Two group of spinal interneurones that respond to stimulation of the abdominal viscera of the cat.

1. Recordings have been made from sixty-three spinal interneurones that received inputs from the abdominal viscera. These were divided into two groups: group A were sacral interneurones without either significant ascending projections or afferent input from the lumbar splanchnic nerves and group B were neurones with short ascending axons and afferent inputs from the lumbar splanchnic nerves, and often also from the pelvic and pudendal nerves.2. Somato-visceral convergence was common in both groups, and the somatic receptive fields of group A interneurones were in muscle and joints and less commonly skin innervated by sacral segments, whereas those of group B were innervated by lumbar and sacral segments.3. Within both groups of neurones there was a proportion that showed no response to distension or contractions of the viscera, (despite the fact that they responded to visceral nerve stimulation), and those which did respond showed graded changes in discharge rate as intravesical pressure was raised. Some cells received inputs from only one viscus: the receptive fields of these cells are described as simple and the effect of raising the pressure in that viscus was either to excite or to inhibit the central neurone. Other cells received convergent inputs from two viscera: their receptive fields are described as compound and the effect of raising intraluminal pressures in the viscera allowed a further subdivision of these cells into two types. Type I cells were either excited or inhibited by distension or contractions of either viscus. Type II cells were excited by natural stimulation of one of the innervated viscera but inhibited by similar changes in the other. The static pressure thresholds of these cells were 9+/-5 mmHg intravesical pressure, and 24+/-11 mmHg intracolonic pressure (means+/-s.d.).4. It is proposed that within the population of neurones described there are cells (group A interneurones) that either mediate the vesico- and colono-sphincteric reflexes or the colono-vesical interactions that can be seen in the micturition reflex pathway.5. It is proposed that the group B interneurones with short ascending projections mediate the vesico- and colono-sympathetic reflexes because their patterns of convergence, position in the spinal cord, latencies, and static pressure thresholds are consistent with those reflexes.