Neurophysiological modeling of voiding in rats: urethral nerve response to urethral pressure and flow.

In male urethan-anesthetized rats, activity was measured in nerves that run over the proximal urethra. The urethral nerve response to stepwise urethral perfusion could be described by a four-parameter model (fit error < 6%). At the onset of perfusion, the urethra was closed and the pressure increased with the infused volume. The nerve activity (NA) increased linearly with this inserted volume to a maximum (NAmax), which was proportional to the instantaneous pressure. The duration of this first episode (delta t) was inversely proportional to the perfusion rate. After infusion of a fixed volume, the urethra opened and the NA decreased with a time constant phi -1 (approximately 1.8 s) to an elevated level (NAlevel). NAlevel was linearly related to the steady-state pressure. Accordingly, sensors in the urethra are sensitive to pressure rather than to the perfusion rate. The parameters NAmax, NAlevel, and delta t showed very good reproducibility (SD approximately 19% of mean). The measured activity was most likely afferent and conducted to the major pelvic ganglion.