Neural mechanisms of swallowing: neurophysiological and neurochemical studies on brain stem neurons in the solitary tract region

Role of 5-HT1A in the nucleus of the solitary tract in the regulation of swallowing activities evoked by electroacupuncture in anesthetized rats

Somatic stimulation therapy, such as electroacupuncture (EA), has been widely applied in the clinic to treat dysphagia. However, its underlying mechanism has remained unknown. In the present study, the effect of EA at acupoints Fengfu (DU16) and Lianquan (RN23) on swallowing activities and the involvement of 5-HT_1A in the nucleus of the solitary tract (NTS) were examined in anesthetized rats. EA at DU16 and RN23 significantly evoked myoelectric activity of the mylohyoid muscle, which was attenuated by injection of 10 nmol 5-HT_1A antagonist (WAY-100635) into the NTS. Meanwhile, 5-HT_1A expression in the NTS increased following EA. The results suggested that EA at DU16 and RN23 promotes swallowing activity, and 5-HT_1A in the NTS may play an important role in the excitatory effects.

Pathogenesis of Lethal Aspiration Pneumonia in Mecp2-null Mouse Model for Rett Syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder mainly caused by mutations in the gene encoding the transcriptional regulator Methyl-CpG-binding protein 2 (MeCP2), located on the X chromosome. Many RTT patients have breathing abnormalities, such as apnea and breathing irregularity, and respiratory infection is the most common cause of death in these individuals. Previous studies showed that MeCP2 is highly expressed in the lung, but its role in pulmonary function remains unknown. In this study, we found that MeCP2 deficiency affects pulmonary gene expression and structures. We also found that Mecp2-null mice, which also have breathing problems, often exhibit inflammatory lung injury. These injuries occurred in specific sites in the lung lobes. In addition, polarizable foreign materials were identified in the injured lungs of Mecp2-null mice. These results indicated that aspiration might be a cause of inflammatory lung injury in Mecp2-null mice. On the other hand, MeCP2 deficiency affected the expression of several neuromodulator genes in the lower brainstem. Among them, neuropeptide substance P (SP) immunostaining was reduced in Mecp2-null brainstem. These findings suggest that alteration of SP expression in brainstem may be involved in autonomic dysregulation, and may be one of the causes of aspiration in Mecp2-null mice.

Measurement of cerebral blood volume dynamics during volitional swallowing using functional near-infrared spectroscopy: an exploratory study

The aim of this study was to examine cerebral blood volume dynamics during volitional swallowing using multi-channel functional near-infrared spectroscopy (fNIRS) to understand the basic cortical activation patterns. Fifteen volunteers (age, 26.5±1.3 years, mean±SD) performed volitional swallowing of a 5-ml bolus of water as a task. A 52-channel fNIRS system was used for measuring oxy-Hb levels. We determined the oxy-Hb concentration changes in each channel by calculating the differences between rest and task oxy-Hb levels. Differences in rest and task data were assessed using a paired-t test (p<0.05). A significant increase in oxy-Hb was found in 21 channels. The cortical regions that exhibited increased oxy-Hb concentration included the bilateral precentral gyrus, postcentral gyrus, inferior frontal gyrus, superior temporal gyrus, middle temporal gyrus, and supramarginal gyrus. These data provide a description of cortical activation patterns during volitional swallowing using fNIRS, which will be useful for the evaluation of dysphasia and the effects of the rehabilitation [Corrected].

Diagnosis with manometry and treatment with repetitive transcranial magnetic stimulation in Dysphagia

Videofluoroscopic swallowing study (VFSS) used for the diagnosis of dysphagia has limitations in objectively assessing the contractility of the pharyngeal muscle or the degree of the upper esophageal sphincter relaxation. With a manometer, however, it is possible to objectively assess the pressure changes in the pharynx caused by pharyngeal muscle contraction during swallowing or upper esophageal sphincter relaxation, hence remedying the limitations of VFSS. The following case report describes a patient diagnosed with lateral medullar infarction presenting a 52-year-old male who had dysphagia. We suggested that the manometer could be used to assess the specific site of dysfunction in patients with dysphagia complementing the limitations of VFSS. We also found that repetitive transcranial magnetic stimulation was effective in treating patients refractory to traditional dysphagia rehabilitation.

Raphé tauopathy alters serotonin metabolism and breathing activity in terminal Tau.P301L mice: possible implications for tauopathies and Alzheimer's disease

Tauopathies, including Alzheimer's disease are the most frequent neurodegenerative disorders in elderly people. Patients develop cognitive and behaviour defects induced by the tauopathy in the forebrain, but most also display early brainstem tauopathy, with oro-pharyngeal and serotoninergic (5-HT) defects. We studied these aspects in Tau.P301L mice, that express human mutant tau protein and develop tauopathy first in hindbrain, with cognitive, motor and upper airway defects from 7 to 8 months onwards, until premature death before age 12 months. Using plethysmography, immunohistochemistry and biochemistry, we examined the respiratory and 5-HT systems of aging Tau.P301L and control mice. At 8 months, Tau.P301L mice developed upper airway dysfunction but retained normal respiratory rhythm and normal respiratory regulations. In the following weeks, Tau.P301L mice entered terminal stages with reduced body weight, progressive limb clasping and lethargy. Compared to age 8 months, terminal Tau.P301L mice showed aggravated upper airway dysfunction, abnormal respiratory rhythm and abnormal respiratory regulations. In addition, they showed severe tauopathy in Kolliker-Fuse, raphé obscurus and raphé magnus nuclei but not in medullary respiratory-related areas. Although the raphé tauopathy concerned mainly non-5-HT neurons, the 5-HT metabolism of terminal Tau.P301L mice was altered. We propose that the progressive raphé tauopathy affects the 5-HT metabolism, which affects the 5-HT modulation of the respiratory network and therefore the breathing pattern. Then, 5-HT deficits contribute to the moribund phenotype of Tau.P301L mice, and possibly in patients suffering from tauopathies, including Alzheimer's disease.

