EXCITATORY EFFECTS ON NECK AND JAW MUSCLE-ACTIVITY OF INFLAMMATORY IRRITANT APPLIED TO CERVICAL PARASPINAL TISSUES

A study was carried out in 19 anaesthetized rats to determine if the e lectromyographic (EMG) activity of jaw and neck muscles could be influ enced by injection of the inflammatory irritant mustard oil into deep paraspinal tissues surrounding the C1-3 vertebrae. The EMG activity wa s recorded ipsilaterally in the digastric, masseter and trapezius musc les and bilaterally in deep neck muscles (rectus capitis posterior). I n comparison with control (vehicle) injections, mustard oil (20 mu 1, 20%) injected into the deep paraspinal tissues induced significant inc reases in EMG activity in the neck muscles in all the animals and in t he jaw muscles in the majority of the animals; the effects of mustard oil were more prominent in the former. The EMG response evoked by must ard oil injection was frequently reflected in two phases of enhanced a ctivity. The early phase of the increase in EMG activity was usually i nitiated immediately following mustard oil injection (mean latency: 20 .4 +/- 17.7 sec) and lasted 1.6 +/- 1.1 min. The second phase occurred 11.3 +/- 7.6 min later and lasted ll.0 +/- 8.1 min. Evans Blue extrav asation was apparent in the deep paraspinal tissues surrounding the C1 -3 vertebrae after mustard oil injection, and histological examination showed that mustard oil injection induced an inflammatory reaction in the rectus capitis posterior muscle. These results document that inje ction of the inflammatory irritant mustard oil into deep paraspinal ti ssues results in a sustained and reversible activation of both jaw and neck muscles. Such effects may be related to the reported clinical oc currence of increased muscle activity associated with trauma to deep t issues.