Diverse physiological properties of hypoglossal motoneurons innervating intrinsic and extrinsic tongue muscles.

The mammalian tongue contains eight muscles that collaborate to ensure that suckling, swallowing, and other critical functions are robust and reliable. Seven of the eight tongue muscles are innervated by hypoglossal motoneurons (XIIMNs). A somatotopic organization of the XII motor nucleus, defined in part by the mechanical action of a neuron's target muscle, has been described, but whether or not XIIMNs within a compartment are functionally specialized is unsettled. We hypothesize that developing XIIMNs are assigned unique functional properties that reflect the challenges that their target muscle faces upon the transition from in utero to terrestrial life. To address this, we studied XIIMNs that innervate intrinsic and extrinsic tongue muscles, because intrinsic muscles play a more prominent role in suckling than the extrinsic muscles. We injected dextran-rhodamine into the intrinsic longitudinal muscles (IL) and the extrinsic genioglossus, and physiologically characterized the labeled XIIMNs. Consistent with earlier work, IL XIIMNs (n = 150) were located more dorsally within the nucleus, and GG XIIMNs (n = 55) more ventrally. Whole cell recordings showed that resting membrane potential was, on average, 9 mV more depolarized in IL than in GG XIIMNs (P = 0.0019), and the firing threshold in response to current injection was lower in IL (-31 ± 23 pA) than in GG XIIMNs (225 ± 39 pA; P < 0.0001). We also found that the appearance of net outward currents in GG XIIMNs occurred at more hyperpolarized membrane potentials than IL XIIMNs, consistent with lower excitability in GG XIIMNs. These observations document muscle-specific functional specializations among XIIMNs.NEW & NOTEWORTHY The hypoglossal motor nucleus contains motoneurons responsible for innervating one of seven different muscles with notably different biomechanics and patterns of use. Whether or not motoneurons innervating the different muscles also have unique functional properties (e.g., spiking behavior, synaptic physiology) is poorly understood. In this work we show that neonatal hypoglossal motoneurons innervating muscles that shape the tongue (intrinsic longitudinal muscles) have different electrical properties than those innervating the genioglossus, which controls tongue position.

Phrenicotomy alters phrenic long-term facilitation following intermittent hypoxia in anesthetized rats.

Intermittent hypoxia (IH) can induce a persistent increase in neural drive to the respiratory muscles known as long-term facilitation (LTF). LTF of phrenic inspiratory activity is often studied in anesthetized animals after phrenicotomy (PhrX), with subsequent recordings being made from the proximal stump of the phrenic nerve. However, severing afferent and efferent axons in the phrenic nerve has the potential to alter the excitability of phrenic motoneurons, which has been hypothesized to be an important determinant of phrenic LTF. Here we test the hypothesis that acute PhrX influences immediate and long-term phrenic motor responses to hypoxia. Phrenic neurograms were recorded in anesthetized, ventilated, and vagotomized adult male rats with intact phrenic nerves or bilateral PhrX. Data were obtained before (i.e., baseline), during, and after three 5-min bouts of isocapnic hypoxia. Inspiratory burst amplitude during hypoxia (%baseline) was greater in PhrX than in phrenic nerve-intact rats (P < 0.001). Similarly, burst amplitude 55 min after IH was greater in PhrX than in phrenic nerve-intact rats (175 + or - 9 vs. 126 + or - 8% baseline, P < 0.001). In separate experiments, phrenic bursting was recorded before and after PhrX in the same animal. Afferent bursting that was clearly observable in phase with lung deflation was immediately abolished by PhrX. The PhrX procedure also induced a form of facilitation as inspiratory burst amplitude was increased at 30 min post-PhrX (P = 0.01 vs. pre-PhrX). We conclude that, after PhrX, axotomy of phrenic motoneurons and, possibly, removal of phrenic afferents result in increased phrenic motoneuron excitability and enhanced LTF following IH.

Control of breathing in children with mild sleep apnoea: a 6-year follow-up study.

We have previously shown that children (average age 9 yrs) with mildly elevated obstructive apnoea/hypopnoea indices (OAHI) retained CO(2) at rest. Here, we report the results of a 6-yr follow-up study on 14 children from that study. Minute ventilation (V'(E)) and end-tidal CO(2) partial pressure (P(ET,CO(2))) were measured during hypercapnic challenge. OAHI decreased from 7.5+/-4.7 events x h(-1) at age 9 yrs to 2.5+/-1.8 events x h(-1) at age 15 yrs (p<0.001), despite an increase in body mass index from 20+/-4.6 kg x m(-2) to 26+/-5.7 kg x m(-2) (p<0.0001). Eupneic V'(E) increased from 4.1+/-0.31 L x min(-1) x m(-2) to 5.9+/-0.4 L x min(-1) x m(-2) (p<0.01), while P(ET,CO(2)) fell from 44.1+/-0.8 to 33+/-1.0 mmHg (p<0.001). The V'(E)-P(ET,CO(2)) obtained during hypercapnia was left shifted, such that V'(E) at a P(ET,CO(2)) of 50 mmHg increased from 24 L x min(-1) at age 9 yrs to 36 L x min(-1) at age 15 yrs. Central respiratory drive did not change. We hypothesise that somatic growth of the pharynx coupled with a regression of tonsillar tissue mass with age leads to enlargement of the upper airway lumen, a reduction in airway resistance and increased respiratory airflow at a given level of ventilatory drive.

Pharyngeal critical pressure in children with mild sleep-disordered breathing.

There is evidence that narrowing or collapse of the pharynx can contribute to obstructive sleep-disordered breathing (SDB) in adults and children. However, studies in children have focused on those with relatively severe SDB who generally were recruited from sleep clinics. It is unclear whether children with mild SDB who primarily have hypopneas, and not frank apnea, also have more collapsible airways. We estimated airway collapsibility in 10 control subjects (9.4 +/- 0.5 yr old; 1.9 +/- 0.2 hypopneas/h) and 7 children with mild SDB (10.6 +/- 0.5 yr old; 11.5 +/- 0.1 hypopneas/h) during stable, non-rapid eye movement sleep. None of the subjects had clinically significant enlargement of the tonsils or adenoids, nor had any undergone previous tonsillectomy or adenoidectomy. Airway collapsibility was measured by brief (2-breath duration) and sudden reductions in pharyngeal pressure by connecting the breathing mask to a negative pressure source. Negative pressure applications ranging from -1 to -20 cmH(2)O were randomly applied in each subject while respiratory airflow and mask pressure were measured. Flow-pressure curves were constructed for each subject, and the x-intercept gave the pressure at zero flow, the so-called critical pressure of the upper airway (Pcrit). Pcrit was significantly higher in children with SDB than in controls (-10.8 +/- 2.8 vs. -15.7 +/- 1.2 cmH(2)O; P < 0.05). There were no significant differences in the slopes of the pressure-flow relations or in baseline airflow resistance. These data support the concept that intrinsic pharyngeal collapsibility contributes to mild SDB in children.

Coordination of intrinsic and extrinsic tongue muscles during spontaneous breathing in the rat.

The muscular-hydrostat model of tongue function proposes a constant interaction of extrinsic (external bony attachment, insertion into base of tongue) and intrinsic (origin and insertion within the tongue) tongue muscles in all tongue movements (Kier WM and Smith KK. Zool J Linn Soc 83: 207-324, 1985). Yet, research that examines the respiratory-related effects of tongue function in mammals continues to focus almost exclusively on the respiratory control and function of the extrinsic tongue protrusor muscle, the genioglossus muscle. The respiratory control and function of the intrinsic tongue muscles are unknown. Our purpose was to determine whether intrinsic tongue muscles have a respiration-related activity pattern and whether intrinsic tongue muscles are coactivated with extrinsic tongue muscles in response to respiratory-related sensory stimuli. Esophageal pressure and electromyographic (EMG) activity of an extrinsic tongue muscle (hyoglossus), an intrinsic tongue muscle (superior longitudinal), and an external intercostal muscle were studied in anesthetized, tracheotomized, spontaneously breathing rats. Mean inspiratory EMG activity was compared at five levels of inspired CO2. Intrinsic tongue muscles were often quiescent during eupnea but active during hypercapnia, whereas extrinsic tongue muscles were active in both eupnea and hypercapnia. During hypercapnia, the activities of the airway muscles were largely coincident, although the onset of extrinsic muscle activity generally preceded the onset of intrinsic muscle activation. Our findings provide evidence, in an in vivo rodent preparation, of respiratory modulation of motoneurons supplying intrinsic tongue muscles. Distinctions noted between intrinsic and extrinsic activities could be due to differences in motoneuron properties or the central, respiration-related control of each motoneuron population.

