Intracellular recording from posterior cricoarytenoid motoneurons in the rat

Laryngeal and swallow dysregulation following acute cervical spinal cord injury

Laryngeal function is vital to airway protection. While swallow is mediated by the brainstem, mechanisms underlying increased risk of dysphagia after cervical spinal cord injury (SCI) are unknown. We hypothesized that loss of descending phrenic drive affects swallow and breathing differently, and loss of ascending spinal afferent information alters swallow regulation. We recorded electromyograms from upper airway and chest wall muscles in freely breathing pentobarbital-anesthetized cats and rats. Inspiratory laryngeal activity increased ~two-fold following C2 lateral-hemisection. Ipsilateral to the injury, crural diaphragm EMG amplitude was reduced during breathing (62 ± 25% change post-injury), but no animal had complete termination of activity; 75% of animals increased contralateral diaphragm recruitment, but this did not reach significance. During swallow, laryngeal adductor and pharyngeal constrictor muscles increased activity, and diaphragm activity was bilaterally suppressed. This was unexpected because of the ipsilateral-specific response during breathing. Swallow-breathing coordination was also disrupted and more swallows occurred during early expiration. Finally, to determine if the chest wall is a major source of feedback for laryngeal regulation, we performed T1 total transections in rats. As in the C2 lateral-hemisection, inspiratory laryngeal recruitment was the first feature noted. In contrast to the C2 lateral-hemisection, diaphragmatic drive increased after T1 transection. Overall, we found that SCI alters laryngeal drive during swallow and breathing, and reduced swallow-related diaphragm activity. Our results show behavior-specific effects, suggesting SCI affects swallow more than breathing, and emphasizes the need for additional studies on the effects of ascending afferents from the spinal cord on laryngeal function.

Respiration-Related Laryngeal Electromyography in Children with Bilateral Vocal Fold Paralysis

We present 2 case reports to demonstrate the relationship between laryngeal muscle activity and respiration in children with bilateral vocal fold paralysis (BVFP) by simultaneous laryngeal electromyography (EMG) with recording of chest wall movement and intercostal muscle EMG. Laryngeal EMG was performed together with recording of chest wall movement in a 55-day-old girl who was undergoing tracheostomy for idiopathic congenital BVFP. Normal phasic activity was observed, ie, the thyroarytenoid (TA) muscle was active during expiration and the posterior cricoarytenoid (PCA) muscle during inspiration, suggesting a good prognosis for recovery. The child was decannulated at 11 months. Laryngeal EMG together with recording of chest wall movement and intercostal EMG in a 5-year-old girl who was tracheostomy-dependent following tracheoesophageal fistula repair due to BVFP showed phasic activity during expiration for both the TA and PCA muscles, indicating aberrant regeneration of the PCA motor nerve. The timing of laryngeal muscle activity with respiration in the assessment of pediatric congenital BVFP is essential to demonstrate the presence of normal or abnormal medullary respiratory neuronal input to laryngeal motoneurons. In cases in which BVFP is due to recurrent laryngeal nerve injury, respiration-related laryngeal EMG will identify aberrant regeneration. Laryngeal EMG should be combined with intercostal muscle EMG in the evaluation of children with significant vocal fold dysfunction of either central or peripheral origin.

Cholinergic inputs to laryngeal motoneurons functionally identified in vivo in rat: a combined electrophysiological and microscopic study

The intrinsic laryngeal muscles are differentially modulated during respiration as well as other states and behaviors such as hypocapnia and sleep. Previous anatomical and pharmacological studies indicate a role for acetylcholine at the level of the nucleus ambiguus in the modulation of laryngeal motoneuron (LMN) activity. The present study investigated the anatomical nature of cholinergic input to inspiratory- (ILM) and expiratory-modulated (ELM) laryngeal motoneurons in the loose formation of the nucleus ambiguus. Using combined in vivo intracellular recording, dye filling, and immunohistochemistry, we demonstrate that LMNs identified in Sprague-Dawley rat receive several close appositions from vesicular acetylcholine transporter-immunoreactive (VAChT-ir) boutons. ELMs receive a significantly greater number of close appositions (mean ± standard deviation [SD]: 47 ± 11; n = 5) than ILMs (32 ± 9; n = 8; t-test P < 0.05). For both LMN types, more close appositions were observed on the cell soma and proximal dendrites compared to distal dendrites (two-way analysis of variance [ANOVA], P < 0.0001). Using fluorescence confocal microscopy, almost 90% of VAChT-ir close appositions (n = 45 boutons on n = 4 ELMs) were colocalized with the synaptic marker synaptophysin. These results support a strong influence of cholinergic input on LMNs and may have implications in the differential modulation of laryngeal muscle activity.

