(A) CRH stimulates zona fasiculata leading to increased cortisol production.# KEY ELEMENTS OF THE HPA AXIS are:> Paraventricular nucleus of the hypothalamus, contains neuroendocrine neurons that synthesize and secrete vasopressin & corticotropin-releasing hormone (CRH).> These two peptides regulate: Anterior lobe of the pituitary gland. In particular, CRH and vasopressin stimulate the secretion of adrenocorticotropic hormone (ACTH), once known as corticotropin. ACTH in turn acts on: Adrenal cortex, which produces glucocorticoid hormones (mainly cortisol in humans) in response to stimulation by ACTH. Glucocorticoids in turn act back on the hypo- thalamus and pituitary (to suppress CRH and ACTH production) in a negative feedback cycle.> CRH & Vasopressin are released from neurosecretory nerve terminals at the median eminence.> CRH is transported to the anterior pituitary through the portal blood vessel system of the hypophyseal stalk & vasopressin is transported by axonal trans- port to the posterior pituitary gland.> CRH and vasopressin act synergistically to stimulate the secretion of stored ACTH from corticotrope cells.> ACTH is transported by the blood to the adrenal cortex of the adrenal gland, and 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 adrenal cortex will negatively feedback to inhibit both the hypothalamus and the pituitary gland. Reduces the secretion of CRH and vasopressin, and also directly reduces the cleavage of propiomelanocortin (POMC) into ACTH and b-endorphins. Epinephrine and norepinephrine (E/NE) are produced by the adrenal medulla through sympathetic stimulation and the local effects of cortisol (upregulation enzymes to make E/NE). E/NE will positively feedback to the pituitary and increase the breakdown of POMCs into ACTH and b-endorphins.