Heterogeneity within geniohyoid motor unit subpopulations in firing patterns during breathing

Oscillation patterns are enhanced and firing threshold is lowered in medullary respiratory neuron discharges by threshold doses of a μ-opioid receptor agonist

μ-Opioid receptors are distributed widely in the brain stem respiratory network, and opioids with selectivity for μ-type receptors slow in vivo respiratory rhythm in lowest effective doses. Several studies have reported μ-opioid receptor effects on the three-phase rhythm of respiratory neurons, but there are until now no reports of opioid effects on oscillatory activity within respiratory discharges. In this study, effects of the μ-opioid receptor agonist fentanyl on spike train discharge properties of several different types of rhythm-modulating medullary respiratory neuron discharges were analyzed. Doses of fentanyl that were just sufficient for prolongation of discharges and slowing of the three-phase respiratory rhythm also produced pronounced enhancement of spike train properties. Oscillation and burst patterns detected by autocorrelation measurements were greatly enhanced, and interspike intervals were prolonged. Spike train properties under control conditions and after fentanyl were uniform within each experiment, but varied considerably between experiments, which might be related to variability in acid-base balance in the brain stem extracellular fluid. Discharge threshold was shifted to more negative levels of membrane potential. The effects on threshold are postulated to result from opioid-mediated disinhibition and postsynaptic enhancement of N-methyl-d- aspartate receptor current. Lowering of firing threshold, enhancement of spike train oscillations and bursts and prolongation of discharges by lowest effective doses of fentanyl could represent compensatory adjustments in the brain stem respiratory network to override opioid blunting of CO₂/pH chemosensitivity.

Transcranial direct current stimulation enhances sucking of a liquid bolus in healthy humans

BACKGROUND

Transcranial direct current stimulation (tDCS) is a non-invasive technique used for modulating cortical excitability in vivo in humans. Here we evaluated the effect of tDCS on behavioral and electrophysiological aspects of physiological sucking and swallowing.

METHODS

Twelve healthy subjects underwent three tDCS sessions (anodal, cathodal and sham stimulation) on separate days in a double-blind randomized order. The active electrode was placed over the right swallowing motor cortex. Repeated sucking and swallowing acts were performed at baseline and at 15 and 60 min after each tDCS session and the mean liquid bolus volume ingested at each time point was measured. We also calculated average values of the following electrophysiological parameters: 1) area and 2) duration of the rectified EMG signal from the suprahyoid/submental muscles related to the sucking and swallowing phases; 3) EMG peak amplitude for the sucking and swallowing phases; 4) area and peak amplitude of the laryngeal-pharyngeal mechanogram; 5) oropharyngeal delay.

RESULTS

The volume of the ingested bolus significantly increased (by an average of about 30% compared with the baseline value) both at 15 and at 60 min after the end of anodal tDCS. The electrophysiological evaluation after anodal tDCS showed a significant increase in area and duration of the sucking phase-related EMG signal.

CONCLUSIONS

Anodal tDCS leads to stronger sucking of a liquid bolus in healthy subjects, likely by increasing recruitment of cortical areas of the swallowing network. This finding might open up interesting perspectives for the treatment of patients suffering from dysphagia due to various pathological conditions.