Impaired opening of the upper esophageal sphincter in patients with medullary infarctions

The aim of this study was to report on nine dysphagic patients with medullary infarction and to evaluate swallowing characteristics based on the location of the lesions.We retrospectively reviewed the medical records of these nine patients. The medullary lesions were midlateral (three patients), dorsolateral (one patient), inferodorsolateral (four patients), and paramedian (one patient). The levels of the lesions were upper (four patients), middle (two patients), upper and middle (two patients), and middle and lower medulla (one patient). Dysphagia after medullary infarction was more common in patients with upper or middle medullary level and dorsolateral medullary level lesions. The common findings on videofluoroscopic swallowing studies in patients with lateral medullary infarctions were impaired upper esophageal sphincter opening, aspiration from pyriform sinuses' residue caused by pharyngeal weakness, and multiple swallowing to clear boluses from the pharynx to the esophagus. In patients with medullary infarctions, the lesion levels and loci and their related clinical findings can be useful in predicting dysphagia and aspiration. Because severe dysphagia with serious complication is very common in patients with medullary infarctions, active diagnostic and therapeutic approaches are needed.

Symptom management and supportive care of the patient with brain metastases

The focus of care for patients with brain metastases will always be on therapeutic options such as surgery, radiotherapy, and chemotherapy. However, proper symptom management and supportive care of non-therapeutic issues will be equally as important, including treatment of seizures, use of anticonvulsants, corticosteroids, and gastric acid inhibitors, assessment of swallowing dysfunction, treatment of thromboembolic events, appropriate use, and safe application of anticoagulation, and evaluation of psychiatric issues. Appropriate management of these supportive aspects of patient care will improve overall quality of life and allow the patient and family to more easily concentrate on treatment.

Quantitative analysis of surface EMG activity of cranial and leg muscles across sleep stages in human

OBJECTIVE

The aim of this study was to make a quantitative analysis of the changes in cranial and limb muscle activity from wakefulness to light and deep sleep stages and during rapid eye movement (REM) sleep of normal subjects.

METHODS

Polysomnographic recordings were made of the sleep of 9 healthy human subjects, including electromyograms of the suprahyoid, temporalis and masseter cranial muscles and the anterior tibialis limb muscle. Quantitative assessments of EMG activity were carried out with root mean square (RMS) and frequency-spectral analysis (FSA) methods.

RESULTS

From wakefulness to sleep, a significant reduction (-25.2 to -71.2%; P < 0.01) was observed in EMG activity (for both RMS and FSA) of the 3 cranial muscles using both methods of analysis. The EMG activity of suprahyoid muscle further decreased from non-REM to REM sleep (-17.8 to -43.0%; P < 0.01). In contrast, the EMG activity of the anterior tibialis muscle was only slightly reduced across sleep stages and did not further reduce during REM sleep. During REM sleep, all the 4 muscles maintained minimal activity.

CONCLUSIONS

The maintenance of muscle activity during REM sleep suggests that a minimal level of activity is required to preserve physiological functions (e.g. airway patency, posture) related to homeostasis and bodily protection.

SIGNIFICANCE

This study suggests that quantitative sleep EMG analysis is important for understanding the mechanisms of sleep-related movement disorders or when objective assessment of changes in EMG activity are needed for diagnostic purposes or for the assessment of drug efficiency.

Neurobiological mechanisms involved in sleep bruxism

Sleep bruxism (SB) is reported by 8% of the adult population and is mainly associated with rhythmic masticatory muscle activity (RMMA) characterized by repetitive jaw muscle contractions (3 bursts or more at a frequency of 1 Hz). The consequences of SB may include tooth destruction, jaw pain, headaches, or the limitation of mandibular movement, as well as tooth-grinding sounds that disrupt the sleep of bed partners. SB is probably an extreme manifestation of a masticatory muscle activity occurring during the sleep of most normal subjects, since RMMA is observed in 60% of normal sleepers in the absence of grinding sounds. The pathophysiology of SB is becoming clearer, and there is an abundance of evidence outlining the neurophysiology and neurochemistry of rhythmic jaw movements (RJM) in relation to chewing, swallowing, and breathing. The sleep literature provides much evidence describing the mechanisms involved in the reduction of muscle tone, from sleep onset to the atonia that characterizes rapid eye movement (REM) sleep. Several brainstem structures (e.g., reticular pontis oralis, pontis caudalis, parvocellularis) and neurochemicals (e.g., serotonin, dopamine, gamma aminobutyric acid [GABA], noradrenaline) are involved in both the genesis of RJM and the modulation of muscle tone during sleep. It remains unknown why a high percentage of normal subjects present RMMA during sleep and why this activity is three times more frequent and higher in amplitude in SB patients. It is also unclear why RMMA during sleep is characterized by co-activation of both jaw-opening and jaw-closing muscles instead of the alternating jaw-opening and jaw-closing muscle activity pattern typical of chewing. The final section of this review proposes that RMMA during sleep has a role in lubricating the upper alimentary tract and increasing airway patency. The review concludes with an outline of questions for future research.

