Motor unit regulation of mammalian pharyngeal dilator muscle activity

Sex-specific vagal and spinal modulation of swallow and its coordination with breathing

Swallow-breathing coordination is influenced by changes in lung volume, which is modulated by feedback from both vagal and spinal sensory afferents. The purpose of this study was to manipulate feedback from these afferents, with and without a simultaneous mechanical challenge (chest compression), in order to assess the influence of each sensory pathway on swallow in rats. We hypothesized that manipulation of afferent feedback would shift the occurrence of swallow toward the inspiratory phase of breathing. Afferent feedback was perturbed by lidocaine nebulization, extra-thoracic vagotomy, or lidocaine administration to the pleural space in sodium pentobarbital anesthetized rats (N = 43). These different afferent perturbations were performed both in control conditions (no chest compression), and with chest compression. Manipulating pulmonary stretch receptor-mediated volume feedback in male animals decreased swallow occurrence. Female rats appear to rely more on spinal afferent feedback, as swallow occurrence shifted to late expiration with chest compression and vagotomy or lidocaine injections. Results suggest that sex-specific mechanisms modulate swallow-breathing coordination, and that vagal feedback is inhibitory to swallow-related muscles, while spinal feedback from pleural afferents has excitatory effects. This study supports the theory that a balance of vagal and spinal afferent feedback is necessary to maintain an optimal swallow pattern and swallow-breathing coordination.

Activities of human genioglossus motor units

Upper airway muscles play an important role in regulating airway lumen and in increasing the ability of the pharynx to remain patent in the face of subatmospheric intraluminal pressures produced during inspiration. Due to the considerable technical challenges associated with recording from muscles of the upper airway, much of the experimental work conducted in human subjects has centered on recording respiratory-related activities of the extrinsic tongue protudor muscle, the genioglossus (GG). The GG is one of eight muscles that invest the human tongue (Abd-El-Malek, 1939). All eight muscles are innervated by the hypoglossal nerve (cranial nerve XII) the cell bodies of which are located in the hypoglossal motor nucleus (HMN) of the caudal medulla. Much of the earlier work on the respiratory-related activity of XII motoneurons was based on recordings obtained from single motor axons dissected from the whole XII nerve or from whole muscle GG EMG recordings. Detailed information regarding respiratory-related GG motor unit activities was lacking until as recently as 2006. This paper examines key findings that have emerged from the last decade of work conducted in human subjects. Wherever appropriate, these results are compared with results obtained from in vitro and in vivo studies conducted in non-human mammals. The review is written with the objective of facilitating some discussion and some new thoughts regarding future research directions. The material is framed around four topics: (a) motor unit type, (b) rate coding and recruitment, (c) motor unit activity patterns, and (d) a compartment based view of pharyngeal airway control.

Atypical antipsychotic medications are independently associated with severe obstructive sleep apnea

BACKGROUND

Atypical antipsychotic (AA) medications are widely prescribed for their Food and Drug Administration-approved uses (acute mania, bipolar mania, psychotic agitation, bipolar maintenance, etc) and off-label indications. Although AA medications are associated with substantial weight gain, their tranquilizing effects may independently contribute to risk of obstructive sleep apnea (OSA) perhaps, by a reduction in activity of hypoglossal or recurrent activity of laryngeal nerve on the upper motor airway musculature.

METHODS

We hypothesized that AA medications are associated with more severe OSA independent of weight and neck circumference. Medical intake data and polysomnographic studies of patients referred to community hospital sleep disorders center were analyzed retrospectively.

RESULTS

Mean age of patients was 49.1 years, 55.1% were male, and mean body mass index (BMI) was 33.8 kg/m. Sixty-eight patients (8.1%) were taking AA at the time of polysomnography. There were no differences in age, sex, neck circumference and BMI of AA versus non-AA patients. The mean (SE) apnea-hypopnea index values were 29.2 (3.5)/h in AA patients and 21.3 (0.8)/h in non-AA patients (P = 0.03). Thirty-four percent of AA patients had severe OSA (apnea-hypopnea index > 30/h) compared with 23% of non-AA patients (P = 0.04). When adjusted for BMI, sex, and use of benzodiazepines and sleeping aids, the odds ratios of severe OSA in AA patients were 1.9 times in non-AA patients (95% confidence interval, 1.1-3.3).

CONCLUSIONS

Atypical antipsychotic medication use may increase the risk of more severe OSA independent of weight and neck circumference.

