Bipolar disorder is thought to be associated with abnormalities in the structure and function of certain brain areas responsible for cognitive tasks and the processing of emotions. A neurologic model for bipolar disorder proposes that the emotional circuitry of the brain can be divided into two main parts. The ventral system (regulates emotional perception) includes brain structures such as the amygdala, insula, ventral striatum, ventral anterior cingulate cortex, and the prefrontal cortex. The dorsal system (responsible for emotional regulation) includes the hippocampus, dorsal anterior cingulate cortex, and other parts of the prefrontal cortex. The model hypothesizes that bipolar disorder may occur when the ventral system is overactivated and the dorsal system is underactivated. Other models suggest the ability to regulate emotions is disrupted in people with bipolar disorder and that dysfunction of the ventricular prefrontal cortex (vPFC) is crucial to this disruption.Meta-analyses of structural MRI studies have shown that certain brain regions (e.g., the left rostral anterior cingulate cortex, fronto-insular cortex, ventral prefrontal cortex, and claustrum) are smaller in people with bipolar disorder, whereas other regions are larger (lateral ventricles, globus pallidus, subgenual anterior cingulate, and the amygdala). Additionally, these meta-analyses found that people with bipolar disorder have higher rates of deep white matter hyperintensities.Functional MRI findings suggest that the vPFC regulates the limbic system, especially the amygdala. In people with bipolar disorder, decreased vPFC activity allows for the dysregulated activity of the amygdala, which likely contributes to labile mood and poor emotional regulation.
Of the 16 genes identified in these pathways, three were found to be dysregulated in the dorsolateral prefrontal cortex portion of the brain in post-mortem studies: CACNA1C, GNG2, and ITPR2.Bipolar disorder is associated with reduced expression of specific DNA repair enzymes and increased levels of oxidative DNA damages. Environmental
Psychosocial factors play a significant role in the development and course of bipolar disorder, and individual psychosocial variables may interact with genetic dispositions. Recent life events and interpersonal relationships likely contribute to the onset  and recurrence of bipolar mood episodes, just as they do for unipolar depression. In surveys, 30–50% of adults diagnosed with bipolar disorder report traumatic/abusive experiences in childhood, which is associated with earlier onset, a higher rate of suicide attempts, and more co-occurring disorders such as post-traumatic stress disorder. The number of reported stressful events in childhood is higher in those with an adult diagnosis of bipolar spectrum disorder than in those without, particularly events stemming from a harsh environment rather than from the childs own behavior. Acutely, mania can be induced by sleep deprivation in around 30% of people with bipolar disorder. Neurological
Less commonly, bipolar disorder or a bipolar-like disorder may occur as a result of or in association with a neurological condition or injury including stroke, traumatic brain injury, HIV infection, multiple sclerosis, porphyria, and rarely temporal lobe epilepsy. Proposed mechanisms
The precise mechanisms that cause bipolar disorder are not well understood.