Afferent excitability compared using pulsating versus constant direct current electroanalgesia in cat tooth pulp

Non-pain and pain sensations evoked by tooth pulp stimulation

This study investigated the quality and magnitude of sensations evoked by electrical tooth pulp stimulation. Detection threshold (the minimum current intensity that evoked a sensation) and pain threshold were determined for tooth pulp stimuli varying in frequency from 5 to 500 Hz. The effect of frequency and intensity of tooth pulp stimulation on the magnitude of sensations was assessed using visual analog scales and verbal descriptor scales. Detection thresholds were stable over experimental sessions and independent of the frequency of the stimulating current. Pain threshold varied as a function of frequency with a minimum value at 100 Hz. Stimuli that evoked non-pain sensations at low frequencies evoked pain sensations when frequency was increased from 5 to 100 Hz. Subjects were able to scale non-pain sensations over a range of stimulus intensities and frequencies. The lowest currents evoked sensations that were non-painful and were of constant magnitude despite changes in the frequency of stimulation. Higher stimulus currents evoked sensations that were non-painful at low stimulus frequencies and painful at high stimulus frequencies. Sensation magnitude at each stimulus intensity increased as a function of frequency. Temporal summation occurred in proportion to stimulus intensity. These findings suggest that the non-pain sensations evoked in tooth pulp are mediated by a distinct population of afferents that are not involved in the coding of pain. High frequency stimulation that increased the discharge rate of the lowest threshold pulpal afferents resulted in no summation of non-pain sensation and never produced pain. However, high frequency stimulation evoked greater magnitude sensations at higher stimulus currents, indicating that central summation mechanisms were critical for higher threshold afferents signaling more intense non-pain and pain sensations.

Some anatomical and electrophysiological properties of tooth-pulp afferents in the cat

1. The responses of nerves which could be excited by electrical stimulation of the canine tooth-pulp were recorded from the trigeminal ganglion. 2. Preliminary experiments showed that the responses of mandibular canine pulpal afferents were recorded from the postero-lateral part of the trigeminal ganglion whereas the responses of maxillary canine pulpal afferents were recorded from the central part of the ganglion. 3. The conduction velocity differed in various parts of these nerves; the conduction velocity inside the tooth tended to be slower than that from the tooth to the trigeminal ganglion; the mean conduction velocity in the part of the nerve peripheral to the trigeminal ganglion was faster than that in the part central to the ganglion, and in the central part the conduction velocity decreased caudally. 4. Collision experiments combining tooth stimulation and stimulation of sites in the trigeminal sensory nuclear complex were carried out to investigate the projections of individual pulpal afferents. 5. The results of these experiments showed that some tooth-pulp afferents bifurcate and project to both the main sensory nucleus and the nucleus caudalis. For other pulpal afferents only a projection to the main sensory nucleus or the nucleus caudalis could be demonstrated. 6. Pulpal afferents that bifurcated had faster conduction velocities than those for which no bifurcation could be shown.