EMG activity in hyoid muscles during pig suckling

Infant suckling is a complex behavior that includes cycles of rhythmic sucking as well as intermittent swallows. This behavior has three cycle types: 1) suck cycles, when milk is obtained from the teat and moved posteriorly into the valleculae in the oropharynx; 2) suck-swallow cycles, which include both a rhythmic suck and a pharyngeal swallow, where milk is moved out of the valleculae, past the larynx, and into the esophagus; and 3) postswallow suck cycles, immediately following the suck-swallow cycles. Because muscles controlling these behaviors are active in all three types of cycles, we tested the hypothesis that different patterns of electromyographic (EMG) activity in the mylohyoid, hyoglossus, stylohyoid, and thyrohyoid muscles of the pig characterized each cycle type. Anterior mylohyoid EMG activity occurred regularly in every cycle and was used as a cycle marker. Thyrohyoid activity, indicating the pharyngeal swallow, was immediately preceded by increased stylohyoid and hyoglossus activity; it divided the suck-swallow cycle into two phases. Timed from the onset of the suck-swallow cycle, the first phase had a relatively fixed duration while the duration of the second phase, timed from the thyrohyoid, varied directly with cycle duration. In short-duration cycles, the second phase could have a zero duration so that thyrohyoid activity extended into the postswallow cycle. In such cycles, all swallowing activity that occurred after the thyrohyoid EMG and was associated with bolus passage through the pharynx fell into the postswallow cycle. These data suggest that while the activity of some muscles, innervated by trigeminal and cervical plexus nerves, may be time locked to the cycle onset in swallowing, the cycle period itself is not. The postswallow cycle consequently contains variable amounts of pharyngeal swallowing EMG activity. The results exemplify the complexity of the relationship between rhythmic sucking and the swallow.

The digastric muscle is less involved in pharyngeal swallowing in rabbits

The swallowing reflex is centrally programmed by the lower brain stem, the so-called swallowing central pattern generator (CPG), and once the reflex is initiated, many muscles in the oral, pharyngeal, laryngeal, and esophageal regions are systematically activated. The mylohyoid (MH) muscle has been considered to be a "leading muscle" according to previous studies, but the functional role of the digastric (DIG) muscle in the swallowing reflex remains unclear. In the present study, therefore, the activities of single units of MH and DIG neurons were recorded extracellularly, and the functional involvement of these neurons in the swallowing reflex was investigated. The experiments were carried out on eight adult male Japanese white rabbits anesthetized with urethane. To identify DIG and MH neurons, the peripheral nerve (either DIG or MH) was stimulated to evoke action potentials of single motoneurons. Motoneurons were identified as such if they either (1) responded to antidromic nerve stimulation of DIG or MH in an all-or-none manner at threshold intensities and (2) followed stimulation frequencies of up to 0.5 kHz. As a result, all 11 MH neurons recorded were synchronously activated during the swallowing reflex, while there was no activity in any of the 7 DIG neurons recorded during the swallowing reflex. All neurons were anatomically localized ventromedially at the level of the caudal portion of the trigeminal motor nucleus, and there were no differences between the MH and DIG neuron sites. The present results strongly suggest that at least in the rabbit, DIG motoneurons are not tightly controlled by the swallowing CPG and, hence, the DIG muscle is less involved in the swallowing reflex.

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.

Single motor unit recordings in human geniohyoid reveal minimal respiratory activity during quiet breathing

Maintenance of airway patency during breathing involves complex interactions between pharyngeal dilator muscles. The few previous studies of geniohyoid activity using multiunit electromyography (EMG) have suggested that geniohyoid shows predominantly inspiratory phasic activity. This study aimed to quantify geniohyoid respiration-related activity with single motor unit (SMU) EMG recordings. Six healthy subjects of normal body mass index were studied. Intramuscular EMG recordings of geniohyoid activity were made with a monopolar needle with subjects in supine and seated positions. The depth of the geniohyoid was identified by ultrasound, and the electrode position was confirmed with maneuvers to isolate activity in geniohyoid and genioglossus. Activity was recorded at 85 sites in the geniohyoid during quiet breathing (45 supine and 40 seated). When subjects were supine, 33 sites (73%) showed no activity during breathing and 10 (22%) showed tonic activity. In addition, one site showed a tonic SMU with increased expiratory discharge, and one site in another subject had one unit with expiratory phasic activity. When subjects were seated, 27 sites (68%) in the geniohyoid showed no activity, 12 sites (30%) showed tonic activity that was not respiration related, and one unit at one site showed phasic expiratory activity. The average peak discharge frequency of geniohyoid motor units was 16.2 ± 3.1 impulses/s during the “geniohyoid maneuver,” which was the first part of a swallow. In contrast to previous findings, the geniohyoid shows some tonic activity but minimal respiration-related activity in healthy subjects in quiet breathing. The geniohyoid has little active role in airway stability under these conditions.

