Neuronal activation in the medulla oblongata during selective elicitation of the laryngeal adductor response

Chronic intermittent hypoxia reveals role of the Postinspiratory Complex in the mediation of normal swallow production

Obstructive sleep apnea (OSA) is a prevalent sleep-related breathing disorder that results in multiple bouts of intermittent hypoxia. OSA has many neurological and systemic comorbidities, including dysphagia, or disordered swallow, and discoordination with breathing. However, the mechanism in which chronic intermittent hypoxia (CIH) causes dysphagia is unknown. Recently, we showed the postinspiratory complex (PiCo) acts as an interface between the swallow pattern generator (SPG) and the inspiratory rhythm generator, the preBötzinger complex, to regulate proper swallow-breathing coordination (Huff et al., 2023). PiCo is characterized by interneurons co-expressing transporters for glutamate (Vglut2) and acetylcholine (ChAT). Here we show that optogenetic stimulation of ChATcre:Ai32, Vglut2cre:Ai32, and ChATcre:Vglut2FlpO:ChR2 mice exposed to CIH does not alter swallow-breathing coordination, but unexpectedly disrupts swallow behavior via triggering variable swallow motor patterns. This suggests that glutamatergic-cholinergic neurons in PiCo are not only critical for the regulation of swallow-breathing coordination, but also play an important role in the modulation of swallow motor patterning. Our study also suggests that swallow disruption, as seen in OSA, involves central nervous mechanisms interfering with swallow motor patterning and laryngeal activation. These findings are crucial for understanding the mechanisms underlying dysphagia, both in OSA and other breathing and neurological disorders.

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Chronic Intermittent Hypoxia reveals role of the Postinspiratory Complex in the mediation of normal swallow production

Obstructive sleep apnea (OSA) is a prevalent sleep-related breathing disorder that results in multiple bouts of intermittent hypoxia. OSA has many neurologic and systemic comorbidities including dysphagia, or disordered swallow, and discoordination with breathing. However, the mechanism in which chronic intermittent hypoxia (CIH) causes dysphagia is unknown. Recently we showed the Postinspiratory complex (PiCo) acts as an interface between the swallow pattern generator (SPG) and the inspiratory rhythm generator, the preBötzinger Complex, to regulate proper swallow-breathing coordination (Huff et al., 2023). PiCo is characterized by interneurons co-expressing transporters for glutamate (Vglut2) and acetylcholine (ChAT). Here we show that optogenetic stimulation of ChATcre:Ai32, Vglut2cre:Ai32, and ChATcre:Vglut2FlpO:ChR2 mice exposed to CIH does not alter swallow-breathing coordination, but unexpectedly disrupts swallow behavior via triggering variable swallow motor patterns. This suggests, glutamatergic-cholinergic neurons in PiCo are not only critical for the regulation of swallow-breathing coordination, but also play an important role in the modulation of swallow motor patterning. Our study also suggests that swallow disruption, as seen in OSA, involves central nervous mechanisms interfering with swallow motor patterning and laryngeal activation. These findings are crucial for understanding the mechanisms underlying dysphagia, both in OSA and other breathing and neurological disorders.

Laryngeal Sensory Symptoms in Spasmodic Dysphonia

OBJECTIVE

The purpose of this research was to determine whether an association between laryngeal hypersensitivity (LH) and spasmodic dysphonia (SD) exists using the LH Questionnaire (LHQ). This study also explored the prevalence of self-reported upper respiratory infection (URI) at the time of SD onset across SD phenotypes.

METHODS

Individuals with and without SD were recruited to complete an online survey measure. All respondents provided demographic information and completed the LHQ. Participants with SD were also asked to provide information about their diagnosed SD subtype and pattern of onset, including whether onset was associated with a URI. The percentage of respondents with and without SD who were classified with LH was determined based on the LHQ. Scores on the LHQ were also compared between the non-SD and the SD groups, as well as between SD phenotypes (adductor SD, abductor SD, and mixed)).

RESULTS AND CONCLUSIONS

Significant associations were found between ADSD and LH, mixed SD and LH, and URI at time of SD onset and increased severity of LH symptoms. These findings suggest that laryngeal sensory symptoms may potentially contribute to or result from motor spasms in SD and/or have implications for its pathophysiology.

What Can Altered Auditory Feedback Paradigms Tell Us About Vocal Motor Control in Individuals With Voice Disorders?

