CONTROL OF SIZE AND EXCITABILITY OF MECHANOSENSORY RECEPTIVE-FIELDS IN DORSAL COLUMN NUCLEI BY HOMOLATERAL DORSAL HORN NEURONS

Both accidental and experimental lesions of the spinal cord suggest th at neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such int eractions might occur has not been adequately explained. To address th is issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spina l dorsal hem. After either a cervical or lumbar laminectomy and exposu re of the dorsal column nuclei in anesthetized cats, the representatio n of the hindlimb or of the forelimb was defined by multiunit recordin gs in both the dorsal column nuclei and in the ipsilateral spinal cord . Next, a single cell was isolated in the dorsal column nuclei, and it s receptive field carefully defined. Each cell could be activated by l ight mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine o r cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord correspond ing to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine o r cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nucle i. The experiment was repeated 16 times, and receptive field enlargeme nts of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory s ignals relayed through the dorsal column-lemniscal pathway. Because pu blished data from other laboratories have shown that receptive field s ize is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location fo r lemniscal neurons is determined by tonic afferent activity that is r elayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the le mniscal pathways must modify our traditional views concerning the rela tive independence of these two systems.