The Internal Superior Laryngeal Nerve in Humans: Evidence for Pure Sensory Function.

OBJECTIVES: To determine if the internal branch of the superior laryngeal nerve (iSLN) provides direct motor innervation to the interarytenoid muscle, a laryngeal adductor critical for airway protection. We studied the iSLN-evoked motor response in the interarytenoid and other laryngeal muscles. If the iSLN is purely sensory, there will be no detectable short latency motor response upon supramaximal stimulation, indicating the absence of a direct efferent conduction path. STUDY DESIGN: Intraoperative case series. METHODS: In seven anesthetized patients undergoing laryngectomy for unilateral laryngeal carcinoma, the iSLN of the unaffected side was electrically stimulated intraoperatively with 0.1-ms pulses of progressive intensities until supramaximal stimulation was reached. Electromyographic responses were measured in the ipsilateral interarytenoid, thyroarytenoid, and cricothyroid muscles. RESULTS: None of the subjects exhibited short-latency interarytenoid motor responses to iSLN stimulation. Supramaximal electrical stimulation of the intact iSLN evoked ipsilateral motor responses with long latencies: 18.7-38.5 ms in the interarytenoid (n = 6) and 17.8-24.9 ms in the thyroarytenoid (n = 5). Supramaximal stimulation of the recurrent laryngeal nerve evoked ipsilateral motor responses with short latencies: 1.6-3.9 ms in the interarytenoid (n = 6) and 1.6-2.7 ms in the thyroarytenoid (n = 6). CONCLUSION: The iSLN provides no functional efferent motor innervation to the interarytenoid muscles. The iSLN exclusively evokes an interarytenoid motor response via afferent activation of central neural circuits that mediate the laryngeal reflex arc. These findings suggest that the role of the iSLN in vital laryngopharyngeal functions, such as normal swallowing and protection of the airway from aspiration, is purely sensory. LEVEL OF EVIDENCE: 4 Laryngoscope, 131:E207-E211, 2021.

Sensory regulation of swallowing and airway protection: a role for the internal superior laryngeal nerve in humans.

During swallowing, the airway is protected from aspiration of ingested material by brief closure of the larynx and cessation of breathing. Mechanoreceptors innervated by the internal branch of the superior laryngeal nerve (ISLN) are activated by swallowing, and connect to central neurones that generate swallowing, laryngeal closure and respiratory rhythm. This study was designed to evaluate the hypothesis that the ISLN afferent signal is necessary for normal deglutition and airway protection in humans. In 21 healthy adults, we recorded submental electromyograms, videofluoroscopic images of the upper airway, oronasal airflow and respiratory inductance plethysmography. In six subjects we also recorded pressures in the hypopharynx and upper oesophagus. We analysed swallows that followed a brief infusion (4-5 ml) of liquid barium onto the tongue, or a sip (1-18 ml) from a cup. In 16 subjects, the ISLN was anaesthetised by transcutaneous injection of bupivacaine into the paraglottic compartment. Saline injections using the identical procedure were performed in six subjects. Endoscopy was used to evaluate upper airway anatomy, to confirm ISLN anaesthesia, and to visualise vocal cord movement and laryngeal closure. Comparisons of swallowing and breathing were made within subjects (anaesthetic or saline injection vs. control, i.e. no injection) and between subjects (anaesthetic injection vs. saline injection). In the non-anaesthetised condition (saline injection, 174 swallows in six subjects; no injection, 522 swallows in 20 subjects), laryngeal penetration during swallowing was rare (1.4 %) and tracheal aspiration was never observed. During ISLN anaesthesia (16 subjects, 396 swallows), all subjects experienced effortful swallowing and an illusory globus sensation in the throat, and 15 subjects exhibited penetration of fluid into the larynx during swallowing. The incidence of laryngeal penetration in the anaesthetised condition was 43 % (P < 0.01, compared with either saline or no injection) and of these penetrations, 56 % led to tracheal aspiration (without adverse effects). We further analysed the swallow cycle to evaluate the mechanism(s) by which fluid entered the larynx. Laryngeal penetration was not caused by premature spillage of oral fluid into the hypopharynx, delayed clearance of fluid from the hypopharynx, or excessive hypopharyngeal pressure generated by swallowing. Furthermore, there was no impairment in the ability of swallowing to halt respiratory airflow during the period of pharyngeal bolus flow. Rather, our observations suggest that loss of airway protection was due to incomplete closure of the larynx during the pharyngeal phase of swallowing. In contrast to the insufficient closure during swallowing, laryngeal closure was robust during voluntary challenges with the Valsalva, Müller and cough manoeuvres under ISLN anaesthesia. We suggest that an afferent signal arising from the ISLN receptor field is necessary for normal deglutition, especially for providing feedback to central neural circuits that facilitate laryngeal closure during swallowing. The ISLN afferent signal is not essential for initiating and sequencing the swallow cycle, for co-ordinating swallowing with breathing, or for closing the larynx during voluntary manoeuvres.