Reflex excitation of masticatory muscles induced by algesic chemicals applied to the temporomandibular joint of the cat.

Algesic chemicals (7 per cent NaCl, KCl and histamine) applied to this joint of anaesthetized cats evoked reflexes in the ipsilateral anterior digastric, temporalis and genioglossus muscles. Whereas the application of isotonic saline was only briefly and weakly effective, and only consistently so in the genioglossus, a single application of each chemical could produce a sustained increase in electromyographic activity of all the muscles. The excitatory effects usually lasted 30 s or more, and the onset latency and latency to peak activity were usually less than 10 s and 20 s, respectively. These reflex excitatory effects and their temporal characteristics are consistent with recent findings of the effects of these algesic chemicals on trigeminal brainstem nociceptive neurones, and provide support for concepts of temporomandibular joint pain and dysfunction that are based on reflexly induced increases in masticatory muscle activity.