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 (NA(max)), which was proportional to the instantaneous pressure. The duration of this first episode (δt) was inversely proportional to the perfusion rate. After infusion of a fixed volume, the urethra opened and the NA decreased with a time constant φ^-1 (~1.8 s) to an elevated level (NA(level)). NA(level) 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 NA(max), NA(level), and δt showed very good reproducibility (SD ~19% of mean). The measured activity was most likely afferent and conducted to the major pelvic ganglion.