How does the distention of urinary bladder cause arousal?

Under urethane anesthesia, the urinary bladder of male rats was distended by 0.3-0.5 mL of saline. When the animals displayed large amplitude delta waves, the distention caused tonic excitation in 76% (19/25) of the noradrenergic neurons in the locus coeruleus and in 53% (16/30) of the cholinergic neurons in the laterodorsaltegmental nucleus. The response was followed by a shift of electroencephalogram (EEG) pattern to faster waves of smaller amplitude. When EEG amplitude was faster and smaller, the same stimuli caused no response. Neurons in Barrington's nucleus were excited by a small amount of bladder distention independent of EEG patterns. These results suggest that the transition of state, caused by bladder distention, from deep sleep to light sleep is mediated by noradrenergic and cholinergic neurons in the brainstem.