The key elements of the HPA axis are: * The paraventricular nucleus of the hypothalamus, which contains neuroendocrine neurons which synthesize and secrete vasopressin and corticotropin-releasing hormone (CRH). These two peptides regulate: * The anterior lobe of the<unk>  gland. In particular, CRH and vasopressin stimulate the secretion of adrenocorticotropic hormone (ACTH), once known as corticotropin. ACTH in turn acts on: * the<unk>  cortex, which produces glucocorticoid hormones (mainly cortisol in humans) in response to stimulation by ACTH. Glucocorticoids in turn act back on the hypothalamus and<unk>  (to suppress CRH and ACTH production) in a negative feedback cycle. CRH and vasopressin are released from neurosecretory nerve terminals at the median eminence. CRH is transported to the anterior<unk>  through the portal blood vessel system of the hypophyseal stalk and vasopressin is transported by axonal transport to the posterior<unk>  gland. There, CRH and vasopressin act synergistically to stimulate the secretion of stored ACTH from corticotrope cells. ACTH is transported by the blood to the<unk>  cortex of the<unk>  gland, where it rapidly stimulates biosynthesis of corticosteroids such as cortisol from cholesterol. Cortisol is a major stress hormone and has effects on many tissues in the body, including the brain. In the brain, cortisol acts on two types of receptor – mineralocorticoid receptors and glucocorticoid receptors, and these are expressed by many different types of neurons. One important target of glucocorticoids is the hypothalamus, which is a major controlling centre of the HPA axis. Vasopressin can be thought of as "water conservation hormone" and is also known as "antidiuretic hormone." It is released when the body is dehydrated and has potent water-conserving effects on the kidney. It is also a potent vasoconstrictor. Important to the function of the HPA axis are some of the feedback loops: * Cortisol produced in the<unk>  cortex will negatively feedback to inhibit both the hypothalamus and the<unk>  gland. This reduces the secretion of CRH and vasopressin, and also directly reduces the cleavage of proopiomelanocortin (POMC) into ACTH and β-endorphins. * Epinephrine and norepinephrine (E/NE) are produced by the<unk>  medulla through sympathetic stimulation and the local effects of cortisol (upregulation enzymes to make E/NE). E/NE will positively feedback to the<unk>  and increase the breakdown of POMCs into ACTH and β-endorphins.
Hypothalamic–pituitary–adrenal axis
Hypothalamic–pituitary–somatotropic axis
Hypothalamic–pituitary–thyroid axis
Sympathoadrenal system
Hypothalamic–pituitary–gonadal axis
Hypothalamic–pituitary–prolactin axis
Parasympathetic nervous system
Autonomic nervous system