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

Mifflin, Steven W. Intensity and frequency dependenceof laryngeal afferent inputs to respiratory hypoglossal motoneurons. J. Appl. Physiol. 83(6):1890-1899, 1997.Inspiratory hypoglossal motoneurons (IHMs)mediate contraction of the genioglossus muscle and contribute to theregulation of upper airway patency. Intracellular recordings wereobtained from antidromically identified IHMs in anesthetized,vagotomized cats, and IHM responses to electrical activation ofsuperior laryngeal nerve (SLN) afferent fibers at various frequenciesand intensities were examined. SLN stimulus frequencies <2 Hz evokedan excitatory-inhibitory postsynaptic potential (EPSP-IPSP) sequence oronly an IPSP in most IHMs that did not change in amplitude as thestimulus was maintained. During sustained stimulus frequencies of5-10 Hz, there was a reduction in the amplitude of SLN-evokedIPSPs with time with variable changes in the EPSP. At stimulusfrequencies >25 Hz, the amplitude of EPSPs and IPSPs was reduced overtime. At a given stimulus frequency, increasing stimulus intensityenhanced the decay of the SLN-evoked postsynaptic potentials (PSPs).Frequency-dependent attenuation of SLN inputs to IHMs also occurred innewborn kittens. These results suggest that activation of SLN afferentsevokes different PSP responses in IHMs depending on the stimulusfrequency. At intermediate frequencies, inhibitory inputs areselectively filtered so that excitatory inputs predominate. At higherfrequencies there was no discernible SLN-evoked PSP temporally lockedto the SLN stimuli. Alterations in SLN-evoked PSPs could play a role inthe coordination of genioglossal contraction during respiration,swallowing, and other complex motor acts where laryngeal afferents areactivated.

     

No evidence for long-term facilitation after episodic hypoxia in spontaneously breathing, anesthetized rats.

Repeated electrical or hypoxic stimulation of peripheral chemoreceptors has been shown to cause a persistent poststimulus increase in respiratory motoneuron activity, termed long-term facilitation (LTF). LTF after episodic hypoxia has been demonstrated most consistently in anesthetized, vagotomized, paralyzed, artificially ventilated rats. Evidence for LTF in spontaneously breathing animals and humans after episodic hypoxia is equivocal and may have been influenced by the awake state of the subjects in these studies. The present study was designed to test the hypothesis that LTF is evoked in respiratory-related tongue muscle and inspiratory pump muscle activities after episodic hypoxia in 10 spontaneously breathing, anesthetized, vagotomized rats. The animals were exposed to three (5-min) episodes of isocapnic hypoxia, separated by 5 min of hyperoxia (50% inspired oxygen). Genioglossus, hyoglossus, and inspiratory intercostal EMG activities, along with respiratory-related tongue movements and esophageal pressure, were recorded before, during, and for 60 min after the end of episodic isocapnic hypoxia. We found no evidence for LTF in tongue muscle (genioglossus, hyoglossus) or inspiratory pump muscle (inspiratory intercostal) activities after episodic hypoxia. Rather, the primary poststimulus effect of episodic hypoxia was diminished respiratory frequency, which contributed to a reduction in ventilatory drive.     

Functional role and structure of the scalene: an accessory inspiratory muscle in hamster

Fournier, Mario, and Michael I. Lewis. Functional roleand structure of the scalene: an accessory inspiratory muscle inhamster. J. Appl. Physiol. 81(6):2436-2444, 1996.Although the scalene muscle (Sca) is a primaryinspiratory muscle in humans, its respiratory function in other speciesis less clear. The electromyographic (EMG) activity of the Sca wasstudied during resting ventilation (eupnea) in both the awake andanesthetized hamster and after a variety of respiratory challenges inthe anesthetized animal. The EMG activities of the medial Sca and thecostal diaphragm were compared. The medial Sca, the major component ofthe Sca, originates from cervical transverse processes 2 to 5 andinserts primarily onto rib 4, with a small segment onto rib 3. In both the anesthetized and awake animal, the Sca was always silent during quiet breathing. WithCO₂-stimulated hyperpnea, the Scawas always recruited during inspiration in phase with the diaphragm.Active recruitment of the Sca was also observed after resistive loading and total airway occlusion. After ipsilateral phrenicotomy, the Sca waspersistently recruited during eupnea. The specificity of the EMGsignals was tested both by excluding cross contamination from other ribcage muscles and by selective denervation studies. Muscle spindles wereidentified in the medial Sca histochemically, suggesting that therespiratory activity of the Sca can also be modulated by changes inmuscle length and/or load. These results indicate that the Scafunctions as an accessory inspiratory muscle in the hamster and mayplay an important role in conditions of chronic load.

