Hypothalamic–pituitary–adrenal axis

The hypothalamic–pituitary–adrenal axis (HPA axis or HTPA axis) is a complex set of direct influences and feedback interactions among three components: the hypothalamus (a part of the brain located below the thalamus), the pituitary gland (a pea-shaped structure located below the hypothalamus), and the adrenal (also called "suprarenal") glands (small, conical organs on top of the kidneys). These organs and their interactions constitute the HPA axis. The HPA axis is a major neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, immune responses, mood and emotions, sexual activity, and energy storage and expenditure. It is the common mechanism for interactions among glands, hormones, and parts of the midbrain that mediate the general adaptation syndrome (GAS). While steroid hormones are produced mainly in vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well. The HPA axis, hypothalamic–pituitary–gonadal (HPG) axis, hypothalamic–pituitary–thyroid (HPT) axis, and the hypothalamic–neurohypophyseal system are the four major neuroendocrine systems through which the hypothalamus and pituitary direct neuroendocrine function. The key elements of the HPA axis are: The paraventricular nucleus of the hypothalamus: It contains neuroendocrine neurons which synthesize and secrete vasopressin and corticotropin-releasing hormone (CRH). The anterior lobe of the pituitary gland: CRH and vasopressin stimulate the anterior lobe of pituitary gland to secrete adrenocorticotropic hormone (ACTH), once known as corticotropin. The adrenal cortex: It produces glucocorticoid hormones (mainly cortisol in humans) in response to stimulation by ACTH. Glucocorticoids in turn, act back on the hypothalamus and pituitary (to suppress CRH and ACTH production) in a negative feedback cycle. CRH and vasopressin are released from neurosecretory nerve terminals at the median eminence.