activity relative to the onset of IIC muscle activity advanced with increases in P(a),CO(2) but was unaffected by lung inflation. This suggests that hypoglossal and external intercostal motoneuron pools are controlled by different circuits or have different sensitivities to CO(2), lung inflation and/or anaesthetic agents. 6. We conclude that hypoglossal motoneuronal activity is more strongly influenced by chemoreceptor-mediated facilitation than by lung volume-mediated inhibition. Hypoglossal motoneurons driving tongue protrudor and retractor muscles respond identically to these stimuli.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Synaptic potentials in respiratory neurones during evoked phase switching after NMDA receptor blockade in the cat

1. Blockade of NMDA receptors by dizocilpine impairs the inspiratory off-switch (IOS) of central origin but not the IOS evoked by stimulation of sensory afferents. To investigate whether this difference was due to the effects of different patterns of synaptic interactions on respiratory neurones, we stimulated electrically the superior laryngeal nerve (SLN) or vagus nerve in decerebrate cats before and after i.v. administration of dizocilpine, whilst recording intracellularly. 2. Phrenic nerve responses to ipsilateral SLN or vagal stimulation were: at mid-inspiration, a transient inhibition often followed by a brief burst of activity; at late inspiration, an IOS; and at mid-expiration, a late burst of activity. 3. In all neurones (n = 16), SLN stimulation at mid-inspiration evoked an early EPSP during phase 1 (latency to the arrest of phrenic nerve activity), followed by an IPSP in inspiratory (I) neurones (n = 8) and by a wave of EPSPs in post-inspiratory (PI) neurones (n = 8) during phase 2 (inhibition of phrenic activity). An EPSP in I neurones and an IPSP in PI neurones occurred during phase 3 (brief phrenic burst) following phase 2. 4. Evoked IOS was associated with a fast (phase 1) activation of PI neurones, whereas during spontaneous IOS, a progressive (30-50 ms) depolarization of PI neurones preceded the arrest of phrenic activity. 5. Phase 3 PSPs were similar to those occurring during the burst of activity seen at the start of spontaneous inspiration. 6. Dizocilpine did not suppress the evoked phrenic inhibition and the late burst of activity. The shapes and timing of the evoked PSPs and the changes in membrane potential in I and PI neurones during the phase transition were not altered. 7. We hypothesize that afferent sensory pathways not requiring NMDA receptors (1) terminate inspiration through a premature activation of PI neurones, and (2) evoke a late burst of phrenic activity which might be the first stage of the inspiratory on-switch.

Intensity and frequency dependence of laryngeal afferent inputs to respiratory hypoglossal motoneurons

Inspiratory hypoglossal motoneurons (IHMs) mediate contraction of the genioglossus muscle and contribute to the regulation of upper airway patency. Intracellular recordings were obtained from antidromically identified IHMs in anesthetized, vagotomized cats, and IHM responses to electrical activation of superior laryngeal nerve (SLN) afferent fibers at various frequencies and intensities were examined. SLN stimulus frequencies <2 Hz evoked an excitatory-inhibitory postsynaptic potential (EPSP-IPSP) sequence or only an IPSP in most IHMs that did not change in amplitude as the stimulus was maintained. During sustained stimulus frequencies of 5-10 Hz, there was a reduction in the amplitude of SLN-evoked IPSPs with time with variable changes in the EPSP. At stimulus frequencies >25 Hz, the amplitude of EPSPs and IPSPs was reduced over time. At a given stimulus frequency, increasing stimulus intensity enhanced the decay of the SLN-evoked postsynaptic potentials (PSPs). Frequency-dependent attenuation of SLN inputs to IHMs also occurred in newborn kittens. These results suggest that activation of SLN afferents evokes different PSP responses in IHMs depending on the stimulus frequency. At intermediate frequencies, inhibitory inputs are selectively filtered so that excitatory inputs predominate. At higher frequencies there was no discernible SLN-evoked PSP temporally locked to the SLN stimuli. Alterations in SLN-evoked PSPs could play a role in the coordination of genioglossal contraction during respiration, swallowing, and other complex motor acts where laryngeal afferents are activated.