     

Effects of transverse fiber stiffness and central tendon on displacement and shape of a simple diaphragm model

Boriek, Aladin M., and Joseph R. Rodarte. Effects oftransverse fiber stiffness and central tendon on displacement and shapeof a simple diaphragm model. J. Appl. Physiol. 82(5): 1626-1636, 1997.Our previous experimental results (A. M. Boriek, S. Lui, and J. R. Rodarte. J. Appl. Physiol. 75:527-533, 1993 and A. M. Boriek, T. A. Wilson, and J. R. Rodarte.J. Appl. Physiol. 76: 223-229, 1994) showed that1) costal diaphragm shape is similar at functional residualcapacity and end inspiration regardless of whether the diaphragm muscleshortens actively (increased tension) or passively (decreased tension);2) diaphragmatic muscle length changes minimally in thedirection transverse to the muscle fibers, suggesting the diaphragm maybe inextensible in that direction; and 3) the central tendon isnot stretched by physiological stresses. A two-dimensional orthotropicmaterial has two different stiffnesses in orthogonal directions. In theplane tangent to the muscle surface, these directions are along thefibers and transverse to the fibers. We wondered whether orthotropicmaterial properties in the muscular region of the diaphragm andinextensibility of the central tendon might contribute to the constancyof diaphragm shape. Therefore, in the present study, we examined theeffects of stiffness transverse to muscle fibers and inextensibility ofthe central tendon on diaphragmatic displacement and shape. Finiteelement hemispherical models of the diaphragm were developed by usingpressurized isotropic and orthotropic membranes with a wide range ofstiffness ratios. We also tested heterogeneous models, in which themuscle sheet was an orthotropic material, having transverse fiberstiffness greater than that along the fibers, with the central tendonbeing an inextensible isotropic cap. These models revealed thatincreased transverse stiffness limits the shape change of thediaphragm. Furthermore, an inextensible cap simulating the centraltendon dramatically limits the change in shape as well as the membrane displacement in response to pressure. These findings provide a plausible mechanism by which the diaphragm maintains similar shapes despite different physiological loads. This study suggests that changesof diaphragm shape are restricted because the central tendon isessentially inextensible and stiffness in the direction transverse tothe muscle fibers is greater than stiffness along the fibers.

     

Involvement of pharyngeal muscles in compensatory responses of the respiration system to inspiration resistance load]

The present study was undertaken to test the hypothesis that recruitment of upper airway muscles in loaded breathing is a result of integration of peripheral chemoreceptor and pulmonary mechanoreceptor afferents. Experiments were performed in spontaneously breathing tracheostomized anesthetized rabbits. It had been studied the effects of inspiratory resistive loading to EMG activity of genioglossus muscle. In the intact rabbits the peak value and duration of inspiratory activity of genioglossus increased in loading. Imposition of resistive load in vagotomized animals did not evoke alteration in inspiratory activity of genioglossus in the first loaded breath. Hyperoxia decreased the response of genioglossus muscle to inspiratory loading and vagatomy. We conclude that hypoxic stimulation of peripheral chemoreceptors and decrease in volume-related afferent activity from pulmonary stretch receptors are major mechanisms of the upper airway muscle recruiment in inspiratory resistive loading.     

Contractile properties of feline genioglossus, sternohyoid, and sternothyroid muscles.

