Is the bite the only thing that can throw the muscles into spasm with resulting pain?

No. Torn cartilage, bony degeneration, cervical issues, neurologic issues, Lymes disease and many other inputs can also cause muscular spasms and pain in the TMD patient. The more a practitioner looks into this with objective tools such as T-Scan, EMG, JVA, CBCT and MRI, the more likely they will find out why and if their patents pain(s) is (are) due to an occlusally (bite) based muscular TMD issue, or not.