MECHANOSENSITIVE PROPERTIES OF PELVIC NERVE AFFERENT-FIBERS INNERVATING THE URINARY-BLADDER OF THE RAT

1. Single-unit activity was recorded from S-1 dorsal root afferent fib ers in pentobarbital-anesthetized rats. In 25 experiments, 245 afferen t fibers were identified by electrical stimulation of the pelvic nerve . Fifty-two percent were C fibers (conduction velocity: 1.70 +/- 0.04 m/s; mean +/- SE) and 48% were A delta-fibers (conduction velocity: 11 .07 +/- 0.95 m/s). 2. Of 245 pelvic nerve afferent fibers, 92 (38%) re sponded to noxious urinary bladder distension (UBD; 80 mmHg); 57 of th ese fibers were C fibers and 35 were A delta-fibers. Forty-four fibers responded to colorectal distension (CRD; 80 mmHg); 32 were C fibers a nd 12 were A delta-fibers. A total of 39 fibers were identified in the anal mucosa; 3 were C fibers and 36 were A delta-fibers. Seventy fibe rs (28%) in these experiments were unresponsive to either UBD or CRD o r to probing of the anal mucosa; 32 were unmyelinated C fibers and 38 were A delta-fibers. 3. Reproducibility of responses to repeated UBD l ess than or equal to 80 mmHg, 20 s; 8 trials at 4-min intervals) was t ested in 10 fibers. In nine fibers, responses to repeated distension d id not change; one fiber exhibited a progressive decrease in response magnitude after the third trial. 4. Of the 92 afferent fibers that res ponded to UBD, 45 were further characterized for responses to graded i ntensities of UBD. Forty fibers had some resting activity (1.7 +/- 0.3 impulses/s) and five fibers exhibited no ongoing activity. The respon se to UBD adapted slowly during the 20-s period of phasic UBD or durin g slow isotonic filling of the bladder. 5. The stimulus-response funct ion (SRF) of fibers (n = 45) to graded UBD was monotonic (80 mmHg. Thr esholds for responses were determined after extrapolation of the least -squares linear regression line to the ordinate, and varied between 0 and 45 mmHg. The frequency distribution profile of thresholds revealed two populations of pelvic nerve afferent fibers in the urinary bladde r: a larger group (n = 36) of low-threshold (LT) fibers (5.7 +/- 1.0 m mHg) and a smaller group (n = 9) of high-threshold (HT) fibers (34 +/- 2.5 mmHg). 6. Responses of four LT fibers to graded UBD were tested b efore and 30 min after instillation of 0.5 ml of 0.5% acetic acid (pH 3) into the bladder. The mean threshold for response of these fibers b efore instillation of acetic acid (9.4 +/- 3.1 mmHg) more than doubled (to 22.3 +/- 6.7 mmHg) after instillation of acetic acid. Acetic acid also significantly reduced the maximum response magnitude of the fibe rs to UBD and reduced the slope of the SRF. 7. The present study chara cterized responses of mechanosensitive pelvic nerve afferent fibers in the S. dorsal root innervating the urinary bladder of the rat. The re sults document the existence of a larger population (80%) of LT fibers and a smaller population (20%) of HT fibers. There was no correlation between conduction velocity and response threshold; approximately one -half of the sample of HT fibers were A delta-fibers and one-half were C fibers. The fibers studied responded reproducibly to repeated UBD, encoded the intensity of UBD, and were desensitized after instillation of acetic acid into the urinary bladder.