Abstract
1. In cats anaesthetized with urethane, extracellular micro-electrode recordings were made in the marginal layer of the trigeminal nucleus caudalis between the level of the obex and the Cl rootlets. 2. Nearly 300 neurones were found whose discharge rate increased with a reduction of facial temperature and a few which were excited by an increase in temperature. Over half of the neurons in each group were specifically sensitive to temperature and the remainder had a weak input from mechanical stimulation of the face. 3. Thermal receptive fields were all ipsilateral and found most frequently on the nose, lips, lower eyelid and pinna. There was a somatotopic organization of the receptive fields according to the three divisions of the trigeminal nerve. 4. On quantitative thermal testing, the neurones showed responses which were similar to those of the skin temperature sensors. 5. The thermal neurones could be antidromically fired by a stimulating electrode in the thalamus.