Endocrinology



Overview: The secretion and Action of Hormones in our body

Endocrine system is a collection of 'glands' that produce hormones. These hormones are very important for regulating metabolism process, growth of the body and sexual development. Each individual type of hormone affects only those body cells that have a genetic program that allows them to react only to those hormones that are related to them. The glands release the hormones into the blood stream and are transported to the various cells and body parts. When the hormones reach the target site, they bind to the receptor cells with a lock and key mechanism. The hormone may be present within the nucleus or on the surface of the cells. Once bound to the receptor, the hormones transmit a signal that triggers an action by the site. Hormones control the organs function and affect the growth and development of the organs. It is due to the hormones that the sexual characteristics of the organs develop and act accordingly. They also determine the use and storage of energy in the body, regulate the fluid, salt and sugar levels in the blood, stress, infection, etc. Minute amount of hormones trigger large reactions within the body. All hormones are proteins, but all proteins are not hormones. Steroids are not derived from proteins, but from the fatty substances- cholesterol.

The body has a well-controlled feedback system that manages the on/off button of the endocrine gland. When the chemical level or the nutrient level in the body is abnormally high or low, the endocrine glands secrete the hormones. Once the levels of the body fluids are normal the hormones secretions is shut down. When the glands receive information to secrete hormones, it is a positive feedback mechanism. When the glands receive information to stop the secretions of the hormones, it is negative feedback. Which was discussed in the Negative feedback page.



What is the endocrine system and how does it work?







Concepts:

1.Endocrine Glands And Hormones

Endocrine glands: "Glands that secrete hormones into the circulation rather than into a duct." Stuart I. Fox

Hormones: "A regulatory chemical produced in an endocrine gland that is secreted into the blood and carried to target cells that respond to the hormone by an alteration in their metabolism." Stuart I. Fox


Major Endocrine Glands
Major Endocrine Glands
Endocrine_Gland_chart.jpg


The chemical classes of hormones include amines, polypeptides, glycoproteins and steroids.

Amines- are hormones that come from the amino acids tyrosine and tryptophan. This hormones is secreted by the thyroid, adrenal medulla, and pineal glands.
Polypeptides- from previous pages we know proteins are large polypeptides consisting of over 100 amino acids. Example for this section is the Growth hormone; which is made up of 191 amino acids which is a protein.
Glycoproteins- these are molecules that consist of a protein connected to one or more carbohydrate groups. Example of this group is (FSH) follicle-stimulating hormone.
polypeptide_&_glycoportein_hormones.jpg

Steroids- are hormones that come from cholesterol. Examples of this hormone are testosterone, progesterone, estradiol, and cortisol.

steroid_hormones.jpg

Nonpolar hormones are non-soluble unlike Polar hormones which are water-soluble, This means that nonpolar hormones can pass through the cell membrane of their target cells, are called lipophilic hormones because they are soluble in lipids.

Precursors of active hormones may be classified as either prohormones or prehormones. Prohormones are relatively inactive precursor molecules made in the endocrine cells. Prehormones are the normal secretions of an endocrine gland that in order to be active must be converted to other derivatives by target cells.

Hormones can interact in permissive, synergistic, or antagonistic ways.

Permissive- is the effect on an action from a 2nd hormone when it increases the activity or responsiveness of the second hormone, or when it enhances the responsiveness of a target organ.
Synergistic- is the effect when 2 or more hormones work together to produce a certain result.
Antagonistic- is the effect when the actions of a hormone antagonizes the effect of another hormone. Example is Insulin and Glucagon on adipose tissue: the formation of fat is promoted by insulin, but with glucagon it promotes fat breakdown.



The effects of a hormone in the body depend on its concentration. Abnormally high amounts of a hormone can result in irregular effects. Hormones have a half-life: which is the time it takes for the plasma concentration to reduce by half. Ranges can be a few minutes to hours for most hormones. But the thyroid hormone however takes several days to reach its' half-life. With Steroids the concentration in a pill form can cause abnormal effects. Such as a drug concentration amount of androgens which can produces an abnormal amount of estrogen. Steroids in particular can have widespread and often damaging side effects. For example people with inflammatory diseases that are treated with high doses of a steroid called cortisone for long periods of time, can develop osteoporosis, and changes in their soft tissues. Target tissues can become desensitized by high hormone concentrations when exposed over time. This causes less receptor proteins in the target cells which produces less of a target tissue response-- called downregulation.



