Properties of different functional types of neurones in the cat's rostral trigeminal nuclei responding to sinus hair stimulation

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Abstract
Properties of neurons in the trigeminal nuclei principalis and oralis responding to movements of facial sinus hairs were studied in cats anesthetized by i.v. infusion of pentobarbitone. Using electrophysiological methods trigeminal neurons were classified into primary afferent fibers, trigeminothalamic relay neurons, interneurons and other unspecified higher order neurons. When receptive fields of synaptically activated neurons were compared with those of primary afferent fibers, an often extensive convergence from 1st order on to higher order neurons was established. Of 119 relay neurons, 6 received input from 1 sinus hair only. Spontaneous activity was encountered about twice as often in synaptically activated neurons than in primary afferent fibers. The responsiveness of single neurons was unstable over time in about 1/5 of the population and then the total number of impulses discharged in successive responses could vary by as much as 500%. Unstable responsiveness occurred sometimes alone but was often accompanied by marked changes in size or configuration of the receptive field. Such instabilities were observed in all kinds of synaptically activated neurons but not in primary afferent fibers. Afferent inhibition in relay neurons could be elicited from within the excitatory receptive field and appeared related to the activation of distinct receptor populations responding to specific stimulus parameters. Inhibition was also seen in interneurons following both mechanical stimulation of the skin and electrical stimulation of lemniscal fiber terminals in the contralateral ventromedial thalamus. The results were discussed and compared with previous findings about sinus hair representation in the trigeminal nucleus and the ascending lemniscal projection. Apparently the concept of the static properties of relay neurons is not adequate for all trigeminothalamic relay neurons and may require a critical reconsideration. The afferent input from sinus hairs may be effectively controlled at the level of the rostral trigeminal nuclei. This control may affect the spatial input to relay neurons, the temporal components of their responses and the intensity dimension of their transmission capacity. By these mechanisms tactile information from the sinus hair system is apparently modulated according to the instantaneous sensory requirements of the behaving cat.