Threshold for efferent bladder nerve firing in the rat

In this study, the mechanism involved in the initiation of voiding was investigated. Bladder pressure and bladder and urethral nerve activity were recorded in the anesthetized rat. Bladder nerve activity was resolved into afferent and efferent activity by means of a theoretical model. The beginning of an active bladder contraction was defined as the onset of bladder efferent firing at a certain time (t₀). From t₀ onward, bladder efferent activity increased linearly during δt seconds (rise time) to a maximum. The pressure at to was 1.0 ± 0.4 kPa, the afferent nerve activity at to was 2.0 ± 0.6 μV (53 ± 15% of maximum total nerve activity), and δt was 11 ± 13 s. Between contractions the afferent activity at t₀ was never exceeded. Urethral afferent nerve activity started at bladder pressures of 2.1 ± 1.1 kPa. Therefore, we concluded that urethral afferent nerve activity does not play a role in the initiation of bladder contractions; voiding contractions pre sumably are initiated by bladder afferent nerve activity exceeding a certain threshold.