Influence of lung volume on respiratory responses to spontaneous bladder contractions

Spontaneous bladder contractions (SBCs) in decerebrate, vagotomized, paralyzed, ventilated cats have been shown to decrease phrenic and hypoglossal inspiratory nerve activities, as well as the activities of other respiratory motor nerves. To determine whether vagal afferents from the lung influence the respiratory inhibition associated with SBCs, we recorded phrenic and hypoglossal nerve activities in decerebrate, paralyzed, vagally intact cats. The animals were ventilated by a servo-respirator, which inflated the lungs in accordance with integrated phrenic nerve activity. Maintained increases in end-expiratory lung volume were produced by the application of 2-10 cm H2O positive end-expiratory pressure (PEEP). SBCs were accompanied by decreases in both phrenic and hypoglossal peak integrated nerve activities, as well as by marked decreases in respiratory frequency. The reduction of respiratory frequency was greater with higher levels of PEEP, a few animals becoming apneic during SBCs. After bilateral vagotomy, SBCs continued to decrease phrenic and hypoglossal peak integrated nerve activities as previously reported, but the reduction of respiratory frequency was much less striking than when the vagi were intact. These results indicate that activity of vagal afferents from the lung augments the respiratory influence of SBCs. Furthermore, SBCs in vagally intact animals can induce periodic breathing.

Respiratory-related activity of the pharyngeal nerves in the rat

In anesthetized (n = 26) or decerebrate (n = 15) rats, vagotomized, paralyzed and artificially ventilated, we recorded efferent respiratory-related discharges of nerves supplying the pharyngeal muscles, i.e. the glossopharyngeal nerve (IX; n = 30) and the pharyngeal ramus of the vagus nerve (PH.X; n = 33). In both types of preparation, all IX nerves fired during inspiration (I); most PH.X fired either during I (n = 11), expiration (E, n = 12), or both phases (n = 4); some of them were continuously active, without any respiratory modulation (n = 6). We also examined the timing of inspiratory pharyngeal bursts in relation to the phrenic (inspiratory) bursts. We found that the burst onset occurred significantly earlier in pharyngeal nerves than in phrenic ones (pharyngeal-to-phrenic delay). In anesthetized animals, this pharyngeal-to-phrenic delay was long enough to reveal that inspiratory activity appeared first on IX, then on PH.X, and then on phrenic nerves. Since hypoxia did not significantly alter the pharyngeal-to-phrenic delay, we conclude that a reliable organization of the inspiratory drive on pharyngeal and diaphragmatic muscles should exist, in the rat.

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)

Respiratory electromyographic estimates of ventilatory functions in piglets

The best electromyographic (EMG) predictors of respiratory drive (P100), tidal volume (VT) and ventilation (VE) were determined from diaphragmatic (DI) and posterior cricoarytenoid (PCA) EMG measures in 8-48-day-old, anesthetized piglets. Progressive hypercapnia was employed to obtain a wide range of muscle activity. A custom-designed, microcomputer-based system was employed to measure the duration, peak amplitude, rate of rise (initial slope) as well as the summed total and initial (first 100 ms) EMG activity from the DI and the PCA. For each respiratory function, the following combinations of EMG measures were identified as significant predictors using regression analyses: (1) for P100, DI amplitude, PCA initial area and PCA rate of rise; (2) for VT, DI amplitude, PCA duration and DI duration; (3) for VE, DI amplitude, DI initial area, PCA initial area, PCA rate of rise, PCA duration, DI area and DI rate of rise. Thus, whereas the traditionally employed measure of DI amplitude is an important correlate of P100, VT or VE, a complete estimate of these respiratory functions requires the inclusion of initial EMG measures and duration.

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.

Synaptic events in ventral respiratory neurones during apnoea induced by laryngeal nerve stimulation in neonatal pig

1. Postsynaptic potentials evoked by electrical stimulation of superior laryngeal nerve (SLN) were recorded during SLN-induced apnoea from the respiratory neurones of the ventral respiratory group (VRG) in pentobarbitone-anaesthetized, vagotomized and artificially ventilated newborn piglets (n = 14, 4-7 days old). All recorded inspiratory (n = 10), post-inspiratory (n = 10) and expiratory (n = 20) neurones had a triphasic pattern of membrane potential and were identified for their projections to the spinal cord or cervical vagus nerve. 2. During long-lasting apnoea, induced by SLN stimulation, the membrane potential trajectory of each type of recorded neurone was held at the level corresponding approximately to the membrane potential reached during stage I of expiration. Compound postsynaptic potentials evoked in most respiratory-related neurones had an early short-lasting and a late long-lasting component. 3. Postsynaptic potentials in four out of seven inspiratory neurones, in which postsynaptic potentials were well demonstrated, were characterized by an early depolarization followed by long-lasting hyperpolarization. In three other inspiratory neurones only late hyperpolarization was present. The reversal of the late hyperpolarization by intracellular chloride injection was achieved to a different degree in the early and late portions of late hyperpolarization. 4. Postsynaptic potentials evoked in expiratory neurones were studied in sixteen neurones and displayed two patterns: early hyperpolarization followed by long-lasting hyperpolarization (n = 7, six were not antidromically activated after spinal cord stimulation) or early hyperpolarization followed by late depolarization (n = 9, eight projected to the spinal cord). The early hyperpolarization was readily reversed by chloride injection. The late hyperpolarization was more difficult to reverse and usually the reversal was not completed. 5. Postsynaptic potentials evoked in post-inspiratory neurones showed a pattern of two consecutive phases of depolarization. 6. The present study revealed that during long-lasting apnoea evoked by SLN stimulation each category of VRG respiratory neurones received a temporally synchronized combination of an initial fast input derived reflexly from laryngeal afferents, and of late inputs representing involvement of the whole respiratory network in the response.

