This study examined the responses to natural cutaneous stimuli of neurons in the dorsal horn of the lumbosacral spinal cord that project to the dorsal accessory portion of the inferior olive (DAO) in cats anesthetized with pentobarbital sodium. Extracellular activity was recorded from single units antidromically activated by currents of less than or equal to 70 muA applied to DAO. 2. A total of 119 antidromically activated neurons was examined. Their antidromic activation latencies displayed a wide range (2.5-24.6 ms). The average latency corresponds to a conduction velocity of 24 m/s. 3. Collision was demonstrated for 24 neurons. All responded to some form of natural cutaneous stimulation. Their receptive fields encompassed some portion of the hind limb, particularly the toes; one-third displayed gradients of sensitivity. 4. Based on their thresholds to peripheral stimulation, the 24 neurons fell into five categories, those sensitive to light cutaneous stimuli (i.e., hair movement or light touch; 37.5%), rub (21%), tap (21%), pressure (12.5%), or noxious stimuli (8%). 5. Comparison of these results with data on the other major source of somatosensory information for DAO, the gracile nucleus (examined previously with the same methods), suggests that the sensitivity of neurons in DAO to light cutaneous stimuli is mediated primarily by neurons in the dorsal horn. The sensitivity of neurons in DAO to tap, rub, or pressure, on the other hand, might be mediated by neurons in either the dorsal horn, the gracile nucleus, or both.