Purpose

The goal of this review article is to provide a summary of the progression of altered auditory feedback (AAF) as a method to understand the pathophysiology of voice disorders. This review article focuses on populations with voice disorders that have thus far been studied using AAF, including individuals with Parkinson's disease, cerebellar degeneration, hyperfunctional voice disorders, vocal fold paralysis, and laryngeal dystonia. Studies using AAF have found that individuals with Parkinson's disease, cerebellar degeneration, and laryngeal dystonia have hyperactive auditory feedback responses due to differing underlying causes. In persons with PD, the hyperactivity may be a compensatory mechanism for atypically weak feedforward motor control. In individuals with cerebellar degeneration and laryngeal dystonia, the reasons for hyperactivity remain unknown. Individuals with hyperfunctional voice disorders may have auditory-motor integration deficits, suggesting atypical updating of feedforward motor control.

Conclusions

These findings have the potential to provide critical insights to clinicians in selecting the most effective therapy techniques for individuals with voice disorders. Future collaboration between clinicians and researchers with the shared objective of improving AAF as an ecologically feasible and valid tool for clinical assessment may provide more personalized therapy targets for individuals with voice disorders.

Auditory Feedback Control of Vocal Pitch in Spasmodic Dysphonia

OBJECTIVES/HYPOTHESIS

Hearing plays an important role in the maintenance of vocal control in normal individuals. In patients with spasmodic dysphonia (SD), however, the ability to maintain sustained control of phonation is impaired. The origins of SD are unknown, and it is unclear whether auditory feedback-dependent vocal control is compromised in these patients.

STUDY DESIGN

Prospective case-control study.

METHODS

We tested 15 SD patients and 11 age-matched controls. Voice recordings were performed while subjects repeated the vowel /e/ and auditory feedback of their vocal sounds was altered in real-time to introduce a pitch-shift (±2 semitones), presented back to subjects using headphones. Recordings were analyzed to determine voice changes following the pitch-shifted feedback. Results were further compared with patient demographics and subjective measures of dysphonia, including the Voice Handicap Index (VHI).

RESULTS

Despite considerable pitch variability and vocal breaks, SD patients exhibited significantly higher average vocal pitch compensation than control subjects. SD patients also exhibited greater variability than controls. However, there were no significant correlations between vocal compensation and patient demographics, although there was a significant inverse correlation with VHI.

CONCLUSIONS

In this pilot study, patients with SD exhibited increased sensitivity to altered auditory feedback during sustained phonation. These results are consistent with recent theories of SD as a disorder of sensory-motor feedback processing, and suggest possible avenues for future investigation.

LEVEL OF EVIDENCE

3 Laryngoscope, 131:2070-2075, 2021.

Intermittent theta burst stimulation over right somatosensory larynx cortex enhances vocal pitch-regulation in nonsingers

While the significance of auditory cortical regions for the development and maintenance of speech motor coordination is well established, the contribution of somatosensory brain areas to learned vocalizations such as singing is less well understood. To address these mechanisms, we applied intermittent theta burst stimulation (iTBS), a facilitatory repetitive transcranial magnetic stimulation (rTMS) protocol, over right somatosensory larynx cortex (S1) and a nonvocal dorsal S1 control area in participants without singing experience. A pitch-matching singing task was performed before and after iTBS to assess corresponding effects on vocal pitch regulation. When participants could monitor auditory feedback from their own voice during singing (Experiment I), no difference in pitch-matching performance was found between iTBS sessions. However, when auditory feedback was masked with noise (Experiment II), only larynx-S1 iTBS enhanced pitch accuracy (50-250 ms after sound onset) and pitch stability (>250 ms after sound onset until the end). Results indicate that somatosensory feedback plays a dominant role in vocal pitch regulation when acoustic feedback is masked. The acoustic changes moreover suggest that right larynx-S1 stimulation affected the preparation and involuntary regulation of vocal pitch accuracy, and that kinesthetic-proprioceptive processes play a role in the voluntary control of pitch stability in nonsingers. Together, these data provide evidence for a causal involvement of right larynx-S1 in vocal pitch regulation during singing.

Central voice production and pathophysiology of spasmodic dysphonia

OBJECTIVE

Our ability to speak is complex, and the role of the central nervous system in controlling speech production is often overlooked in the field of otolaryngology. In this brief review, we present an integrated overview of speech production with a focus on the role of central nervous system. The role of central control of voice production is then further discussed in relation to the potential pathophysiology of spasmodic dysphonia (SD).

DATA SOURCES

Peer-review articles on central laryngeal control and SD were identified from PUBMED search. Selected articles were augmented with designated relevant publications.

REVIEW METHODS

Publications that discussed central and peripheral nervous system control of voice production and the central pathophysiology of laryngeal dystonia were chosen.

