Capsaicin-induced Muscle Pain Alters the Excitability of the Human Jaw-stretch Reflex
K. Wang1,*,
L. Arendt-Nielsen1, and
P. Svensson2,3
1 Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg S, Denmark;
2 Department of Clinical Oral Physiology, Royal Dental College, University of Aarhus; and
3 Department of Maxillofacial Surgery, Aarhus University Hospital, Denmark;
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Figure 1. Averaged reflex response (12 sweeps) in the right masseter evoked by fast stretch (ramp time: 10 ms) in a single subject. Arrow shows the onset of the stretch stimulus. The amplitude was measured as the peak-to-peak value. The normalized peak-to-peak amplitude was calculated as the pre-stimulus EMG divided by peak-to-peak amplitude.
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Figure 2. Normalized peak-to-peak amplitude of stretch reflexes in left (A) and right (B) masseter and left (C) and right (D) temporalis muscles evoked at different stretch velocities and the effect of injection of capsaicin into the right masseter. The ordinate axis represents the percentage of normalized peak-to-peak amplitude. For clarity, the stretch velocity is displayed on a logarithmic scale. Mean values + SEM (n = 11). * indicates significant difference between conditions (SNK: P < 0.05).
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Figure 3. Normalized peak-to-peak amplitude of stretch reflexes in left (A) and right (B) masseter and left (C) and right (D) temporalis muscles evoked at different displacements. Note: The stretch velocity of 25 mm/sec is constant, since the ramp time is adjusted accordingly. Mean values + SEM (n = 11). Two-way ANOVAs indicated significantly higher amplitudes during the pain condition compared with the pre- and post-pain conditions in the right masseter.
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