Central distribution of neuronal cell bodies innervating the levator veli palatini muscle and associated pattern of myosin heavy chain isoform expression in rat

The levator veli palatini (LVP) is a muscle that plays a very important role in the complex functions regulating velopharyngeal function. Although previous studies have indicated that the contraction properties of the LVP closely resemble those of the intrinsic laryngeal muscle, histological evidence has not yet been obtained. The LVP is generally considered to be innervated by the glossopharyngeal nerve, which contains efferent and afferent components. LVP motoneurons are localized in the nucleus ambiguus (Amb), and afferent neurons project into the bilateral regions of the nucleus of the solitary tract (NST). However, the position of neuronal cell bodies on afferent neurons has remained unknown. The present study examined serial muscle cross-sections using monoclonal antibodies specific for myosin heavy chain (MyHC), to characterize muscle fibers of the LVP, clarify the central distribution of LVP motoneurons within the Amb and afferent terminals within the NST, and elucidate the location of LVP afferent neuronal cell bodies. Clear separation was observed within the LVP between fibers containing only fast MyHC and others positive for both slow and fast MyHC. Horseradish peroxidase (HRP)-labeled motoneurons in the Amb were separated into rostral and caudal divisions, corresponding to the Bötzinger complex and the rostral ventral respiratory group, respectively. HRP-labeled afferent neuronal cell bodies were observed in a glossopharyngo-vagal complex ganglion, and HRP-labeled afferent terminals were observed in bilateral lateral regions of the NST. These results suggest a relationship between MyHC isoform expression and the central distribution of LVP motoneurons or central projections of afferent neurons, with regard to activity of the LVP during both inspiration and expiration.

Effects on mastication of reversible bilateral inactivation of the lateral pericentral cortex in the monkey (Macaca fascicularis)

It is known that intracortical microstimulation (ICMS) of the lateral pericentral cortex can evoke masticatory movements and swallowing in awake monkeys. The aim was to determine if the ability of monkeys to carry out mastication is affected by reversible bilateral cold block of the ICMS-defined cortical masticatory area/swallow cortex. A cranial chamber was implanted bilaterally in two monkeys and a warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the ICMS-defined cortical masticatory area/swallow cortex while monkeys chewed standardised amounts of fruit during pre-cool (thermode temperature, 37 degrees C), cool (0-4 degrees C), and post-cool (37 degrees C) trials. Electromyographic (EMG) activity was recorded from masseter, genioglossus, anterior digastric, geniohyoid and thyrohyoid or perilaryngeal muscles. Vertical and horizontal jaw movements were recorded with a photodiode position transducer, which monitored movements of a light-emitting diode fixed to the mandible. Each masticatory period was divided into a food-preparatory phase, a rhythmic chewing phase and a preswallow phase. Both monkeys could readily accept and ingest the foodstuffs during pre-cool and post-cool trials. In contrast, cold block was associated with masticatory deficits, reflected in both monkeys as impaired food intake or manipulation and difficulty in carrying out a sequence of masticatory cycles, alterations in of the food-preparatory phase, and alterations in masticatory-related EMG patterns of the jaw and tongue muscles. The cold block-induced changes included significant (P<0.05) prolongations of the total masticatory time, the food-preparatory phase duration, and burst durations of the jaw and tongue muscle EMG activities; furthermore, the amplitudes and temporal correlations of the EMG activities of the jaw and tongue muscles were significantly (P<0.05) changed by cold block. These findings provide further evidence that the lateral pericentral cortex has a critical role in the initiation and regulation of masticatory movements in the primate, and that the programming of masticatory muscle activities may be dependent upon corticofugal influences for engaging masticatory motor activities appropriate to the masticatory conditions.

Effects of reversible bilateral inactivation of face primary motor cortex on mastication and swallowing

The effects of reversible cold block-induced bilateral inactivation of the face primary motor cortex (face MI) on mastication and swallowing were studied in awake monkeys. A warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the intracortical microstimulation-defined face MI while the monkey chewed and swallowed food during pre-cool (thermode temperature 37 degrees C), cold block (4 degrees C), and post-cool (37 degrees C) sessions. Vertical and horizontal jaw movements and electromyographic (EMG) activity of several muscles were monitored. Each masticatory sequence was divided into three masticatory phases (i.e. food preparatory, rhythmic chewing, preswallow). The cold block markedly affected the ability of the monkey to carry out mastication although it did not prevent mastication from occurring. The masticatory deficit was characterized by a significant elongation of the total masticatory time, including in particular elongation of the food preparatory phase. The coordination of the jaw- and tongue-muscle activities was severely disrupted during the food preparatory phase. Face MI cold block also significantly affected the duration of some masticatory-related EMG activities and had some limited effects on the temporal relationships of the EMG activities during mastication. Although cold block significantly affected the duration and some EMG parameters of the preswallow phase, it had no significant effect on swallow duration or the EMG parameters during swallowing. These findings provide further evidence that the primate face MI plays a critical role in the regulation of mastication and that it plays a role in the preparation for swallowing.

Non-invasive monitoring of functionally distinct muscle activations during swallowing

OBJECTIVES

Dysphagia is an important consequence of many diseases. As some of the muscles of deglutition tend to be deep to the surface, needle electrodes are typically used, but this limits the number of muscles that can be simultaneously recorded. Since control of swallowing involves central pattern generators (CPGs) which distribute commands to several muscles, monitoring several muscles simultaneously is desirable. Here we describe a novel method, based on computing the independent components (ICs) of the simultaneous sEMG recordings (Muscle Nerve Suppl 9 (2000) 9) to detect the underlying functional muscle activations during swallowing using only surface EMG (sEMG) electrodes.

METHODS

Seven normal subjects repeatedly swallowed liquids of varying consistency while sEMG was recorded from 15 electrodes from the face and throat. Active areas of EMG were excised from the recordings and the ICs of the sEMG were calculated.