RESULTS

Our ability to speak is regulated by specialized complex mechanisms coordinated by high-level cortical signaling, brainstem reflexes, peripheral nerves, muscles, and mucosal actions. Recent studies suggest that SD results from a primary central disturbance associated with dysfunction at our highest levels of central voice control. The efficacy of botulinum toxin in treating SD may not be limited solely to its local effect on laryngeal muscles and also may modulate the disorder at the level of the central nervous system.

CONCLUSION

Future therapeutic options that target the central nervous system may help modulate the underlying disorder in SD and allow clinicians to better understand the principal pathophysiology.

LEVEL OF EVIDENCE

NA.Laryngoscope, 128:177-183, 2018.

Physiology Of Drowning: A Review

Drowning physiology relates to two different events: immersion (upper airway above water) and submersion (upper airway under water). Immersion involves integrated cardiorespiratory responses to skin and deep body temperature, including cold shock, physical incapacitation, and hypovolemia, as precursors of collapse and submersion. The physiology of submersion includes fear of drowning, diving response, autonomic conflict, upper airway reflexes, water aspiration and swallowing, emesis, and electrolyte disorders. Submersion outcome is determined by cardiac, pulmonary, and neurological injury. Knowledge of drowning physiology is scarce. Better understanding may identify methods to improve survival, particularly related to hot-water immersion, cold shock, cold-induced physical incapacitation, and fear of drowning.

Laryngeal Reflexes: Physiology, Technique, and Clinical Use

This review examines the current level of knowledge and techniques available for the study of laryngeal reflexes. Overall, the larynx is under constant control of several systems (including respiration, swallowing and cough) as well as sensory motor reflex responses involving glossopharyngeal, pharyngeal, laryngeal, and tracheobronchial sensory receptors. Techniques for the clinical assessment of these reflexes are emerging and need to be examined for sensitivity and specificity in identifying laryngeal sensory disorders. Quantitative assessment methods for the diagnosis of sensory reductions and sensory hypersensitivity may account for laryngeal disorders, such as chronic cough, paradoxical vocal fold disorder, and muscular tension dysphonia. The development of accurate assessment techniques could improve our understanding of the mechanisms involved in these disorders.

Medullary mediation of the laryngeal adductor reflex: A possible role in sudden infant death syndrome

The laryngeal adductor reflex (LAR) is a laryngeal protective reflex. Vagal afferent polymodal sensory fibres that have cell bodies in the nodose ganglion, originate in the sub-glottal area of the larynx and upper trachea. These polymodal sensory fibres respond to mechanical or chemical stimuli. The central axons of these sensory vagal neurons terminate in the dorsolateral subnuclei of the tractus solitarius in the medulla oblongata. The LAR is a critical, reflex in the pathways that play a protective role in the process of ventilation, and the sychronisation of ventilation with other activities that are undertaken by the oropharyngeal systems including: eating, speaking and singing. Failure of the LAR to operate properly at any time after birth can lead to SIDS, pneumonia or death. Despite the critical nature of this reflex, very little is known about the central pathways and neurotransmitters involved in the management of the LAR and any disorders associated with its failure to act properly. Here, we review current knowledge concerning the medullary nuclei and neurochemicals involved in the LAR and propose a potential neural pathway that may facilitate future SIDS research.

Distribution of Fos-Like Immunoreactivity, Catecholaminergic and Serotoninergic Neurons Activated by the Laryngeal Chemoreflex in the Medulla Oblongata of Rats

The laryngeal chemoreflex (LCR) induces apnea, glottis closure, bradycardia and hypertension in young and maturing mammals. We examined the distribution of medullary nuclei that are activated by the LCR and used immunofluorescent detection of Fos protein as a cellular marker for neuronal activation to establish that the medullary catecholaminergic and serotoninergic neurons participate in the modulation of the LCR. The LCR was elicited by the infusion of KCl-HCl solution into the laryngeal lumen of adult rats in the experimental group, whereas the control group received the same surgery but no infusion. In comparison, the number of regions of Fos-like immunoreactivity (FLI) that were activated by the LCR significantly increased in the nucleus of the solitary tract (NTS), the vestibular nuclear complex (VNC), the loose formation of the nucleus ambiguus (AmbL), the rostral ventral respiratory group (RVRG), the ventrolateral reticular complex (VLR), the pre-Bötzinger complex (PrBöt), the Bötzinger complex (Böt), the spinal trigeminal nucleus (SP5), and the raphe obscurus nucleus (ROb) bilaterally from the medulla oblongata. Furthermore, 12.71% of neurons with FLI in the dorsolateral part of the nucleus of the solitary tract (SolDL) showed tyrosine hydroxylase-immunoreactivity (TH-ir, catecholaminergic), and 70.87% of neurons with FLI in the ROb were serotoninergic. Our data demonstrated the distribution of medullary nuclei that were activated by the LCR, and further demonstrated that catecholaminergic neurons of the SolDL and serotoninergic neurons of the ROb were activated by the LCR, indicating the potential central pathway of the LCR.

