Bladder motility and efferent nerve activity during isotonic and isovolumic recording in the cat

1. Bladder motility and pelvic nerve efferent activity were recorded in anaesthetized cats under isotonic or isovolumic conditions. In isotonic conditions the bladder was connected to a reservoir and fluid entering or leaving the bladder was monitored for 8 min using different heads of pressure (isotonic recording). Under isovolumic conditions fixed volumes were injected into the bladder and pressure changes recorded (micturition contractions). 2. In isotonic conditions, at low pressures the bladder filled to a roughly constant level, and then developed small amplitude oscillatory contractions. At higher pressures, after the initial filling, the bladder contracted, expelling part of its volume. Depending on the pressure this contraction was either sustained or led to a gradual expansion of the bladder. In both cases transient periods of expansion were superimposed. In contrast after an initial slow build up, the efferent nerve activity increased reaching a sustained level of activity during which transient decreases in rate were seen. 3. The firing frequency of the efferent nerve activity during the sustained phase increased as the intravesical pressure was raised until a plateau was reached at high pressures. The plateau pressure (Pplateau) in a bladder was positively correlated with the peak pressure reached by the bladder during micturition contractions under isovolumic conditions, suggesting that micturition contractions were produced by the maximum output of the reflex pathway. 4. Section of the hypogastric nerves had relatively little effect, but additional section of the pelvic nerves abolished the contractile phase of the bladder response to filling, and resulted in continuous filling of the bladder. 5. The transient bladder expansions (duration, 3-33 s) seen when the innervation was intact were closely related to the transient decreases in efferent nerve activity, suggesting involvement of a central inhibitory mechanism. The threshold pressure for triggering transient expansions was similar to the pressure shown to activate myelinated afferent fibres. 6. Under isovolumic conditions the frequency of micturition contractions increased with increasing vesical volume, and the relaxed interval between contractions shortened. This response could also be evoked by stimulation of the central cut end of the pelvic nerves, suggesting that the central inhibitory mechanism could also be active during isovolumic conditions.