Direct projections from the periaqueductal gray to the pontine micturition center (M-region). An anterograde and retrograde tracing study in the cat.

Micturition is a spino-bulbo-spinal reflex. The bulbospinal part of this reflex is formed by the projections from the M-region, also called the pontine micturition center or Barrington's nucleus, to the preganglionic parasympathetic motoneurons in the sacral cord innervating the bladder. In respect to the spino-bulbar part of the micturition reflex, our group recently showed that the sacral cord projections to the brainstem terminate mainly in the periaqueductal gray (PAG). In this study it was investigated whether the PAG might serve as a link between the sacral cord and the M-region, by examining the possible connections using the tracers wheat germ-agglutin horseradish peroxidase and tritiated leucine. The results demonstrate that a specific circumscribed rostrocaudally oriented cell group within the ventrolateral PAG and parts of the dorsomedial PAG project specifically to the M-region. A concept is put forward in which specific parts of the PAG are involved in the control of micturition and that information concerning bladder filling is conveyed via the PAG to the M-region.