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

Abstract

Single unit electrical activity was recorded extracellularly from 133 neurons in the gray matter of the 8th and 9th thoracic segments of the spinal cord in chloralose anesthetized cats. The responses of these neurons to electrical stimulation of the ipsilateral splanchnic nerve, to natural stimulation of the skin and to distension of the biliary system were studied. Of the neurons studied, 75% responded to electrical stimulation of the splanchnic nerve and had a cutaneous receptive field in the coastal region (viscerosomatic neurons). Of the neurons, 23% had a cutaneous receptive field but no visceral input (somatic neurons) and 2% had a visceral input but no cutaneous field. Somatic neurons 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 neurons projected to supraspinal levels via the contralateral ventrolateral funiculus. Viscerosomatic neurons were excited by small myelinated and nonmyelinated afferent fibers in the splanchnic nerve. Most viscerosomatic neurons had cutaneous inputs from nociceptors either exclusively (38%) or in addition to nonnoxious inputs (53%). The recording sites of somatic neurons were located almost exclusively in laminae II and IV and dorsal V of the dorsal horn. In contrast, viscerosomatic neurons were located in lamina I and in laminae V-IX of the gray matter. No differential distribution of recording sites according to type of cutaneous receptive field was found within the viscerosomatic group of neurons. About 1/3 of all viscerosomatic neurons could be excited by distensions of the biliary system. In all cases, intensities of visceral stimulation above physiological levels were necessary to activate the neurons. 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 specifically mediated by receptors in the biliary system. Of the neurons, 16% were projected to supraspinal levels via crossed ventrolateral pathways. All of these neurons were viscerosomatic with axonal conduction velocities between 12-68 m s-1. These results are discussed in relation to the postulates of the convergence-projection theory of referred pain.