17.2 Hypothalamus and Pituitary Gland

hypothalamus

shaped like a flattened funnel, forms floor and walls of third ventricle of brain; Regulates primitive functions from water balance and thermoregulation to sex drive and childbirth; Many of its functions carried out by pituitary gland

pituitary gland

suspended from hypothalamus by a stalk (infundibulum); Size and shape of kidney bean; housed in sella turcica of sphenoid bone; Composed of two structures: anterior and posterior pituitary

anterior pituitary

constitutes anterior three-quarters of pituitary; Linked to hypothalamus by blood vessels of the hypophyseal portal system; Hypothalamic hormones these cells

hypophyseal portal system

Primary capillaries in hypothalamus connected to secondary capillaries in anterior pituitary by portal venules

posterior pituitary

constitutes the posterior one-quarter of the pituitary; Nerve tissue, not a true gland; Nerve cell bodies in hypothalamus pass down the stalk as hypothalamo–hypophyseal tract and end in posterior lobe; stores hypothalamic hormones

releasing hormones

typically hypothalamic

stimulating hormones

typically anterior pituitary

Gonadotropin-releasing hormone (GnRH)

stimulates release of gonadotropins (FSH, LH) from anterior pituitary

Thyrotropin-releasing hormone (TRH)

stimulates release of thyroid-stimulating hormone (TSH, thyrotropin) from anterior pituitary

Corticotropin-releasing hormone (CRH)

stimulates release of adrenocorticotropic hormone (ACTH, corticotropin) from anterior pituitary

Prolactin-inhibiting hormone (PIH)

inhibits release of prolactin from anterior pituitary

Growth hormone-releasing hormone (GHRH)

stimulates release of growth hormone (GH) from anterior pituitary

somatostatin

inhibits release of GH and TSH from anterior pituitary

oxytocin (OT)

has a variety of reproductive functions; Released during sexual arousal and orgasm; promotes feelings of sexual satisfaction and emotional bonding between partners; Stimulates labor contractions during childbirth; Stimulates flow of milk during lactation, and may promote emotional bonding between lactating mother and infant

antidiuretic hormone (ADH)

increases water retention by kidneys, thus reducing urine volume and preventing dehydration; Also called arginine vasopressin (AVP) because it can cause vasoconstriction, but only at unnaturally high concentrations

Follicle-stimulating hormone (FSH)

stimulates: secretion of ovarian sex hormones, development of ovarian follicles, and sperm production

Luteinizing hormone (LH)

stimulates: ovulation, corpus luteum secretion of progesterone, and testosterone secretion by testes

gonadotropins

hormones that stimulate gonadal function, including FSH and LH

Thyroid-stimulating hormone (TSH) or thyrotropin

stimulates secretion of thyroid hormone (TH) from thyroid gland

Adrenocorticotropic hormone (ACTH), or corticotropin

stimulates adrenal cortex to secrete glucocorticoids

Prolactin (PRL)

after birth, stimulates mammary glands to synthesize milk

Growth hormone (GH), or somatotropin

stimulates mitosis and cellular differentiation

antidiuretic hormone (ADH) and oxytocin (OT)

which two hormones are produced in hypothalamus, transported to the posterior pituitary, and released when hypothalamic neurons are stimulated:

Hypothalamic and cerebral control

Brain monitors conditions and influences anterior pituitary accordingly; Posterior pituitary is controlled by neuroendocrine reflexes

corticotropin (ACTH)

In times of stress the pituitary gland releases this

prolactin

during pregnancy, the hypothalamus stimulates secretion of this

hypothalamic osmoreceptors

trigger release of ADH when they detect a rise in blood osmolarity

oxytocin

Infant suckling triggers hypothalamic response to release this

negative feedback inhibition

increased target organ hormone levels inhibit release of hypothalamic and/or pituitary hormones

growth hormone

has widespread effects on the body tissues, Especially cartilage, bone, muscle, and fat; Induces liver to produce growth stimulants

liver produced growth stimulants

Insulin-like growth factors (IGF-I and IGF-II), or somatomedins; Stimulate target cells in diverse tissues; IGF-I prolongs the action of GH

GH half life

6 to 20 min

IGF half life

about 20 hrs

GH-IGF protein synthesis

boosts transcription and translation; increases amino acid uptake into cells; suppresses protein catabolism

GH-IGF lipid metabolism

stimulates adipocytes to catabolize fats, release fatty acids for body cells to use as fuel instead of proteins (protein-sparing effect)

GH-IGF carbohydrate metabolism

mobilizing fatty acids reduces dependence of most cells on glucose, freeing more for the brain (glucose-sparing effect); stimulates glucose secretion by liver

GH-IGF electrolyte balance

promotes Na⁺ retention by kidneys, enhances absorption in intestine; makes electrolytes available to growing tissues

GH secretion

fluctuates with time of day, activity level, and age; Secretion high during first 2 hours of sleep; Can peak in response to vigorous exercise; Also activated by ghrelin, a hormone released by the stomach

GH levels with age

Average 6 ng/mL during adolescence, 1.5 ng/mg in old age; Lack of protein synthesis contributes to aging of tissues and wrinkling of the skin