Despite a wealth of information about the respiratory behavior of pharyngeal dilator muscles such as the genioglossus, sternohyoid, and sternothyroid muscles, little is known about their contractile and endurance properties. Strips of these muscles (as well as of the diaphragm) were surgically removed from anesthetized cats and studied in vitro at 37 degrees C. The isometric contraction times of the muscles were 38 +/- 1, 31 +/- 1, 28 +/- 2, and 35 +/- 1 ms for genioglossus, sternothyroid, sternohyoid, and diaphragm, respectively. Contraction times were significantly longer for the genioglossus than for the sternohyoid and sternothyroid muscles and significantly longer for the diaphragm than for the sternohyoid muscle. Twitch-to-tetanic ratios were largest for the diaphragm and lowest for the sternohyoid muscle, and the force-frequency relationship of the sternohyoid was most rightward positioned and that of the diaphragm was most leftward positioned. During repetitive stimulation, the decrement in force was greatest for the diaphragm and least for the genioglossus muscle, with the force loss of the two hyoid muscles being intermediate in magnitude. The Burke fatigue index was significantly greater for the genioglossus than for the diaphragm, despite similar tension-time indexes during repetitive stimulation. These data indicate heterogeneity among pharyngeal dilator muscles in their contractile and endurance properties, that certain pharyngeal dilator muscle properties differ from diaphragmatic properties, and that pharyngeal muscles have relatively fast contractile kinetics yet reasonable endurance characteristics.     

Differential respiratory muscle recruitment induced by clonidine in awake goats

The purpose of this study was to test thehypothesis that dysrhythmic breathing induced by the₂-agonist clonidine isaccompanied by differential recruitment of respiratory muscles. Inadult goats (n = 14) electromyographic(EMG) measurements were made from inspiratory muscles (diaphragm andparasternal intercostal) and expiratory muscles [triangularissterni (TS) and transversus abdominis (Abd)]. EMG of thethyroarytenoid (TA) muscle was used as an index of upper airway(glottal) patency. Peak EMG activities of all spinal inspiratory andexpiratory muscles were augmented by central and peripheralchemoreceptor stimuli. Phasic TA was apparent in the postinspiratoryphase of the breathing cycle under normoxic conditions. Duringdysrhythmic breathing episodes induced by clonidine, TS and Abdactivities were attenuated or abolished, whereas diaphragm andparasternal intercostal activities were unchanged. There was no tonicactivation of TS or Abd EMG during apneas; however, TA activity becametonic throughout the apnea. We conclude that1) ₂-adrenoceptor stimulationresults in differential recruitment of respiratory muscles duringrespiratory dysrhythmias and 2) apneas are accompanied by active glottic closure in the awake goat.

     

Inspiratory and expiratory patterns of the pectoralis major muscle during pulmonary defensive reflexes

Experiments wereconducted to determine the discharge pattern of the pectoralis majormuscle during pulmonary defensive reflexes in anesthetized cats(n = 15). Coughs andexpiration reflexes were elicited by mechanical stimulation of theintrathoracic trachea or larynx. Augmented breaths occurredspontaneously or were evoked by the same mechanical stimuli.Electromyograms (EMGs) were recorded from the diaphragm, rectusabdominis, and pectoralis major muscles. During augmented breaths, thepectoralis major had inspiratory EMG activity similar to that of thediaphragm, but during expiration reflexes the pectoralis major also hadpurely expiratory EMG activity similar to the rectus abdominis. Duringtracheobronchial cough, the pectoralis major had an inspiratory patternsimilar to that of the diaphragm in 10 animals, an expiratory patternsimilar to that of the rectus abdominis in 3 animals, and a biphasicpattern in 2 animals. The pectoralis major was active during both the inspiratory and expiratory phases during laryngeal cough. We conclude that, in contrast to the diaphragm or rectus abdominis muscles, thepectoralis major is active during both inspiratory and expiratory pulmonary defensive reflexes.

     

Respiratory-related pharyngeal constrictor muscle activity in decerebrate cats

Kuna, Samuel T., and Christi R. Vanoye.Respiratory-related pharyngeal constrictor muscle activity indecerebrate cats. J. Appl. Physiol.83(5): 1588-1594, 1997.Respiratory-related activity of thehyopharyngeus (middle pharyngeal constrictor) and thyropharyngeus(inferior pharyngeal constrictor) muscles was determined indecerebrate, tracheotomized adult cats and compared with theelectromyographic activity of the thyroarytenoid, a vocal cordadductor. During quiet breathing, the hyopharyngeus and usually thethyroarytenoid exhibited phasic activity during expiration and tonicactivity throughout the respiratory cycle. Respiratory-related thyropharyngeus activity was absent under these conditions. Progressive hyperoxic hypercapnia and progressive isocapnic hypoxia increased phasic expiratory activity in both pharyngeal constrictor (PC) musclesbut tended to suppress thyroarytenoid activity. Passively inducedhypocapnia and the central apnea that followed the cessation of themechanical hyperventilation were associated with tonic activation ofthe hyopharyngeus and thyroarytenoid but no recruitment inthyropharyngeus activity. The expiratory phase of a sigh and progressive pneumothorax were associated with an increase in phasic thyroarytenoid activity but no change in phasic PC activity. The results indicate that a variety of stimuli modulate respiratory-related PC activity, suggesting that the PC muscles may have a role in theregulation of upper airway patency during respiration.

