Responses to spinal microstimulation in the chronically spinalized rat andtheir relationship to spinal systems activated by low threshold cutaneous stimulation

We describe the responses evoked by microstimulation of interneuronal regio ns of the spinal cord in unanesthetized rats chronically spinalized at T10- T12. One to three weeks after spinalization, sites in the lumbar spinal cor d were stimulated using trains of low current microstimulation. The isometr ic force produced by stimulation of a spinal site was measured at the ankle . Responses were reliably observed from stimulation of a region within the first 1250 mu m from the dorsal surface of the spinal cord. These responses were clearly not due to direct motoneuronal activation and were maintained after chronic deafferentation. The force evoked by microstimulation and me asured at the ankle varied smoothly across the workspace. Simultaneous stim ulation of two sites in the spinal cord produced a response that was a simp le linear summation of the responses evoked from each of the sites alone. M icrostimulation generally produced a highly non-uniform distribution of res ponse directions, biased toward responses which pulled the limb toward the body. Within these distributions there appeared to be two main types of res ponses. These different types of responses were preferentially evoked by mi crostimulation of different rostrocaudal regions of the spinal cord. This a natomical organization paralleled the spinal cutaneous somatotopy, as asses sed by recording cutaneous receptive fields of neurons at sites to which th e microstimulation was applied. This relationship was maintained after chro nic deafferentation. The findings described here in the rat spinal cord in large part replicate those previously described in the frog.