Superior laryngeal nerve stimulation in the cat: effect on oropharyngeal swallowing, oesophageal motility and lower oesophageal sphincter activity.

Superior laryngeal nerve (SLN) stimulation can activate the brainstem swallowing mechanism to produce a complete swallowing sequence consisting of oropharyngeal, oesophageal and lower oesophageal sphincter (LOS) components. However, little is known of the effect of SLN stimulation (peripheral-sensory input from the pharynx) on the characteristics of oesophageal motor activity, especially in the smooth muscle portion. The present study examined the effect of varying stimulus train length and frequency on each of the three components of the reflex. Acute studies were performed in urethane anaesthetized cats. Oesophageal motility was monitored using conventional manometric techniques, and oropharyngeal swallowing by the mylohyoid electromyogram. SLN stimulus train length (1-10 sec) and frequency (5-30 Hz) were varied independently. Increased train length or frequency resulted in (1) an increase in oropharyngeal swallowing and incidence of the complete swallowing response, (2) an increase in latency to onset of the oesophageal peristaltic wave, (3) reduction of the amplitude of the evoked peristaltic contraction in the smooth muscle portion, without altering its velocity, (4) increased LOS relaxation, and increased LOS after-contraction. The LOS contraction was abolished by atropine (100 micrograms kg-1). Therefore, increased SLN stimulation not only results in excitation of the central swallowing program and the oropharyngeal stage of swallowing, but has major effects on the oesophageal and LOS stages of swallowing. Afferent SLN stimuli can impact on the control mechanisms for each stage, to inhibit or excite the stages in different ways.