While medical science has not determined the exact cause of altered serotonin levels that are suspected to produce depression and bipolar disorder, recent research has pointed towards a likely trauma-induced origin for certain cases of mood disorders. Evidence supports that trauma (in particular mild concussive injury to the head, neck or upper back) increases the risk of onset of depression. Following the trauma, mood disorders can be triggered immediately or can take months or years to develop. Chiropractic care may reverse the trauma-induced injury; thereby reducing irritation to the injured nerves in the central nervous system (brain and spinal cord). Numerous research studies have documented improvement in depression and related symptoms as a result of chiropractic adjustments which reduce pressure on the brain stem and spinal cord caused by misaligned vertebrae.