Somatic and visceral inputs to the thoracic spinal cord of the cat: effects of noxious stimulation of the biliary system.

1. Single unit electrical activity has been recorded extracellularly from 133 neurones in the grey matter of the 8th and 9th thoracic segments of the spinal cord in chloralose anaesthetized cats. The responses of these neurones to electrical stimulation of the ipsilateral splanchnic nerve, to natural stimulation of the skin and to distension of the biliary system have been studied.2. Of the neurones studied, 75% responded to electrical stimulation of the splanchnic nerve and had a cutaneous receptive field in the costal region (viscero-somatic neurones). Twenty-three per cent of the neurones had a cutaneous receptive field but no visceral input (somatic neurones) and 2% had a visceral input but no cutaneous field.3. Somatic neurones had well-localized receptive fields from which they could be driven by innocuous stimulation of the skin (52%), by noxious (7%) or by both forms of stimulation (41%). No somatic neurones projected to supraspinal levels via the contralateral ventro-lateral funiculus.4. Viscero-somatic neurones were excited by small myelinated and non-myelinated afferent fibres in the splanchnic nerve. Most viscero-somatic neurones had cutaneous inputs from nociceptors either exclusively (38%) or in addition to non-noxious inputs (53%).5. The recording sites of somatic neurones were located almost exclusively in laminae II and IV and dorsal V of the dorsal horn. In contrast, viscero-somatic neurones were located in lamina I and in laminae V-IX of the grey matter. No differential distribution of recording sites according to type of cutaneous receptive field has been found within the viscero-somatic group of neurones.6. About one-third of all viscero-somatic neurones could be excited by distensions of the biliary system. In all cases, intensities of visceral stimulation above physiological levels were necessary to activate the neurones. Most units driven by biliary afferents were located in or ventral to lamina V of the dorsal horn. The excitation of these units by biliary distension was found to be specifically mediated by receptors in the biliary system.7. Sixteen per cent of the neurones were found to project to supraspinal levels via crossed ventro-lateral pathways. All of these neurones were viscero-somatic with axonal conduction velocities between 12 and 68 m sec(-1).8. These results are discussed in relation to the postulates of the ;Convergence-projection' theory of referred pain.