Post on 03-Jun-2018
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HYPOTHALAMUS ANDPITUITARY GLAND
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O For many years, the pituitary gland orhypophysis was called the masterendocrine gland because it secretesseveral hormones that control otherendocrine glands.
O We now know that the pituitary gland itselfhas a masterthe hypothalamus.
O This small region of the brain below thethalamus is the major link between thenervous and endocrine systems.
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O Cells in the hypothalamus synthesize at
least nine different hormones, and the
pituitary gland secretes seven.O Together, these hormones play important
roles in the regulation of virtually all
aspects of growth, development,
metabolism, and homeostasis.
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Pituitary GlandO The pituitary gland is a pea-shaped
structure that measures 11.5 cm (0.5 in.)
in diameter and lies in the hypophysealfossa of the sella turcica of the sphenoid
bone.
O It attaches to the hypothalamus by a stalk,
the infundibulum and has two
anatomically and functionally separate
portions: the anterior pituitary and the
posterior pituitary.
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TheAnterior PituitaryO The anterior pituitary (anterior lobe),
also called the adenohypophysis,
accounts for about 75% of the total weightof the gland.
O The anterior pituitary consists of two parts
in an adult: The pars distalis is the larger
portion, and the pars tuberalis forms a
sheath around the infundibulum.
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The Posterior PituitaryO The posterior pituitary (posterior
O lobe), also called the neurohypophysis,
also consists of two parts: the parsnervosa, the larger bulbar portion, and
the infundibulum.
O A third region of the pituitary gland called
thepars intermedia atrophies duringhuman fetal development and ceases to
exist as a separate lobe in adults.
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ANTERIOR PITUITARY
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Anterior Pituitary
The anterior pituitary or adenohypophysissecretes hormones that regulate a wide range of
bodily activities, from growth to reproduction.
Release of anterior pituitary hormones is stimulatedby releasing hormones and suppressed byinhibiting hormones from the hypothalamus.
Thus, the hypothalamic hormones are an important
link between the nervous and endocrine systems.
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Hypophyseal Portal System
Hypothalamic hormones reach the anterior pituitarythrough a portal system.
In aportal system,blood flows from one capillary
network into a portal vein, and then into a secondcapillary network without passing through the heart.
In the hypophyseal portal system, blood flowsfrom capillaries in the hypothalamus into portal
veins that carry blood to capillaries of the anteriorpituitary.
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The superior hypophyseal arteries,branchesof the internal carotid arteries, bring blood into thehypothalamus.
At the junction of the median eminence of thehypothalamus and the infundibulum, these arteriesdivide into a capillary network called the primaryplexus of the hypophyseal portal system.
From the primary plexus, blood drains into thehypophyseal portal veins that pass down theoutside of the infundibulum. In the anteriorpituitary, the hypophyseal portal veins divide again
and form another capillary network called thesecondary plexus of the hypophyseal portalsystem.
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Relation between Putuitary and Hypothalamus
Near the median eminence and above the opticchiasm are clusters of specialized neurons, calledhypothalamic neurosecretory cells.
They synthesize the hypothalamic releasing and
inhibiting hormones in their cell bodies and packagethe hormones inside vesicles, which reach the axonterminals by axonal transport.
Nerve impulses stimulate the vesicles to undergo
exocytosis. The hormones then diffuse into theprimary plexus of the hypophyseal portalsystem.
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Relation between Pituitary and Hypothalamus
hypothalamic hormones flowwith the blood through theportal veins and into thesecondary plexus.
hypothalamic hormones actimmediately on anteriorpituitary cells.
Hormones secreted by anteriorpituitary drain into the anteriorhypophyseal veins and out intothe general circulation
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Types of Anterior Pituitary Cells
4. Lactotrophs secrete prolactin (PRL),whichinitiates milk production in the mammary glands.
5. Corticotrophs secrete adrenocorticotropic
hormone (ACTH) or corticotropin , whichstimulates the adrenal cortex to secreteglucocorticoids such as cortisol. Some corticotrophs,remnants of the pars intermedia, also secrete
melanocyte-stimulating hormone (MSH).
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Types of Anterior Pituitary Cells
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Hormone of Anterior Pituitary
1. Human Growth Hormone and InsulinlikeGrowth Factors
The main function of hGH is to promote synthesis and
secretion of small protein hormones called insulinlikegrowth factors (IGFs) or somatomedins.
