HYPOTHALAMUS & PITUITARY George Liapakis, PhD
The pituitary gland or hypophysis is located in a bony cavity at the base of the brain just below the hypothalamus
The pituitary is connected with the hypothalamus by a thin stalk Pituitary
The pituitary gland has two anatomically and functionally distinct lobes, the posterior pituitary and the anterior pituitary
Hormones secreted by the posterior pituitary Vasopressin Oxytocin
Posterior pituitary or neurohypophysis: It is composed of nervous tissue, which is connected to the hypothalamus by a neural pathway.
HOW ARE VASOPRESSIN AND OXYTOCIN SECRETED? • They are synthesizedin the cell bodies of neurosecretory neurons, which are located in the hypothalamus. The synthesized hormones are packaged in secretory granules • These neurons have axons that through a thin connecting stalk terminate on blood capillaries in the posterior pituitary • The hormones, packaged in secretory granules, are transported by specific mechanisms, through the axons of the neurosecretoryneurons to their nerve terminals where they are stored. • Each terminal stores either vasopressin or oxytocin and thus these hormones can be released independently. • The nerve terminals of neurosecretoryneurons secretethe hormones into blood.
CONCLUSIONS The posterior pituitary is simply an anatomical and functional extension of the hypothalamus. The posterior pituitary does not produce any hormones. It simply stores and releases into blood the hormones which are synthesized by the neuronal cell bodies in the hypothalamus Without the hypothalamus the posterior pituitary is not functional because all the hormones released from this gland are synthesized in specific neurons of the hypothalamus
What do vasopressin and oxytocin do? Vasopressin, or antidiuretic hormone or ADH: It increases water reabsorption in kidney tubules thus conserving water during urine formation, and decreasing urine output. It is the primary endocrine factor that regulates overall H2O balance. It causes contraction of smooth muscle in arterioles, thus producing vasoconstriction.
Oxytocin: It stimulates contraction of uterine smooth muscle, thus helping to expel the infant during birth. It promotes ejection of milk from mammary glands in breasts, during breast-feeding.
Anterior pituitary or adenohypophysis: It consists of glandular tissue, and connects to the hypothalamus by a unique blood-vessel link.
In marked contrast to posterior pituitary the anterior pituitary itself synthesizes the hormones, which are released into the blood. The anterior pituitary contains five different cell populations that secrete peptide hormones Somatotropes Thyrotropes Corticotropes Gonadotropes Lactotropes.
Somatotropes secrete the Growth Hormone (GH) Somatotropes
Thyrotropes Thyrotropessecrete the TSH orthyrotropin
Corticotropes secrete the ACTH or adrenocorticotropin Corticotropes
Gonadotropessecrete the gonadotropins (FSH or follicle stimulating hormone, LH or luteinizing hormone ) Gonadotropes Gonadotropins
Lactotropessecrete the prolactin Lactotropes
The hormones of the anterior pituitary, except prolactin are tropic hormones, stimulating the secretion of other hormones.
ACTH is synthesized as part of a large precursor molecule known as pro-opiomelancortin (POMC) • PMOC is also found, in different cells • Depending on the stimulus and the cell type the POMC can be cleaved into different products, ACTH, melanocyte-stimulating hormone (MSH) and endorphin . Skin MSH Brain Endorphin PMOC Hypothalamus MSH Pituitary ACTH
Pituitary POMC is converted to ACTH INFLAMMATION POMC Brain POMC is converted to endorphin PAIN
POMC Hypothalamus It controls food intake MSH Skin It promotes dispersion of the pigment melanin
POMC • An example of the economy of nature. • One single precursor molecule in different cells and under different stimuli can give different products with different physiological actions.
Without the hypothalamus the anterior pituitary is not functional. • The axons of the hypothalamic neurons which control the release of hormones from the anterior pituitary do not reach this gland. • Instead the hypothalamic neurons release specific hormones, which reach the anterior pituitary through the hypothalamic–hypophyseal portal system and regulate the secretion of the anterior pituitary hormones. • The hypothalamic–hypophyseal portal system is a unique capillary-to capillary connection system
Thehypothalamic–hypophyseal portal system It beginsin the base of the hypothalamus with a group of hypothalamic capillaries. Specific hormones from the hypothalamus enter the hypothalamic capillaries Hypothalamic capillaries recombineinto small portal vessels. The portal vessels pass down through a connecting stalk into the anterior pituitary The portal vessels branchto form the anterior pituitary capillaries, from which the hypothalamic hormones leave the blood and act on anterior pituitary cells (they control the release of anterior pituitary hormones) The anterior pituitary capillaries rejoin to form a vein, through which the hormones of anterior pituitary leave the gland
Thehypothalamic–hypophysealportal system is physiologically very important WHY? The specific hormones secreted by the hypothalamus are delivered immediately and directly to the anterior pituitary at relatively high concentrations, bypassing the general circulation.
Hypophysiotropichormones Specific hormones, which are secreted from hypothalamus, reach the anterior pituitary through the hypothalamic–hypophyseal portal system and regulate the secretion of pituitary hormones Releasing hormones : They stimulate the secretion of anterior pituitary hormones. Inhibiting hormones: They inhibit the secretion of anterior pituitary hormones
INPUTS OF INFORMATION which are received and integrated by hypothalamic neurons HORMONAL Secretion is controlled by target-gland hormones that reach the hypothalamus through the blood NEURONAL Secretion is influenced by emotions • Negative feedback mechanism • A target hormone suppress the secretion of the hypophysiotropic hormone, which results in the suppression of secretion of the pituitary hormone that is driving it • Example: CRH-ACTH-cortisol EXAMPLE Menstrual irregularities in emotionally upset women
Negative feedback mechanism A target hormone inhibits secretion of the hypophysiotropic hormone from the hypothalamus, which results in the suppression of secretion of the pituitary hormone that is driving it A target hormone could also inhibit directly the release of the pituitary hormone that is driving it. target hormone
SUMMARY Location of the hypothalamus and the pituitary gland or hypophysis The pituitary is connected with the hypothalamus by a thin stalk Two distinct lobes exist, the posterior pituitary and the anterior pituitary Posterior pituitary is composed of nervous tissue: hormones are synthesized in the hypothalamus and are secreted from the posterior pituitary Posterior pituitary connects to the hypothalamus by a neural pathway Anterior pituitary is composed of glandular tissue: hormones are synthesized in anterior pituitary and are secreted from this gland Anterior pituitary connects to the hypothalamus by a unique blood-vessel link
SUMMARY Posterior pituitary: Vasopressin, Oxytocin Vasopressin: primary endocrine factor that regulates overall H2O balance Oxytocin: contraction of uterine smooth muscle, ejection of milk Anterior pituitary: Five different cell populations that secrete peptide hormones An example of the economy of nature: POMC-ACTH-MSH-endorphin The hypothalamic–hypophyseal portal system and its physiological significance. Hypophysiotropichormones Regulation of the secretion of hypophysiotropic hormones
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