RESULTS

The ICs demonstrated less swallow-to-swallow variability than the raw sEMG. The ICs, each consisting of a unique temporal waveform and a spatial distribution, provided a means to segregate the complex sequence of muscle activation into rigorously defined separate functional units. The temporal profiles of the ICs and their spatial distribution were consistent with prior needle EMG studies of the cricopharyngeal, superior pharyngeal constrictor, submental and possibly arytenoid muscles. Other components appeared to correspond to EKG artifact contaminating the EMG recordings, laryngeal excursion, tongue movement and activation of the buccal and/or masseter musculature At least two of the components were affected by the consistency of the liquids swallowed. Re-testing one subject a week later demonstrated good intertrial reliability.

CONCLUSIONS

We propose that the ICs of the sEMG provide a non-invasive means to assess the complex muscle sequence activation of deglutition.

Selective suppression of late laryngeal adductor responses by N-methyl-D-aspartate receptor blockade in the cat

Laryngeal adductor responses to afferent stimulation play a key role in airway protection. Although vital for protection during cough and swallow, these responses also must be centrally controlled to prevent airway obstruction by laryngospasm during prolonged stimulation. Our purpose was to determine the role of N-methyl-D-aspartate (NMDA) receptors in modulating early R1 responses (at 9 ms) and/or later more prolonged R2 responses (at 36 ms) during electrical stimulation of the laryngeal afferent fibers contained in the internal branch of the superior laryngeal nerve in the cat. The percent occurrence, amplitude, and conditioning of muscle responses to single superior laryngeal nerve (SLN) stimuli presented in pairs at interstimulus intervals of 250 ms were measured in three experiments: 1) animals that had ketamine as anesthetic premedication were compared with those who did not, when both were maintained under alpha-chloralose anesthesia. 2) The effects of administering ketamine in one group of animals were compared with increasing the depth of alpha-chloralose anesthesia without NMDA receptor blockade in another group of animals. 3) The effects of dextromethorphan (without anesthetic effects) were examined in another group of animals. In the first experiment, the occurrence of R2 responses were reduced from 95% in animals without ketamine premedication to 25% in animals with ketamine premedication (P = 0.015). No differences occurred in the occurrence, amplitude, latency, or conditioning effects on R1 responses between these groups. In the second experiment, the occurrence of R2 responses was reduced from 96 to 79% after an increase in the depth of anesthesia with alpha-chloralose in contrast with reductions in R2 occurrence from 98 to 19% following the administration of ketamine to induce NMDA receptor blockade along with increased anesthesia (P = 0.025). In the third experiment, R2 occurrence was reduced from 89 to 27% (P = 0.017) with administration of dextromethorphan while R1 response occurrence and amplitude did not change. In each of these experiments, NMDA receptor blockade did not have significant effects on cardiac or respiratory rates in any of the animals. The results demonstrate that NMDA receptors play an essential role in long latency R2 laryngeal responses to laryngeal afferent stimulation. On the other hand, early R1 laryngeal adductor responses are likely to involve non-NMDA receptor activation.

Neural control of tongue movement with respect to respiration and swallowing

The tongue must move with remarkable speed and precision between multiple orofacial motor behaviors that are executed virtually simultaneously. Our present understanding of these highly integrated relationships has been limited by their complexity. Recent research indicates that the tongue s contribution to complex orofacial movements is much greater than previously thought. The purpose of this paper is to review the neural control of tongue movement and relate it to complex orofacial behaviors. Particular attention will be given to the interaction of tongue movement with respiration and swallowing, because the morbidity and mortality associated with these relationships make this a primary focus of many current investigations. This review will begin with a discussion of peripheral tongue muscle and nerve physiology that will include new data on tongue contractile properties. Other relevant peripheral oral cavity and oropharyngeal neurophysiology will also be discussed. Much of the review will focus on brainstem control of tongue movement and modulation by neurons that control swallowing and respiration, because it is in the brainstem that orofacial motor behaviors sort themselves out from their common peripheral structures. There is abundant evidence indicating that the neural control of protrusive tongue movement by motoneurons in the ventral hypoglossal nucleus is modulated by respiratory neurons that control inspiratory drive. Yet, little is known of hypoglossal motoneuron modulation by neurons controlling swallowing or other complex movements. There is evidence, however, suggesting that functional segregation of respiration and swallowing within the brainstem is reflected in somatotopy within the hypoglossal nucleus. Also, subtle changes in the neural control of tongue movement may signal the transition between respiration and swallowing. The final section of this review will focus on the cortical integration of tongue movement with complex orofacial movements. This section will conclude with a discussion of the functional and clinical significance of cortical control with respect to recent advances in our understanding of the peripheral and brainstem physiology of tongue movement.

Brain stem control of swallowing: neuronal network and cellular mechanisms

Swallowing movements are produced by a central pattern generator located in the medulla oblongata. It has been established on the basis of microelectrode recordings that the swallowing network includes two main groups of neurons. One group is located within the dorsal medulla and contains the generator neurons involved in triggering, shaping, and timing the sequential or rhythmic swallowing pattern. Interestingly, these generator neurons are situated within a primary sensory relay, that is, the nucleus tractus solitarii. The second group is located in the ventrolateral medulla and contains switching neurons, which distribute the swallowing drive to the various pools of motoneurons involved in swallowing. This review focuses on the brain stem mechanisms underlying the generation of sequential and rhythmic swallowing movements. It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs which shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed.

