Neck muscle length modulates nociceptive reflex evoked by noxious irritant application to rat neck tissues

The application of mustard oil (MO), a small-fibre excitant and inflammatory irritant, into neck paraspinal muscles of the rat has been shown to produce a significant reflexive increase in electromyographic (EMG) activity in both neck and jaw muscles. It is possible that this nociceptive reflex activity is influenced by muscle length since recent evidence indicates that abnormal neck posture may be associated with cervical musculoskeletal disorders and pain. Therefore, the aim of this study was to test if muscle length modulates this nociceptive reflex response. Three different experimental procedures were employed in rats under halothane anesthesia: (1) MO injected into the left deep neck muscles with the rat placed in a straight body position (Straight group, n = 7); (2) MO injected into lengthened left deep neck muscles with the rat's neck rotated 45 degrees to the right with respect to the head (Stretched group, n = 11); and (3) MO injected into the right deep neck muscles with the rat's neck rotated 45 degrees to the right (Relaxed group, n = 9). The EMG activity of the deep neck, trapezius, and digastric muscles was bilaterally recorded, rectified and integrated into area under the curve (AUC). Control injections of the vehicle, mineral oil, did not evoke any muscle activity but MO evoked EMG activity in the ipsilateral deep neck and trapezius muscles of the Stretched group that was significantly greater than that evoked in the same muscles in the Straight and Relaxed groups. Also, the MO-evoked EMG activity in the contralateral deep neck muscles of the Stretched and Relaxed groups was greater than that of the corresponding muscles in the Straight group. The MO-evoked activity in the digastric, a jaw muscle whose length was not changed, did not show any significant difference between the three groups. These findings indicate that MO application to the rat deep neck muscles results in a larger nociceptive reflex in deep neck and trapezius muscles when they are stretched. This enhanced muscle activity could be associated with changes in the susceptibility of the neck muscles to pain or damage.