Cortically evoked depolarization of trigeminal cutaneous afferent fibers in the cat.

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Abstract
Primary afferent depolariza-tion (PAD) of the central terminals of trigeminal cutaneous afferents produced by cortical stimulation was investigated. Excitability changes in these terminals were used as an index of the depolarization. Cortically evoked PAD was greatest in terminals within the trigeminal main sensory nucleus and nucleus tractus spinalis oralis. Comparison of the cortical regions from which PAD was evoked, with the specific somatic sensory cortex, revealed a somatotopic organization of both the contralateral and ipsilateral effective cortex. Additional effective regions were in the anterior ectosylvian and anterior sigmoid gyri. It was concluded that a direct corticobular projection, linked to the primary afferent fibers via bulbar interneurons, constituted the pathway mediating the cortically evoked PAD.