Evaluation of data from clinical studies show that chronic muscle pain is not associated with tonic hyperactivity, as postulated by the vicious cycle theory. Instead, resting EMG activity appears normal. During function, agonist activity is less than normal, while antagonist activity tends to rise slightly.
Experimental pain causes similar changes in muscle activity in man and animals.
The specific aim of the present study was to study how nociceptive muscle inputs act on the brainstem oro-facial motor circuitry during masticatory movements. Rabbits were anaesthetized with halothane, decerebrated at the precollicular level and paralyzed. Fictive mastication was evoked by repetitive stimulation of the cortico-bulbar tract and monitored in the trigminal motoneuron subnuclei. Changes in the masticatory motor pattern and the firing properties of interneurons located in the oral subnucleus of the spinal trigeminal tract, were simultaneously analyzed before, during and after fictive mastication induced in the presence or absence of the noxious stimulations. Our results show that muscle nociception reduces the masticatory frequency and the frequency of discharge of the jaw motoneurons.