     

Muscle fiber architecture of the dog diaphragm

Boriek, Aladin M., Charles C. Miller III, and Joseph R. Rodarte. Muscle fiber architecture of the dog diaphragm.J. Appl. Physiol. 84(1): 318-326, 1998.Previous measurements of muscle thickness and length ratio ofcostal diaphragm insertions in the dog (A. M. Boriek and J. R. Rodarte.J. Appl. Physiol. 77: 2065-2070,1994) suggested, but did not prove, discontinuous muscle fiberarchitecture. We examined diaphragmatic muscle fiber architecture usingmorphological and histochemical methods. In 15 mongrel dogs, transversesections along the length of the muscle fibers were analyzedmorphometrically at ×20, by using the BioQuant System IVsoftware. We measured fiber diameters, cross-sectional fiber shapes,and cross-sectional area distributions of fibers. We also determinednumbers of muscle fibers per cross-sectional area and ratio ofconnective tissue to muscle fibers along a course of the muscle fromnear the chest wall (CW) to near the central tendon (CT) for midcostalleft and right hemidiaphragms, as well as ventral, middle, and dorsalregions of the left costal hemidiaphragm. In six other mongrel dogs,the macroscopic distribution of neuromuscular junctions (NMJ) onthoracic and abdominal diaphragm surfaces was determined by stainingthe intact diaphragmatic muscle for acetylcholinesterase activity. Theaverage major diameter of muscle fibers was significantly smaller, andthe number of fibers was significantly larger midspan between CT and CWthan near the insertions. The ratio of connective tissues to musclefibers was largest at CW compared with other regions along the lengthof the muscle. The diaphragm is transversely crossed by multiplescattered NMJ bands with fairly regular intervals offset in adjacentstrips. Muscle fascicles traverse two to five NMJ, consistent withfibers that do not span the entire fascicle from CT to CW. Theseresults suggest that the diaphragm has a discontinuous fiberarchitecture in which contractile forces may be transmitted among themuscle fibers through the connective tissue adjacent to the fibers.

     

Expression of nitric oxide synthase isoforms in normal ventilatory and limb muscles

Hussain, Sabah N. A., Qasim El-Dwairi, Mohammed N. Abdul-Hussain, and Dalia Sakkal. Expression of nitric oxidesynthase isoforms in normal ventilatory and limb muscles.J. Appl. Physiol. 83(2): 348-353, 1997.Nitric oxide (NO), an important messenger molecule withwidespread actions, is synthesized by NO synthases (NOS). In thisstudy, we investigated the correlation between fiber type and NOSactivity among ventilatory and limb muscles of various species. We alsoassessed the presence of the three NOS isoforms in normal skeletalmuscles and how various NOS inhibitors influence muscle NOS activity.NOS activity was detected in various muscles; however, NOS activity inrabbits and rats varied significantly among different muscles.Immunoblotting of muscle samples indicated the presence of both theneuronal NOS and the endothelial NOS isoforms but not thecytokine-inducible NOS isoform. However, these isoforms were expressedto different degrees in various muscles. Although the neuronal NOSisoform was detectable in the canine diaphragm, very weak expressionwas detected in rabbit, rat, and mouse diaphragms. The endothelial NOSisoform was detected in the rat and mouse diaphragms but not in thecanine and rabbit diaphragms. We also found thatN^G-nitro-L-arginine methyl ester,7-nitroindazole, andS-methylisothiourea werestronger inhibitors of muscle NOS activity than was aminoguanidine. These results indicate the presence of different degrees ofconstitutive NOS expression in normal ventilatory and limb muscles ofvarious species. Our data also indicate that muscle NOS activity is not determined by fiber type distribution but by other not yet identified factors. The functional significance of this expression remains to beassessed.

