Alterations of hypoglossal motoneuronal activities during pulmonary inflations

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.

Preinspiratory and inspiratory hypoglossal motor output during hypoxia-induced plasticity in the rat

Respiratory-related discharge in the hypoglossal (XII) nerve is composed of preinspiratory (pre-I) and inspiratory (I) activity. Our first purpose was to test the hypothesis that hypoxia-induced plasticity in XII motor output is differentially expressed in pre-I vs. I XII bursting. Short-term potentiation (STP) of XII motor output was induced in urethane-anesthetized, vagotomized, and ventilated rats by exposure to isocapnic hypoxia (PaO2 of approximately 35 Torr). Both pre-I and I XII discharge abruptly increased at beginning of hypoxia (i.e., acute hypoxic response), and the relative increase in amplitude was much greater for pre-I (507+/-46% baseline) vs. I bursting (257+/-16% baseline; P<0.01). In addition, STP was expressed in I but not pre-I bursting following hypoxia. Specifically, I activity remained elevated following termination of hypoxia but pre-I bursting abruptly returned to prehypoxia levels. Our second purpose was to test the hypothesis that STP of I XII activity results from recruitment of inactive or "silent" XII motoneurons (MNs) vs. rate coding of active MNs. Single fiber recordings were used to classify XII MNs as I, expiratory-inspiratory, or silent based on baseline discharge patterns. STP of I XII activity following hypoxia was associated with increased discharge frequency in active I and silent MNs but not expiratory-inspiratory MNs. We conclude that the expression of respiratory plasticity is differentially regulated between pre-I and I XII activity. In addition, both recruitment of silent MNs and rate coding of active I MNs contribute to increases in XII motor output following hypoxia.

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.

Uncoupling of upper airway motor activity from phrenic bursting by positive end-expired pressure in the rat

Phasic bursting in the hypoglossal nerve can be uncoupled from phrenic bursting by application of positive end-expired pressure (PEEP). We wished to determine whether similar uncoupling can also be induced in other respiratory-modulated upper airway (UAW) motor outputs. Discharge of the facial, hypoglossal, superior laryngeal, recurrent laryngeal, and phrenic nerves was recorded in anesthetized, ventilated rats during stepwise changes in PEEP with a normocapnic, hyperoxic background. Application of 3- to 6-cmH2O PEEP caused the onset inspiratory (I) UAW nerve bursting to precede the phrenic burst but did not uncouple bursting. In contrast, application of 9- to 12-cmH2O PEEP uncoupled UAW neurograms such that rhythmic bursting occurred during periods of phrenic quiescence. Single-fiber recording experiments were conducted to determine whether a specific population of UAW motoneurons is recruited during uncoupled bursting. The data indicate that expiratory-inspiratory (EI) motoneurons remained active, while I motoneurons did not fire during uncoupled UAW bursting. Finally, we examined the relationship between motoneuron discharge rate and PEEP during coupled UAW and phrenic bursting. EI discharge rate was linearly related to PEEP during preinspiration, but showed no relationship to PEEP during inspiration. Our results demonstrate that multiple UAW motor outputs can be uncoupled from phrenic bursting, and this response is associated with bursting of EI nerve fibers. The relationship between PEEP and EI motoneuron discharge rate differs during preinspiratory and I periods; this may indicate that bursting during these phases of the respiratory cycle is controlled by distinct neuronal outputs.

Neural drive to tongue protrudor and retractor muscles following pulmonary C-fiber activation