Modulation of upper airway muscle activities by bronchopulmonary afferents

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

Sternohyoid muscle fatigue properties of dy/dy dystrophic mice, an animal model of merosin-deficient congenital muscular dystrophy

Humans with merosin-deficient congenital muscular dystrophy have both sucking problems during infancy and sleep-disordered breathing during childhood. We hypothesized that merosin-deficient pharyngeal muscles fatigue faster than normal muscles. This was tested in vitro using sternohyoid muscle from an animal model of this disease, the dy/dy dystrophic mouse. Isometric twitch contraction and half-relaxation times were similar for dy/dy and normal sternohyoid. However, rate of force loss during repetitive 25-Hz train stimulation was markedly diminished in dystrophic compared with normal sternohyoid muscle. Furthermore, force potentiation, which occurred during the early portion of the fatigue-inducing stimulation, had a longer duration in dystrophic compared with normal muscle (approximately 60 versus 20 s). As a result of these two processes, at the end of 2 min of stimulation, force of dystrophic muscle had decreased by 8 +/- 5% and that of normal muscle by 69 +/- 4% (p < 0.0001). The potassium-channel blocker, 3,4-diaminopyridine, increased force of dy/dy sternohyoid muscle during twitch and 25-Hz contractions by 148 +/- 20% (p < 0.00001) and 109 +/- 18% (p < 0.00002), respectively. During repetitive 25-Hz stimulation, force of 3,4-diaminopyridine-treated dystrophic muscle remained significantly higher than that of untreated muscle, despite the early force potentiation being eliminated and fatigue being accelerated. Thus, merosin deficiency reduces fatigue and prolongs the duration of force potentiation. The latter alterations may partially preserve the integrity of upper airway muscle function, without which the severity of pharyngeal complications (feeding problems, sleep-related respiratory dysfunction) might be even more pronounced in the human merosin-deficient congenital muscular dystrophies.

Heterogeneity within geniohyoid motor unit subpopulations in firing patterns during breathing

Respiratory motor units (MU) segregate into subpopulations, which differ in firing patterns during resting and stimulated breathing. For phrenic/diaphragm MUs, diversity also exists within subpopulations, and is greater for late than early-onset MUs. The present study characterized the extent of diversity within upper airway respiratory MU subpopulations by recording geniohyoid MUs in anesthetized cats. Inspiratory MUs (I-MU, n=21) had a wide range of firing durations (coefficient of variation (CV)=42%). In contrast, inspiratory-expiratory MUs (I/E-MU, n=19) had a narrow range of firing durations during inspiration (CV=13%), but a wide range of firing durations during expiration (CV=36%). Mean firing frequency had similar degrees of diversity among units for I-MU and I/E-MU (CV=31-40%). For I-MU firing duration correlated with mean firing frequency, whereas no such relationship was apparent for I/E-MU. Single-breath end-expiratory airway occlusion decreased heterogeneity in firing duration during inspiration and increased it during expiration, whereas end-inspiratory airway occlusion decreased heterogeneity during expiration. In conclusion, (a) there is considerable diversity within geniohyoid MU subpopulations receiving respiratory drive; (b) the degree of diversity within subpopulations differs for I-MU and I/E-MU; and (c) diversity within subpopulations in timing of activity is modulated by single-breath airway occlusion.

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.

Changes in pharyngeal respiratory muscle force produced by K+ channel blockade

The purpose of the present study was to determine whether the contractility of pharyngeal respiratory muscles can be augmented by altering membranous K+ channel conductance. The effects on twitch force of two K+ channel blockers, tetraethylammonium (TEA, 10 mM) and 4-aminopyridine (4-AP, 0.3 mM), were examined in vitro for sternohyoid and diaphragm muscle strips. Both agents augmented isometric twitch force of both muscles. In response to TEA twitch force of the sternohyoid muscle increased significantly more than that of the diaphragm (by 33 +/- 7 vs. 9 +/- 1%, P = 0.004), whereas with 4-AP the increase in twitch force of the sternohyoid muscle was comparable to that of the diaphragm (55 +/- 15 vs. 64 +/- 6%, P = 0.50). 4-AP shifted the force-frequency relationship of both muscles leftward but did not alter peak tetanic force, so that force with 4-AP exceeded that without drug at stimulation frequencies below 60 Hz. In contrast TEA reduced force at stimulation frequencies > 20 Hz. The isometric contraction times of both muscles was variably prolonged, more so with 4-AP (by 30 +/- 15% for the sternohyoid and 32 +/- 3% for the diaphragm) than with TEA (by 9 +/- 2% for the sternohyoid and 5 +/- 2% for the diaphragm). For the group of muscles and K+ channel blockers, the degree of augmentation of twitch force correlated with the degree of prolongation of contraction time (r = 0.82, P < 0.001), consistent with blocking delayed rectifier K+ channels as the mechanism of increasing muscle force.