     

Respiratory function of pharyngeal muscles

This article reviews experimental studies of pharyngeal muscles with emphasis on m. genioglossus as a major muscle dilating pharynx and discusses neuromuscular mechanisms that maintains patency of upper airway. Mechanisms of inspiratory activation of genioglossus muscle in comparative with diaphragm are also discussed. Experimental data suggesting that upper airway muscles have a significant role in compensation of added inspiratory load are presented. It allows to regard pharyngeal dilating muscles as accessory muscles of respiration. Activation of m. genioglossus (together with others muscles dilating the pharynx) decreases airway resistance and thereby facilitates the load compensation function of "pumping" muscles. Similar to diaphragm involvement of the pharynx dilating muscles in the load compensatory response is resulted from a complex integration of several influences originating from mechano- and chemoreceptors.     

Airway smooth muscle orientation in intraparenchymal airways

Lei, M., H. Ghezzo, M. F. Chen, and D. H. Eidelman.Airway smooth muscle orientation in intraparenchymal airways.J. Appl. Physiol. 82(1): 70-77, 1997.Airway smooth muscle (ASM) shortening is the central eventleading to bronchoconstriction. The degree to which airway narrowingoccurs as a consequence of shortening is a function of both themechanical properties of the airway wall as well as the orientation ofthe muscle fibers. Although the latter is theoretically important, ithas not been systematically measured to date. The purpose of this studywas to determine the angle of orientation of ASM () in normal lungs by using a morphometric approach. We analyzed the airway tree of theleft lower lobes of four cats and one human. All material was fixedwith 10% buffered Formalin at a pressure of 25 cmH₂O for 48 h. The fixed materialwas dissected along the airway tree to permit isolation ofgenerations 4-18 in the cats andgenerations 5-22 in the humanspecimen. Each airway generation was individually embedded in paraffin.Five-micrometer-thick serial sections were cut parallel to the airwaylong axis and stained with hematoxylin-phloxine-saffron. Each blockyielded three to five sections containing ASM. To determine , wemeasured the orientation of ASM nuclei relative to the transverse axisof the airway by using a digitizing tablet and a light microscope (×250) equipped with a drawing tube attachment. Inspection of thesections revealed extensive ASM crisscrossing without a homogeneous orientation. The  was clustered between 20° and 20°in all airway generations and did not vary much between generations inany of the cats or in the human specimen. When  was expressedwithout regard to sign, the mean values were 13.2° in the cats and13.1° in the human. This magnitude of obliquity is not likely toresult in physiologically important changes in airway length duringbronchoconstriction.

     

Endurance training affects myosin heavy chain phenotype in regenerating fast-twitch muscle

Bigard, Xavier A., Chantal Janmot, Danièle Merino,Françoise Lienhard, Yannick C. Guezennec, and Anne D'Albis.Endurance training affects myosin heavy chain phenotype inregenerating fast-twitch muscle. J. Appl.Physiol. 81(6): 2658-2665, 1996.The aim of thisstudy was to analyze the effects of treadmill training (2 h/day, 5 days/wk, 30 m/min, 7% grade for 5 wk) on the expression of myosinheavy chain (MHC) isoforms during and after regeneration of afast-twitch white muscle [extensor digitorum longus (EDL)]. Male Wistar rats were randomly assigned to a sedentary(n = 10) or an endurance-trained (ET;n = 10) group. EDL muscle degeneration and regeneration were induced by two subcutaneous injections of a snaketoxin. Five days after induction of muscle injury, animals were trainedover a 5-wk period. It was verified that ~40 days after venomtreatment, central nuclei were present in the treated EDL muscles fromsedentary and ET rats. The changes in the expression of MHCs in EDLmuscles were detected by using a combination of biochemical andimmunocytochemical approaches. Compared with contralateral nondegenerated muscles, relative concentrations of types I, IIa, andIIx MHC isoforms in ET rats were greater in regenerated EDL muscles(146%, P < 0.05; 76%,P < 0.01; 87%,P < 0.01, respectively). Their elevation corresponded to a decreasein the relative concentration of type IIb MHC (36%,P < 0.01). Although type I accountedfor only 3.2% of total myosin in regenerated muscles from the ETgroup, the cytochemical analysis showed that the proportion of positive staining with the slow MHC antibody was markedly greater in regenerated muscles than in contralateral ones. Collectively, these results demonstrate that the regenerated EDL muscle is sensitive to endurance training and suggest that the training-induced shift in MHC isoforms observed in these muscles resulted from an additive effect of regeneration and repeated exercise.

