Stimulus-response function studies of esophageal mechanosensitive nociceptors in sympathetic afferents of opossum.

1. Single-unit activity was recorded from afferent fibers in either the paravertebral sympathetic chain or the splanchnic nerves. Forty-three fibers that responded to distension of the smooth-muscle portion of esophagus were selected for further study. 2. Out of the 43 fibers, 27 (62%) had ongoing resting activity, and 16 (38%) were silent. The mean resting activity of these fibers was 0.28 +/- 0.06 imp/s (range, 0-2.6 imp/s). 3. Repeated distensions of the esophagus to a fixed pressure (80 mmHg) evoked spike discharge at a reproducible rate. There was no evidence of facilitation or inhibition of subsequent responses. The mean coefficient of variation (CV) was 0.17 +/- 0.05 imp/s (n = 5). 4. The stimulus-response function (SRF) to intraluminal graded distension was studied in 35 fibers. Thirty-three fibers showed linear increase in firing that did not saturate up to 120 mmHg. Two fibers reached maximal discharge rate at 60 mmHg. The mean discharge rate at 120 mmHg pressure was 14.87 +/- 1.52 imp/s. 5. Threshold pressure for activation of each fiber was calculated by a least-squares linear-regression plot of the SRF. The threshold pressure varied from 0 to 50 mmHg (mean, 16.21 +/- 2.86 mmHg). The distribution profile of the threshold values showed that there were two separate populations of mechanosensitive receptors: 1) wide-dynamic-range mechanonociceptors (WDR-MN) with mean threshold pressure of 2.89 +/- 0.75 mmHg (range, 0-7 mmHg; n = 22); and 2) high-threshold mechanonociceptors (HT-MN) with mean threshold of 33.26 +/- 2.52 mmHg (range, 19-50 mmHg; n = 13). 6. Discharge evoked by esophageal peristalsis was studied in five WDR-MN and five HT-MN units. Whereas all five WDR-MN units (threshold value, 2.6 +/- 0.96 mmHg) responded to peristalsis, none of the HT-MN units (threshold value, 31.2 +/- 4.01 mmHg) did so. The mean response to peristaltic contraction in WDR-MN units was 5.32 +/- 1.36 imp/s, which was 24.7% of the maximal response (21.53 +/- 1.92 imp/s; n = 5) at 120 mmHg. The duration of evoked response to peristaltic contraction was 10-12 s. 7. The activity profile of the units to 60-s balloon distension at near threshold and higher pressures showed three patterns: 1) rapid adaptation, 2) slow adaptation, and 3) slow adaptation with after discharge. The rapidly adapting fibers became slowly adapting with greater degrees of esophageal distension. The units that showed rapid or slow adaptation at low-distension pressure became indistinguishable from one another at high-distension pressure.(ABSTRACT TRUNCATED AT 400 WORDS)