Elevated intramuscular concentration of bradykinin in law muscle increasesthe fusimotor drive to neck muscles in the cat

Patients suffering from temporomandibular dysfunction exhibit clinical find ings such as fatigue, painful muscles, and muscles that are tender to palpa tion, not only in the temporomandibular area, but also in the neck/shoulder region. The mechanisms behind this are not known, although previous studie s of similar spreading phenomena have revealed a possible involvement of th e fusimotor system. In the present study, we evaluated the activity of this system by recording the activity of muscle spindle afferents from dorsal n eck muscles after intramuscular injections of 6 mug to 25 mug bradykinin in the ipsilateral masseter muscle. A total of 23 muscle spindle afferents fr om the trapezius and splenius muscles was recorded at the C-3-C-4 level in 7 adult cats anesthetized with alpha -chloralose. Of these 23 afferents, 17 (74%) showed significantly different responses to bradykinin injections co mpared with control injections (dissolving agent, Tyrode), and the majority of the effects were compatible with an increased static fusimotor drive to the muscle spindle system. Thus, the results demonstrate potent reflex con nections from groups III and IV masseter muscle afferents to fusimotor neur ones on the C-3-C-4 level. It is concluded that the fusimotor system might play a significant role in the mechanisms behind the spread of muscle pain and tension from the temporomandibular region to the neck.