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1. Properties of neurones in the trigeminal nuclei principalis and oralis responding to movements of facial sinus hairs were studied in cats anaesthetized by I. V. infusion of pentobarbitone.

2. Using electrophysiological methods trigeminal neurones were classified into primary afferent fibres, trigeminothalamic relay neurones, interneurones and other unspecified higher order neurones.

3. When receptive fields of synaptically activated neurones were compared with those of primary afferent fibres, an often extensive convergence from first order on to higher order neurones was established. Out of 119 relay neurones six received input from one sinus hair only. Spontaneous activity was encountered about twice as often in synaptically activated neurones than in primary afferent fibres.

4. The responsiveness of single neurones was unstable over time in about one fifth 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 the size or the configuration of the receptive field. Such instabilities were observed in all kinds of synaptically activated neurones but not in primary afferent fibres.

5. Afferent inhibition in relay neurones could be elicited from within the excitatory receptive field and appeared to be related to the activation of distinct receptor populations responding to specific stimulus parameters. Inhibition was also seen in interneurones following both mechanical stimulation of the skin and electrical stimulation of lemniscal fibre terminals in the contralateral ventromedial thalamus.

6. The results are discussed and compared with previous findings about sinus hair representation in the trigeminal nucleus and the ascending lemniscal projection. The findings indicate that the concept of the `static properties' of relay neurones is not adequate for all trigeminothalamic relay neurones and may require a critical reconsideration.

7. It is suggested that the afferent input from sinus hairs is effectively controlled at the level of the rostral trigeminal nuclei. This control may affect the spatial input to relay neurones, the temporal components of their responses and the intensity dimension of their transmission capacity. It is postulated that by these mechanisms tactile information from the sinus hair system is modulated according to the instantaneous sensory requirements of the behaving cat.