In response to human growth hormone, cells in the liver,
skeletal muscles, cartilage, bones, and other tissues secreteIGFs, which may either enter the bloodstream from theliver or act locally in other tissues as autocrines orparacrines.
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Hormone of Anterior Pituitary
2.Follicle-stimulating HormoneIn females, the ovaries are the targets for follicle-stimulating hormone (FSH). Each month FSH initiatesthe development of several ovarian follicles, saclike
arrangements of secretory cells that surround adeveloping oocyte
In males,FSH stimulates sperm production in thetestes.Gonadotropinreleasing hormone (GnRH) from thehypothalamus stimulates FSH release.
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Hormone of Anterior Pituitary
3.Luteinizing HormoneIn females, luteinizing hormone (LH) triggers ovulation,the release of a secondary oocyte (future ovum) by anovary.
Together, FSH and LH also stimulate secretion ofestrogens by ovarianbcells. In males, LH stimulates cells inthe testes to secrete testosterone.
Secretion of LH, like that of FSH, is controlled bygonadotropin-releasing hormone (GnRH).
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Hormone of Anterior Pituitary
4.Prolactin
Prolactin (PRL), together with other hormones,initiates and maintains milk secretion by the
mammary glands.
5. Thyroid-stimulating Hormone
Thyroid-stimulating hormone (TSH) stimulates thesynthesis and secretion of the two thyroid hormones,triiodothyronine (T3) and thyroxine (T4), bothproduced by the thyroid gland
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Hormone of Anterior Pituitary
6.Adrenocorticotropic Hormone
Corticotrophs secrete mainly adrenocorticotropichormone (ACTH). ACTH controls the production and
secretion of cortisol and other glucocorticoids by thecortex (outer portion) of the adrenal glands.
Corticotropin-releasing hormone (CRH) from the
hypothalamus stimulates secretion of ACTH bycorticotrophs.
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Hormone of Anterior Pituitary
7.Melanocyte-stimulating Hormone
Melanocyte-stimulating hormone (MSH) increasesskin pigmentation in amphibians by stimulating the
dispersion of melanin granules in melanocytes.
Its exact role in humans is unknown, but the presenceof MSH receptors in the brain suggests it may
influence brain activity.
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Although the posterior pituitary or neurohypophysis
does not synthesize hormones, it does store and release twohormones.
It consists of axons and axon terminals of more than10,000 hypothalamic neurosecretory cells.
The cell bodies of the neurosecretory cells are in theparaventricular and supraoptic nuclei of thehypothalamus; their axons form thehypothalamohypophyseal tract.
This tract begins in the hypothalamus and ends nearblood capillaries in the posterior pituitary.
Posterior Pituitary
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The paraventricular nucleus synthesizes thehormone oxytocin and the supraoptic nucleus
produces antidiuretic Hormone also calledvasopressin
The axon terminals in the posterior pituitary areassociated with specialized neuroglia called
pituicytes. These cells have a supporting role similarto that of astrocytes.
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After their production in the cell bodies ofneurosecretory cells, oxytocin and antidiuretic
hormone are packaged into secretory vesicles, whichmove by fast axonal transport to the axon terminalsin the posterior pituitary, where they are stored untilnerve impulses trigger exocytosis and release of thehormone.
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During and after delivery of a baby, oxytocin affectstwo target tissues: the mothers uterus and breasts.
During delivery, oxytocin enhances contraction ofsmooth muscle cells in the wall of the uterus;
After delivery, it stimulates milk ejection(letdown) from the mammary glands in response
to the mechanical stimulus provided by a sucklinginfant.
Oxytocin
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an antidiuretic is a substance that decreases urine production.
ADH causes the kidneys to return more water to the blood,
thus decreasing urine volume. In the absence of ADH, urine output increases more than
tenfold, from the normal 1 to 2 liters to about 20 liters a day.
Drinking alcohol often causes frequent and copious urinationbecause alcohol inhibits secretion of ADH. ADH also decreases
the water lost through sweating and causes constriction ofarterioles, which increases blood pressure.
This hormones other name, vasopressin, reflects this effect onblood pressure.
Antidiuretic Hormone
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The amount of ADHsecreted varies withblood osmoticpressure and blood
volume
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