Regional differences in hyoid muscle activity and length dynamics during mammalian head shaking

The sternohyoid (SH) and geniohyoid (GH) are antagonist strap muscles that are active during a number of different behaviors, including sucking, intraoral transport, swallowing, breathing, and extension/flexion of the neck. Because these muscles have served different functions through the evolutionary history of vertebrates, it is quite likely they will have complex patterns of electrical activity and muscle fiber contraction. Different regions of the SH exhibit different contraction and activity patterns during a swallow. We examined the dynamics of the SH and GH muscles during an unrestrained, and vigorous head shaking behavior in an animal model of human head, neck, and hyolingual movement. A gentle touch to infant pig ears elicited a head shake of several revolutions. Using sonomicrometry and intramuscular EMG, we measured regional (within) muscle strain and activity in SH and GH. We found that EMG was consistent across three regions (anterior, belly, and posterior) of each muscle. Changes in muscle length, however, were more complex. In the SH, mid-belly length-change occurred out-of-phase with the anterior and posterior end regions, but with a zero lag timing; the anterior region shortened before the posterior. In the GH, the anterior region shortened before and out-of-phase with the mid-belly and posterior regions. Head shaking is a relatively simple reflex behavior, yet the underlying patterns of muscle length dynamics and EMG activity are not. The regional complexity in SH and GH, similar to regionalization of SH during swallowing, suggests that these anatomically simple hyoid strap muscles have more complex function than textbooks often suggest.

Regional differences in length change and electromyographic heterogeneity in sternohyoid muscle during infant mammalian swallowing

A complex sling of muscles moves and stabilizes the hyoid bone during many mammalian behaviors. One muscle in this sling, the sternohyoid, is recruited during food acquisition, processing, and swallowing, and also during nonfeeding behaviors. We used synchronous sonomicrometry and electromyography to investigate regional (intramuscular) changes in length and electromyographic (EMG) activity of the sternohyoid during swallowing in the infant pig. The simple straplike architecture of the sternohyoid led us to hypothesize that limited regional variation in length and muscle activity would be present. We found statistically significant regional differences in EMG activity, and, with respect to length dynamics, the sternohyoid did not behave homogeneously during swallowing. The midbelly region typically shortened while the anterior and posterior regions lengthened, although in a minority of swallows (12.5%) the midbelly lengthened simultaneously with the end-regions. Despite its nonpennate architecture and evolutionarily conservative innervation, the mammalian sternohyoid appears to contain previously unrecognized populations of regionally specialized motor units. It also displays differential contraction patterns, very similar to the sternohyoid of nonmammalian vertebrates.

Impact of rhythmic oral activity on the timing of muscle activation in the swallow of the decerebrate pig

The pharyngeal swallow can be elicited as an isolated event but, in normal animals, it occurs within the context of rhythmic tongue and jaw movement (RTJM). The response includes activation of the multifunctional geniohyoid muscle, which can either protract the hyoid or assist jaw opening; in conscious nonprimate mammals, two bursts of geniohyoid EMG activity (GHemg) occur in swallow cycles at times consistent with these two actions. However, during experimentally elicited pharyngeal swallows, GHemg classically occurs at the same time as hyoglossus and mylohyoid activity (short latency response) but, when the swallow is elicited in the decerebrate in the absence of RTJM, GHemg occurs later in the swallow (long latency response). We tested the hypothesis that it was not influences from higher centers but a brain stem mechanism, associated with RTJM, which caused GHemg to occur earlier in the swallow. In 38 decerebrate piglets, RTJM occurred sporadically in seven animals. Before RTJM, GHemg had a long latency, but, during RTJM, swallow related GHemg occurred synchronously with activity in hyoglossus and mylohyoid, early in the swallow. Both early and late responses were present during the changeover period. During this changeover period, duplicate electrodes in the geniohyoid could individually detect either the early or the late burst in the same swallow. This suggested that two sets of geniohyoid task units existed that were potentially active in the swallow and that they were differentially facilitated or inhibited depending on the presence or absence of rhythmic activity originating in the brain stem.

