Facial thermal input to the trigeminal spinal nucleus of rabbits and rats.

In rabbits and rats under urethane anesthesia, a systematic survey was made of the caudal trigeminal nucleus, using glass coated tungsten microelectrodes. This revealed many neurons in the marginal layer sensitive either to warming or cooling the facial skin. The majority of these neurons were specifically temperature sensitive. In rats, a somatotopic arrangement of cold receptive fields was evident within the marginal layer of the trigeminal nucleus, with the ophthalmic division of the trigeminal nerve represented most laterally, the mandibular most medially and the maxillary division represented over most of the recording region. This arrangement is similar to the mediolateral distribution seen in cats. In rabbits, the distribution was less rigorous but the ophthalmic division tended to be represented caudally and the mandibular division rostrally. The maxillary division was represented over most of the recording region. All receptive fields were ipsilateral and showed spatial convergence of input. Both rabbits and rats possessed a concentration of thermal receptive-fields around the nose, whisker pad and mouth. At steady skin temperatures, marginal units gave bell-shaped intensity functions which were very similar to those reported for the cold and warm receptors. With rapid changes of temperature, neurons responded with a dynamic outburst which corresponded to the equivalent receptor response.