Depression is not a sign of weakness or a mood you can will yourself out of: it is a biological condition that involves the brain's chemistry, circuitry, and stress-response systems. Neurotransmitters such as serotonin, norepinephrine, and dopamine help regulate mood, motivation, and pleasure, and when signaling within these systems becomes disrupted, symptoms of depression emerge.
Research also points to reduced activity in the prefrontal cortex and altered function in the limbic system, which together govern emotional regulation, decision-making, and memory. These changes help explain why depression often feels cognitive (trouble focusing, ruminating thoughts) in addition to emotional, and why TMS therapy for depression can be effective by directly stimulating these underactive regions.
Chronic stress compounds the picture by elevating cortisol, which over time can shrink the hippocampus and impair the brain's ability to form new neural connections. This is one reason combined care, such as medication plus psychotherapy for depression, tends to produce stronger, more durable results than either alone.
