Two pontine micturition centers in the cat are not interconnected directly: implications for the central organization of micturition.

The urinary bladder muscle and its external urethral sphincter are innervated, respectively, by the parasympathetic preganglionic motoneurons in the sacral intermediolateral cell column and somatic motoneurons in Onuf's nucleus. Neurons coordinating the activity of these muscles during micturition and urinary continence are not located in the sacral cord but in two pontine regions, the medial (M)-region (or pontine micturition center) and the lateral (L)-region (or pontine storage center). The M-region excites the bladder muscle through projections to its motoneurons and inhibits the urethral sphincter through excitatory projections to sacral cord gamma-amino butyric acid (GABA)-immunoreactive interneurons, which, in turn, inhibit urethral sphincter motoneurons. The L-region, through direct projections, excites urethral sphincter motoneurons. The present study investigated whether there are interconnections between the M- and L-regions. Anterograde tracing injections in the M-region resulted in labeled fibers to the intermediolateral cell column containing bladder motoneurons but not to Onuf's nucleus. No specific projections were found to the L-regions or to the contralateral M-region. L-region injections resulted in distinct projections to the Onuf's nucleus but not to the sacral intermediolateral cell column. No specific projections were observed either to the M-region or to the contralateral L-region. In conclusion, the M- and L-regions have direct long fiber projections, respectively, to the motoneurons of the bladder muscle and the external urethral sphincter, but they do not influence one another through direct pathways. The results strongly suggest that the M- and L-regions represent separate functional systems that act independently.