Ascending projections of nociceptor-driven Lamina I neurones in the cat.

Single unit activity has been recorded from nociceptor-driven Lamina I neurones in the lumbar spinal cord of chloralose anaesthetized and gallamine paralysed cats. Ninety-four nociceptor-driven Lamina I neurones were identified by their superficial location in the dorsal horn and their ability to respond only to noxious stimulation of their cutaneous receptive fields. One-third of the Lamina I neurones responded only to noxious mechanical stimulation of the skin (Class 3a) and two-thirds responded to both mechanical and thermal noxious stimulation (Class 3b). Lissauer's tract was stimulated electrically two and three segments rostral to the recording sites. Ninety percent of the neurones tested showed a post-synaptic excitation mediated by fibres conducting at a mean velocity of 5.2 m/s (range 0.9--13.3 m/s). It is concluded that A delta and C afferent fibres running in Lissauer's tract excite nociceptor-driven Lamina I neurones. Ninety-six percent of the neurones tested showed a long period of inhibition (100--200 ms) following stimulation of large afferent fibres in the dorsal column. This inhibition was increased when the intensity of stimulation recruited Lissauer's tract fibres. Fifteen percent of the neurones tested were antidromically activated by Lissauer's tract stimulation from up to 3 segments rostal to their origin. A further 18.5% were antidromically excited by stimulation of deeper tracts. The mean conduction velocity of the axons of these projecting neurones was 8.6 m/s (range 3.8--16.5 m/s) and thus are small myelinated axons. The Class 3b neurones exhibited a significantly lower conduction velocity (7.5 +/- 2.8 (S.D.) m/s) than the Class 3a neurones (10.7 +/- 3.7 (S.D.) m/s). It is concluded that at least two-thirds of the population of nociceptor-driven Lamina I neurones are segmental interneurones.