Properties of the descending limb of the spinobulbospinal micturition reflex pathway in the cat

The micturition reflex is thought to be mediated by a spinobulbospinal reflex pathway passing through the rostral pons. This study examined the properties of the descending limb of the reflex pathway by monitoring the responses of the lower urinary tract to stimulation of the pons in the decerebrate cat. Electrical stimulation (300 μs pulses at 50 Hz intratrain frequencies, 300-500 ms trains, 0.5-15 V) in the region of the locus coeruleus (P 0.5-3.1/L 2-4/H to -2.75) was used to activate the descending excitatory pathway to the sacral parasympathetic nucleus. Low intensity stimulation induced small amplitude, short duration (14 ± 11 cm H₂O, 10 ± 3 s) bladder contractions in a partially full bladder, whereas higher intensity stimulation induced large amplitude, long duration (69 ± 29 cm H₂O, 70 ± 44 s) contractions which were similar to distension-induced reflex micturition contractions. The evoked bladder contractions coincided with a reduction in external urethral sphincter (EUS) EMG activity. Following bilateral L₇-S₃ dorsal root transection, electrical stimulation of the pons still elicited the small amplitude bladder contractions, but the larger amplitude, long duration micturition contractions were abolished. During these small evoked bladder contractions, a suppression of EUS activity still occurred following deafferentation, indicating a pontine mediated bladder/EUS synergy. It is concluded that the pons can initiate bladder contractions and coordinated bladder-sphincter activity, but that afferent feedback (via the dorsal roots) is needed to maintain the large amplitude micturition contractions.