Abstract
1. In anaesthetized cats recordings have been made in the mesencephalic nucleus of the fifth cranial nerve and the trigeminal ganglion from neurones that respond when forces are applied to the mandibular canine tooth. The site of the mechanoreceptors in the periodontal ligament and their distribution around the tooth root have been determined. 2. Receptors with their cell bodies in the mesencephalic nucleus were found to be situated in the periodontal ligament in a discrete area intermediate between the fulcrum and apex of the tooth, while those in the trigeminal ganglion were situated in the whole area of the periodontal ligament between the fulcrum and apex of the tooth. 3. All of the located mechanoreceptors responded maximally when that part of the ligament in which they lay was put under tension. 4. The directional sensitivities of the mechanoreceptors suggested that there was an uneven distribution around the tooth root of receptors with cell bodies in the mesencephalic nucleus. In contrast mechanoreceptors with cell bodies in the trigeminal ganglion were distributed more equally around the tooth root. The rationale for the differences requires further investigation.