Hypoglossal (XII) nerve recordings indicate that pulmonary C-fiber (PCF) receptor activation reduces inspiratory bursting and triggers tonic discharge. We tested three hypotheses related to this observation: 1) PCF receptor activation inhibits inspiratory activity in XII branches innervating both tongue protrudor muscles (medial branch; XIImed) and retractor muscles (lateral branch; XIIlat); 2) reduced XII neurogram amplitude reflects decreased XII motoneuron discharge rate; and 3) tonic XII activity reflects recruitment of previously silent motoneurons. Phrenic, XIImed, and XIIlat neurograms were recorded in anesthetized, paralyzed, and ventilated rats. Capsaicin delivered to the jugular vein reduced phrenic bursting at doses of 0.625 and 1.25 μg/kg but augmented bursting at 5 μg/kg. All doses reduced inspiratory amplitude in XIImed and XIIlat ( P < 0.05), and these effects were eliminated following bilateral vagotomy. Single-fiber recordings indicated that capsaicin causes individual XII motoneurons to either decrease discharge rate ( n = 101/153) or become silent ( n = 39/153). Capsaicin also altered temporal characteristics such that both XIImed and XIIlat inspiratory burst onset occurred after the phrenic burst ( P < 0.05). Increases in tonic discharge after capsaicin were greater in XIImed vs. XIIlat ( P < 0.05); single-fiber recordings indicated that tonic discharge reflected recruitment of previously silent motoneurons. We conclude that PCF receptor activation reduces inspiratory XII motoneuron discharge and transiently attenuates neural drive to both tongue protrudor and retractor muscles. However, tonic discharge appears to be selectively enhanced in tongue protrudor muscles. Accordingly, reductions in upper airway stiffness associated with reduced XII burst amplitude may be offset by enhanced tonic activity in tongue protrudor muscles.

Capsaicin administration inhibits the abducent branch but excites the thyroarytenoid branch of the recurrent laryngeal nerves in the rat

Our recent study showed that both inspiratory and expiratory activities of the recurrent laryngeal nerve (RLN) were enhanced by capsaicin administration in the rat (Lu IJ, Ku LC, Lin JT, Lee KZ, and Hwang JC. Chin J Physiol 45: 143–154, 2002). There are two intralaryngeal branches of the RLN: one innervates the thyroarytenoid (TA) muscle and the other innervates the abductor (Abd) muscles. To examine whether these two intralaryngeal branches respond similarly to capsaicin administration, their discharges as well as activities of the phrenic nerve (PNA) and the superior laryngeal nerve (SLNA) were monitored in anesthetized and ventilated rats at normocapnia in hyperoxia. The low dose of capsaicin (0.625 μg/kg) produced a cardiopulmonary chemoreflex, showing apnea, a decrease in PNA, hypotension, and bradycardia, and significant decreases in SLNA and the activity of the Abd branch. Concurrently, there was an increase in the intralaryngeal TA activity during both apnea and the recovery from apnea. The high dose of capsaicin (1.25 μg/kg) evoked larger chemoreflexive responses and laryngeal nerve activities. In addition, both doses of capsaicin initiated a similar delay in the onset of Abd activity and SLNA but an earlier onset for the TA branch to commence during inspiration. A bilateral vagotomy abolished the laryngeal responses to capsaicin administration. However, PNA and blood pressure were enhanced with capsaicin administration after the vagotomy. These results suggest that laryngeal adduction in response to capsaicin administration is vagal afferent dependent and that it may also represent reflexive protection for the airway and lungs.

Phasic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

The perfused in situ juvenile rat preparation produces patterns of phrenic discharge comparable to eupnea and gasping in vivo. These ventilatory patterns differ in multiple aspects, including most prominently the rate of rise of inspiratory activity. Although we have recently demonstrated that both eupnea and gasping are similarly modulated by a Hering-Breuer expiratory-promoting reflex to tonic pulmonary stretch, it has generally been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, we recorded eupneic and gasplike efferent activity of the phrenic nerve in the in situ juvenile rat perfused brain stem preparation, with and without phrenic-triggered phasic pulmonary inflation. We tested the hypothesis that phasic pulmonary inflation produces reflex responses in situ akin to those in vivo and that both eupnea and gasping are similarly modulated by phasic pulmonary stretch. In eupnea, we found that phasic pulmonary inflation decreases inspiratory burst duration and the period of expiration, thus increasing burst frequency of the phrenic neurogram. Phasic pulmonary inflation also decreases the duration of expiration and increases the burst frequency during gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation and that the brain stem circuitry generating the respiratory patterns respond to phasic activation of pulmonary stretch receptors in both eupnea and gasping. These findings support the homology of eupneic phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.

