Bipolar disorder is a serious mood disorder that is composed of unstable manic (highest energy) and depressive (lowest energy) states. Mania and depression each lie on a spectrum, and the severity of the two dictates which type of bipolar disorder you have.

Type 1: Full-blown mania (often consisting of hallucinations, delusions, and psychosis) as well as dysthymia (mild depression - sadness, low energy, and trouble concentrating) or full-blown depression (associated with feelings of hopelessness, self-loathing, and suicidal ideation).

Type 2: Hypomania (milder mania: hyper-productivity, and little-to-no sleep) and full-blown depression.

Cyclothymia: Fluctuations between hypomania and dysthymia.

If you think you fit multiple of these diagnoses at different times, you might be right. The DSM-5 assumes that we permanently fit inside a tightly-constructed box of symptoms, but I am very skeptical of this.

Small hippocampus: Poor emotional control and memory.
An unstable amygdala: Intense emotional reactions.
Overactive Nucleus Accumbens: Heightened reward during mania.
Smaller and weaker prefrontal cortex: Poor executive functions.
Weaker fornix/thalamus: Impulsivity.
Disrupted Cingulate Gyrus: Unawareness.

My PhD advisor (before I left the program to focus on my recovery) showed me this presentation and asserted that I am also a strong research student, regardless of my mood-related struggles. This presentation gave me so much hope and allowed me to accept and embrace the gifts that come with this curse. If you are also struggling with mood-related symptoms, please do not give up, and I assure you that you are not on your own. It is important to find this sense of community. I hope it helps you in the same way it helped me.

Schizophrenia has similarities with bipolar disorder (such as hallucinations and delusions experienced during mania), but these abnormalities in thought occur regardless of one's mood state. Structural abnormalities with the anterior cingulate cortex can lead to flat affect (or lacking emotional expression), and a disturbed dorsolateral prefrontal cortex abolishes motivation and the ability to make sound decisions.

Schizoaffective disorder is a combination of both schizophrenia and a mood disorder (whether bipolar disorder or depression).

PTSD is a mental health condition that's triggered by a terrifying event, and this trauma may seem small to someone else, but it is significant to the person with the condition. Symptoms may include uncontrollable thoughts surrounding the event, difficulty concentrating, and engaging in self-destructive behaviors (a disrupted prefrontal cortex), unbearable anxiety and panic (an overactive amygdala), or a replay of the haunting memories in the form of flashbacks or nightmares (a damaged hippocampus). 

ADHD can be very debilitating for people. The prefrontal cortex is very underactive, which leads to difficulties focusing, following instructions, remaining patient, poor decision making, and staying organized. The limbic system, which contains the hippocampus and amygdala, is also impaired, which affects memory, troubling regulating emotions, and motivation. With a dysfunctional basal ganglia, one can have trouble sitting still, and the conscious awareness of the reticular activating system is not working properly as well.

For people with depression, many areas of the brain are underactive: it is difficult to stay awake or present (thalamus), escape negative emotions (amygdala and hypothalamus), and cope with anger or pain of any kind, whether physical, emotional, or mental (anterior cingulate cortex). Since the prefrontal cortex is the main executor in our personality, self-discipline, and concentration, a person with depression may feel helpless. Increasing the excitatory neurotransmitters serotonin (involved in mood) and norepinephrine (crucial for alertness) can help these individuals regain their sense of self.

Whether someone has social anxiety, specific phobias, or generalized feelings of fear and uneasiness, anxiety disorders are real and crippling. The limbic system is overactive with a mind looping on pain and emotional discomfort (with the anterior cingulate cortex serving as the broken motor). Meanwhile, the prefrontal cortex cannot get a grip over the anxiety and thought looping, which keeps the anxious mind stuck in worrying, rather than thinking calmly and rationally.