Ontogeny of sleep/wake and cardiorespiratory behavior in unanesthetized piglets

Young and older piglets (2-15, 25-35 days old) underwent chronic recording of electrocorticogram, vertical and horizontal electrooculograms, electromyograms of submental muscles, diaphragm (EMGdi) and posterior cricoarytenoid (EMGpca), and heart rate, arterial pressure, pH and gas tensions. With age, (1) the distribution of percent time spent in various sleep-wake states differed; (2) heart rate decreased in all S/W, arterial pressure increased in wakefulness (W), transitional sleep (TS) and quiet sleep (QS); (3) respiratory frequency decreased, EMGdi and EMGpca duration and EMGpca amplitude increased in all S/W, EMGdi amplitude decreased in TS and QS and rate of rise of EMGdi and EMGpca decreased in W, TS and QS. Active sleep was characterized by smaller normalized EMGpca amplitudes in the young, short EMGpca to EMGdi intervals in both ages and predominance of prolonged diminished muscle activity (DMA) of either muscle. Discoordination between EMGpca and EMGdi activation and the occurrence of DMA were influenced by youth and male gender. These results provide insight into subtle expressions of gender and sleep influences on developmental respiratory control.

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.

Synaptic inhibition of phrenic motoneurones evoked by stimulation of the superior laryngeal nerve

Synaptic responses evoked in phrenic motoneurones (PMNs) by stimulation of the superior laryngeal nerve (SLN) were analysed in anaesthetised cats. Stimulation of the SLN was followed by inhibition of ipsilateral phrenic nerve discharge with the latency of 9.5 +/- 2.3 ms (mean +/- S.D.) and hyperpolarizations of ipsilateral PMN membrane potentials (latency, 8.4 +/- 2.1 ms) which were observed after stimuli applied both during inspiration and expiration. During the injection of Cl ions, the hyperpolarizations were either reversed or flattened in all 28 tested PMNs, thus indicating a direct inhibition. The possibility that the inhibitory postsynaptic potentials are produced by segmental respiratory interneurones is discussed.

Inspiratory-modulated neurons of the rostrolateral pons: effects of pulmonary afferent input

In decerebrate, paralyzed cats ventilated with a cycle-triggered pump, firing of inspiratory (I) and I-modulated neurons in the pontine respiratory group was markedly increased by withholding lung inflation, indicating strong inhibition by lung afferents. Spectral analysis showed that only a small minority of I-modulated neurons had high-frequency oscillations (HFO), in contrast to medullary I neurons, indicating that the pontine neurons are not closely linked to medullary I networks.

Characterization of respiratory-related activity of the facial nerve

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

Respiratory-modulated activities of motor units of the facial nerve

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

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

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

Intracellular analysis of respiratory-modulated hypoglossal motoneurons in the cat

Intracellular recordings from hypoglossal motoneurons in the brainstem of cats are described, along with postsynaptic potentials evoked by superior laryngeal, vagal and carotid sinus nerve stimulation. The study concentrates on hypoglossal motoneurons with respiratory-related discharge, which can be categorized into inspiratory, inspiratory/early-expiratory and expiratory patterns. Seven cells were labelled with horseradish peroxidase, their location and morphology are described. Stimulation of laryngeal receptors by balloon inflation or by water injection into the larynx, or mimicked by electrical stimulation of the superior laryngeal nerve results in enhanced postinspiratory activity in those cells (inspiratory/early-expiratory, expiratory) already receiving postinspiratory excitation; or actually produces a wave of postinspiratory depolarization in cells (inspiratory) previously quiescent during that period. It is concluded that the firing pattern of the respiratory-modulated hypoglossal motoneurons is unlikely to be static but depends on other factors, one of these being the level of ongoing, or previous laryngeal receptor stimulation.

