The hypothalamic-pituitary-adrenal (HPA) axis is the primary biological system through which environmental stressors are transduced into neurological and psychiatric disease. Chronic environmental stressors — poverty, violence, discrimination, climate threat — produce sustained cortisol elevation that structurally reshapes the brain over years.
1
Stressor Perception
The prefrontal cortex and amygdala appraise a threat. The amygdala activates the hypothalamus via corticotropin-releasing hormone (CRH).
2
Pituitary Activation
CRH signals the anterior pituitary to release adrenocorticotropic hormone (ACTH) into the bloodstream.
3
Adrenal Response
ACTH reaches the adrenal glands, triggering cortisol release. Cortisol mobilizes glucose, suppresses immune function, and heightens alertness.
4
Hippocampal Feedback
Cortisol normally feeds back to suppress further CRH release. In chronic stress, hippocampal glucocorticoid receptors downregulate — the feedback fails. The axis stays activated.
5
Structural Damage
Sustained cortisol suppresses hippocampal neurogenesis, reduces BDNF, shrinks PFC gray matter, and enlarges the amygdala. These changes increase reactivity to future stressors — allostatic load accumulates.