Cerebral cortical representation of reflexive and volitional swallowing in humans

The purpose of this study was to compare cerebral cortical representation of experimentally induced reflexive swallow with that of volitional swallow. Eight asymptomatic adults (24-27 yr) were studied by a single-trial functional magnetic resonance imaging technique. Reflexive swallowing showed bilateral activity concentrated to the primary sensory/motor regions. Volitional swallowing was represented bilaterally in the insula, prefrontal, cingulate, and parietooccipital regions in addition to the primary sensory/motor cortex. Intrasubject comparison showed that the total volume of activity during volitional swallowing was significantly larger than that activated during reflexive swallows in either hemisphere (P < 0.001). For volitional swallowing, the primary sensory/motor region contained the largest and the insular region the smallest volumes of activation in both hemispheres, and the total activated volume in the right hemisphere was significantly larger compared with the left (P < 0.05). Intersubject comparison showed significant variability in the volume of activity in each of the four volitional swallowing cortical regions. We conclude that reflexive swallow is represented in the primary sensory/motor cortex and that volitional swallow is represented in multiple regions, including the primary sensory/motor cortex, insular, prefrontal/cingulate gyrus, and cuneus and precuneus region. Non-sensory/motor regions activated during volitional swallow may represent swallow-related intent and planning and possibly urge.

Modulation of laryngeal responses to superior laryngeal nerve stimulation by volitional swallowing in awake humans

Laryngeal sensori-motor closure reflexes are important for the protection of the airway and prevent the entry of foreign substances into the trachea and lungs. The purpose of this study was to determine how such reflexes might be modulated during volitional swallowing in awake humans, when persons are at risk of entry of food or liquids into the airway. The frequency and the amplitude of laryngeal adductor responses evoked by electrical stimulation of the internal branch of the superior laryngeal nerve (ISLN) were studied during different phases of volitional swallowing. Subjects swallowed water on command while electrical stimuli were presented to the ISLN at various intervals from 500 ms to 5 s following the command. Laryngeal adductor responses to unilateral ISLN stimulation were recorded bilaterally in the thyroarytenoid muscles using hooked wire electrodes. Early ipsilateral R1 responses occurred at 17 ms, and later bilateral R2 began around 65 ms. The muscle responses to stimuli occurring during expiration without swallowing were quantified as control trials. Responses to stimulation presented before swallowing, during the swallow, within 3 s after swallowing, and between 3 and 5 s after a swallow were measured. The frequency and amplitude of three responses (ipsilateral R1 and bilateral R2) relative to the control responses were compared across the different phases relative to the occurrence of swallowing. Results demonstrated that a reduction occurred in both the frequency and amplitude of the later bilateral R2 laryngeal responses to electrical stimulation for up to 3 s after swallowing (P = 0.005). The amplitude and frequency of ipsilateral R1 laryngeal responses, however, did not show a significant main effect following the swallow (P = 0.28), although there was a significant time by measure interaction (P = 0.006) related to reduced R1 response amplitude up to 3 s after swallowing (P = 0.021). Therefore, the more rapid and shorter unilateral R1 responses continued to provide some, albeit reduced, laryngeal protective functions after swallowing, whereas the later bilateral R2 responses were suppressed both in occurrence and amplitude for up to 3 s after swallowing. The results suggest that R2 laryngeal adductor responses are suppressed following swallowing when residues may remain in the laryngeal vestibule putting persons at increased risk for the entry of foreign substances into the airway.

Features of cortically evoked swallowing in the awake primate (Macaca fascicularis)

Although the cerebral cortex has been implicated in the control of swallowing, the output organization of the cortical swallowing representation, and features of cortically evoked swallowing, remain unclear. The present study defined the output features of the primate "cortical swallowing representation" with intracortical microstimulation (ICMS) applied within the lateral sensorimotor cortex. In four hemispheres of two awake monkeys, microelectrode penetrations were made at </=1-mm intervals, initially within the face primary motor cortex (face-MI), and subsequently within the cortical regions immediately rostral, lateral, and caudal to MI. Two ICMS pulse trains [35-ms train, 0.2-ms pulses at 333 Hz, </=30 microA (short train stimulus, T/S); 3- to 4-s train, 0.2-ms pulses at 50 Hz, </=60 microA (continuous stimulus, C/S)] were applied at </=500-micron intervals along each microelectrode penetration to a depth of 8-10 mm, and electromyographic (EMG) activity was recorded simultaneously from various orofacial and laryngeal muscles. Evoked orofacial movements, including swallowing, were verified by EMG analysis, and T/S and C/S movement thresholds were determined. Effects of varying ICMS intensity on swallow-related EMG properties were examined by applying suprathreshold C/S at selected intracortical sites. EMG patterns of swallows evoked from various cortical regions were compared with those of natural swallows recorded as the monkeys swallowed liquid and solid material. Results indicated that swallowing was evoked by C/S at approximately 20% of 1,569 intracortical sites where ICMS elicited an orofacial motor response in both hemispheres of the two monkeys, typically at C/S intensities </=30 microA. In contrast, swallowing was not evoked by T/S in either monkey. Swallowing was evoked from four cortical regions: the ICMS-defined face-MI, the face primary somatosensory cortex (face-SI), the region lateral and anterior to face-MI corresponding to the cortical masticatory area (CMA), and an area >5 mm deep to the cortical surface corresponding to both the white matter underlying the CMA and the frontal operculum; EMG patterns of swallows elicited from these four cortical regions showed some statistically significant differences. Whereas swallowing ONLY was evoked at some sites, particularly within the deep cortical area, swallowing was more frequently evoked together with other orofacial responses including rhythmic jaw movements. Increasing ICMS intensity increased the magnitude, and decreased the latency, of the swallow-related EMG burst in the genioglossus muscle at some sites. These findings suggest that a number of distinct cortical foci may participate in the initiation and modulation of the swallowing synergy as well as in integrating the swallow within the masticatory sequence.

