The most clinically important hypothalamic axis is the hypothalamic-pituitary-adrenal axis, abbreviated HPA axis. This is the stress response. Memorize the cascade because it underlies depression, PTSD, anxiety disorders, Cushing's syndrome, Addison's disease, and the long-term physiological cost of chronic stress.
Acute stress — physical or psychological — activates the paraventricular nucleus of the hypothalamus. The PVN releases corticotropin-releasing hormone (CRH) into the hypothalamic-pituitary portal system. CRH travels a short distance to the anterior pituitary, where it stimulates the release of adrenocorticotropic hormone (ACTH) into the systemic circulation. ACTH travels through the bloodstream to the adrenal cortex, where it stimulates the release of cortisol.
Once released, cortisol does three things. First, it mobilizes glucose — gluconeogenesis in the liver, glucose release into the bloodstream, fuel for the body's response. Second, it suppresses inflammation — useful in the short term, problematic in chronic excess. Third, it feeds back to the hypothalamus and pituitary to inhibit further CRH and ACTH release, closing the loop. This negative feedback prevents the axis from running indefinitely.
The system is exquisitely calibrated for acute, time-limited stress. A perceived threat activates the axis; the threat passes; cortisol levels return to baseline; the body returns to homeostasis. The diurnal pattern of cortisol — high in early morning to wake the body, declining through the day, lowest in the early evening — reflects healthy axis function.
Cortisol does its work by binding glucocorticoid receptors throughout the body. The receptor density is particularly high in the hippocampus, which is one reason chronic cortisol elevation produces measurable hippocampal atrophy. We will return to this in Stage 6. For now, hold the cascade: hypothalamus, pituitary, adrenal, cortisol, feedback.
In the next concept, we will see what happens when the loop dysregulates and cortisol stays elevated for years — the physiological substrate of chronic depression, PTSD, and the cumulative wear of unrelenting stress.