Effect of experimental posterior temporalis muscle pain on human brainstem reﬂexes Anitha Peddireddy a , Kelun Wang a , Peter Svensson a,b , Lars Arendt-Nielsen a, * a Orofacial Pain Laboratory, Center for Sensory-Motor Interaction, Aalborg University, Fredrik Bajars Vej 7D-3, 9220 Aalborg, Denmark b Department of Clinical Oral Physiology, School of Dentistry, University of Aarhus, Aarhus, Denmark Accepted 23 February 2005 Available online 29 April 2005 Abstract Objective: To study the modulation of jaw-stretch and blink reﬂexes by experimental posterior temporalis muscle pain. Methods: Thirty healthy volunteers (15 males, 25.5G0.6 years and 15 females, 27.4G1.2 years) were included. Short-latency stretch reﬂex responses were evoked in the masseter and temporalis muscles by fast stretches (1 mm displacement, 10 ms ramp time) and the blink reﬂexes were evoked by painful electrical pulses (0.5 ms duration), delivered by a concentric electrode placed on the left lower forehead close to the supraorbital foramen before, during and 15 min after a period with experimentally induced muscle pain. Results: The normalized peak-to-peak amplitude of the stretch reﬂex in the painful temporalis was signiﬁcantly higher during pain in both males and females compared with pre- and post-pain conditions (P!0.004). The R2 root mean square (RMS) of the blink reﬂex decreased signiﬁcantly during muscle pain as compared to the pre-pain (P!0.03) in both males and females. Conclusions: The present results indicated that experimental posterior temporalis muscle pain facilitates the jaw-stretch reﬂex, whereas the nociceptive speciﬁc blink reﬂex is inhibited. Signiﬁcance: Present study suggested that these reﬂexes are suitable models for probing pontine and medullary pain processing. q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Human experimental muscle pain; Stretch reﬂex; Blink reﬂex; Trigeminal brainstem reﬂexes 1. Introduction Some of the most common acute and chronic pain conditions occur in the face and mouth (for example, toothaches, migraine headaches, tension-type headaches, temporomandibular disorders (TMD)). The masseter stretch reﬂexes, and the blink reﬂexes are useful diagnostic tools for evaluation of brain stem disorders (Hopf, 1994). Monitoring jaw stretch reﬂex parameters makes it possible to look into the excitability of the trigeminal motoneuron pool. The level of this excitability may be modiﬁed by inputs from the orofacial area during function and dysfunction (Murray and Klineberg, 1984). Sudden stretches of the jaw-closing muscle can elicit short-latency excitatory response in the muscles, so called jaw-jerk reﬂexes or stretch reﬂexes. The stretch reﬂex in jaw-closing muscles is the trigeminal equivalent of the monosynaptic, myotatic spinal reﬂexes of the limb muscles (Lund et al., 1983). One of the functions of the stretch reﬂex in the jaw-closing muscles is to maintain and restore the postural position of the mandible when it is perturbed during rapid head movements (Miles et al., 2003). It has recently been shown that injection of algogenic substances such as hypertonic saline, capsaicin and glutamate into the human masseter muscle increases the short-latency reﬂex response to stretch in both the ipsi- and contra-lateral masseter muscles, when measured in the surface electromyogram (EMG) (Cairns et al., 2003; Svensson et al., 2000; Wang et al., 2000, 2001, 2002, 2004). However, no studies have so far examined the inﬂuence of posterior temporalis experimental pain on the human jaw-stretch reﬂex. There is evidence that tension Clinical Neurophysiology 116 (2005) 1611–1620 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2005.02.022 * Corresponding author. Tel.: C45 9635 8830; fax: C45 9815 4008. E-mail address: lan@miba.auc.dk (L. Arendt-Nielsen).