Sleep-disordered breathing, pharyngeal size and soft tissue anatomy in children.

We tested the hypothesis that pharyngeal geometry and soft tissue dimensions correlate with the severity of sleep-disordered breathing. Magnetic resonance images of the pharynx were obtained in 18 awake children, 7-12 yr of age, with obstructive apnea-hypopnea index (OAHI) values ranging from 1.81 to 24.2 events/h. Subjects were divided into low-OAHI (n = 9) and high-OAHI (n = 9) groups [2.8 +/- 0.7 and 13.5 +/- 4.9 (SD) P < 0.001]. The OAHI correlated positively with the size of the tonsils (r2 = 0.42, P = 0.024) and soft palate (r2 = 0.33, P = 0.049) and inversely with the volume of the oropharyx (r2 = 0.42, P = 0.038). The narrowest point in the pharyngeal airway was smaller in the high-compared with the low-OAHI group (4.4 +/- 1.2 vs. 6.0 +/- 1.3 mm; P = 0.024), and this point was in the retropalatal airway in all but two subjects. The airway cross-sectional area (CSA)-airway length relation showed that the high-OAHI group had a narrower retropapatal airway than the low-OAHI group, particularly in the retropalatal region where the soft palate, adenoids, and tonsils overlap (P = 0.001). The "retropalatal air space," which we defined as the ratio of the retropalatal airway CSA to the CSA of the soft palate, correlated inversely with the OAHI (r2 = 0.49, P = 0.001). We conclude that 7- to 12-yr-old children with a narrow retropalatal air space have significantly more apneas and hypopneas during sleep compared with children with relatively unobstructed retropalatal airways.

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.

MRI study of pharyngeal airway changes during stimulation of the hypoglossal nerve branches in rats.

The medial branch (Med) of the hypoglossal nerve innervates the tongue protrudor muscles, whereas the lateral branch (Lat) innervates tongue retractor muscles. Our previous finding that pharyngeal airflow increased during either selective Med stimulation or whole hypoglossal nerve (WHL) stimulation (coactivation of protrudor and retractor muscles) led us to examine how WHL, Med, or Lat stimulation affected tongue movements and nasopharyngeal (NP) and oropharyngeal (OP) airway volume. Electrical stimulation of either WHL, Med, or Lat nerves was performed in anesthetized, tracheotomized rats while magnetic resonance images of the NP and OP were acquired (slice thickness 0.5 mm, in-plane resolution 0.25 mm). NP and OP volume was greater during WHL and Med stimulation vs. no stimulation (P < 0.05). Ventral tongue depression (measured in the midsagittal images) and OP volume were greater during Med stimulation than during WHL stimulation (P < 0.05). Lat stimulation did not alter NP volume (P = 0.39). Our finding that either WHL or Med stimulation dilates the NP and OP airways sheds new light on the control of pharyngeal airway caliber by extrinsic tongue muscles and may lead to new treatments for patients with obstructive sleep apnea.

Feasibility of using unattended polysomnography in children for research--report of the Tucson Children's Assessment of Sleep Apnea study (TuCASA).

STUDY OBJECTIVES: The Tucson Children's Assessment of Sleep Apnea study (TuCASA) is designed to investigate the prevalence and correlates of objectively measured sleep-disordered breathing in pre-adolescent children. This paper documents the methods and feasibility of attaining quality unattended polysomnograms in the first 162 TuCASA children recruited. DESIGN: A prospective cohort study projected to enroll 500 children between 5 and 12 years of age who will undergo unattended polysomnography, neurocognitive evaluation, and physiological and anatomical measurements thought to be associated with sleep-disordered breathing. SETTING: Children are recruited through the Tucson Unified School District. Polysomnograms and anthropometric measurements are completed in the child's home. PARTICIPANTS: Of the 157 children enrolled in TuCASA, there were 100 children (64%) between 5-8 years old and 57 children (36%) between the ages of 9 to 12. There were 74 (47%) Hispanic children, and 68 (43%) female participants. INTERVENTIONS: N/A. MEASUREMENTS & RESULTS: Technically acceptable studies were obtained in 157 children (97%). The initial pass rate was 91%, which improved to 97% when 9 children who failed on the first night of recording completed a second study which was acceptable. In 152 studies (97%), greater than 5 hours of interpretable respiratory, electroencephalographic, and oximetry signals were obtained. The poorest signal quality was obtained from the chin electromyogram and from the combination thermister/nasal cannula. Parents reported that 54% of children slept as well as, or better than usual, while 40% reported that their child slept somewhat worse than usual. Only 6% were observed to sleep much worse than usual. Night-to-night variability in key polysomnographic parameters (n=10) showed a high degree of reproducibility on 2 different nights of study using identical protocols in the same child. In 5 children, polysomnograms done in the home were comparable to those recorded in a sleep laboratory. CONCLUSIONS: The high quality of data collected in TuCASA demonstrates that multi-channel polysomnography data can be successfully obtained in children aged 5-12 years in an unattended setting under a research protocol.

