Horseradish peroxidase was applied either to severed sciatic nerves or to individual muscle groups in the lower leg of the rat. This horseradish peroxidase was taken up by sensory nerves and transported across the dorsal root ganglion and then anterogradely as far as central sensory terminals in the spinal cord and medulla. The dorsal root, dorsal column and the tract of Lissauer contained horseradish peroxidase labeled fibers. Following horseradish peroxidase application to the cut sciatic nerve, extraperikaryal reaction-product within the central processes of the sensory neurons was seen in the upper dorsal horn (substantia gelatinosa), lower dorsal horn, intermediate gray, ventral horn and nucleus of Clarke of the ipsilateral spinal cord; in the contralateral dorsal and ventral horns, and in the ipsilateral nucleus gracilis. Following intramuscular injections of horseradish peroxidase, the reaction-product was virtually absent in the upper dorsal horn and was concentrated in the lower dorsal horn, intermediate gray and ventral horn. These experiments show that horseradish peroxidase is transported anterogradely in significant quantities along the intact central processes of the dorsal root ganglion so that sensory terminations belonging to individual peripheral nerves or muscle groups can be traced. The demonstration of this complex transport was made possible with a sensitive histochemical method that used tetramethylbenzidine as the chromogen. Alternate sections processed with a commonly employed diaminobenzidine procedure did not show any of these central sensory connections.
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