Effects of functional disruption of lateral pericentral cerebral cortex on primate swallowing

Bilateral cold block of the intracortical microstimulation (ICMS)-defined swallow cortex markedly affected the ability of monkeys to carry out swallowing. Significant changes also occurred in swallow-related electromyographic (EMG) activity patterns. These findings provide further evidence that the lateral pericentral cortex plays a critical role in the initiation and regulation of swallowing in the primate.

Solitarial premotor neuron projections to the rat esophagus and pharynx: implications for control of swallowing

BACKGROUND & AIMS

The buccopharyngeal and esophageal phases of swallowing are controlled by distinct networks of premotor neurons localized in the nucleus tractus solitarius. The neuronal circuitry coordinating the two phases was investigated using a combination of central and peripheral tracing techniques.

METHODS

Using pseudorabies virus, a transsynaptic tracer, in anesthetized rats, third-order esophageal neurons (neurons projecting to premotor neurons) were identified. In a separate protocol that combined transsynaptic and retrograde fluorescent tracing, third-order esophageal neurons projecting to pharyngeal motoneurons (buccopharyngeal premotor neurons) were then identified.

RESULTS

Third-order esophageal neurons were identified in the interstitial and intermediate subnuclei of the nucleus tractus solitarius and in other medullary, pontine, midbrain, and forebrain nuclei. A subpopulation of these neurons (double labeled) in the interstitial and intermediate subnuclei were found to project to pharyngeal motoneurons (buccopharyngeal premotor neurons) and to be linked synaptically to esophageal premotor neurons.

CONCLUSIONS

The synaptic link between buccopharyngeal and esophageal premotor neurons provides an anatomic pathway for the central initiation of esophageal peristalsis and its coordination with the pharyngeal phase of swallowing. This neural circuitry within the nucleus tractus solitarius is consistent with a complex central control mechanism for the swallowing motor sequence that can function independently of afferent feedback.

Functional properties of neurons in the primate tongue primary motor cortex during swallowing

Recent studies conducted in our laboratory have suggested that the tongue primary motor cortex (i.e., tongue-MI) plays a critical role in the control of voluntary tongue movements in the primate. However, the possible involvement of tongue-MI in semiautomatic tongue movements, such as those in swallowing, remains unknown. Therefore the present study was undertaken in attempts to address whether tongue-MI plays a role in the semiautomatic tongue movements produced during swallowing. Extracellular single neuron recordings were obtained from tongue-MI, defined by intracortical microstimulation (ICMS), in two awake monkeys as they performed three types of swallowing (swallowing of a juice reward after successful tongue task performance, nontask-related swallowing of a liquid bolus, and nontask-related swallowing of a solid bolus) as well as a trained tongue-protrusion task. Electromyographic activity was recorded simultaneously from various orofacial and laryngeal muscles. In addition, the afferent input to each tongue-MI neuron and ICMS-evoked motor output characteristics at each neuronal recording site were determined. Neurons were considered to show swallow and/or tongue-protrusion task-related activity if a statistically significant difference in firing rate was seen in association with these behaviors compared with that observed during a control pretrial period. Of a total of 80 neurons recorded along 40 microelectrode penetrations in the ICMS-defined tongue-MI, 69% showed significant alterations of activity in relation to the swallowing of a juice reward, whereas 66% exhibited significant modulations of firing in association with performance of the trained tongue-protrusion task. Moreover, 48% showed significant alterations of firing in relation to both swallowing and the tongue-protrusion task. These findings suggest that the region of cortex involved in swallowing includes MI and that tongue-MI may play a role in the regulation of semiautomatic tongue movement, in addition to trained motor behavior. Swallow-related tongue-MI neurons exhibited a variety of swallow-related activity patterns and were distributed throughout the ICMS-defined tongue-MI at sites where ICMS evoked a variety of types of tongue movements. These findings are consistent with the view that multiple efferent zones for the production of tongue movements are activated in swallowing. Many swallow-related tongue-MI neurons had an orofacial mechanoreceptive field, particularly on the tongue dorsum, supporting the view that afferent inputs may be involved in the regulation of the swallowing synergy.

Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 microm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS.

Quantitative assessment of oral and pharyngeal function in Parkinson's disease

Oral and pharyngeal dysfunction is common in Parkinson's disease. To reveal the frequency of swallowing dysfunction and correlate swallowing dysfunction with locomotor disturbances, we studied 75 patients with Parkinson's disease staged I-IV according to the Hoehn and Yahr score. We assessed oral and pharyngeal swallow during optimal medication by a quantitative test of swallowing (the ROSS test) measuring the suction pressure, bolus volume, swallowing capacity, and time for important events in the swallowing cycle. We found abnormal results in 7/12 patients (58%) in stage 1 of the Hoehn and Yahr score, in 13/14 patients (93%) in stage 2, in 29/32 patients (91%) in stage 3, and in 16/17 patients (94%) in stage 4. Abnormal test results in stages, 1, 2, and 3 were seldom related to swallowing difficulties noticed by the patients. In advanced disease (Hoehn and Yahr stage 4), the abnormal results were often considerable, with swallowing difficulties obvious to the patient. Two of 17 patients coughed during or immediately after the test and 3/ 17 patients were unable to complete the test. The degree of swallowing disturbance increased during stress (forced, repetitive swallow). The Hoehn and Yahr score and the results in the ROSS test did not correlate, indicating that swallowing disturbances are due to nondopaminergic degeneration. Silent swallowing impairment may interfere with the nutrition and quality of life in Parkinson's disease, thus it is of interest to monitor this in clinical practice.