Clinical manifestations of choanal atresia in infancy

The clinical features of 41 infants with various types of nasal obstruction were reviewed to study the correlation between the degree of nasal obstruction and clinical manifestations. Twenty-one subjects had bilateral choanal atresia/stenosis; 12 had unilateral choanal atresia/stenosis, and eight who were referred with suspected choanal atresia had a simple inflammatory nasal obstruction. Patients with bilateral choanal obstruction and patients with unilateral choanal obstruction or rhinitis showed no differences in clinical picture or in referral age. Many infants with either unilateral or bilateral choanal obstruction had one or more symptoms not fully relieved after surgical repair, although the choanae were widely patent. At long-term follow-up the surviving patients showed spontaneous recovery and good nasal respiration. Overall, five patients died. Since the common syndrome appeared to be related to a dysfunction of the autonomic nervous system, we conclude that any type of nasal obstruction may exacerbate or precipitate an underlying maturational autonomic disturbance.

Activity of medullary respiratory neurons regenerating axons into peripheral nerve grafts in the adult rat

Autologous segments of peroneal nerve were implanted into the medulla oblongata of young adult rats. To investigate activity of medullary respiratory neurons regenerating axons into these grafts, unitary recording from single fibers was performed on small strands teased from the grafts. Spontaneous activity was observed in teased fibers in 7 of 9 grafts recorded 2-5 months after graft implantation. Respiratory-related activity was found in 5 of these grafts and could in most cases be characterized as emanating from medullary respiratory neurons other than cranial motoneurons. The integrity of the input connections to the neurons that had regenerated axons was manifested by normal patterns of unitary respiratory-related activity and by the responsiveness of firing patterns of these neurons to lung hyperinflation and to the inspiratory off-switch effect induced by vagal stimulation. No spontaneous respiratory activity was found in fibers teased from any of the 10 grafts studied 9-11 months after implantation. Five of these grafts were blind-ended as were the 2-5-month grafts; the other 5 grafts formed bridges between the medulla and C4 ventral horn. No physiologic evidence of functional connections with phrenic motoneurons was found in these bridge grafts. These experiments indicate that physiologic function is maintained or regained in some respiratory neurons regenerating axons into peripheral nerve grafts but that this function is not indefinitely preserved in the absence of functional reconnection with an appropriate target.

Alterations of hypoglossal motoneuronal activities during pulmonary inflations

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

High-frequency and medium-frequency components of different inspiratory nerve discharges and their modification by various inputs

In decerebrate paralyzed cats, spectral analysis was performed on simultaneous recordings of efferent inspiratory nerves (phrenic, recurrent laryngeal, hypoglossal). Spectral peaks were present both in the high-frequency (HFO) range (50-100 Hz) and the medium-frequency (MFO) range (20-50 Hz). Different activities were coherent only in the HFO range, indicating that the HFOs arise in a common inspiratory pattern generator that drives the different motoneuron populations, whereas the MFOs are specific to different systems.

Neural and anatomic factors related to upper airway occlusion during sleep

Both neural and anatomical factors play an important role in the maintenance of upper airway patency. An abnormality in one or both of these factors is felt to be the underlying cause of obstructive sleep apnea.

Responses of recurrent laryngeal motoneurons to changes of pulmonary afferent inputs

In decerebrate, paralyzed cats ventilated with a cycle-triggered pump, the discharges of the recurrent laryngeal (whole nerve or single fibers) and phrenic nerves, and the changes produced by pulmonary afferent inputs (lung inflation), were compared. When lung inflation was in phase with neural inspiration, four types of laryngeal fiber activities were observed: (a) phasic-inspiratory; (b) tonic-inspiratory; (c) expiratory-inspiratory; (d) early-expiratory. The firing patterns during inspiration were plateau-like, whereas the phrenic pattern was augmenting. When inflation was withheld, the plateau patterns usually became augmenting, indicating inhibition of laryngeal inspiratory activity by pulmonary afferents. Secondary effects of withholding inflation were (a) increases of early-expiratory activity (both whole nerve and individual fiber), indicating increased post-inhibitory rebound excitation; (b) decreased activity of tonic-inspiratory and expiratory-inspiratory fibers during early neural expiration, indicating increased inhibition by early-expiratory neurons. The discharge patterns of different types of laryngeal motoneuron, as well as their changes with inflation, are interpreted in relation to the function of regulating airway resistance.