Characteristics of vagal esophageal tension-sensitive afferent fibers in the opossum.

1. Single-unit vagal afferent activity was recorded from 35 fibers that demonstrated evoked response to distension in the smooth muscle portion of the esophagus in anesthetized opossums. 2. The conduction velocities, measured in 22 fibers, varied from 1.0 to 21.33 m/s. Eight fibers (36%) had conduction velocities in the range of C-fibers (less than 2.5 m/s), whereas 14 (64%) had velocities in the range of A delta-fibers (3.16-21.33 m/s). All fibers were spontaneously active with an average discharge rate of 7.3 +/- 1.0 imp/s (mean +/- SE; range, 1.2-23 imp/s). 3. Esophageal distension produced a reproducible increase in discharge rate that adapted slowly to sustained distension. The average threshold pressure of the endings was 10 mmHg. The saturation pressure was 70 mmHg with cumulative, stepwise distension and 56 mmHg with graded, discrete distensions, respectively. 4. The discharge rate at the saturation pressure was 46 +/- 7 imp/s with cumulative, stepwise distensions and was 59 +/- 4 imp/s with graded, discrete distensions. The difference in maximum discharge between these two modes of distension was not statistically significant (P greater than 0.05). 5. Esophageal peristaltic contraction was associated with bursts of spike discharge with an average rate of 53.6 +/- 4.7 imp/s. 6. Two types of fibers were identified based on their duration of spike discharge associated with peristaltic contraction. The short-activity fibers showed a short duration of response consisting of approximately 3 s of spike bursts in response to swallows. These short-activity fibers were not activated by either stretch or contraction of the longitudinal esophageal muscle. The long-activity fibers showed a long duration of response consisting of approximately 10 s of activity in response to swallows. These long-activity fibers could be activated by longitudinal muscle stretch or contraction. 7. It is concluded that esophageal tension-sensitive mechano-receptors associated with vagal afferents are activated by physiological peristalsis and are present "in series" with either circular or longitudinal muscle layers.