Variation in EMG activity: a hierarchical approach

Recordings of naturally occurring Electromyographic (EMG) signals are variable. One of the first formal and successful attempts to quantify variation in EMG signals was Shaffer and Lauder's (1985) study examining several levels of variation but not within muscle. The goal of the current study was to quantify the variation that exists at different levels, using more detailed measures of EMG activity than did Shaffer and Lauder (1985). The importance of accounting for different levels of variation in an EMG study is both biological and statistical. Signal variation within the same muscle for a stereotyped action suggests that each recording represents a sample drawn from a pool of a large number of motor units that, while biologically functioning in an integrated fashion, showed statistical variation. Different levels of variation for different muscles could be related to different functions or different tasks of those muscles. The statistical impact of unaccounted or inappropriately analyzed variation can lead to false rejection (type I error) or false acceptance (type II error) of the null hypothesis. Type II errors occur because such variation will accrue to the error, reducing power, and producing an artificially low F-value. Type I errors are associated with pseudoreplication, in which the replicated units are not truly independent, thereby leading to inflated degrees of freedom, and an underestimate of the error mean square. To address these problems, we used a repeated measures, nested multifactor model to measure the relative contribution of different hierarchical levels of variation to the total variation in EMG signals during swallowing. We found that variation at all levels, among electrodes in the same muscle, in sequences of the same animal, and among individuals and between differently named muscles, was significant. These findings suggest that a single intramuscular electrode, recording from a limited sample of the motor units, cannot be relied upon to characterize the activity of an entire muscle. Furthermore, the use of both a repeated-measures model, to avoid pseudoreplication, and a nested model, to account for variation, is critical for a correct testing of biological hypotheses about differences in EMG signals.

Electromyographic activity during the reflex pharyngeal swallow in the pig: Doty and Bosma (1956) revisited

The currently accepted description of the pattern of electromyographic (EMG) activity in the pharyngeal swallow is that reported by Doty and Bosma in 1956; however, those authors describe high levels of intramuscle and of interindividual EMG variation. We reinvestigated this pattern, testing two hypotheses concerning EMG variation: 1) that it could be reduced with modern methodology and 2) that it could be explained by selective detection of different types of motor units. In eight decerebrate infant pigs, we elicited radiographically verified pharyngeal swallows and recorded EMG activity from a total of 16 muscles. Synchronization signals from the video-radiographic system allowed the EMG activity associated with each swallow to be aligned directly with epiglottal movement. The movements were highly stereotyped, but the recorded EMG signals were variable at both the intramuscle and interanimal level. During swallowing, some muscles subserved multiple functions and contained different task units; there were also intramuscle differences in EMG latencies. In this situation, statistical methods were essential to characterize the overall patterns of EMG activity. The statistically derived multimuscle pattern approximated to the classical description by Doty and Bosma (Doty RW, Bosma JF. J Neurophysiol 19: 44-60, 1956) with a leading complex of muscle activities. However, the mylohyoid was not active earlier than other muscles, and the geniohyoid muscle was not part of the leading complex. Some muscles, classically considered inactive, were active during the pharyngeal swallow.

Respiratory-related discharge of genioglossus muscle motor units

RATIONALE

Little is known about the respiratory-related discharge properties of motor units driving any of the eight muscles that control the movement, shape, and stiffness of the mammalian tongue.

