HPA Axis

entityUpdated 2026-06-14
hpa-axiscortisolhypothalamushpa-axissympathetic-nervous-systemallostatic-load
Related: Sympathetic Nervous System, Allostasis and Allostatic Load, Parasympathetic Branch

HPA Axis

The HPA axis (Hypothalamic-Pituitary-Adrenal axis) is the body's slow, sustained stress-response pathway — the "marathon runner" to adrenaline's "sprinter." (Card: "Sympathetic Branch")

Key Functional Role

The HPA axis is a three-step cascade:

  1. The hypothalamus — described as the "CEO" of the body, a pea-sized structure monitoring "corporate metrics" like blood temperature, plasma salt, and hormone levels — releases CRH (corticotropin-releasing hormone) when it detects a deviation from the optimal range or receives a threat signal.
  2. CRH triggers the pituitary gland to release ACTH (adrenocorticotropic hormone).
  3. ACTH triggers the adrenal cortex to release cortisol.

Cortisol's job is energy management: it raises blood glucose (triggering the liver to break down glycogen) and inhibits "non-essential" functions — growth, reproduction, immune activity — so calories go to muscle and brain. Unlike the immediate adrenaline/noradrenaline surge (seconds), the HPA axis cascade unfolds over minutes to hours and can sustain elevated arousal for much longer. (Card: "Sympathetic Branch")

How It Appears in This Domain

The HPA axis is the second of the two SNS activation waves described in sympathetic-nervous-system, and it is the central mechanism behind chronic allostatic-load: under sustained stress, the "catecholamine cascade" (adrenaline) and the HPA axis "never fully shut off." McEwen's Four Pathways to Allostatic Load describe the ways this axis can become pathologically engaged — repeated hits, lack of habituation, prolonged response, or (the fourth, less intuitive pathway) an inadequate HPA/SNS response that shifts the load onto the immune system instead. (Card: "Allostasis and the Baseline")

The hypothalamus and brainstem (medulla, pons) together form the ANS's "command centers" — the hypothalamus makes executive decisions, while brainstem nuclei (cardiovascular center, respiratory center) act as "middle management," relaying signals to organs via the spinal cord and cranial nerves and receiving constant feedback in return. (Card: "ANS as Automatic Pilot")

Sources

  • Card: "Sympathetic Branch"
  • Card: "Allostasis and the Baseline"
  • Card: "ANS as Automatic Pilot"