Inhibition and excitation of primate spinothalamic tract neurons by stimulation in region of nucleus reticularis gigantocellularis

Electrical stimulation in the region of the nucleus reticularis gigantocellularis (NGc) in anesthetized monkeys inhibited or excited spinothalamic tract neurons in the lumbosacral and cervical enlargements. The descending effects were generally more pronounced for activity evoked by cutaneous A-δ-fibers than activity produced by large myelinated cutaneous afferents. Nevertheless, the responses to all types of natural stimuli used could be inhibited or facilitated. The excitation from repeated brief stimulus trains to the NGc sometimes increased progressively, suggesting the existence of a positive feedback system. Occasionally, repeated stimulation of the NGc produced a progressively greater inhibition. The threshold stimulus strength to elicit the inhibitory and excitatory actions was usually less than 50 μA, and in some cases less than 25 μA. The inhibition and excitation increased as the stimulus intensity was raised above the threshold value, or as the number and/or frequency of pulses in the stimulus train was increased. The strongest inhibition and excitation was produced by stimulation within the NGc on either side of the brain stem. There was no obvious topographic organization of inhibitory and excitatory zones. Dorsolateral tractotomies in the high cervical spinal cord did not prevent the effects of NGc stimulation, indicating that the inhibitory and excitatory pathways descend in the ventral parts of the white matter. It is suggested that the inhibition and excitation are mediated by the medullary reticulospinal system.