Respiration-Related Laryngeal Electromyography in Children with Bilateral Vocal Fold Paralysis

We present 2 case reports to demonstrate the relationship between laryngeal muscle activity and respiration in children with bilateral vocal fold paralysis (BVFP) by simultaneous laryngeal electromyography (EMG) with recording of chest wall movement and intercostal muscle EMG. Laryngeal EMG was performed together with recording of chest wall movement in a 55-day-old girl who was undergoing tracheostomy for idiopathic congenital BVFP. Normal phasic activity was observed, ie, the thyroarytenoid (TA) muscle was active during expiration and the posterior cricoarytenoid (PCA) muscle during inspiration, suggesting a good prognosis for recovery. The child was decannulated at 11 months. Laryngeal EMG together with recording of chest wall movement and intercostal EMG in a 5-year-old girl who was tracheostomy-dependent following tracheoesophageal fistula repair due to BVFP showed phasic activity during expiration for both the TA and PCA muscles, indicating aberrant regeneration of the PCA motor nerve. The timing of laryngeal muscle activity with respiration in the assessment of pediatric congenital BVFP is essential to demonstrate the presence of normal or abnormal medullary respiratory neuronal input to laryngeal motoneurons. In cases in which BVFP is due to recurrent laryngeal nerve injury, respiration-related laryngeal EMG will identify aberrant regeneration. Laryngeal EMG should be combined with intercostal muscle EMG in the evaluation of children with significant vocal fold dysfunction of either central or peripheral origin.

Differential regulation of the central neural cardiorespiratory system by metabotropic neurotransmitters

Central neurons in the brainstem and spinal cord are essential for the maintenance of sympathetic tone, the integration of responses to the activation of reflexes and central commands, and the generation of an appropriate respiratory motor output. Here, we will discuss work that aims to understand the role that metabotropic neurotransmitter systems play in central cardiorespiratory mechanisms. It is well known that blockade of glutamatergic, gamma-aminobutyric acidergic and glycinergic pathways causes major or even complete disruption of cardiorespiratory systems, whereas antagonism of other neurotransmitter systems barely affects circulation or ventilation. Despite the lack of an 'all-or-none' role for metabotropic neurotransmitters, they are nevertheless significant in modulating the effects of central command and peripheral adaptive reflexes. Finally, we propose that a likely explanation for the plethora of neurotransmitters and their receptors on cardiorespiratory neurons is to enable differential regulation of outputs in response to reflex inputs, while at the same time maintaining a tonic level of sympathetic activity that supports those organs that significantly autoregulate their blood supply, such as the heart, brain, retina and kidney. Such an explanation of the data now available enables the generation of many new testable hypotheses.

Response of laryngeal motoneurons to hyperventilation induced apnea in the rat

Central apnea is common in the premature newborn. To explain the upper airway findings in different clinical conditions characterized by central apnea, we made single unit recordings from laryngeal motoneurons during normal and hyperventilation. Posterior cricoarytenoid (n = 4) and cricothyroid (n = 4) motoneurons displayed an inspiratory pattern during normal ventilation and remained synchronous with phrenic nerve discharge (PND) during hyperventilation. Laryngeal constrictor motoneurons (LCon) displayed a post-inspiratory pattern during normal ventilation, but lost phasic activity during early hyperventilation (the period after the onset of hyperventilation but before cessation of PND; n = 12). There was a nearly linear relationship between the post-inspiratory activity and strength of PND. Six LCon motoneurons remained silent throughout hyperventilation, while the other six developed a tonic activity during cessation of PND. Further analysis suggested that the silent and tonic LCon motoneurons are likely to share a similar mechanism in their post-inspiratory pattern generation, but differ from each other in their responses to CO2 inputs. In addition, strong inhibition of the LCon tonic activity by the early return of PND could be an important factor in recovery following a period of apnea. Failure of this inspiratory inhibition to occur might explain certain clinical situations, where obstructive apnea supervenes following a period of central apnea.