GABA receptor-mediated inhibition of reflex deglutition in the cat

In anesthetized cats, swallowing elicited by electrical stimulation of the superior laryngeal nerves (SLNs) was inhibited by the GABA-mimetic muscimol and by diazepam, an action that was reversed by picrotoxin and bicuculline. This inhibition supports the involvement of GABA receptors, specifically those of the GABAA subtype which both antagonists have been shown to block in various areas of the central nervous system. The inhibition of reflex swallowing and its reversal were unaltered by a transection of the brainstem at a midcollicular level. Stimulation of the SLNs also caused a bradycardia that was inhibited by both muscimol and diazepam and was restored by both GABA antagonists. Data from these experiments provide suggestive evidence for a role of GABA-ergic transmission in the central control of the deglutitory reflex.

Dysphagia in drug-induced parkinsonism: a case report

Dysphagia complicates both idiopathic Parkinson's disease (IPD) and drug-induced parkinsonism (DIP). Although parkinsonism of DIP and IPD are often clinically indistinguishable, there is no assurance that their abnormalities of swallowing will be similar. We evaluated a patient with DIP who complained of difficulty chewing and swallow initiation. The dysphagia evaluation demonstrated abnormalities during all stages of ingestion. However, the prepharyngeal stages were disproportionately affected when compared with patients with IPD and similar levels of parkinsonian functional disability. This case gives additional support for a significant basal ganglia influence on motor deglutitive functions.

Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord

Visceral feedback circuits in lower brainstem were elucidated with retrograde tracers by mapping neurons that issue local projections to the general visceral afferent division of the nucleus tractus solitarii (NTS) and dorsomotor vagal nucleus (DMX) in adult male rats. In study 1, spinal and intramedullary afferents to the visceral-sensorimotor complex (NTS-X) were traced to contiguous populations of cell bodies arranged in cylindrical segmental organization. NTS-X afferents derive from curvilinear arrays of neurons that parallel the efferent radiations of the solitariotegmental tract. Newly discovered afferents arise from circumscribed cell groups in the dorsal reticular formation and periventricular zone. Another source was traced to a paraambigual cell column in the apex of the rostral ventrolateral reticular nucleus (n.RVL). In study 2, catecholaminergic afferents were initially defined with combined retrograde transport-immunocytochemical methods. Deposits of retrograde tracers into NTS-X transported to neurons containing tyrosine hydroxylase (TH) in the A1, C1, and C3 areas or phenylethanolamine N-methyltransferase (PNMT) in the C1 area of the n.RVL and C3 area. In study 3, it was revealed that NTS-X afferents arise, in part, as collaterals of thoracic reticulospinal neurons. Deposits of the retrograde fluorescent tracer Fluorogold into the upper thoracic cord and rhodamine-labeled microbeads into NTS-X transported to the same neurons within a subambigual locus in n.RVL and parts of nucleus raphe magnus. In study 4, dual retrograde tracer-immunocytochemical analysis demonstrated that catecholamines are synthesized by a subset of neurons in the n.RVL that issue collaterals to the NTS-X and thoracic cord. Double retrogradely labeled TH- or PNMT-immunoreactive cell bodies were restricted to the C1 area within a 450-microns column bordered rostrally by the facial nucleus and ventrally by the medullary subpial surface. We conclude that visceral reflex arcs are reciprocally organized. Targets of NTS projection are also sources of local NTS-X afferent innervation. Catecholaminergic and other local afferents from reticular formation, periventricular, and spinal gray may, via collaterals, simultaneously modulate visceral reflex excitability at the level of NTS and the outflow of autonomic and respiratory motoneurons.

Projections from the nucleus tractus solitarii to the spinal cord

Projections from the nucleus tractus solitarii (NTS) to the spinal cord were demonstrated in the male Sprague-Dawley rat. In retrograde transport studies, a horseradish peroxidase conjugate or a fluorescent dye, FluoroGold, were injected into midcervical or upper thoracic spinal segments. Most solitariospinal neurons were multipolar or bipolar and located between the obex and spinomedullary junction. Solitariospinal neurons were concentrated in proximity to the ventral border of the solitary tract and extended dorsally into the intermediate division and ventrolaterally into the intermediate reticular zone (IRt) of the lateral tegmental field. This subgroup predominantly projects to midcervical spinal segments. A subset of small neurons was retrogradely labeled from cervical or thoracic spinal segments in the medial commissural nucleus and contiguous with a periventricular group surrounding the central canal. In anterograde transport studies, iontophoretic deposits of Phaseolus vulgaris leucoagglutinin were centered stereotaxically on sites in NTS identified by retrograde transport data. The lectin was incorporated by neurons of the solitary complex and transported bilaterally by axons that emerged from the nucleus and entered the reticular formation. The solitario-reticular (transtegmental) pathway irradiated diagonally across the IRt and extended caudally into the cervical lateral funiculus and spinal gray. A small periventricular-spinal pathway also descended longitudinally to the neuraxis. Solitariospinal neurons project to superficial lamina of the dorsal horn, laminae VII and X and ventral horn. The projections are predominantly contralateral to phrenic and intercostal motor nuclei and ipsilateral to the intermediolateral cell column. The solitariospinal projection represents the shortest route in the central nervous system, other than the local intraspinal reflex, through which first order visceral afferents signal cardiorespiratory and alimentary motor nuclei.

