Jw. Hu et al., EXCITATORY EFFECTS ON NECK AND JAW MUSCLE-ACTIVITY OF INFLAMMATORY IRRITANT APPLIED TO CERVICAL PARASPINAL TISSUES, Pain, 55(2), 1993, pp. 243-250
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.