Evidence for the persistence of axons at the apex of the cat's lower canine tooth after section of the inferior alveolar nerve

Abstract

The aim of this investigation was to establish the degree of denervation produced by inferior alveolar nerve section and to provide histological evidence for the presence of pulpal nerve fibres supplying the teeth which do not travel with the inferior alveolar nerve. Four adult cats were used. Each stage of the experiment was carried out under general anesthesia. The left inferior alveolar nerve was exposed and sectioned near the mandibular foramen. After 56 hours and 7 days, respectively, the jaw opening reflex to electrical stimulation of each lower canine was tested. Recordings were made from the left canine during electrical stimulation of the ipsilateral inferior alveolar nerve central and peripheral to the site of section as well as from the ipsilateral and contralateral inferior alveolar nerve during electrical stimulation of the left canine. Recordings were also made from the lingual nerve. After the recordings were completed two animals were perfused 56 hours after inferior alveolar nerve section, two more 7 days after section. Ultrathin sections of the apices of the lower canine teeth were examined in the electron microscope and each nerve fibre photographed. Each axon was examined to determine whether it was degenerating or normal. A jaw-opening reflex could not be elicited by stimulation of the left canine either 2 or 7 days after nerve section, whereas a normal response was evoked by stimulation of the right, control canine. At 2 days small responses could be recorded from the left canine teeth during stimulation of the left inferior alveolar nerve peripheral to the point of section. In one 2-day animal, responses could be recorded in the lingual nerve during stimulation. No pulpal fibres could be recorded in the inferior alveolar nerve central to the point of section nor from the contralateral inferior alveolar nerve. No pulpal fibres supplying the left canine could be recorded in any of the nerves examined at 7 days. Extensive degeneration was seen histologically even at 2 days. The canine pulp on the operated side contained only 31%, in one animal, and 26% in the other, of the number of axons of normal appearance that were present on the control side. At 7 days the number of remaining normal axons on the operated side were 5% and 13% of the numbers on the control side. All the axons of normal appearance were nonmyelinated. It is possible that the remaining axons represent fibres carried by the lingual nerve or some other alternative pathway that could not be detected electro-physiologically. Alternatively they may be a collateral innervation from adjacent tissues.