Fictive locomotion and scratching inhibit dorsal horn neurons receiving thin fiber afferent input

In decerebrate paralyzed cats, we examined the effects of two central motor commands (fictive locomotion and scratching) on the discharge of dorsal ho rn neurons receiving input from group III and IV tibial nerve afferents. We recorded the impulse activity of 74 dorsal horn neurons, each of which rec eived group III input from the tibial nerve. Electrical stimulation of the mesencephalic locomotor region (MLR), which evoked fictive static contracti on or fictive locomotion, inhibited the discharge of 44 of the 64 dorsal ho rn neurons tested. The mean depth from the dorsal surface of the spinal cor d of the 44 neurons whose discharge was inhibited by MLR stimulation was 1. 77 +/- 0.04 mm. Fictive scratching, evoked by topical application of bicucu lline to the cervical spinal cord and irritation of the ear, inhibited the discharge of 22 of the 29 dorsal horn neurons tested. Fourteen of the twent y-two neurons whose discharge was inhibited by fictive scratching were foun d to be inhibited by MLR stimulation as well. The mean depth from the dorsa l surface of the cord of the 22 neurons whose discharge was inhibited by fi ctive scratching was 1.77 +/- 0.06 mm. Stimulation of the MLR or the elicit ation of fictive scratching had no effect on the activity of 22 dorsal horn neurons receiving input from group III and IV tibial nerve afferents. The mean depth from the dorsal surface of the cord was 1.17 +/- 0.07 mm, a valu e that was significantly (P< 0.05) less than that for the neurons whose dis charge was inhibited by either MLR stimulation or fictive scratching. We co nclude that centrally evoked motor commands can inhibit the discharge of do rsal horn neurons receiving thin fiber input from the periphery.