The generation of pharyngeal phase of swallow and its coordination with breathing: interaction between the swallow and respiratory central pattern generators

Swallowing and breathing utilize common muscles and an anatomical passage: the pharynx. The risk of aspiration of ingested material is minimized not only by the laryngeal adduction of the vocal folds and laryngeal elevation but also by the precise coordination of swallows with breathing. Namely, swallows: (1) are preferentially initiated in the postinspiratory/expiratory phase, (2) are accompanied by a brief apnea, and (3) are often followed by an expiration and delay of the next breath. This review summarizes the expiratory evidence on the brainstem regions comprising the central pattern generator (CPG) that produces the pharyngeal stage of swallow, how the motor acts of swallowing and breathing are coordinated, and lastly, brainstem regions where the swallowing and respiratory CPGs may interact in order to ensure "safe" swallows.

Proposed toxic and hypoxic impairment of a brainstem locus in autism

Electrophysiological findings implicate site-specific impairment of the nucleus tractus solitarius (NTS) in autism. This invites hypothetical consideration of a large role for this small brainstem structure as the basis for seemingly disjointed behavioral and somatic features of autism. The NTS is the brain's point of entry for visceral afference, its relay for vagal reflexes, and its integration center for autonomic control of circulatory, immunological, gastrointestinal, and laryngeal function. The NTS facilitates normal cerebrovascular perfusion, and is the seminal point for an ascending noradrenergic system that modulates many complex behaviors. Microvascular configuration predisposes the NTS to focal hypoxia. A subregion--the "pNTS"--permits exposure to all blood-borne neurotoxins, including those that do not readily transit the blood-brain barrier. Impairment of acetylcholinesterase (mercury and cadmium cations, nitrates/nitrites, organophosphates, monosodium glutamate), competition for hemoglobin (carbon monoxide, nitrates/nitrites), and higher blood viscosity (net systemic oxidative stress) are suggested to potentiate microcirculatory insufficiency of the NTS, and thus autism.

The normal swallow: muscular and neurophysiological control

Swallowing is a complex physiologic function that involves precisely coordinated movements within the oral cavity, pharynx, larynx, and esophagus. This article reviews the anatomy, muscular control, and neurophysiological control of normal, healthy swallowing.

Laryngeal and tracheal afferent nerve stimulation evokes swallowing in anaesthetized guinea pigs

  We describe swallowing reflexes evoked by laryngeal and tracheal vagal afferent nerve stimulation in anaesthetized guinea pigs. The swallowing reflexes evoked by laryngeal citric acid challenges were abolished by recurrent laryngeal nerve (RLN) transection and mimicked by electrical stimulation of the central cut ends of an RLN. By contrast, the number of swallows evoked by upper airway/pharyngeal distensions was not significantly reduced by RLN transection but they were virtually abolished by superior laryngeal nerve transection. Laryngeal citric acid-evoked swallowing was mimicked by laryngeal capsaicin challenges, implicating transient receptor potential vanilloid 1 (TRPV1)-expressing laryngeal afferent nerves arising from the jugular ganglia. The swallowing evoked by citric acid and capsaicin and evoked by electrical stimulation of either the tracheal or the laryngeal mucosa occurred at stimulation intensities that were typically subthreshold for evoking cough in these animals. Swallowing evoked by airway afferent nerve stimulation also desensitized at a much slower rate than cough. We speculate that swallowing is an essential component of airway protection from aspiration associated with laryngeal and tracheal afferent nerve activation.