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.

Influence of posture and breathing route on neural drive to upper airway dilator muscles during exercise.

Our purpose was to determine the influence of posture and breathing route on electromyographic (EMG) activities of nasal dilator (NDM) and genioglossus (GG) muscles during exercise. Nasal and oral airflow rates and EMG activities of the NDM and GG were recorded in 10 subjects at rest and during upright and supine incremental cycling exercise to exhaustion. EMG activities immediately before and after the switch from nasal to oronasal breathing were also determined for those subjects who demonstrated a clear switch point (n = 7). NDM and GG EMG activities were significantly correlated with increases in nasal, oral, and total ventilatory rates during exercise, and these relationships were not altered by posture. In both upright and supine exercise, NDM activity rose more sharply as a function of nasal inspired ventilation compared with total or oral inspired ventilation (P < 0.01), but GG activity showed no significant breathing-route dependence. Peak NDM integrated EMG activity decreased (P = 0.008), and peak GG integrated EMG activity increased (P = 0.032) coincident with the switch from nasal to oronasal breathing. In conclusion, 1) neural drive to NDM and GG increases as a function of exercise intensity, but the increase is unaltered by posture; 2) NDM activity is breathing-route dependent in steady-state exercise, but GG activity is not; and 3) drive to both muscles changes significantly at the switch point, but the change in GG activity is more variable and is often transient. This suggests that factors other than the breathing route dominate drive to the GG soon after the initial changes in the configuration of the oronasal airway are made.

Fatiguing contractions of tongue protrudor and retractor muscles: influence of systemic hypoxia.

The influence of systemic hypoxia on the endurance performance of tongue protrudor and retractor muscles was examined in anesthetized, ventilated rats. Tongue protrudor (genioglossus) or retractor (hyoglossus and styloglossus) muscles were activated via medial or lateral XII nerve branch stimulation (0.1-ms pulse; 40 Hz; 330-ms trains; 1 train/s). Maximal evoked potentials (M waves) of genioglossus and hyoglossus were monitored with electromyography. Fatigue tests were performed under normoxic and hypoxic (arterial PO(2) = 50 +/- 1 Torr) conditions in separate animals. The fatigue index (FI; %initial force) after 5 min of normoxic stimulation was 85 +/- 6 and 79 +/- 7% for tongue protrudor and retractor muscles, respectively; these values were significantly lower during hypoxia (protrudor FI = 52 +/- 10, retractor FI = 18 +/- 6%; P < 0.05). Protrudor and retractor muscle M-wave amplitude declined over the course of the hypoxic fatigue test but did not change during normoxia (P < 0.05). We conclude that hypoxia attenuates tongue protrudor and retractor muscle endurance performance; potential mechanisms include neuromuscular transmission failure and/or diminished sarcolemmal excitability.

Consequences of periodic augmented breaths on tongue muscle activities in hypoxic rats.

