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Department of Physiology and Biophysics, Mayo Medical School, Rochester, MN 55905, U.S.A.

1. Segmental, lumbar sympathetic outflow to neurones in the cat inferior mesenteric ganglion and to the large intestine were studied. Synaptic responses of neurones in the inferior mesenteric ganglion were recorded intracellularly, in vitro, during electrical stimulation of preganglionic fibres in the lumbar white rami. Synaptic responses consisted of excitatory post-synaptic potentials and/or action potentials.

2. None of the neurones tested received synaptic input from spinal cord segment L₁. There was synaptic input from segments L₂-L₅ of the spinal cord. The strongest synaptic input arose from spinal cord segments L₃ and L₄.

3. 42% of the neurones tested received synaptic input from only one spinal cord segment. 54% of the neurones tested received convergent synaptic input from two, three or four adjacent lumbar segments.

4. Electrophysiological measurements indicated that the number of preganglionic fibres in any lumbar white ramus communicans which provided synaptic input ranged from one to thirteen. Each lumbar white ramus contained, on average, five preganglionic fibres which provided synaptic input to neurones in the inferior mesenteric ganglion.

5. Changes in intraluminal colonic pressure were measured in vivo during electrical stimulation of preganglionic fibres in the different lumbar white rami and lumbar ventral roots. Electrical stimulation of white rami L₃ and L₄ abolished phasic changes in intraluminal colonic pressure and reduced basal pressure to near zero. Electrical stimulation of preganglionic fibres in lumbar ventral roots L₃ and L₄ abolished phasic changes in intraluminal colonic pressure and reduced basal pressure to near zero. Stimulation of ventral roots L₁, L₂ and L₅ had little to no effect on intraluminal pressure.

6. Based on the data obtained in this study, two hypotheses are proposed. First, spinal cord segments L₃, L₄ and L₅ are the primary sources of central synaptic input to neurones in the inferior mesenteric ganglion. Secondly, spinal cord segments L₃ and L₄ control colonic motility.