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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 also controls the glandular secretion
of the pituitary gland.
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: The Pituitary gland is found in the inferior part of the brain and
is connected by the Pituitary Stalk. It can be referred to as the master gland because it is the main place for
everything that happens within the endocrine system. It is divided into two sections: the anterior lobe
(adenohypophysis) and the posterior lobe (neurohypophysis). The Anterior pituitary is involved in sending
hormones that control all other hormones of the body.
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Communication between the hypothalamus and the posterior pituitary occurs through neurosecretory cells that span
the short distance between the hypothalamus and the posterior pituitary. Hormones produced by the cell bodies of
the neurosecretory cells are packaged in vesicles and transported through the axon and stored in the axon terminals
that lie in the posterior pituitary. When the neurosecretory cells are stimulated, the action potential generated triggers
the release of the stored hormones from the axon terminals to a capillary network within the posterior pituitary. Two
hormones, oxytocin and antidiuretic hormone (ADH), are produced and released this way. Decreased ADH release
or decreased renal sensitivity to ADH produces a condition known as diabetes insipidus. Diabetes insipidus is
characterised by polyuria (excess urine production), hypernatremia (increased blood sodium content) and polydipsia (thirst).
The posterior lobe is composed of neural tissue [neural ectoderm] and is derived from hypothalamus. Its function is
to store oxytocin and antidiuretic hormone. When the hypothalamic neurons fire these hormones are release into the
capillaries of the posterior lobe.
The posterior pituitary is, in effect, a projection of the hypothalamus. It does not produce its own hormones, but only
stores and releases the hormones oxytocin and antidiuretic hormone. ADH is also known as arginine vasopressin
(AVP) or simply vasopressin.
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The anterior lobe is derived from oral ectoderm and is composed of glandular epithelium. Communication between
the hypothalamus and the anterior pituitary occurs through hormones(releasing hormones and inhibiting hormones)
produced by the hypothalamus and delivered to the anterior pituitary via a portal network of capillaries. The
releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus called neurosecretory
cells. The hormones are released into a capillary network or primary plexus, and transported through veins or
hypophyseal portal veins, to a second capillary network or 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. Many of the hormones produced by the anterior pituitary are tropic
hormones or tropins, which are hormones that stimulate other endocrine glands to secrete their hormones.
The anterior pituitary lobe receives releasing hormones from the hypothalamus via a portal vein system known as the
hypothalamic-hypophyseal portal system.
The anterior pituitary secretes:
- thyroid-stimulating hormone (TSH)
- adrenocorticotropic hormone (ACH)
- prolactin
- follicle-stimulating hormone (FSH)
- luteinizing hormone (LH)
- growth hormone (GH)
- endorphins
- and other hormones
It does this in response to a variety of chemical signals from the hypothalamus, which travels to the anterior lobe by
way of a special capillary system from the hypothalamus, down the median eminence, to the anterior lobe. These
include:
- thyrotropin-releasing hormone (TRH)
- corticotropin-releasing hormone (CRH)
- dopamine (DA), also called 'prolactin inhibiting factor' (PIF)
- gonadotropin-releasing hormone (GnRH)
- growth hormone releasing hormone (GHRH)
These hormones from the hypothalamus cause release of the respective hormone from the pituitary. The control of
release of hormones from the pituitary is via negative feedback from the target gland. For example homeostasis of
thyroid hormones is achieved by the following mechanism; TRH from the hypothalamus stimulates the release of
TSH from the anterior pituitary. The TSH, in turn, stimulates the release of thyroid hormones form the thyroid gland.
The thyroid hormones then cause negative feedback, suppressing the release of TRH and TSH.
The heart, gastrointestinal tract, the placenta, the kidneys and the skin, whose major function is not the secretion of
hormones, also contain some specialized cells that produce hormones.
In addition, all cells, except red blood cells secrete a class of hormones called eicosanoids. These hormones are
paracrines, or local hormones, that primarily affect neighboring cells. Two groups of eicosanoids, the prostaglandins
(PGs) and the leukotrienes (LTs), have a wide range of varying effects that depend upon the nature of the target cell.
Eicosanoid activity, for example, may impact blood pressure, blood clotting, immune and inflammatory responses,
reproductive processes, and the contraction of smooth muscles.
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Maintaining homeostasis often requires conditions to be limited to a narrow range. When conditions exceed the
upper limit of homeostasis, specific action, usually the production of a hormone is triggered. When conditions return
to normal, hormone production is discontinued. If conditions exceed the lower limits of homeostasis, a different
action, usually the production of a second hormone is triggered. Hormones that act to return body conditions to
within acceptable limits from opposite extremes are called antagonistic hormones. The two glands that are the most
responsible for homeostasis is the thyroid and the parathyroid.
The regulation of blood glucose concentration (through negative feedback) illustrates how the endocrine system
maintains homeostasis by the action of antagonistic hormones. Bundles of cells in the pancreas called the islets of
Langerhans contain two kinds of cells, alpha cells and beta cells. These cells control blood glucose concentration by
producing the antagonistic hormones insulin and glucagon.
Beta cells secrete insulin. When the concentration of blood glucose rases such in after eating, beta cells secret
insulin into the blood. Insulin stimulates the liver and most other body cells to absorb glucose. Liver and muscle cells
convert glucose to glycogen, for short term storage, and adipose cells convert glucose to fat. In response, glucose
concentration decreases in the blood, and insulin secretion discontinues through negative feedback from declining
levels of glucose.
Alpha cells secrete glucagon. When the concentration of blood glucose drops such as during exercise, alpha cells
secrete glucagon into the blood. Glucagon stimulates the liver to release glucose. The glucose in the liver originates
from the breakdown of glycogen. Glucagon also stimulates the production of ketone bodies from amino acids and
fatty acids. Ketone bodies are an alternative energy source to glucose for some tissues. When blood glucose levels
return to normal, glucagon secretion discontinues through negative feedback.
Another example of antagonistic hormones occurs in the maintenance of Ca2+ ion concentration in the blood.
Parathyroid hormone (PTH) from the parathyroid glands increases Ca2+ in the blood by increasing Ca2+ absorption
in the intestines and reabsorption in the kidneys and stimulating Ca2+ release from bones. Calcitonin (CT) produces
the opposite effect by inhibiting the breakdown of bone matrix and decreasing the release of calcium in the blood.
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