The main part of the brain that bipolar disorder affects is the prefrontal cortex. The prefrontal cortex is smaller in people with bipolar disorder, notes the National Institute of Mental Health.
Studies show that in some people with the disorder, the prefrontal cortex does not functional as well as it does in people without the condition. The neurotransmitters in the brain also play a part in bipolar disorder, as stated by WebMD. The main contributor is serotonin and this natural chemical can also be responsible for other mood disorders, including depression. People with relatives suffering from bipolar disorder are more likely to have it than people without a family history.