People with bipolar disorder show deficits in the connectivity between the parietal regions of the brain and the limbic system. The limbic system, responsible for regulating emotional states, from relaxation to alertness, is closely linked to the autonomic nervous system, which controls sleep cycles, digestion, hunger, and other bodily functions. Disruptions in these connections can cause extreme mood swings, as the limbic system acts like a volume control for emotional activation.
In individuals with bipolar disorder, the parietal lobe, which communicates bidirectionally with the limbic system, seems to exert less top-down control, meaning it can suppress certain aspects of the limbic system less effectively. This results in the system becoming overactive, causing the emotional extremes associated with the condition. Essentially, two major deficits in neural circuits are present in bipolar disorder: one affecting self-awareness and the other affecting energy regulation, both of which are influenced by disrupted neural pathways in the brain.
Source: The American Journal of Psychiatry