Effects of chronic descending tractotomy on the response patterns of neurons in the trigeminal nuclei principalis and oralis.

Unilateral, descending, trigeminal tractotomy was carried out on adult cats using aseptic technique. Unit activity, evoked by bipolar noxious stimulation of the tooth pulp and by innocuous mechanical stimulation of other receptive fields, was recorded from neurons in the trigeminal principalis and oralis nuclei, 7 to 13 weeks after tractotomy. Seventy neurons from the lesion side of the brain stem and 59 from the control side were studied. All units responded to pulpal stimuli and the great majority to mechanical stimuli. Three general response configurations were produced by each of the two forms of stimulation: brief bursts of 1 to 5 spikes, trains of 6 to 20 spikes, and multiple component discharges consisting of sequences of brief bursts. No statistically significant differences between the two populations were found for peripheral field size or location, for discharge characteristics such as latency, configuration, or mean density, or for responsiveness to the two types of stimuli. However, a statistically greater proportion of control units fired dense impulse trains (more than eight spikes) to intense pulpal stimuli. The results provide evidence that the response patterns of most somatosensory neurons in nuclei principalis and oralis are unaffected by descending trigeminal tractotomy and that stimulus modality is not coded by the response characteristics of polymodal units in these nuclei. Although some modulatory influence is exerted on polymodal neurons in the rostral nuclei by the trigeminal nucleus caudalis, it appears doubtful that this is a significant factor in determining whether a peripheral stimulus is perceived as noxious or innocuous.