ULTRASTRUCTURAL EVIDENCE FOR A PAUCITY OF PROJECTIONS FROM THE LUMBOSACRAL CORD TO THE PONTINE MICTURITION CENTER OR M-REGION IN THE CAT - A NEW CONCEPT FOR THE ORGANIZATION OF THE MICTURITION REFLEX WITH THE PERIAQUEDUCTAL GRAY AS CENTRAL RELAY

Information concerning the rate of bladder filling is determined by re ceptors in the bladder wall and conveyed via afferent fibers in the pe lvic nerve to sensory neurons in the lumbosacral cord. It was assumed that this information is relayed from the lumbosacral cord to a medial cell group in the dorsolateral pontine tegmentum, called the M-region , the pontine micturition center, or Barrington's nucleus. The M-regio n, in turn, projects via long descending pathways to the sacral parasy mpathetic motoneurons. In the present electron microscopic study, it w as investigated in cats whether monosynaptic projections from lumbosac ral neurons to the M-region indeed exist. Wheat-germ agglutinin-horser adish peroxidase injections were made into the lumbosacral, cord. Many retrogradely labeled dendrites and somata were found in the M-region, but no labeled terminals were found on retrogradely labeled dendrites or somata. Only a small number of anterogradely labeled terminals, wh ich were filled with mainly round vesicles, contacted unlabeled dendri tes in the M-region. In contrast, many more anterogradely labeled term inals, which were filled with mainly round and, to a limited extent, d ense core vesicles and with asymmetrical synapses, were found on dendr ites in the lateral part of the periaqueductal gray (PAG). Previously (Blok and Holstege [1994] Neurosci. Lett. 166:93-96), it was demonstra ted that the lateral part of the FAG contains neurons projecting to th e M-region. A concept for the central organization of the micturition reflex is presented in which ascending projections from the lumbosacra l cord convey information on bladder filling to the FAG. When the blad der contains so much urine that voiding is necessary, the FAG, in turn , triggers the M-region. The M-region, however, also receives afferent s from the preoptic area, which might be involved in the final decisio n to start micturition. (C) 1995 Wiley-Liss, Inc.