This study was designed to investigate the influence of hypoxia-evoked augmented breaths (ABs) on respiratory-related tongue protrudor and retractor muscle activities and inspiratory pump muscle output. Genioglossus (GG) and hyoglossus (HG) electromyogram (EMG) activities and respiratory-related tongue movements were compared with peak esophageal pressure (Pes; negative change in pressure during inspiration) and minute Pes (Pes x respiratory frequency = Pes/min) before and after ABs evoked by sustained poikilocapnic, isocapnic, and hypercapnic hypoxia in spontaneously breathing, anesthetized rats. ABs evoked by poikilocapnic and isocapnic hypoxia triggered long-lasting (duration at least 10 respiratory cycles) reductions in GG and HG EMG activities and tongue movements relative to pre-AB levels, but Pes was reduced transiently (duration of <10 respiratory cycles) after ABs. Adding 7% CO(2) to the hypoxic inspirate had no effect on the frequency of evoked ABs, but this prevented long-term declines in tongue muscle activities. Bilateral vagotomy abolished hypoxia-induced ABs and stabilized drive to the tongue muscles during each hypoxic condition. We conclude that, in the rat, hypoxia-evoked ABs 1) elicit long-lasting reductions in protrudor and retractor tongue muscle activities, 2) produce short-term declines in inspiratory pump muscle output, and 3) are mediated by vagal afferents. The more prolonged reductions in pharyngeal airway vs. pump muscle activities may lead to upper airway narrowing or collapse after spontaneous ABs.

Effect of co-activation of tongue protrudor and retractor muscles on tongue movements and pharyngeal airflow mechanics in the rat.

1. The purpose of these experiments was to examine the mechanisms by which either co-activation or independent activation of tongue protrudor and retractor muscles influence upper airway flow mechanics. We studied the influence of selective hypoglossal (XIIth) nerve stimulation on tongue movements and flow mechanics in anaesthetized rats that were prepared with an isolated upper airway. In this preparation, both nasal and oral flow pathways are available. 2. Inspiratory flow limitation was achieved by rapidly lowering hypopharyngeal pressure (Php) with a vacuum pump, and the maximal rate of flow (VI,max) and the nasopharyngeal pressure associated with flow limitation (Pcrit) were measured. These experimental trials were repeated while nerve branches innervating tongue protrudor (genioglossus; medial XIIth nerve branch) and retractor (hyoglossus and styloglossus; lateral XIIth nerve branch) muscles were stimulated either simultaneously or independently at frequencies ranging from 20-100 Hz. Co-activating the protrudor and retractor muscles produced tongue retraction, whereas independently activating the genioglossus resulted in tongue protrusion. 3. Co-activation of tongue protrudor and retractor muscles increased VI, max (peak increase 44 %, P < 0.05), made Pcrit more negative (peak decrease of 44 %, P < 0.05), and did not change upstream nasopharyngeal resistance (Rn). Independent protrudor muscle stimulation increased VI,max (peak increase 61 %, P < 0.05), did not change Pcrit, and decreased Rn (peak decrease of 41 %, P < 0.05). Independent retractor muscle stimulation did not significantly alter flow mechanics. Changes in Pcrit and VI,max at all stimulation frequencies were significantly correlated during co-activation of protrudor and retractor muscles (r2 = 0.63, P < 0.05), but not during independent protrudor muscle stimulation (r2 = 0.09). 4. These findings indicate that either co-activation of protrudor and retractor muscles or independent activation of protrudor muscles can improve upper airway flow mechanics, although the underlying mechanisms are different. We suggest that co-activation decreases pharyngeal collapsibility but does not dilate the pharyngeal airway. In contrast, unopposed tongue protrusion dilates the oropharynx, but has a minimal effect on pharyngeal airway collapsibility.

Co-activation of tongue protrudor and retractor muscles during chemoreceptor stimulation in the rat.

1. Our primary purpose was to test the hypothesis that the tongue protrudor (genioglossus, GG) and retractor (styloglossus, SG and hyoglossus, HG) muscles are co-activated when respiratory drive increases, and that co-activation will cause retraction of the tongue. This was addressed by performing two series of experiments using a supine, anaesthetized, tracheotomized rat in which tongue muscle force and the neural drive to the protrudor and retractor muscles could be measured during spontaneous breathing. In the first series of experiments, respiratory drive was increased progressively by occluding the tracheal cannula for thirty respiratory cycles; in the second series of experiments, the animals were subjected to hyperoxic hypercapnia and poikilocapnic hypoxia. 2. Airway occlusion for thirty breaths caused progressive, quantitatively similar increases in efferent motor nerve activity to protrudor and retractor tongue muscles. Net tongue muscle force was always consistent with tongue retraction during occlusion, and peak force rose in parallel with the neural activites. When airway occlusion was repeated following section of the lateral XIIth nerve branch (denervation of retractor muscles) the tongue either protruded (15/21 animals; 10 +/- 2 mN at the 30th occluded breath) or retracted weakly (6/21 animals; 6 +/- 2 mN at 30th occluded breath). 3. To ensure that our findings were not the result of damage to the muscle nerves, occlusion experiments were also done in eight animals in which GG EMG activity was recorded instead of nerve activities. Changes in peak integrated GG electryomyogram (EMG) activity and peak retraction force during occlusion were highly correlated (r2 = 0.86, slope = 1.05). 4. In separate experiments in fourteen rats, we found that hyperoxic hypercapnia and poikilocapnic hypoxia also result in parallel increases in the respiratory-related EMG activity of the GG and HG muscles. Also, as in the occlusion experiments, augmentations of protrudor and retractor muscle EMG activities were associated with parallel changes in tongue retraction force. 5. These studies in anaesthetized rats demonstrate that tracheal occlusion and independent stimulation of central or peripheral chemoreceptors results in inspiratory-related co-activation of the protrudor and retractor muscles, and proportional changes in tongue retraction force. These observations also demonstrate that recording GG EMG activity in isolation could lead to erroneous conclusions about respiratory-related movements of the tongue.

