Bladder response to urethral flow in the awake ewe

The flowing of fluid along the urethra can facilitate the bladder micturition contraction, a reflex appropriate to achieve complete bladder emptying. The response has been observed in animals studied in the decerebrate, spinal, or anesthetized intact state but is difficult to demonstrate in awake, healthy humans. The aim of this study was to investigate this reflex in the awake, intact state using urodynamics in the ewe. The animals were fully awake and lay gently restrained on a recording table. The bladder was catheterized, and urethral flows were obtained by slowly injecting saline at the level of the proximal urethra. Bladder and rectal pressure were monitored, and the responses to urethral flow were tested at various bladder volumes. Urethral flows consistently evoked detrusor contractions in all animals. The response started approximately 2 seconds after the beginning of the flow and largely outlasted the stimulus. Its amplitude was similar to that of the bladder-to-bladder micturition reflex, and, like the latter, it always required some degree of bladder filling to appear. The reflex was selectively suppressed by local anesthesia of the urethral mucosa, indicating that the effect must originate from urethral receptors. Small urethral flows thus appear to elicit large bladder micturition contractions in awake healthy animals, and the reflex could be quite significant during the normal micturition. That this reflex is found in all studied species further supports the notion that the negative results in normal awake humans are owing to descending inhibitory controls rather than to the lack of appropriate pathways.