Hypothalamic-Pituitary Axis

Our response to stress, physical or psychological, begins in the brain with the activation of the neuroendocrine system, the hypothalamic-pituitary-adrenal, or HPA, axis.

The hypothalamus, the H, releases a corticotropin-releasing hormone, or CRH, which activates the pituitary gland, the P in the axis. While this neuroendocrine gland has many functions, during times of stress it releases the adrenal corticotropic hormone, or ACTH, into the blood.

ACTH stimulates the adrenal cortices, the A, small structures on the surface of the kidneys. The adrenals release two types of hormones.

First are norepinephrine and epinephrine, two sympathomimetic compounds responsible for arousing the body to action by initiating the sympathetic nervous system and causing increased heart rate and blood flow.

Second are glucocorticoids, specifically cortisol. They activate systems in the body designed to sustain stress reactions by mobilizing energy reserves via increased glucogenesis and suppressing non-vital processes like the immune system.

When cortisol levels get high, a negative feedback loop onto both the pituitary and hypothalamus is initiated. This reaction stops the release of ACTH and CRH, putting an end to the stress response.