This contributing factor occurs when the brainstem is excited by pain, producing protective co-contraction of muscles. This phenomenon is not limited to the masticatory musculature. It occurs throughout the body in response to pain and is a protective response of the body to injury. It can be seen in the masticatory system when, for example, in the presence of a necrotic, painful pulp, the patient may exhibit a limited ability to open the mouth. Once the tooth pain is resolved, the ability to open the mouth typically returns to normal.

The clinician diagnosing a patient with muscle pain and limited range of motion needs to be very aware of this. In addition to pain from an abscessed tooth, other forms of oral pain might include an inflamed operculum, a periodontal abscess, or any other type of common painful dental conditions. When there is reason to suspect a condition of this kind, before prescribing treatment for a TMD, the offending dental condition should be addressed and the patient should be re-evaluated for their original presenting symptoms after the dental condition is resolved.

This same response is seen with pain originating from the temporomandibular joints. As one of several causes of masticatory muscle hypertonicity, the masticatory muscles will tighten in response to the joint pain. So even in the presence of obvious pain from the TM joints, when there is another source of dental pain present, care must be taken in assuming that the response seen in the muscles is entirely due to pain from the joints. Because resolving the dental pain may be the quickest and easiest problem to address, it is usually best to make that a priority before getting extensively involved in treatment of the TMD.