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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