Effects of aging and levodopa on the laryngeal adductor reflex in rats

Dopaminergic neurotransmission plays an essential role in sensorimotor function, and declines with age. Previously, we found the laryngeal adductor reflex (LAR) was increased in excitation by a dopamine receptor antagonist. If this airway-protective reflex is similarly affected by aging, it will interfere with volitional control in older adults. The current study tested whether the LAR was affected by aging, and whether such deficits were reversed by levodopa administration in aging rats. We recorded thyroarytenoid (TA) muscle activity at rest and during elicitation of LAR responses by stimulation of the internal branch of the superior laryngeal nerve (iSLN) in 6-, 18- and 30-month-old rats under alpha-chloralose anesthesia. Using paired stimuli at different inter-stimulus intervals (ISIs), LAR central conditioning, resting muscle activity, and reflex latency and amplitudes were quantified. Numbers of dopaminergic neurons in the substantia nigra pars compacta (SNpc) were measured using tyrosine hydroxylase staining. We found: (1) increased resting TA muscle activity and LAR amplitude occurred with fewer dopaminergic neurons in the SNpc in 18- and 30-month-old rats; (2) decreases in LAR latency and increases in amplitude correlated with reduced numbers of dopaminergic neurons in the SNpc; (3) test responses were greater at 1000ms ISI in 18-month-old rats compared with 6-month-old rats; and (4) levodopa administration further increased response latency but did not alter muscle activity, response amplitude, or central conditioning. In conclusion, increases in laryngeal muscle activity levels and reflex amplitudes accompanied age reductions in dopaminergic neurons but were not reversed with levodopa administration.

Combined laryngeal inflammation and trauma mediate long-lasting immunoreactivity response in the brainstem sensory nuclei in the rat

Somatosensory feedback from the larynx plays a critical role in regulation of normal upper airway functions, such as breathing, deglutition, and voice production, while altered laryngeal sensory feedback is known to elicit a variety of pathological reflex responses, including persistent coughing, dysphonia, and laryngospasm. Despite its clinical impact, the central mechanisms underlying the development of pathological laryngeal responses remain poorly understood. We examined the effects of persistent vocal fold (VF) inflammation and trauma, as frequent causes of long-lasting modulation of laryngeal sensory feedback, on brainstem immunoreactivity in the rat. Combined VF inflammation and trauma were induced by injection of lipopolysaccharide (LPS) solution and compared to VF trauma alone from injection of vehicle solution and to controls without any VF manipulations. Using a c-fos marker, we found significantly increased Fos-like immunoreactivity (FLI) in the bilateral intermediate/parvicellular reticular formation (IRF/PCRF) with a trend in the left solitary tract nucleus (NTS) only in animals with combined LPS-induced VF inflammation and trauma. Further, FLI in the right NTS was significantly correlated with the severity of LPS-induced VF changes. However, increased brainstem FLI response was not associated with FLI changes in the first-order neurons of the laryngeal afferents located in the nodose and jugular ganglia in either group. Our data indicate that complex VF alterations (i.e., inflammation/trauma vs. trauma alone) may cause prolonged excitability of the brainstem nuclei receiving a direct sensory input from the larynx, which, in turn, may lead to (mal)plastic changes within the laryngeal central sensory control.

Recurrent laryngeal nerve activity exhibits a 5-HT-mediated long-term facilitation and enhanced response to hypoxia following acute intermittent hypoxia in rat

A progressive and sustained increase in inspiratory-related motor output (“long-term facilitation”) and an augmented ventilatory response to hypoxia occur following acute intermittent hypoxia (AIH). To date, acute plasticity in respiratory motor outputs active in the postinspiratory and expiratory phases has not been studied. The recurrent laryngeal nerve (RLN) innervates laryngeal abductor muscles that widen the glottic aperture during inspiration. Other efferent fibers in the RLN innervate adductor muscles that partially narrow the glottic aperture during postinspiration. The aim of this study was to investigate whether or not AIH elicits a serotonin-mediated long-term facilitation of laryngeal abductor muscles, and if recruitment of adductor muscle activity occurs following AIH. Urethane anesthetized, paralyzed, unilaterally vagotomized, and artificially ventilated adult male Sprague-Dawley rats were subjected to 10 exposures of hypoxia (10% O2 in N2, 45 s, separated by 5 min, n = 7). At 60 min post-AIH, phrenic nerve activity and inspiratory RLN activity were elevated (39 ± 11 and 23 ± 6% above baseline, respectively). These responses were abolished by pretreatment with the serotonin-receptor antagonist, methysergide ( n = 4). No increase occurred in time control animals ( n = 7). Animals that did not exhibit postinspiratory RLN activity at baseline did not show recruitment of this activity post-AIH ( n = 6). A repeat hypoxia 60 min after AIH produced a significantly greater peak response in both phrenic and RLN activity, accompanied by a prolonged recovery time that was also prevented by pretreatment with methysergide. We conclude that AIH induces neural plasticity in laryngeal motoneurons, via serotonin-mediated mechanisms similar to that observed in phrenic motoneurons: the so-called “Q-pathway”. We also provide evidence that the augmented responsiveness to repeat hypoxia following AIH also involves a serotonergic mechanism.

