The temporalis muscle is a very large fan shaped muscle, which is found within the area of the red circle in the image on the right. It is one of the four closing muscles of the jaw. The other three are the superficial masseter, the deep masseter, and the medial pterygoid muscles.
When any muscle hits its tipping point, it will start shooting off pain. The pain can be caused by overusing the muscle due to clenching, grinding, chewing gum, and any stressful event that would cause a patient to clench. The pain can also be caused from long dental appointments due to the patient being open for an extended period of time. It is not usually caused by occlusion, which is a dental term that refers to how the teeth meet but it can be caused by excessive occlusion, which means the patient is holding their teeth together too much.
I always recommend having patients stretch using the gentle jaw before trying trigger points and/or Botox. Stretching is always beneficial for any tight muscle but when it doesn't help enough then procedures are indicated. This procedure is done using 1cc of 1% plain lidocaine. It is not a painful procedure but it has to be done properly. It is not a traditional injection at all. Once the lidocaine is injected, the muscles must be "needled" to break up the tigger points, or knots, that are causing the pain so needling takes about 30 to 60 seconds but since patients are numb, they rarely feel anything. As you can see the procedure start to finish takes about 2 minutes per injection.
Here is a page that shows a trigger point injection of the superficial masseter muscle.
Dr. Hirschinger is the inventor of the gentle jaw®, a passive stretching device for treatment of acute and chronic TMJ and muscle pain. gentle jaw is "yoga for the jaw®."