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Department of Physiology, University of Leeds, Leeds LS2 9JT

1. Recordings have been made from seventy-three neurones responding to electrical stimulation of pelvic, hypogastric or lumbar colonic nerves, in decerebrate or anaesthetized cats. Fifty-two of the units had long projections that ascended to the first cervical segment, and no units with visceral inputs were found to belong to the spino-cervical tract. Twenty-one units had long descending projections.

2. Twenty percent (i.e. 11/46) responded to parasympathetic (pelvic) nerve stimulation (group 1) whilst 80% (35/46) responded to stimulation of hypogastric and/or lumbar colonic nerves (group 2). Ninety percent of group 2 neurones also responded to pelvic nerve stimulation.

3. The electrical thresholds for activation of the units indicated that the largest peripheral nerve fibres responsible for the response were of the A size.

4. Thirty-one of the neurones had visceral mechanosensitive receptive fields; twenty-one had simple receptive fields in the bladder (seven) or in the colon (fourteen), ten units had compound receptive fields. The response of units with simple receptive fields to mechanical stimulation were either inhibitory or excitatory, and slowly adapting or rapidly adapting. Forty-two units appeared to have no visceral mechano-sensitive receptive fields in spite of showing responses to visceral nerve stimulation.

5. Fifty percent of the units tested responded to innocuous somatic stimuli, mostly derived from muscle or joint receptors. Some of the units were found to respond to injections of bradykinin (10-15 µg) into a hindlimb artery.

6. Group 1 had predominantly inhibitory visceral receptive fields, and somatic receptive fields in structures innervated from sacral segments of the spinal cord. Group 2 units all received inputs from visceral nerves entering the spinal cord over lumbar segments; many also received projections from sacral segmental inputs. These inputs showed an equal mixture of excitatory and inhibitory visceral receptive fields and convergence from somatic inputs arising from lumbar as well as sacral dermatomes. It seems likely that this group represents units originating in lumbar as well as sacral segments of the cord.

7. The possible role of these neurones as mediators of visceral sensations and visceral reflexes is discussed.

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