Effects of lesions to rat spinal cord lamina I cell projection pathways on reactions to acute and chronic noxious stimuli.

(1) Lamina I contains large numbers of nociceptive specific cells and wide-dynamic-range (WDR) cells which respond to both noxious and innocuous stimuli. Many of the cells project to the brain. 82% of the projecting axons travel by way of the contralateral dorsolateral funiculus (DLF). (2) Section of the contralateral DLF produced no change in behavioural response to brief mechanical or thermal or chemical stimuli. However, section of the contralateral DLF greatly accelerated the slow onset autotomy in response to section of the sciatic and saphenous nerves. (3) Section of the ipsilateral DLF or bilateral section produce the same acceleration of onset of autotomy as is produced by contralateral DLF section. Section of dorsal columns does not change the long-term onset of autotomy. (4) Destruction of cells with ibotenic acid in the contralateral parabrachial area where many lamina I cells are known to terminate produced the same acceleration of autotomy as was observed after DLF lesions. Contralateral cortical lesions were without effect on autotomy. (5) It is proposed that the lamina I projection system is more concerned with long-latency long-duration reactions to prolonged events than to abrupt reactions to brief stimuli. The behavioural results reported here are consistent with McMahon and Wall's physiological hypothesis that a contralateral DLF projecting pathway excites cells in the parabrachial area which in turn excite descending control systems running mainly in the ipsilateral DLF to affect spinal dorsal horn cells.