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 deltat seconds (rise time) to
a maximum. The pressure at t0 was 1.0 +/- 0.4 kPa, the afferent nerve
activity at t0 was 2.0 +/- 0.6 microV (53 +/- 15% of maximum total nerve
activity), and deltat 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 presumably are
initiated by bladder afferent nerve activity exceeding a certain
threshold