I think it's both. The nerves and muscles are alll located so close together in the jaw joint area that everything is affected. For instance, my jaw makes a grinding sound every time I open it and when i experience earaches (one of my many tmjd side effects) they are remedied by opening my mouth and having a loud pop in the joint and instantly the pain goes away and the surrounding muscle tension is relieved ( mind u, the joint then makes a Squeaking noise for several hours afterward when I try to open it) My biggest issue is ear popping and fullness. That is a muscular issue because it is caused by the eustachian tube which is controlled by the muscles in there. When my muscle tension and jaw pain is worse then the ear popping and fullness is worse. Also I get frequent muscle spasms in my ears usually caused by my jaw feeling like it's locked. I'm a hairstylist and my problems have definitely coincided with my job. My neck, back and shoulders are all mangled from my job and when I'm focusing on a client I clench my teeth.... Both these things are obvious contributing factors. The point here is that its probably both muscle and joint because they are so closely related in potion that one will ultimately have an effect on the other.