Forces have been applied to the lower canine teeth of cats, and the range of directions over which single periodontal mechanoreceptors are sensitive (arc of sensitivity) has been determined. We have further studied the response of each unit to forces applied in the direction of maximum sensitivity and investigated whether there is any relationship between the properties of the unit and its location within the periodontal ligament. 2. Recordings were made from 91 single units dissected from the inferior alveolar nerve in six cats. The mean arc of sensitivity to a 0.5 N force applied at right angles to the long axis of the tooth was 312 degrees. The arc of sensitivity was dependent on the force applied and the threshold of the receptor. 3. More units responded maximally to forces applied in a distolingual direction than would have been expected had the responses been evenly distributed around the tooth. Thirty-four of the units were bidirectionally sensitive, 28 became active when the force was removed, and 20 were spontaneously active. 4. The force threshold decreased with increasing rates of force application. 5. The majority of the units (81%) were slowly adapting. The rapidly adapting units had significantly higher force thresholds. 6. Twenty-eight of the receptors could be localized in the periodontal ligament after removing bone overlying the tooth root. There was no apparent relationship between the distance of the receptor from the fulcrum and its arc of sensitivity, threshold, maximum frequency of discharge, or adaptation properties.
