Factors that determine the excitability of parasympathetic reflexes to the cat bladder.

1. Spino-bulbo-spinal reflex responses could be recorded from the vesical branches of the pelvic nerve following electrical stimulation of afferents in the vesical or colonic branches of the pelvic nerves, the hypogastric or pudendal nerves. The latencies of responses from these different sources were similar.2. Short latency responses could be recorded from the vesical branches of the pelvic nerve on electrical stimulation of descending pathways in the spinal cord and ipsilateral to and just below a hemitransection at the second cervical segment.3. These responses were facilitated by increases in intravesical pressure, maximal facilitation occurring at about 30 mmHg. Conversely, increases in intracolonic pressure inhibited these parasympathetic evoked responses, the maximum effect being seen with intracolonic pressures of 40-50 mmHg.4. These results suggest that the spino-bulbo-spinal responses that are recorded from the vesical parasympathetic efferents can be elicited from nerves innervating viscera other than the bladder, and also somatic structures. The effects of these spino-bulbo-spinal pathways on vesical parasympathetic efferents are dependent on antagonistic influences of intravesical and intracolonic pressure.5. The pathways that mediate the changes in excitability in these reflexes appear to act at least in part at the termination of the bulbo-spinal limb of the reflex, and involve afferents in the pelvic nerves but not the hypogastric or lumbar colonic nerves.6. It is proposed that the neurones that mediate this ;gating' action on the excitability of pelvic nerve reflexes are located in the sacral cord, and form a proportion of the population of interneurones described by McMahon & Morrison (1982b). In addition it is proposed that the neurones which mediate the ascending limb of the spino-bulbo-spinal reflexes are the long ascending neurones described by McMahon & Morrison (1982a).