     

Kinematics and mechanics of midcostal diaphragm of dog

Boriek, Aladin M., Joseph R. Rodarte, and Theodore A. Wilson. Kinematics and mechanics of midcostal diaphragm of dog. J. Appl. Physiol. 83(4):1068-1075, 1997.Radiopaque markers were attached to theperitoneal surface of three neighboring muscle bundles in the midcostaldiaphragm of four dogs, and the locations of the markers were trackedby biplanar video fluoroscopy during quiet spontaneous breathing andduring inspiratory efforts against an occluded airway at three lungvolumes from functional residual capacity to total lung capacity inboth the prone and supine postures. Length and curvature of the musclebundles were determined from the data on marker location. Musclelengths for the inspiratory states, as a fraction of length atfunctional residual capacity, ranged from 0.89 ± 0.04 at endinspiration during spontaneous breathing down to 0.68 ± 0.07 duringinspiratory efforts at total lung capacity. The muscle bundles werefound to have the shape of circular arcs, with the three bundlesforming a section of a right circular cylinder. With increasing lungvolume and diaphragm displacement, the circular arcs rotate around theline of insertion on the chest wall, the arcs shorten, but the radiusof curvature remains nearly constant. Maximal transdiaphragmaticpressure was calculated from muscle curvature and maximaltension-length data from the literature. The calculated maximaltransdiaphragmatic pressure-length curve agrees well with the data ofRoad et al. (J. Appl. Physiol. 60:63-67, 1986).

     

Superior laryngeal nerve section alters responses to upper airway distortion in sleeping dogs

Curran, Aidan K., Peter R. Eastwood, Craig A. Harms, CurtisA. Smith, and Jerome A. Dempsey. Superior laryngeal nerve sectionalters responses to upper airway distortion in sleeping dogs.J. Appl. Physiol. 83(3): 768-775, 1997.We investigated the effect of superior laryngeal nerve (SLN)section on expiratory time(TE) and genioglossuselectromyogram (EMGgg) responses to upper airway (UA) negative pressure(UANP) in sleeping dogs. The same dogs used in a similar intact study(C. A. Harms, C. A., Y.-J. Zeng, C. A. Smith, E. H. Vidruk, and J. A. Dempsey. J. Appl. Physiol. 80:1528-1539, 1996) were bilaterally SLN sectioned. After recovery,the UA was isolated while the animal breathed through a tracheostomy.Square waves of negative pressure were applied to the UA from below thelarynx or from the mask (nares) at end expiration and held until thenext inspiratory effort. Section of the SLN increased eupneicrespiratory frequency and minute ventilation. Relative to the same dogsbefore SLN section, sublaryngeal UANP caused lessTE prolongation while activation of the genioglossus required less negative pressures. Mask UANP had noeffect on TE or EMGgg activity.We conclude that the SLN 1) is notobligatory for the reflex prolongation ofTE and activation of EMGggactivity produced by UANP and 2)plays an important role in the maintenance of UA stability and thepattern of breathing in sleeping dogs.

     

刺激中缝背核不同区域对颏舌肌和膈肌的影响

本实验在４８只氨基甲酸惭酯麻醉、断双侧迷走神经的健康家兔上观察了电、化学刺激中缝背核背侧区（ｄＮＲＤ）和腹侧区（ｖＮＲＤ）对颏舌肌和膈肌肌电积分活动的影响。结果如下：（１）长串电脉冲刺激ｄＮＲＤ使颏舌肌和膈肌肌电活动均明显增强;（２）长串电脉冲刺激ｖＮＲＤ时,颏舌肌活动被易化,而膈肌活动则被抑制;（３）在ｄＮＲＤ和ｖＮＲＤ分别微量注射谷氨酸钠,其效应与电刺激结果基本一致。结果提示：（３）在ｄＮＲＤ     

Inputs from upper airway affect firing of respiratory-associated midbrain neurons

