A novel type of unmyelinated chemosensitive nociceptor in the acutely inflamed urinary bladder.

Single primary afferents supplying the cat urinary bladder were electrophysiologically recorded in the sacral dorsal roots. Under normal conditions, afferents were not spontaneously active when the bladder was empty and innocuous increases of intravesical pressure excited mainly thin myelinated, low-threshold mechanoreceptors. Less than 2.5% of all unmyelinated visceral afferents responded to a mechanical stimulus, but all had high, presumably noxious thresholds. During an acute inflammation induced with intravesical injections of 2.5% mustard oil or 50-100% turpentine oil both populations developed resting activity and changed their mechanosensitive properties. Out of the many unmyelinated afferents without appreciable mechanosensitivity, an entirely new subpopulation was activated by chemical irritants during an acute inflammation. Subsequently, some of these chemosensitive receptors started to display new mechanosensitive properties. This novel population of sensory neurones may contribute considerably to the pathogenesis of visceral pain states including reflex disturbances of bladder motility.