Abstract
Afferent feedback can appreciably alter the pharyngeal phase of swallow. In order to measure the stability of the swallow motor pattern during several types of alterations in afferent feedback, we assessed swallow during a conventional water challenge in four anesthetized cats, and compared that to swallows induced by fixed (20 Hz) and stochastic (1-20Hz) electrical stimulation applied to the superior laryngeal nerve. The swallow motor patterns were evaluated by electromyographic activity (EMG) of eight muscles, based on their functional significance: laryngeal elevators (mylohyoid, geniohyoid, and thyrohyoid); laryngeal adductor (thyroarytenoid); inferior pharyngeal constrictor (thyropharyngeus); upper esophageal sphincter (cricopharyngeus); and inspiratory activity (parasternal and costal diaphragm). Both the fixed and stochastic electrical stimulation paradigms increased activity of the laryngeal elevators, produced short-term facilitation evidenced by increasing swallow durations over the stimulus period, and conversely inhibited swallow-related diaphragm activity. Both the fixed and stochastic stimulus conditions also increased specific EMG amplitudes, which never occurred with the water challenges. Stochastic stimulation increased swallow excitability, as measured by an increase in the number of swallows produced. Consistent with our previous results, changes in the swallow motor pattern for pairs of muscles were only sometimes correlated with each other. We conclude that alterations in afferent feedback produced particular variations of the swallow motor pattern. We hypothesize that specific SLN feedback might modulate the swallow central pattern generator during aberrant feeding conditions (food/liquid entering the airway), which may protect the airway and serve as potentially important clinical diagnostic indicators.
Highlights
The stochastic stimulation produced significantly more swallows compared to the fixed frequency stimulation (p = 0.005), due to a difference in time from the initiation of the stimulation to the first swallow [fixed (4.2 ± 0.9 s), stochastic (2.7 ± 0.4 sec; p = 0.01)]
There was a significant effect of condition on the EMG amplitude of mylohyoid, geniohyoid, thyrohyoid, thyropharyngeus, parasternal, and costal diaphragm (Table 1)
There was no significant effect of condition on swallow duration (Table 1 and Figure 3)
Summary
The pharyngeal phase of swallow is initiated and regulated by a host of sensory afferents in the oral, pharyngeal, and laryngeal cavities (Pommerenke, 1928; Kahrilas and Logemann, 1993; Ertekin et al, 2000; Hiss et al, 2001; Humbert et al, 2009; Pitts et al, 2013; Spearman et al, 2014; Huff et al, 2018). During normal breathing and swallow, laryngeal afferents are stimulated, producing variable sensory frequency patterns with discharge rates from 10 – 184 Hz, which are transmitted by the SLN (Storey, 1968; Bradley et al, 1983). This corresponds to stochastic-like afferent nerve firing discharge which can stimulate multiple behavior responses including apnea, swallow, and cough. When swallow is induced by delivering milk to the vallecular space of decerebrate piglets (Thexton et al, 2007, 2009), the motor pattern is modified by the presence of other rhythmic oral movements (suckling) This indicates that afferent feedback from natural stimuli can influence pharyngeal swallow motor pattern, and that this is brainstem-mediated
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