Differential expression of voltage-activated calcium channels in III and XII motoneurones during development in the rat

To further our understanding of the role that voltage-activated Ca2+ channels play in the development, physiology and pathophysiology of motoneurones (MNs), we used whole-cell patch-clamp recording to compare voltage-activated Ca2+ currents in oculomotor (III) and hypoglossal (XII) MNs of neonatal [postnatal day (P)1-5] and juvenile (P14-19) rats. In contrast to III MNs that innervate extraocular muscles, XII MNs that innervate tongue muscles mature more rapidly, fire bursts of low frequency action potentials and are vulnerable to degeneration in amyotrophic lateral sclerosis. In neonates, low voltage-activated (LVA) Ca2+ current densities are similar in XII and III MNs but high voltage-activated (HVA) Ca2+ current densities are twofold higher in XII MNs. The HVA Ca2+ channel antagonists (nimodipine and nifedipine for L-type, omega-agatoxin-TK for P/Q-type and omega-conotoxin-GVIA for N-type) revealed that, while N- and P/Q-type HVA Ca2+ channels are present in both MN pools, a 3.5-fold greater P/Q-type Ca2+ current in XII MNs accounts for their greater HVA Ca2+ currents. Developmentally, LVA and HVA Ca2+ current densities decrease in III MNs but remain unchanged in XII MNs. Thus, the differences between these MN pools increase developmentally so that, in juveniles, the LVA Ca2+ current density is twofold greater and the HVA Ca2+ current density is threefold greater in XII compared with III MNs. We propose that this differential expression of LVA and HVA Ca2+ channels in XII and III MNs during development contributes to their distinct physiology and may also be a factor contributing to the greater susceptibility of XII MNs to degeneration as seen in amyotrophic lateral sclerosis.

Olfactory-hypoglossal connections

Natural olfactory stimulation with amyl acetate significantly modulates the electrical activity of hypoglossal neurons and the electromyographic responses of the tongue musculature. The aim of the present study was to identify and characterize, using neuroanatomical and neurophysiological approaches, the pathways involved in the transmission of the olfactory information to the hypoglossal nucleus (XIIn). The neuroanatomical findings provided the initial demonstration that olfactory information is conveyed from the olfactory bulb to the hypoglossal nucleus via the interpeduncular nucleus (IPn) by both fast disynaptic and different polysynaptic pathways. The latter, in particular, involve many of the brain structures that process olfactory information. The electrophysiological studies demonstrated that the IPn neurons respond with a variety of patterns to natural stimulation of the olfactory receptors, thus supporting the hypothesis that the IPn is a crucial relay station for the elaboration and transmission of olfactory stimuli to XIIn.

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.

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.

Motor and premotor mechanisms of licking

The location, organization and anatomical connections of a central pattern generator (CPG) for licking are discussed. Anatomical and physiological studies suggest a brainstem location distributed within several subdivisions of the medullary reticular formation (RF). The involvement of widespread RF regions is evident from brainstem recording experiments in awake freely moving preparations and studies employing electrical stimulation of the frontal cortex to produce ororhythmic activity. The complex multifunctional properties of RF neurons producing licking are indicated by their activity during licking, swallowing and the rejection of an aversive gustatory stimulus. Anatomical studies place descending inputs to a brainstem CPG for licking to widely distributed areas of both the medial and lateral RF. In contrast, most projections originating from brainstem orosensory nuclei terminate primarily within the lateral RF. Because many pre-oromotor neurons appear concentrated largely in the intermediate zone of the RF (IRt), it is hypothesized that neurons from both lateral and medial sites converge within the IRt to control oromotor function.

Preliminary evidence suggesting delayed development in the hypoglossal and vagal nuclei of SIDS infants: a necropsy study

Little neuropathology has been documented in sudden infant death syndrome (SIDS) infants. Two hypotheses predict abnormalities in the hypoglossal nucleus and dorsal motor nucleus of the vagus: first, that upper airways are obstructed as a result of abnormal innervation (principally the hypoglossal nerve), and second, that they are obstructed as a result of abnormal cardiorespiratory control (the vagus nerve). A quantitative morphometric analysis was carried out to test these hypotheses in SIDS infants and controls (who died in accidents). The following nuclei dimensions were analyzed; length, volume, density, and estimated total cell number. In addition, cell size was analyzed. There were no differences in the anatomical distribution, site, or number of neurons between the groups. The most significant difference between the SIDS and control infants was the neuronal size: control infants had significantly larger neurons. In many other variables, there were trends suggesting a difference between the groups: the volume occupied by the neuronal populations was smaller in the SIDS infants, and therefore the neuronal density was increased. These values suggest differences in the development of these nuclei between SIDS and control infants.

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.

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

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