Is the periaqueductal gray an essential relay center for the micturition reflex pathway in the cat?

The periaqueductal gray (PAG), especially in a region between the levels the oculomotor nucleus and the trochlear nucleus, was suggested to be the essential relay center that conveys information of bladder fullness to the pontine micturition center (Barrington's nucleus). The present study examined this hypothesis by transecting the brainstem in anesthetized cats. In eight cases of the midbrain transection, all (n = 3) or most (n = 5) of the PAG between the levels the oculomotor nucleus and the trochlear nucleus was separated from the intact side of the brain. Furthermore, in the former three cases, the PAG at the level caudal to the trochlear nucleus was separated from the intact brain by more than half (n = 2) or completely (n = 1). In all these cases, there were no remarkable differences in the amplitude of the micturition contraction (80–98% of that before transection), irrespective of the levels of the transection. In the cases of the pontine transection, micturition contraction disappeared after transecting through the caudal part of Barrington's nucleus (n = 1) or through regions caudal to this nucleus (n = 5). In the one case that received a transection through the rostral part of Barrington's nucleus, the amplitude of the micturition contraction was 43% of that before transection. This study demonstrates that Barrington's nucleus is essential, but the PAG is not essential, for evoking micturition. Our results suggest that the information of bladder fullness in the cat is conveyed to Barrington's nucleus either directly from the lumbosacral neurons or indirectly via relay neurons located below the midbrain.