Perspective on the human cough reflex

This review dissects the complex human cough reflex and suggests hypotheses about the evolutionary basis for the reflex. A mechanosensory-induced cough reflex conveys through branches of myelinated Aδ nerve fibers is not chemically reactive (i.e., capsaicin, bradykinin); possibly, its evolution is to prevent the harmful effects of aspiration of gastric or particulate contents into the lungs. This became necessary as the larynx moves closer to the opening of the esophagus as human ancestors adapt phonation over olfaction beginning less than 10 million years ago. The second type of cough reflex, a chemosensory type, is carried by unmyelinated C fibers. Supposedly, its origin dates back when prehistoric humans began living in close proximity to each other and were at risk for infectious respiratory diseases or irritant-induced lung injury. The mechanism for the latter type of cough is analogous to induced pain after tissue injury; and, it is controlled by the identical transient receptor potential vanilloid cation channel (TRPV1). The airways do not normally manifest nociceptive pain from a stimulus but the only consistent response that capsaicin and lung inflammation provoke in healthy human airways is cough. TRPA1, another excitatory ion channel, has been referred to as the "irritant receptor" and its activation also induces cough. For both types of cough, the motor responses are identical and via coordinated, precisely-timed and sequential respiratory events orchestrated by complex neuromuscular networking of the diaphragm, chest and abdominal respiratory muscles, the glottis and parts of the brain.

Effects of dopamine D1 and D2 receptor antagonists on laryngeal neurophysiology in the rat

Hypophonia is an early symptom in Parkinson's disease (PD) that involves an increase in laryngeal muscle activity, interfering with voice production. Our aim was to use an animal model to better understand the role of different dopamine receptor subtypes in the control of laryngeal neurophysiology. First, we evaluated the combined effects of SCH23390-a D(1) receptor antagonist with a D(2) receptor antagonist (eticlopride) on laryngeal neurophysiology, and then tested the separate effects of selective receptor antagonists. Thyroarytenoid (TA) and gastrocnemius (GN) muscle activity was measured at rest and while stimulating the internal branch of superior laryngeal nerve to elicit the laryngeal adductor response (LAR) in alpha-chloralose-anesthetized rats. Paired stimuli at different interstimulus intervals between 250 and 5,000 ms measured central conditioning of the LAR. Changes in resting muscle activity, response latency, amplitude, and LAR conditioning after each drug were compared with the saline control. SCH23390 alone increased the resting TA muscle activity (P < 0.05). With the combined SCH23390 + eticlopride or SCH23390 alone, response latency decreased (P < 0.01), amplitude increased (P < 0.01), and the test LAR was reduced at 2,000-ms ISI (P < 0.01). No LAR changes occurred when eticlopride was administered alone at a low dose and only a tendency to suppress responses was found at a high dose. No changes in GN muscle activity occurred in any of the groups. The results suggest that a loss of stimulation of D(1) receptors plays a significant role in laryngeal pathophysiology in PD.

A mouse model of pharyngeal dysphagia in amyotrophic lateral sclerosis

We recently established that the SOD1-G93A transgenic mouse is a suitable model for oral-stage dysphagia in amyotrophic lateral sclerosis (ALS). The purpose of the present study was to determine whether it could serve as a model for pharyngeal-stage dysphagia as well. Electrophysiological and histological experiments were conducted on end-stage SOD1-G93A transgenic mice (n = 9) and age-matched wild-type (WT) littermates (n = 12). Transgenic mice required a twofold higher stimulus frequency (40 Hz) applied to the superior laryngeal nerve (SLN) to evoke swallowing compared with WT controls (20 Hz); transgenic females required a significantly higher (P < 0.05) stimulus frequency applied to the SLN to evoke swallowing compared with transgenic males. Thus, both sexes demonstrated electrophysiological evidence of pharyngeal dysphagia but symptoms were more severe for females. Histological evidence of neurodegeneration (vacuoles) was identified throughout representative motor (nucleus ambiguus) and sensory (nucleus tractus solitarius) components of the pharyngeal stage of swallowing, suggesting that pharyngeal dysphagia in ALS may be attributed to both motor and sensory pathologies. Moreover, the results of this investigation suggest that sensory stimulation approaches may facilitate swallowing function in ALS.