Interaction between central pattern generators for breathing and swallowing in the cat

1. We examined the interaction between central pattern generators for respiration and deglutition in decerebrate, vagotomized, paralysed and ventilated cats (n = 10), by recording activity from the following nerves: hypoglossal, phrenic, thyroarytenoid and triangularis sterni. Fictive breathing was spontaneous with carbon dioxide above the apnoeic threshold (end-tidal PCO2, 32 +/- 4 mmHg) and fictive swallowing was induced by stimulating the internal branch of the left superior laryngeal nerve (SLN) continuously (0.2 ms pulse duration, 10 Hz). 2. In all ten animals, SLN stimulation evoked short bursts of thyroarytenoid and hypoglossal nerve activity indicative of fictive swallowing. In two of ten animals, respiration was inhibited completely during deglutition. In the other eight animals, fictive breathing and swallowing occurred simultaneously. 3. With SLN stimulation below threshold for eliciting swallowing, the respiratory rhythm decreased, the duration of inspiration did not change but the duration of expiration, especially stage II, increased. Integrated nerve activities indicated that the rate of rise and peak of phrenic nerve activity decreased, stage I expiratory activity of the thyroarytenoid and especially that of the hypoglossal nerve increased and stage II expiratory activity of the triangularis sterni nerve was suppressed completely. However, if inspired carbon dioxide was increased, i.e. hypercapnic ventilation, stage II expiratory activity remained partially during continuous SLN stimulation. 4. Fictive-swallowing bursts occurred only at respiratory phase transitions. At the minimal stimulus intensity that evoked repetitive swallowing bursts, the pattern of interaction between breathing and swallowing central pattern generators was consistent for each animal (n = 7) but was different across animals. In four animals, fictive swallows occurred at the phase transition between stage II expiration and inspiration, at the transition between inspiration and stage I expiration in one animal; and in two other animals, at the transition between stage I and II of expiration. 5. The response to SLN stimulation accommodated during the stimulus train. Accommodation was evident in both the interswallow interval (ISI) which lengthened, and the interaction pattern which had fewer swallows per breath as the stimulus period progressed. In contrast to the ISI, characteristics of the fictive swallow did not accommodate. For example, duration of the swallow was constant, distributed over a narrow range throughout the stimulus train. 6. We conclude that the central pattern generators for swallowing and breathing interact. The pattern of interaction supports the three-phase theory of respiratory pattern generation.(ABSTRACT TRUNCATED AT 400 WORDS)

Breath-to-breath variability in hypoglossal motor unit firing

Instability in the magnitude and timing of motor output to pharyngeal dilator muscles occurs during breathing. This contributes to alterations in upper airway resistance, and is one of several factors that play a role in the pathophysiology of obstructive apneas. To define the motor unit mechanisms accounting for such variability, geniohyoid motor unit activity was recorded simultaneously with diaphragm EMG in anesthetized cats spontaneously breathing 7% CO2 in O2. Variability was quantified with the coefficient of variation [CV = (SD/mean) x 100%]. In this preparation, we confirmed greater breath-to-breath variability of geniohyoid compared to diaphragm peak moving average EMGs. During recordings of geniohyoid motor unit activity, average CV of other respiratory parameters were as follows: peak diaphragm EMG 5.8%, inspiratory time 3.5%, expiratory time 3.8%. The average CV for geniohyoid motor unit activity patterns were substantially higher: spikes per breath 15.6%, mean firing frequency 13.3%, peak firing frequency 19.0%, minimal firing frequency 26.3%, onset time 40.9%, offset time 10.0% and duration of firing 12.8%. Values differed considerably among motor units, even when activity was recorded simultaneously. These findings suggest that variability is present in both intensity and timing of geniohyoid motor unit firing during breathing, and that different geniohyoid motor units appear to have varying degrees of stability during breathing.

Phasic respiratory muscle patterns and sleep-disordered breathing during rapid eye movement sleep in the English bulldog

Basic mechanisms of sleep-disordered breathing (SDB) during rapid-eye-movement sleep (REMS) have been little investigated, despite the fact that events are often more prolonged and SaO2 nadirs lower during REMS. We predicted that the mechanisms of SDB in REMS would be related to the normal phasic changes in respiratory control in that state, rather than to cyclic arousals or responses to hypoxia as postulated for non-REMS SDB. Recordings of the EMG of the diaphragm (DIA) and the sternohyoid (SH), an upper airway dilating muscle, were made in five English bulldogs during sleep. We found that, as predicted, SDB events were associated with phasic influences rather than with arousals or response to hypoxia. The onset of SDB was significantly related to suppression of drive to both the DIA (p less than 0.01) and the SH (p less than 0.01). The mean drive of the DIA was suppressed to 42% of normal and of the SH to 17% of normal; the suppression of the SH was significantly greater than that of the DIA (p less than 0.05). Events were associated with changes in respiratory muscle EMG patterns typical of phasic REMS (p less than 0.01 for each muscle). The occurrence and duration of events exhibited no clear pattern or relationship to arousal or SaO2. Rather, as would be expected of phenomena associated with phasic REMS, the onset and termination of events were unpredictable. The association of SDB in REMS with phasic REMS influences rather than arousal or hypoxia suggests new directions for therapeutic approaches.