Evidence on the origin of impulses recorded from dentine in the cat.

1. Recordings have been made from dentine at the tip of the canine teeth of cats using a large Ag/AgCl electrode.2. All-or-none action potentials with complex shapes were recorded when single nerve fibres from the dental pulp were stimulated electrically outside the tooth.3. The wave forms of these action potentials changed when the stimulation rate was increased from 1/sec to 10/sec, when the temperature of the tooth surface was changed between 17 and 42 degrees C, when the thickness of the dentine was reduced, and when local anaesthetic was applied to the dentine. Only a small, monophasic, positive potential remained after transection of the pulp in the crown of the tooth.4. The latency of the action potentials was not affected by these same procedures.5. It is suggested that each wave form represented a compound action potential produced by impulses invading the main branches and terminals of a single nerve in the pulp. Some of the terminals may have penetrated the innermost layers of the dentine.6. There was no spontaneous discharge from pulpal nerves.7. A discharge of impulses was recorded from dentine when 2.5 mol/l. NaCl was applied beneath the electrode at the tip of the tooth. By recording simultaneously from dentine and from single fibres from the tooth pulp, it was shown that impulses recorded from dentine were associated with propagated nerve action potentials.