Substance P inputs to laryngeal motoneurons in the rat

Substance P terminals have previously been demonstrated around retrogradely labelled posterior cricoarytenoid (PCA) motoneurons, but little is known regarding substance P inputs to other functionally identified laryngeal motoneurons. In the present study, we determined the number and distribution of close appositions between substance P immunoreactive boutons and three types of laryngeal motoneuron by using a combination of intracellular recording, dye-filling and immunocytochemistry in the rat. Cricothyroid (CT) motoneurons received 15+/-5 substance P appositions/neuron (mean+/-S.D., n = 6), PCA motoneurons received 13+/-5 (n = 6), and laryngeal constrictor (LCS) motoneurons received 11+/-4 (n = 5). In contrast to our previous finding of a preferential serotonin innervation of CT motoneurons, we found no significant difference between the substance P inputs to CT, PCA and LCS motoneurons. Our results indicate a modest role for substance P in control of laryngeal motoneuronal function.

Serotonin inputs to inspiratory laryngeal motoneurons in the rat

Serotonergic neurons are distributed widely throughout the central nervous system and exert a tonic influence on a range of activities in relation to the sleep-wake cycle. Previous morphologic and functional studies have indicated a role for serotonin in control of laryngeal motoneurons. In the present study, we used a combination of intracellular recording, dye-filling, and immunocytochemistry in rats to demonstrate close appositions between serotonin immunoreactive boutons and posterior cricoarytenoid (PCA) and cricothyroid (CT) motoneurons, both of which are located in the nucleus ambiguus and exhibit phasic inspiratory activity. PCA motoneurons received 29 +/- 5 close appositions/neuron (mean +/- SD, n = 6), with the close appositions distributed more frequently on the distal dendrites, less frequently on the proximal dendrites, and sparsely on the axons and somata. CT motoneurons received 56 +/- 15 (n = 6), with close appositions found on both the somata and dendrites, especially proximal dendrites. Close appositions on the axons were only seen on one CT motoneuron. These results demonstrate a significant serotonin input to inspiratory laryngeal motoneurons, which is more prominent on CT compared with PCA motoneurons, and may reflect the different functional role of the muscles that they innervate during the sleep-wake cycle.

Neuromuscular disorders presenting as congenital bilateral vocal cord paralysis

Congenital bilateral vocal cord paralysis (BVCP) can be associated with an underlying neuromuscular disorder, and may present before other features of the neuromuscular disorder become apparent. All infants less than 12 months of age presenting with BVCP between July 1987 and July 1999 at the Royal Children's Hospital, Melbourne, in whom a neuromuscular disorder was subsequently diagnosed were followed. Three children in whom BVCP was diagnosed soon after birth and before recognition of an underlying neuromuscular disorder were identified. All presented with upper airway obstructive symptoms at birth, had a diagnosis of bilateral abductor vocal cord paralysis made at awake flexible laryngoscopy, and had no underlying structural laryngeal abnormality on microlaryngoscopy and bronchoscopy. Two children required a tracheostomy, and 1 child was weaned from nasopharyngeal continuous positive airway pressure after 3 weeks. Subsequent neuromuscular symptoms were recognized between 4 months and 7 years later, leading to diagnoses of facioscapulohumeral myopathy, spinal muscular atrophy, and congenital myasthenia gravis. In each case, the prognosis for recovery from symptoms related to BVCP reflected that of the underlying neuromuscular disorder. This experience suggests that congenital BVCP may be a feature of an unrecognized neuromuscular condition. This possibility should be considered particularly in the presence of associated neurodevelopmental or neuromuscular dysfunction, or in cases in which BVCP is progressive.