Depression caused by TMJ is suspected to be mediated by numerous mechanisms in the central nervous system. There is extensive medical literature linking depression and TMJ. Depression is so prevalent with TMJ that TMJ is thought to be a psychosomatic disorder by certain schools of thought.
One known pathway in which depression is caused by TMJ is through trigeminal modulation of the limbic part of the brain. The trigeminal nerve (by multisynaptic pathways) has a very large influence on this part of the brain (Pain. 2010 May;149(2):222-8. Epub 2010 Mar 16. Chronic myofascial temporomandibular pain is associated with neural abnormalities in the trigeminal and limbic systems. Younger JW, Shen YF, Goddard G, Mackey SC.).
Another pathway in which depression is caused by TMJ, is by way of substance P which is elevated with jaw misalignment and chronic pain. Substance P is a major neurotransmitter in the brain and is known to effect serotonin and dopamine levels (Mol Pharmacol. 2007 Dec;72(6):1411-8. Epub 2007 Sep 21. Substance P neurokinin 1 receptor activation within the dorsal raphe nucleus controls serotonin release in the mouse frontal cortex. Guiard BP, Guilloux JP, Reperant C, Hunt SP, Toth M, Gardier AM.). Depressed serotonin levels are known to be associated with depression (Nervenarzt. 2007 Nov;78 Suppl 3:531-47; quiz 548-9. [New insights into the pathogenesis and pathophysiology of depression]. [Article in German] Schüle C, Baghai TC, Rupprecht R.).
Depression could occur due to jaw misalignment even when there is no manifestation of head pain or jaw dysfunction. Substance P levels can be covertly effected by proprioceptive pathways of the trigeminal nerve influencing the mesencephalic nucleus in the brain.
There is extensive medical literature showing a very strong correlation between TMJ dysfunction and depression. Clinically, jaw alignment therapy is very effective at reversing depression and other mood disorders.