Depression arises from disruptions in the brain's neural circuitry, particularly within networks connecting the prefrontal cortex, amygdala, and hippocampus. These regions regulate mood, emotional processing, and stress response. Neuroimaging studies reveal that individuals with depression often show reduced activity in the left dorsolateral prefrontal cortex (DLPFC), a region critical for positive emotional regulation, and hyperactivity in the amygdala, which amplifies negative emotional responses.
At the molecular level, depression involves imbalances in multiple neurotransmitter systems, including serotonin, norepinephrine, and dopamine. However, modern neuroscience has moved beyond the simple "chemical imbalance" model. Research now points to impaired neuroplasticity, the brain's ability to form new neural connections, as a central mechanism. Brain-derived neurotrophic factor (BDNF), a protein essential for neuronal growth and survival, is consistently found at reduced levels in patients with depression.
Chronic stress triggers sustained activation of the hypothalamic-pituitary-adrenal (HPA) axis, flooding the brain with cortisol. Over time, elevated cortisol damages hippocampal neurons and suppresses neurogenesis, the production of new brain cells, in key mood-regulating regions. This creates a self-reinforcing cycle where stress-induced brain changes perpetuate depressive symptoms even after the initial stressor has resolved.
