Cortical representation of the urge to void: a functional magnetic resonance imaging study

The urge to void generally increases with bladder distention but the relationship between the 2 factors is complex. When the bladder is moderately filled, the desire to void can be called forth deliberately but it can also be suppressed. To elucidate human brain mechanisms that are active during such intentional modulations of the desire to void we performed functional magnetic resonance imaging in healthy volunteers.

Brain activity was studied in 22 young women. At moderate bladder filling (about 350 ml) they periodically suppressed or enhanced the urge to void without allowing urine to pass. A manual task with a dynamometer, in which the current urge to void was expressed as grip force, demonstrated that the intensity of sensations could be influenced voluntarily. We also examined brain activity during repetitive (1 Hz) contractions of pelvic floor muscles after the bladder was emptied.

Significant brain activity associated with an increased urge to void was found in the insular cortex, frontal opercula, supplementary motor area (SMA), cingulate motor area (CMA), right posterior parietal cortex, left prefrontal cortex and cerebellum. Trends toward activation were detected in the thalamus, peri-aquaeductal gray matter and ventral pons. Suppression of the urge to void significantly activated the left superior frontal lobe. The SMA and CMA were active during voluntary rhythmical contractions of pelvic floor muscles.

Sensation intensity of the desire to void can be influenced intentionally. Frontoparietal cortical areas and the SMA/CMA seem to be involved in this process.