Chen, Zibin, and Frederic L. Eldridge. Inputs fromupper airway affect firing of respiratory-associated midbrain neurons. J. Appl. Physiol. 83(1): 196-203, 1997.In 16 decerebrated unanesthetized cats, we studied effects ofneural inputs from upper airway on firing of 62 mesencephalic neuronsthat also developed respiratory-associated (RA) rhythmic firing whenrespiratory drive was high [Z. Chen, F. L. Eldridge, and P.G.Wagner. J. Physiol. (Lond.) 437:305-325, 1991] and on firing of 16 neurons that did notdevelop the rhythmic firing (non-RA neurons). Activity in RA neuronsincreased after mechanical expansion of pharynx (45% of those tested)or larynx (68%) and after stimulation of glossopharyngeal (50%) orsuperior laryngeal nerves (77%). The increased neuronal firingoccurred despite decreases or abolition of respiratory activity(expressed in phrenic nerve). Neuronal firing also increased aftermechanical stimulation of nasal mucosa (66%) or by jetsof air directed into the nares (48%) and after lightbrushing of nasal skin (~40%). Most stimuli led to decreased firingin a smaller number of neurons, and some neurons showed no response.None of the non-RA neurons developed an increase of firing after any ofthe stimuli, although one had decreased firing after stimulation of thesuperior laryngeal nerve. We conclude that inputs from the upper airwayand nasal skin have independent modulatory effects on the samemesencephalic neurons that are stimulated by ascending rhythmic RAinput from the medulla. These findings may have relevance to generationof the sensation of dyspnea.

     

Corticomotor control of the genioglossus in awake OSAS patients: a transcranial magnetic stimulation study

Background

Upper airway collapse does not occur during wake in obstructive sleep apnea patients. This points to wake-related compensatory mechanisms, and possibly to a modified corticomotor control of upper airway dilator muscles. The objectives of the study were to characterize the responsiveness of the genioglossus to transcranial magnetic stimulation during respiratory and non-respiratory facilitatory maneuvers in obstructive sleep apnea patients, and to compare it to the responsiveness of the diaphragm, with reference to normal controls.

Methods

Motor evoked potentials of the genioglossus and of the diaphragm, with the corresponding motor thresholds, were recorded in response to transcranial magnetic stimulation applied during expiration, inspiration and during maximal tongue protraction in 13 sleep apnea patients and 8 normal controls.

Main Results

In the sleep apnea patients: 1) combined genioglossus and diaphragm responses occurred more frequently than in controls (P < 0.0001); 2) the amplitude of the genioglossus response increased during inspiratory maneuvers (not observed in controls); 3) the latency of the genioglossus response decreased during tongue protraction (not observed in controls). A significant negative correlation was found between the latency of the genioglossus response and the apnea-hypopnea index; 4) the difference in diaphragm and genioglossus cortico-motor responses during tongue protraction and inspiratory loading differed between sleep apnea and controls.

Conclusion

Sleep apnea patients and control subjects differ in the response pattern of the genioglossus and of the diaphragm to facilitatory maneuvers, some of the differences being related to the frequency of sleep-related events.     

Response of genioglossus muscle activity to nasal airway occlusion in normal sleeping adults

To determine the combined effect of increased subatmospheric upper airway pressure and withdrawal of phasic volume feedback from the lung on genioglossus muscle activity, the response of this muscle to intermittent nasal airway occlusion was studied in 12 normal adult males during sleep. Nasal occlusion at end expiration was achieved by inflating balloon-tipped catheters located within the portals of a nose mask. No seal was placed over the mouth. During nose breathing in non-rapid-eye-movement (NREM) sleep, nasal airway occlusion resulted in multiple respiratory efforts before arousal. Mouth breathing was not initiated until arousal. Phasic inspiratory genioglossus activity was present in eight subjects during NREM sleep. In these subjects, comparison of peak genioglossus inspiratory activity on the first three occluded efforts to the value just before occlusion showed an increase of 4.7, 16.1, and 28.0%, respectively. The relative increases in peak genioglossus activity were very similar to respective increases in peak diaphragm activity. Arousal was associated with a large burst in genioglossus activity. During airway occlusion in rapid-eye-movement (REM) sleep, mouth breathing could occur without a change in sleep state. In general, genioglossus responses to airway occlusion in REM sleep were similar in pattern to those in NREM sleep. A relatively small reflex activation of upper airway muscles associated with a sudden increase in subatmospheric pressure in the potentially collapsible segment of the upper airway may help compromise upper airway patency during sleep.