Encoding of tooth loads by human periodontal afferents and their role in jaw motor control

Microneurography has been used to analyze the functional properties of human periodontal mechanoreceptors. Signals were recorded from single afferents in the inferior alveolar nerve while controlled forces were applied to the teeth. We have found that all periodontal afferents adapt slowly to maintained loads. Most afferents are tuned broadly to direction of force application, and about half respond to forces applied to teeth adjacent to the one to which the afferent distributes. Populations of periodontal afferents, nevertheless, reliably encode information about both the teeth stimulated and the direction of forces applied to the individual teeth. Information about the magnitude of steady forces is made available in the mean firing-rate response of periodontal afferents. Most afferents exhibit a marked "hyperbolic" relationship between the static discharge rate and the force amplitude; the highest sensitivity to changes in static force is observed at forces below 1 N. Similarly, the dynamic sensitivity is highest at low forces. These afferents efficiently encode food contact during biting and continuously discharge while food is held between the incisors. Subjects spontaneously exert low contact forces matched to the sensitivity characteristics of these periodontal afferents when holding food substances between the incisors. If periodontal afferent information is not available, the control of the hold forces is severely impaired. Moreover, since only a few afferents encode information about the rapid and strong force increase employed to bite through food, we conclude that subjects rely on signals from periodontal afferents to regulate the jaw muscles primarily when they first contact, manipulate and hold food substances between the teeth. A potential role for periodontal afferents in the spatio-intensive control of jaw actions is discussed.