Sensory fibers of the pelvic nerve innervating the rat's urinary bladder

Much attention has been given to the pelvic nerve afferent innervation of t he urinary bladder; however, reports differ considerably in descriptions of afferent receptor types, their conduction velocities, and their potential roles in bladder reflexes and sensation. The present study was undertaken t o do a relatively unbiased sampling of bladder afferent fibers of the pelvi c nerve in adult female rats. The search stimulus for units to be studied w as electrical stimulation of both the bladder nerves and the pelvic nerve. Single-unit activity of 100 L-6 dorsal root fibers, activated by both pelvi c and bladder nerve stimulation, was analyzed. Sixty-five units had C-fiber and 35 units had A delta-fiber conduction velocities. Receptive characteri stics were established by direct mechanical stimulation, filling of the bla dder with 0.9% NaCl at a physiological speed and by filling the bladder wit h solutions containing capsaicin, potassium, or turpentine oil. The majorit y (61) of these fibers were unambiguously excited by bladder filling with 0 .9% NaCl and were classified as mechanoreceptors. All mechanoreceptors with receptive fields on the body of the bladder had low pressure thresholds (l ess than or equal to 10 mmHg). Receptive fields of units with higher thresh olds were near the ureterovesical junction, on the base of the bladder or c ould not be found. Neither thresholds nor suprathreshold responses could be related to conduction velocity. Bladder compliance and mechanoreceptor thr esholds were influenced by the stage of the estrous cycle: both were lowest in proestrous rats and highest in metaestrous rats. Mechanoreceptors inner vating the body of the bladder and the region near the ureterovesical junct ion showed two patterns of responsiveness to slow bladder filling. One grou p of units exhibited increasing activity with increasing pressure up to 40 mmHg, while the other group showed a peak in activity at pressures below 40 mmHg followed by a plateau or decrease in activity with increasing pressur e. It is proposed that differences in stimulus transduction relate to the d ifferent response patterns. Thirty-nine units failed to respond to bladder filling. Eight of these were excited by intravesical potassium or capsaicin and were classified as chemoreceptors. The remaining 31 units were not exc ited by any stimulus tested. Chemoreceptors and unexcited units had both A delta and C afferent fibers. We conclude that the pelvic nerve sensory inne rvation of the rat bladder is complex, may be sensitive to hormonal status, and that the properties of individual sensory receptors are not related in an obvious manner to the conduction velocity of their fibers.