SEGMENTAL AND INTERSEGMENTAL ORGANIZATION OF NEURONS IN THE SUBSTANTIA GELATINOSA ROLANDI OF THE CATS SPINAL-CORD

Single unit electrical activity has been recorded from neurones in the Substantia Gelatinosa Rolandi (SG) of the lumbar spinal cord of chloralose anaesthetized and gallamine paralysed cats. SG neurones were identified on the basis of five previously defined criteria [Cervero, Iggo and Molony, 1979b]. The surface of the spinal cord containing Lissauer's tract was stimulated electrically in order to excite axons running through Lissauer's tract. In about half of the animals studied Lissauer's tract was in addition surgically isolated by sectioning the adjacent dorsal column and dorso‐lateral funiculus. The responses of 32 intracellularly recorded and 78 extracellularly recorded SG neurones to stimulation of Lissauer's tract and surrounding structures have been analysed. Ten SG neurones had axons in Lissauer's tract that projected for distances between 15 and 41 mm (two and three segments rostral to their origin). The majority of the SG neurones studied did not project beyond their segments of origin. The mean conduction velocity of the long projecting SG neurones was 10·3 m.s‐l (range 4·6–18·3 m.s−1), corresponding to conduction in small myelinated fibres. All 110SG neurones studied were excited post‐synaptically by stimulation of Lissauer's tract and evidence is given to support the view that it was monosynaptically mediated. The latencies and threshold of the post‐synaptic activation indicated that it was mediated by a group of Aδ fibres within Lissauer's tract. All SG neurones studied showed a long period of inhibition (200–500ms) following the initial excitation. This inhibition was mediated by large myelinated fibres since it appeared at stimulus intensities that were threshold for excitation of the largest dorsal column or dorso‐lateral funiculus fibres. When axons in Lissauer's tract were recruited by electrical stimulation the period of inhibition lasted as long as 1s. The segmental somatotopic organization of the SG neurones was found to be similar to the larger dorsal horn neurones of the same spinal cord segments. © 1979 The Physiological Society