Inhibition of spinothalamic tract cells and interneurons by brain stem stimulation in the monkey

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Abstract
Stimulation in or near the raphe nuclei of the caudal brainstem in anesthetized monkeys inhibited spinothalamic tract neurons. The threshold stimulus strength to produce inhibition was sometimes < 25 .mu.A, using brief trains of stimuli. Inhibition increased with higher stimulus strengths and longer periods of repetitive stimulation. The most effective stimuli were long trains of pulses at frequencies of 100 Hz or higher. The latency of inhibition was short enough to indicate that at least some axons of the descending pathway were myleinated. The maximum inhibition after a brief conditioning train was at conditioning-test intervals of 20-40 ms, and the inhibiton lasted over 150 ms. The best inhibition was produced by stimulation within the raphe magnus nucleus. Lesions of the dorsolateral fasciculus showed that the inhibitory pathway descends in this part of the spinal cord white matter. Brainstem stimulation also inhibited the N3 wave and the activity of interneurons, indicating that a variety of cord neurons are inhibited, in addition to spinothalamic tract cells. The inhibition was more effective for activity evoked by A.delta. fibers than for activity produced by large myelinated fibers. The responses to noxious forms of natural stimuli and to tactile stimuli were inhibited. The inhibitory pathway may be the raphe-tract.