Barrington's nucleus contains neurones that decrease their firing during micturition contraction, as well as neurones that increase their firing during this phase. These neurones are commonly termed inverse neurones and direct neurones, respectively. The aims of the present study were to determine whether inverse neurones send descending axons to the spinal cord and to clarify how these neurones regulate bladder contractility. Forty-five single neurones were recorded from the dorsolateral pontine tegmentum. Spinal-projecting neurones were identified by antidromic stimulation of the spinal cord. More than half of inverse neurones were located outside Barrington's nucleus. Only three were spinal-projecting neurones. The results were in marked contrast with direct neurones that we studied previously: the majority of them were located within Barrington's nucleus, and 56% were spinal-projecting neurones. The firing frequency of inverse neurones ranged between 6 and 37 Hz during the relaxation phase of the micturition contraction-relaxation rhythm. The firing of all neurones began to decrease within 8 s after the onset of micturition contraction. During micturition contraction, neurones displayed little firing, being virtually silent (n = 29) or displayed weak tonic firing (3-11 Hz; n = 16). All neurones began to increase their firing within 8 seconds after the onset of bladder relaxation. These results suggest that inverse neurones do not trigger bladder contraction or relaxation, despite the finding that a few of them send descending axons to the spinal cord. One possible role of the inverse neurone is to regulate firing activities of direct neurones in Barrington's nucleus. © 2004 Elsevier B.V. All rights reserved.
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