Vagal afferent input determines the volume dependence of rat esophageal motility patterns.

The volume dependence of balloon distension-evoked esophageal rhythmic motor responses and their neural correlates was investigated in 72 urethane-anesthetized rats. With increasing balloon volume (75--200 microl), distal esophageal rhythmic contractions decreased in rate and became tonic in the range of 150--250 microl. This change in motor pattern involved only the striated musculature of the esophageal body and persisted after acute transection of the spinal cord at C(2). Impulse frequency in single vagal afferents of the distal esophagus increased with intraluminal pressure over the entire range of balloon volumes tested (50--300 microl). Distension-responsive neurons in the nucleus tractus solitarii showed rhythmic burst activity (type I), tonic excitation (type II), or inhibition followed by off bursts (type III). Increasing strength of stimulation changed type I responses to nonrhythmic but intensified type II and III responses. We conclude that load-dependent changes in distal esophagus motility pattern are encoded by vagal afferents alone and do not involve a spinal afferent input even at near-noxious stimulus strengths.