The search for the central swallowing pathway: the quest for clarity

As the work of Dr. Martin Donner has brought a clarity to understanding swallowing, so has the work of various neuroscientists, including that of a Nobel Laureate, in providing us with a better comprehension of this complex motor pattern. Understanding the neural control of swallowing has been a process that has occurred during this century in which several investigators, primarily from Europe, Japan, Canada, and the United States, have brought their perspectives in applying particular techniques to decipher how the central and peripheral nervous system control swallowing. Swallowing represents a complex muscular response of the oral, pharyngeal, and esophageal regions which are integrated to provide an effective functional pattern that prepares and transports food while simultaneously protecting the airway. This adaptation of the upper gastrointestinal tract in mammals has been extensively studied peripherally by two methods: recording from the peripheral nerves and muscles, and stimulating peripheral nerves and their receptive fields that can induce the pharyngeal and esophageal phases of swallowing. The study of the peripheral nervous system has provided insight into the sensory receptive fields that evoke or facilitate swallowing, and has established the first serious evidence of the all-or-none sequential contraction pattern of the oropharyngeal and esophageal muscles. It has been these electromyographic studies of the muscles that has established much of the criteria for evaluating the central swallowing pathway. Five techniques have been applied to the central nervous system to study swallowing and include lesioning or destroying discrete regions to determine how swallowing is impaired or modified, electrically stimulating the central neural tissue to determine the type of effects on swallowing, recording from the central neural tissue with macro- and microelectrodes to ascertain when neurons respond in timing to the peripheral muscle activity during swallowing, applying pharmacological agents through micropipettes which could mimic or inhibit potential transmitters, and using immunochemical techniques to tag specific chemicals that could be transmitters used by the neurons in the central swallowing pathway. These various techniques have provided insight into how the central swallowing pathway is organized but the details of the central control are still in the process of being defined and will require as much effort this next century as has been previously developed over the past 90 years.

The role of the cerebral cortex in swallowing

This paper reviews clinical, neuroanatomical, and neurophysiological studies that have implicated the cerebral cortex in the initiation and/or regulation of swallowing as well as related functions such as mastication. Cortical dysfunction has been reported to result in a variety of swallowing impairments. Furthermore, swallowing can be evoked and/or modulated by stimulation applied to restricted regions of the cortex. Neuroanatomical investigations and single neuron recording studies also provide some insights into the cortical structures, pathways, and mechanisms that may mediate deglutition.

Velar activity and timing of eustachian tube function in swallowing

Velar motion for dry and liquid swallows was investigated. as well as velar activity in speech, based on X-ray microbeam pellet tracking data. Electromyographic recordings for tensor and levator veli palatini were obtained simultaneously. Velar pellet trajectories for swallowing were more complex than for speech, since there was a high-velocity anterior component in swallowing. For some swallows this anterior component was integrated with velar elevation (especially in liquid swallows), but in other cases initial velar elevation occurred considerably earlier (chiefly in dry swallows). The burst of tensor and levator veli palatini activity characteristic of swallowing was associated with the anterior component of velar pellet motion, but not consistently with velar elevation per se. The conventional view on timing of tensor veli palatini contraction in a swallow, which governs Eustachian tube opening, is that this is associated with velar closure. The X-ray microbeam data suggest rather that Eustachian tube ventilation is more closely associated in time to the onset of pharyngeal peristalsis, which may or may not coincide with initial velar elevation.

Dysphagia in Huntington's disease: a 16-year retrospective

Degenerative diseases of the basal ganglia are commonly complicated by dysphagia. In 35 patients with Huntington's disease (HD), a hereditary neurodegenerative basal ganglia disease characterized by chorea, dementia, and emotional changes, an extensive battery of clinical and radiologic procedures helped to identify numerous abnormalities of deglutition. The results permitted the classification of our patients with HD into hyperkinetic (HD-h) or rigid-bradykinetic (HD-rb) groups. Although the two groups share multiple abnormalities, statistically significant intergroup differences were observed. Clinical assessment of the HD-h cohort (30 patients) demonstrated rapid lingual chorea, swallow incoordination, repetitive swallows, prolonged laryngeal elevation, inability to stop respiration, and frequent eructations. In the HD-rb group (five patients), frequently observed abnormalities included mandibular rigidity, slow lingual chorea, coughing on foods, and choking on liquids. Videofluoroscopic swallowing studies (VFSS) using a variety of barium-impregnated foods and liquids confirmed the abnormalities noted on the clinical assessment. Respiratory and laryngeal chorea, pharyngeal space retention, and aspiration were also identified. Numerous compensatory techniques introduced during videofluoroscopy benefited all patients.

Neuropharmacologic correlates of deglutition: lessons from fictive swallowing

Pharmacologic investigations into the transmission processes underlying fictive swallowing in the rat have disclosed the potential diversity of chemical signals used in central deglutitive pathways. Monoaminergic mechanisms appear to serve as links between subcortical structures and the medullary pattern generator of swallowing (PGS), and may play a critical role in maintaining internal facilitatory drive, required by the PGS for optimal responsivity to peripheral sensory input. Cholinergic bulbar interneurons form an integral component of the PGS subnetwork controlling esophageal peristalsis. Local GABA neurons exert a tonic inhibition of the buccopharyngeal stage, may regulate buccopharyngeal-esophageal coupling, and may contribute to peristaltic rhythmic generation at both the premotoneuronal and motoneuronal level. Receptor subtypes for excitatory amino acids (glutamate, aspartate) are differentially associated with deglutitive premotoneurons for both the buccopharyngeal and esophageal stage, as well as with ambiguus motoneurons. Preliminary evidence suggests the existence of excitatory peptidergic mechanisms involving thyrotropin-releasing hormone, vasopressin, oxytocin, and somatostatin, a probable candidate for excitatory transmitter in the solitarioambigual internuncial projection to motoneurons innervating esophageal striated musculature. Further validation of this experimental model may ultimately help to establish a framework for the clinical recognition, management, and exploitation of drug actions on central deglutitive neuroeffectors.