OBJECTIVES

To characterize the respiratory-related discharge of genioglossus motor units as synaptic drive to the hypoglossal motoneuron pool is increased with hypercapnia.

MEASUREMENTS

We recorded airflow, genioglossus muscle EMG activity, and the respiratory-related discharge of 30 genioglossus muscle motor units in spontaneously breathing, urethane-anesthetized rats under control conditions and in hypercapnia (inspired CO2: 3, 6, 9, and 12%, 3-5 min at each level).

MAIN RESULTS

All motor units were active throughout all or most of inspiration. Nine of 30 units showed "preinspiratory" activity (discharge onset within the last 20% of expiration), with continued discharge into inspiration. Six inspiratory units transitioned to a preinspiratory pattern when inspired CO2 exceeded 6%. For the majority of units (23/30), discharge rate increased with hypercapnia, with the maximum increase averaging about 50%. The average variability of interspike intervals within a spike train increased from 33% under baseline conditions to 50% with maximal hypercapnia.

CONCLUSIONS

(1) The discharge pattern of genioglossus muscle motor units can be altered by hypercapnia; (2) most, but not all, genioglossus motor units receive synaptic input from CO2-sensitive chemoreceptors; (3) individual motor units have a wide range of CO2 sensitivities; and (4) hypercapnia significantly increases the variability of motor unit discharge, which may enhance muscle force output.

Discharge patterns of hypoglossal motoneurons during fictive breathing, coughing, and swallowing

We performed a series of experiments to study the intracellular activity of 58 hypoglossal motoneurons (HMs) in decerebrate, paralyzed, and ventilated cats. Changes in membrane potentials (MP) and discharge activities were evaluated during fictive breathing (FB), swallowing (FS), and coughing (FC). FS and FC were elicited by electrical stimulation of the superior laryngeal nerves. FB, FS, and FC all exhibited characteristic discharge patterns of the phrenic, abdominal, pharyngeal branch of the vagus, and hypoglossal nerves. Thirty-nine HMs displayed respiratory modulation, and 19 were nonrespiratory modulated. Nine HMs did not exhibit MP changes during FB, FS, and FC. During FS, 49 HMs exhibited MP changes consisting of depolarization, hyperpolarization or hyperpolarization-depolarization. HMs involved in FS were either respiratory modulated (n = 38) or not (n = 11). Only 20 HMs displayed MP changes and/or discharge activity during FC. All but two HMs fired during the expiratory phase of FC or at the end of this reflex. All HMs involved in FC (n = 20) were also modulated during both FB and FS. Our results suggest that the XII nucleus is functionally divided into common and distinct subsets of HMs based on their spontaneous activities and responses observed during FS and FC. The changes in MP and discharge frequencies observed during the three behaviors also suggest that HMs are driven by specific premotor neurons during FS, whereas a common premotor pathway is involved during FB and FC.

Effect of upper airway negative pressure and lung inflation on laryngeal motor unit activity in rabbit

Distortion of the upper airway by negative transmural pressure (UANP) causes reflex vagal bradycardia. This requires activation of cardiac vagal preganglionic neurons, which exhibit postinspiratory (PI) discharge. We hypothesized that UANP would also stimulate cranial respiratory motoneurons with PI activity. We recorded 32 respiratory modulated motor units from the recurrent laryngeal nerve of seven decerebrate paralyzed rabbits and recorded their responses to UANP and to withholding lung inflation using a phrenic-triggered ventilator. The phasic inspiratory (n = 17) and PI (n = 5) neurons detected were stimulated by -10 cmH(2)O UANP and by withdrawal of lung inflation (P < 0.05, Friedman's ANOVA). Expiratory-inspiratory units (n = 10) were tonically active but transiently inhibited in postinspiration; this inhibition was more pronounced and prolonged during UANP stimuli and during no-inflation tests (P < 0.05). We conclude that, in addition to increasing inspiratory activity in the recurrent laryngeal nerve, UANP also stimulates units with PI activity.