Laryngeal and swallow dysregulation following acute cervical spinal cord injury.

Laryngeal function is vital to airway protection. Although swallow is mediated by the brainstem, the mechanism underlying the increased risk of dysphagia after cervical spinal cord injury (SCI) is unknown. We hypothesized that: 1) loss of descending phrenic drive affects swallow and breathing differently, and 2) loss of ascending spinal afferent information alters swallow regulation. We recorded electromyograms (EMGs) from upper airway and chest wall muscles in freely breathing pentobarbital-anesthetized cats and rats. Laryngeal abductor activity during inspiration increased about twofold following C2 lateral hemisection. Ipsilateral to the injury, the crural diaphragm EMG amplitude was reduced during breathing (62 ± 25% change postinjury), but no animal had complete termination of activity; 75% of animals had 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 disrupted by injury, 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 after injury. 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 alters swallow-related diaphragm activity. Our results show behavior-specific effects, suggesting that swallow is affected more than breathing is by SCI, and emphasizing the need for additional studies on the effect of ascending afferents from the spinal cord on laryngeal function.NEW & NOTEWORTHY This is the first manuscript to determine the impact of cSCI on laryngeal and swallow function, and to describe a possible mechanism for dysphagia and altered airway protection after injury.

Rapid activation of esophageal mechanoreceptors alters the pharyngeal phase of swallow: Evidence for inspiratory activity during swallow.

Swallow is a complex behavior that consists of three coordinated phases: oral, pharyngeal, and esophageal. Esophageal distension (EDist) has been shown to elicit pharyngeal swallow, but the physiologic characteristics of EDist-induced pharyngeal swallow have not been specifically described. We examined the effect of rapid EDist on oropharyngeal swallow, with and without an oral water stimulus, in spontaneously breathing, sodium pentobarbital anesthetized cats (n = 5). Electromyograms (EMGs) of activity of 8 muscles were used to evaluate swallow: mylohyoid (MyHy), geniohyoid (GeHy), thyrohyoid (ThHy), thyropharyngeus (ThPh), thyroarytenoid (ThAr), cricopharyngeus (upper esophageal sphincter: UES), parasternal (PS), and costal diaphragm (Dia). Swallow was defined as quiescence of the UES with overlapping upper airway activity, and it was analyzed across three stimulus conditions: 1) oropharyngeal water infusion only, 2) rapid esophageal distension (EDist) only, and 3) combined stimuli. Results show a significant effect of stimulus condition on swallow EMG amplitude of the mylohyoid, geniohyoid, thyroarytenoid, diaphragm, and UES muscles. Collectively, we found that, compared to rapid cervical esophageal distension alone, the stimulus condition of rapid distension combined with water infusion is correlated with increased laryngeal adductor and diaphragm swallow-related EMG activity (schluckatmung), and post-swallow UES recruitment. We hypothesize that these effects of upper esophageal distension activate the brainstem swallow network, and function to protect the airway through initiation and/or modulation of a pharyngeal swallow response.