Contractile properties of human nasal dilator motor units.

The technique of intramuscular microstimulation was used to activate facial nerve fibers while acquiring simultaneous twitch force measurements to measure the contractile properties and force-frequency responses of human nasal dilator (ND) motor units. Twitch force amplitude (TF), contraction time (CT), half-relaxation time (HRT), and the maximal rate of rise of force normalized to the peak force (maximum contraction rate, MCR) were recorded from 98 ND motor units in 37 subjects. The average CT, HRT, MCR, and TF were 47.9 +/- 1.8 ms, 42.6 +/- 2.1 ms, 28.6 +/- 1.8 s-1, and 1.06 +/- 0.1 mN, respectively. Neither CT nor HRT were significantly correlated with TF. The average CT and HRT were similar to values recorded for small muscles of the hand but were faster than the values recorded from human toe extensor motor units. However the lack of an association between twitch force and CT or HRT was similar to the findings obtained for both human hand and foot muscles. Force-frequency curves were recorded from eight ND motor units. The force produced by the eight motor units was recorded in response to stimuli delivered at 1, 5, 10, 15, 20, 25, 30, 35, and 40 Hz to assess force-frequency relationships. The mean twitch force of the eight motor units was 0.91 +/- 0.3 mN and the average tetanic force was 8.1 +/- 1.8 mN. Therefore the average twitch force was equal to 12.7% of the tetanic force. Fifty percent of the unit tetanic force was achieved at an average frequency of 16. 4 +/- 1.7 Hz, which is greater than the value recorded for human toe extensor motor units (9.6 Hz). Thus the force produced by the ND motor units was more sensitive to changes in discharge frequency over the range of approximately 10-30 Hz and less sensitive to changes in the range of 0-10 Hz because of their fast contractile properties. The mean slope of the regression lines that were fit to the steep portion of each force-frequency curve was 5.15 +/- 0.5% change in force/Hz. This value was greater than the slope measured for human toe extensor muscles (4.2% change in force/Hz). These observations suggest that force gradation by ND motor units is more sensitive to changes in stimulation frequency than human toe extensor motor units. We conclude that most ND motor units have fast contractile properties and that rate coding may play a significant role in the gradation of force produced by the ND muscle. Furthermore, the findings of this investigation have demonstrated that contractile speed and TF in a human facial muscle are not correlated. This supports previous findings obtained from human hand and foot muscles and suggests that there may be a fundamental difference in the contractile speed-twitch force relationship between many human muscles and most muscles of other mammals.

Long-term facilitation of upper airway muscle activities in vagotomized and vagally intact cats.

The primary purpose of the present investigation was to determine whether long-term facilitation (LTF) of upper airway muscle activities occurs in vagotomized and vagally intact cats. Tidal volume and diaphragm, genioglossus, and nasal dilator muscle activities were recorded before, during, and after one carotid sinus nerve was stimulated five times with 2-min trains of constant current. Sixty minutes after stimulation, nasal dilator and genioglossus muscle activities were significantly greater than control in the vagotomized cats but not in the vagally intact cats. Tidal volume recorded from the vagotomized and vagally intact cats was significantly greater than control during the poststimulation period. In contrast, diaphragm activities were not significantly elevated in the poststimulation period in either group of animals. We conclude that 1) LTF of genioglossus and nasal dilator muscle activities can be evoked in vagotomized cats; 2) vagal mechanisms inhibit LTF in upper airway muscles; and 3) LTF can be evoked in accessory inspiratory muscles because LTF of inspired tidal volume was greater than LTF of diaphragm activity.