Multiple forebrain systems converge on motor neurons innervating the thyroarytenoid muscle

The present study investigated the central connections of motor neurons innervating the thyroarytenoid laryngeal muscle that is active in swallowing, respiration and vocalization. In both intact and sympathectomized rats, the pseudorabies virus (PRV) was inoculated into the muscle. After initial infection of laryngomotor neurons in the ipsilateral loose division of the nucleus ambiguus (NA) by 3 days post-inoculation, PRV spread to the ipsilateral compact portion of the NA, the central and intermediate divisions of the nucleus tractus solitarii, the Botzinger complex, and the parvicellular reticular formation by 4 days. Infection was subsequently expanded to include the ipsilateral granular and dysgranular parietal insular cortex, the ipsilateral medial division of the central nucleus of the amygdala, the lateral, paraventricular, ventrolateral and medial preoptic nuclei of the hypothalamus (generally bilaterally), the lateral periaqueductal gray, the A7 and oral and caudal pontine nuclei. At the latest time points sampled post-inoculation (5 days), infected neurons were identified in the ipsilateral agranular insular cortex, the caudal parietal insular cortex, the anterior cingulate cortex, and the contralateral motor cortex. In the amygdala, infection had spread to the lateral central nucleus and the parvicellular portion of the basolateral nucleus. Hypothalamic infection was largely characterized by an increase in the number of infected cells in earlier infected regions though the posterior, dorsomedial, tuberomammillary and mammillary nuclei contained infected cells. Comparison with previous connectional data suggests PRV followed three interconnected systems originating in the forebrain; a bilateral system including the ventral anterior cingulate cortex, periaqueductal gray and ventral respiratory group; an ipsilateral system involving the parietal insular cortex, central nucleus of the amygdala and parvicellular reticular formation, and a minor contralateral system originating in motor cortex. Hypothalamic innervation involved several functionally specific nuclei. Overall, the data imply complex CNS control over the multi-functional thyroarytenoid muscle.

Research priorities in spasmodic dysphonia

OBJECTIVE

To identify research priorities to increase understanding of the pathogenesis, diagnosis, and improved treatment of spasmodic dysphonia.

STUDY DESIGN AND SETTING

A multidisciplinary working group was formed that included both scientists and clinicians from multiple disciplines (otolaryngology, neurology, speech pathology, genetics, and neuroscience) to review currently available information on spasmodic dysphonia and to identify research priorities.

RESULTS

Operational definitions for spasmodic dysphonia at different levels of certainty were recommended for diagnosis and recommendations made for a multicenter multidisciplinary validation study.

CONCLUSIONS

The highest priority is to characterize the disorder and identify risk factors that may contribute to its onset. Future research should compare and contrast spasmodic dysphonia with other forms of focal dystonia. Development of animal models is recommended to explore hypotheses related to pathogenesis. Improved understanding of the pathophysiology of spasmodic dysphonia should provide the basis for developing new treatment options and exploratory clinical trials.

SIGNIFICANCE

This document should foster future research to improve the care of patients with this chronic debilitating voice and speech disorder by otolaryngology, neurology, and speech pathology.

Brainstem regions involved in the expiration reflex. A c-fos study in anesthetized cats

Expression of the immediate-early gene c-fos, a marker of neuronal activation, was employed to localize brainstem neuronal populations functionally related to the expiration reflex (ER). Twelve spontaneously breathing, non-decerebrate, pentobarbital anesthetized cats were used. The level of Fos-like immunoreactivity (FLI) in 6 animals with repetitive ERs mechanically induced from the glottis (296+/-9 ERs) was compared to FLI in 6 control non-stimulated cats. Respiratory rate, arterial blood pressure, and end tidal CO(2) concentration remained stable during the experiment. In the medulla, increased FLI was found in the region of nucleus tractus solitarii (p<0.001), in the ventrolateral medulla along with the lateral tegmental field (p<0.01), and in the vestibular nuclei (p<0.01). In the pons, increased FLI was detected in the caudal extensions of the lateral parabrachial and Kölliker-Fuse nuclei (p<0.05). Within the rostral mesencephalon, FLI was enhanced in the midline area (p<0.05). A lower level of ER-related FLI compared to control animals was detected in the pontine raphe region (p<0.05) and the lateral division of mesencephalic periaqueductal gray (p<0.05). The results suggest that the ER is coordinated by a complex long loop of medullary-pontine-mesencephalic neuronal circuits, some of which may differ from those of other respiratory reflexes. The FLI related to the expulsive behavior ER differs from that induced by laryngeal stimulation and laryngeal adductor responses, particularly in ventrolateral medulla and mesencephalon.

