Modulation of human exteroceptive jaw reflexes during simulated mastication F. Lobbezoo a , P.F. Sowman b,c , K.S. Türker c, * a Department of Oral Function, Section Oral Kinesiology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and VU University, Amsterdam, The Netherlands b Discipline of Physiology, School of Molecular and Biomedical Sciences, University of Adelaide, SA 5005, Australia c E.U. Center for Brain Research, Department of Physiology, Faculty of Medicine, Ege University, Bornova, Izmir, Turkey article info Article history: Accepted 12 November 2008 Available online 3 January 2009 Keywords: Chewing Jaw muscles Electrical stimulation Pain Human abstract Objective: To investigate changes in synaptic input from lower lip afferents to human jaw muscle moto- neurons during simulated mastication. Methods: The lower lip of 14 subjects was stimulated electrically under static and dynamic conditions. In the static condition, subjects bit at mid-open position and received stimuli while keeping the masseteric excitation level at 20%, 40%, 60%, 80%, or 100% of the maximum EMG (generated during simulated chew- ing). In the dynamic condition, the subjects ‘masticated’ at their habitual chewing pace, and stimuli were delivered whenever the jaw crossed a predetermined gape. In both conditions, mildly (scores of 2–3 on a 0–10 rating scale) and moderately (scores of 5–6) painful stimulus intensities were used. Results: Under static conditions, there was no modulation of the inhibitory masseteric reflexes with the level of the background level of excitation used in these experiments. However, under dynamic condi- tions there were significant strength modulations with gape that differed between mildly and moderately painful stimuli. Conclusions: Reflexes in response to mildly painful stimuli were ‘gated’ during simulated mastication: as the teeth moved closer toward occlusion, the inhibitory response was progressively reduced. Conversely, responses to moderately painful stimuli became stronger as the teeth moved closer toward occlusion. Significance: The modulation described allows smooth mastication to occur as it gates out mildly painful signals while responding strongly when the signal indicates potential or actual damage closer to occlusion. Ó 2008 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. 1. Introduction Basic rhythm for mastication is provided by a group of cells in the brain stem, the so called central pattern generator (Dellow and Lund, 1971). Animal studies suggest that the fine control of mastication depends on sensory feedback (Lund et al., 1991). The masticatory system is richly supplied by receptors that do not only supply information regarding static jaw positions and contribute to the development of forces, but also respond to jaw movements and chewing forces dynamically, as to protect the teeth and supporting tissues and to masticate the food bolus effectively (Lund et al., 1991; Türker, 2002). In anaesthetized chewing animals, this peripheral sensory feedback system is responsible for most of the jaw-closing muscle activity (Lavigne et al., 1987; Morimoto et al., 1989). It can be argued however that the contribution from the subrabulbar pathways is not possible in these animals and hence the influence of the peripheral receptors to the development of masticatory forces may have been overestimated. It has been established that sensory feedback is modulated dur- ing movement, so that the muscles are effectively controlled and movement occurs smoothly (Hultborn, 2001). Animal and human work on reflex modulation concludes that synaptic connections of afferents to limb muscles are altered during movement, and that it is not possible to appreciate this flexibility if the reflexes are only studied under static conditions (Stein and Capaday, 1988; Stein et al., 2000). In the masticatory system, evidence for synaptic mod- ulation mostly comes from animal studies. These studies suggest that jaw movement induces modulation on the strength of synap- tic connections between trigeminal afferents and jaw muscle motoneurons (Appenteng et al., 1982; Haraguchi et al., 1994). This modulation allows vigorous mastication to take place, while at the same time acting as a brake to protect the teeth, tongue, and sup- port structures from potentially damaging forces – and thus from pain (Lund and Olsson, 1983; Lund et al., 1991; Lobbezoo et al., 2006). Lund and Olsson’s (1983) experiments are especially rele- vant to the current study. They have shown in anaesthetized chew- ing rabbits that the jaw opening reflex elicited by weak electrical stimuli was reduced in size while the jaw opening reflex elicited by strong stimuli increased during mastication. One of our aims 1388-2457/$34.00 Ó 2008 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2008.11.016 * Corresponding author. Tel.: +90 232 3901810; fax: +90 232 3432653. E-mail address: kemal.turker@ege.edu.tr (K.S. Türker). Clinical Neurophysiology 120 (2009) 398–406 Contents lists available at ScienceDirect Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph