Physiological properties of tooth pulp-driven neurons in the first somatosensory cortex (SI) of the cat

Tooth pulp-driven (TPD) neurons are found in the oral area of the first somatosensory cortex (SI) of the cat. They have been classified according to their discharge patterns in response to electrical stimulation of the tooth pulp: the fast (F) type, slow (S) type, and a fast (Fa) type accompanied by afterdischarges. The characteristics of each type of TPD neuron were investigated in cats anesthetized with nitrous oxide and halothane. In surface distribution, there was no biased localization for any of the types. However, F-type neurons receiving input from the mandibular tooth tended to be found more medially than F-types receiving maxillary input. These TPD neurons did not change their firing pattern even when the stimulus was intensified. Mean threshold of the F-type to tooth pulp stimulation was 7.8 +/- 1.6 V and tended to be lower than that of the S-type (16.3 +/- 3.0 V). Graded increases in tooth pulp stimulation produced progressive increases in discharge frequency of both types of neurons. An analysis of the power function in relation to stimulus vs. response demonstrated that the exponent of the S-type neuron was about 2.0, being significantly larger compared to the 0.8 value for the F-type. The mean number of pulps afferent to an F-type was 1.6, compared to 4.8 for an S-type or 4.3 for an Fa-type. The results suggest that F-type TPD neurons may play a more important part in localizing pulpal pain and in recognizing the intensity than the other types.