How does manic depression affect one's brain and emotions?

Dramatically. Manic depression, aka bipolar disorder, is, a major mood disorder that has profound and variable effects on emotions which may range from elation to range to profound depression. The neurotransmitter systems are altered in many areas throughout the brain. Although most abnormalities are subtle and not structural there is some evidence for subtle structural brain abnormalities as well.
Mood swings. Manic depression, now known as bipolar disorder, is characterized by swings of mood, often extreme, and these swings can happen slowly or quickly, even several times during a day, especially if left unmedicated. Manic phases can even become psychotic. Imagine waking up feeling as though you can beat up the universe with your pinky finger, then a short time later feeling suicidal and despondent.
Manic depression. Bipolar disorder affects brain chemicals known as neurotransmitters, mainly dopamine, norepinephrine and serotonin. Changes in these chemicals affect mood, cognition, perceptions and rational thinking.