The Hypothalamus and Pituitary Glands

The hypothalamus makes up the lower region of the diencephalons and lies just above the brain stem. The pituitary gland (hypophysis) is attached to the bottom of the hypothalamus by a slender stalk called the infundibulum. The pituitary gland consists of two major regions—the anterior pituitary gland (anterior lobe or adenohypophysis) and the posterior pituitary gland (posterior lobe or neurohypophysis).

The hypothalamus oversees many internal body conditions. It receives nervous stimuli from receptors throughout the body and monitors chemical and physical characteristics of the blood, including temperature, blood pressure, and nutrient, hormone, and water content. When deviations from homeostasis occur or when certain developmental changes are required, the hypothalamus stimulates cellular activity in various parts of the body by directing the release of hormones from the anterior and posterior pituitary glands. The hypothalamus communicates directives to these glands by one of the following two pathways:

• Communication between the hypothalamus and the anterior pituitary occurs through chemicals (releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to the anterior pituitary through blood vessels. The releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus called neurosecretory cells. The hormones are released into a capillary network (primary plexus) and transported through veins (hypophyseal portal veins) to a second capillary network (secondary plexus) that supplies the anterior pituitary. The hormones then diffuse from the secondary plexus into the anterior pituitary, where they initiate the production of specific hormones by the anterior pituitary. The releasing and inhibiting hormones secreted by the hypothalamus and the hormones produced in response by the anterior pituitary are listed in Table 1 . Many of the hormones produced by the anterior pituitary are tropic hormones (tropins), hormones that stimulate other endocrine glands to secrete their hormones.
  

  TABLE 1 Hormone Functions Source Hormone (H), Releasing Hormone (RH), Or Inhibiting Hormone (IH) Chemical Form* Target Action Hypothalamus GHRH growth hormone RH PP anterior pituitary inhibits release of hGH GHIH growth hormone IH (somatostatin) PP anterior pituitary stimulates release of hGH TRH thyrotropin RH PP anterior pituitary stimulates release of TSH and hGH GnRH gonadotropin RH PP anterior pituitary stimulates release of LH and FSH PRH prolactin RH PP anterior pituitary stimulates release of PRL PIH prolactin IH (dopanmine) PP anterior pituitary inhibits release of PRL CRH corticotropin RH PP anterior pituitary stimulates release of ACTH Anterior pituitary (tropic hormones) TSH thyroid stimulating H (thyrotropin) GP thyroid stimulates secretion of T3 and T4 ACTH adrenocortico-tropic hormone PP adrenal cortex stimulates secretion of glucocorticoids FSH follicle-stimulating hormone GP ovary, testes regulates oogenesis & spermatogenesis LH luteinizing hormone GP ovary, testes regulates oogenesis & spermatogenesis Anterior pituitary (hormones) PRL prolactin PR mammary glands stimulates production of milk hGH human growth H (somatotropin) PR bone, muscle, various stimulates growth Posterior pituitary OT oxytocin PP uterus, mammary glands uterine contractions, release of milk ADH antidiuretic H (vasopressin) PP kidneys, sweat glands increases water retention Thyroid gland T4 thyroxine AA most body cells increases rate of cellular metabolism T3 triiodothyronine AA bone increases rate of cellular metabolism calcitonin PP bone decreases blood Ca2+ Parathyroid gland PTH parathyroid hormone PP bone, kidneys, intestine increases blood Ca2+ Adrenal medulla NE epinephrine (adrenaline) AA blood vessels, liver, heart increases blood sugar, constricts blood vessels (fight-or-flight response) NE norepinephrine (noradrenaline) AA blood vessels, liver, heart increases blood sugar constricts blood vessels (fight or flight response) Adrenal cortex mineralocorticoids (e.g., aldosterone) S kidneys increase reabsorption of Na+, excretion of K+ glucocorticoids (e.g., cortisol) S most body cells increase blood sugar androgens (e.g., DHEA) S general stimulate onset of puberty, female sex drive Pancreas glucagon (secreted by alpha cells) PP liver increases blood glucose insulin (secreted by beta cells) PP liver, muscle, adipose decreases blood glucose somatostatin (secreted by delta cells) PP alpha & beta cells inhibits insulin & glucagon release pancreatic polypeptide (from F cells) PP delta cells inhibits somato-statin & pancreatic enzymes Ovaries estrogen S uterus, general menstrual cycle, secondary sex characteristics progesterone S uterus regulates menstrual cycle, pregnancy relaxin PP pelvis, cervix dilates cervix & birth canal inhibin PR anterior pituitary inhibits FSH release Testes testosterone S testes, general spermatogenesis, secondary sex characteristics inhibin PR anterior pituitary inhibits FSH release Pineal melatonin AA various regulates biological clock Kidney erythropoietin