Respiratory-related control of extrinsic tongue muscle activity.

The purpose of this brief report is to introduce new evidence showing that the protrudor and retractor muscles of the tongue are co-activated during inspiration in eupnea and hyperpnea in an anesthetized, tracheotomized rat model. We also review previous work on the respiratory related control of the tongue musculature, and briefly consider the clinical significance of this work. The important new findings are that: (1) Both hypoxia and hypercapnia cause parallel increases in drive to the tongue protrudor and retractor muscles (the genioglossus and hyoglossus muscles, respectively); (2) phasic volume feedback inhibits the peak inspiratory activity of both muscles; and (3) the tongue muscles consistently produce a retraction force when the genioglossus and hyoglossus are co-activated, in both animal and human subjects. This latter observation is consistent with previous work showing that the retractor muscles (hyoglossus and styloglossus) develop up to ten times more force than the genioglossus muscle. The possible mechanical consequences of tongue muscle co-activation are briefly considered.

Effect of hypoxia on abdominal motor unit activities in spontaneously breathing cats.

These experiments were designed to examine the behavior of external oblique motor units in spontaneously breathing cats during hypoxia and to estimate the contribution of recruitment and rate coding to changes in the integrated external oblique electromyogram (iEMG). Motor unit activities in the external oblique muscle were identified while the cats expired against a positive end-expiratory pressure (PEEP) of 1-2.5 cmH2O. After localization of unit activity, PEEP was removed, and recordings were made continuously for 3-4 min during hyperoxia, normoxia, and hypoxia. A total of 35 single motor unit activities were recorded from 10 cats. At each level of fractional concentration of end-tidal O2, the motor unit activity was characterized by an abrupt increase in mean discharge frequency, at approximately 30% of expiratory time, which then continued to increase gradually or remained constant before declining abruptly at the end of expiration. The transition from hyperoxia to normoxia and hypoxia was accompanied by an increase in the number of active motor units (16 of 35, 20 of 35, and 29 of 35, respectively) and by an increase in the mean discharge frequency of those units active during hyperoxia. The changes in motor unit activity recorded during hypoxia were accompanied by a significant increase in the average peak amplitude of the abdominal iEMG. Linear regression analysis revealed that motor unit rate coding was responsible for close to 60% of the increase in peak iEMG amplitude. The changes in abdominal motor unit activity and the external oblique iEMG that occurred during hypoxia were abolished if the arterial PCO2 was allowed to fall. We conclude that external oblique motor units are activated during the latter two-thirds of expiration and that rate coding and recruitment contribute almost equally to the increase in expiratory muscle activity that occurs with hypoxia. In addition, the excitation of abdominal motor units during hypoxia is critically dependent on changes in CO2 and/or tidal volume.

Expiratory muscle endurance performance after exhaustive submaximal exercise.

The aim of our study was to describe the endurance capacity of the expiratory muscles and to determine whether it is altered after exhaustive cycling exercise. Subjects performed repeated maximal expiratory efforts against a closed breathing valve, with and without prior exercise performed at a work rate that elicited 75% of the maximum ventilation rate. Each expiratory effort lasted 6 s, was separated by 10 s of rest, and was initiated from the end-expiratory lung volume. Endurance performance was assessed by measuring the decline in area under the pressure*time curve over 39 contractions. Prior exhaustive exercise attenuated the ability to generate and sustain maximal expiratory pressure (P = 0.013) and resulted in significant declines in the integrated electromyogram of the rectus abdominis (P = 0.005) and external oblique (P = 0.036) abdominal muscles. Each subject also performed a handgrip endurance task before and after exhaustive exercise on a separate day. Prior exercise had no effect on handgrip endurance performance, suggesting that the decline in expiratory muscle performance after exercise was not the result of reduced motivation. We conclude that the ability to maximally activate the abdominal expiratory muscles and to generate maximum expiratory pressure is impaired after exhaustive exercise. Declines in the surface integrated electromyogram despite maximal effort is consistent with findings in limb muscles and is thought to be due to a slowing of motoneuron firing rates or to neuromuscular transmission failure.