Laryngeal reflex responses are not modulated during human voice and respiratory tasks

The laryngeal adductor response (LAR) is a protective reflex that prevents aspiration and can be elicited either by electrical stimulation of afferents in the superior laryngeal nerve (SLN) or by deflection of mechanoreceptors in the laryngeal mucosa. We hypothesized that because this reflex is life-sustaining, laryngeal muscle responses to sensory stimuli would not be suppressed during volitional laryngeal tasks when compared to quiet respiration. Unilateral electrical superior laryngeal nerve stimulation was used to elicit early (R1) and late (R2) responses in the ipsilateral thyroarytenoid muscle in 10 healthy subjects. The baseline levels of muscle activity before stimulation, R1 and R2 response occurrence and the integrals of responses were measured during each task: quiet inspiration, prolonged vowels, humming, forced inhalation and effort closure. We tested whether R1 response integrals during tasks were equal to either: (1) baseline muscle activity during the task added to the response integral at rest; (2) the response integral at rest minus the baseline muscle activity during the task; or (3) the response integral at rest. R1 response occurrence was not altered by task from rest while fewer R2 responses occurred only during effort closure and humming compared to rest. Because the R1 response integrals did not change from rest, task increases in motor neuron firing did not alter the LAR. These findings demonstrate that laryngeal motor neuron responses to sensory inputs are not gated during volitional tasks confirming the robust life-sustaining protective mechanisms provided by this airway reflex.

Neuroprostheses for management of dysphagia resulting from cerebrovascular disorders

Swallowing is a complicated process that involves intricate timing between many different muscles in the mouth and neck. The primary purpose of swallowing is to move food through the mouth and pharynx and into the esophagus for transport to the stomach for digestion. Dysphagia is a general term that refers to a disruption in any part of the process. The consequences of dysphagia include social embarrassment; malnutrition; and aspiration. Of these, aspiration is the most significant as it is associated with a significantly greater risk of pneumonia and death. If patients fail to adequately protect the airways with standard exercise and therapy, they are often disallowed from taking food by mouth and receive nutrition by alternate means. If patients still experience frequent pneumonia, more drastic surgical measures that permanently separate the airway from foodway are required. As an alternative to these surgical procedures, neuroprostheses can dynamically restore airway protection. There are two primary protective mechanisms that neuroprostheses seek to restore. The first is laryngeal elevation and the second is vocal fold closure. The present article is an introductory overview of the swallowing process, the primary muscles and nerves related to swallowing, the effects of dysphagia, the standard treatment options, and the neuroprosthetic options.

Electric stimulation approaches to the restoration and rehabilitation of swallowing: a review

In recent years, there has been a proliferation of interest in the use of electric stimulation for the treatment of swallowing disorders. This review explores both the rationale and existing evidence for electric stimulation approaches to swallowing rehabilitation. Although this is an exciting area of research which holds promise for future clinically relevant technology and/or therapy, a critical analysis of the existing literature will be presented to support the argument that implementation of electric stimulation in clinical swallowing rehabilitation settings still remains pre-mature.

Central nervous system control of the laryngeal muscles in humans

Laryngeal muscle control may vary for different functions such as: voice for speech communication, emotional expression during laughter and cry, breathing, swallowing, and cough. This review discusses the control of the human laryngeal muscles for some of these different functions. Sensori-motor aspects of laryngeal control have been studied by eliciting various laryngeal reflexes. The role of audition in learning and monitoring ongoing voice production for speech is well known; while the role of somatosensory feedback is less well understood. Reflexive control systems involving central pattern generators may contribute to swallowing, breathing and cough with greater cortical control during volitional tasks such as voice production for speech. Volitional control is much less well understood for each of these functions and likely involves the integration of cortical and subcortical circuits. The new frontier is the study of the central control of the laryngeal musculature for voice, swallowing and breathing and how volitional and reflexive control systems may interact in humans.

Suppression of thyroarytenoid muscle responses during repeated air pressure stimulation of the laryngeal mucosa in awake humans

Repeated stimulation of the laryngeal mucosa occurs during speech. Single stimuli, however, can elicit the laryngeal adductor response (LAR). Our hypothesis was that the LAR to repeated rapid air pressure stimuli is centrally suppressed in humans. Hooked-wire electrodes were inserted into the thyroarytenoid and cricothyroid muscles on both sides and into the posterior cricoarytenoid muscle on one side. Pairs of air puff stimuli were presented to the mucosa over the arytenoids at pressure levels three times threshold with interstimulus intervals from 250 to 5,000 ms. Bilateral thyroarytenoid responses occurred at around 150 ms to more than 70% of the initial stimuli. With repeated presentation at intervals of 2 seconds or less, the percent occurrence decreased to less than 40% and response amplitudes were reduced by 50%. Central suppression of adductor responses to repeated air puff stimuli may allow speakers to produce voice without eliciting reflexive spasms that could disrupt speech.