Bladder filling attenuates spinal cord nociceptive reflexes in humans.

OBJECTIVE: To examine the viscerosomatic interaction between bladder afferents and somatic nociception we evaluated the effect of bladder filling on the nociceptive withdrawal reflex (NWR) in 21 healthy subjects.

METHODS: NWR was evoked in the lower and upper limbs by stimulating the sural and index finger digital nerves, respectively, while simultaneously recording EMG activity in the biceps femoris and biceps brachialis. NWR pain-related perception was quantified on a 10-point pain scale. Bladder filling was evaluated with suprapubic bladder sonography. Subjects were examined during empty bladder, medium and high level of bladder filling sessions.

RESULTS: NWR magnitude in both upper and lower limbs and perceived pain for the upper limb were significantly decreased at higher levels of bladder filling compared to empty bladder sessions.

CONCLUSIONS: Reduced NWR magnitude in both upper and lower limbs during bladder filling strongly indicates that bladder control and nociception share common modulatory descending pathways. Bladder afferents may activate these pathways to suppress the micturition reflex, but they may also inhibit spinal reflexes to maintain continence during pain stimuli.

SIGNIFICANCE: The effect of bladder filling on the NWR may represent a useful tool to investigate interactions between the neural pathways controlling the bladder and pain.