2.Mechanism Of Hormone Action


The lipophilic hormones (steroids and thyroid hormones) bind to nuclear receptor proteins, which function as ligand-dependant transcription factors. Steroid hormones bind to cytoplasmic receptors, which then move into the nucleus. Or steroids and thyroxine can also bind to receptors already in the nucleus. Each receptor binds to both the hormone and to a region of DNA called a hormone-response element, which locates the part of the gene that will be transcribed.





steroid_hormone.jpg





Two units of the nuclear receptor are needed to bind to the hormone-response element to activate a gene; as a result, the gene is transcribed (makes mRNA). The polar hormones bind to receptors located on the outer surface of the cell membrane. This activates enzymes that enlist second-messenger molecules.

receptors_for_steroid_hormones.jpg


Many hormones activate adenylate cyclase when they bind to their receptors. This enzyme produces cyclic AMP (cAMP), which activates protein kinase enzymes within the cell cytoplasm.
second_messenger_system.jpg


Other hormones may activate phospholipase C when they bind to their receptors. This leads to the release of inositol triphosphate (IP3), which stimulates the endoplasmic reticulum to release Ca2+ into the cytoplasm, activating calmodulin.

2_messenger_system.jpg



The membrane receptors for insulin and various growth factors are tyrosine kinase enzymes that are activated by binding to the hormone. Once activated, the receptor kinase phosphorylates signaling molecules in the cytoplasm that can have many effects.


receptor_for_insulin.jpg


3. Pituitary Gland

The pituitary gland is located on the inferior aspect of the brain in the region of the diencephalon.
It is roughly the size of a pea and it attatched to the hypothalamus by a stalklike structure called the infundibulum.


external image mayo+pituitary+gland.jpg




The pituitary gland includes the anterior pituitary and the posterior pituitary.

1. Anterior lobe (adenohypophysis) is regulated by hormones secreted by the hypothalamus, as well as by feedback from the target gland hormones. It is composed of glandular tissue.

  • Consists of two parts in adults: pars distalis is the rounded portion and the major endocrine part of the gland; and the pars tuberalis is a sheath of tissue that partially wraps around the infundibulum.

2. Posterior lobe (neurohypophysis) stores and releases hormones that are actually produced by the hypothalamus. It is composed largely of neuroglia and nerve fibers.
external image f20-4_pituitary_gland_c.jpg




Pituitary Hormones
The hormones secreted by the anterior pituitary are called trophic hormones.
The hormones of the anterior pitiuitary:
  • Growth hormone (GH or somatotropin)
  • Thyroid-stibulation hormone (TSH or thyrotropin)
  • Adrenocorticotropic hormone (ACTH or corticotropin)
  • Follicle-stimulating hormone (FSH or folliculotropin)
  • Luteinizing hormone (LH or luteotropin)
  • Prolactin (PRL)

The hormones of the posterior pituitary:
  • Antidiuretic hormone (ADH)
  • Oxytocin

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4.Adrenal Glands
Adrenal glands sit right on top of your kidneys. Each gland has an adrenal cortex and an adrenal medulla. Each part has their own function. The adrenal cortex secretes steroid hormones called corticosteroids. Corticosteroids have three functions under it. These include mineralocorticoids, glucocorticoids, and sex steroids. These three hormones are a result from cholesterol.The mineralocorticoids function is to help increase blood pressure and volume. Glucocorticoids has a very important piece in our body known as cortisol which helps raise the blood glucose level and also to promote the breakdown of fat in the body. The adrenal medulla is your fight or flight system, it secretes epinepherine and norepinephrine. These hormones increase your heart rate, dilate blood vessles, higher alertness, increased breating, and elevate the metabolic rate.

adrenal_gland.jpgstructure_of_adrenal_glands.jpg
5.Thyroid And Parathyroid Glands
The thyroid is just below your larynx, where it attaches to the trachea by isthmus, which is a type of throid tissue. The thyroid gland consists of many hollow sacs called thyroid follicles. These follicles store iodide from the blood and brings it into the collioid and once the iodide is inside it attaches to tyrosine which is an amino acid. If you attach one iodine to tyrosine it will produce monoiodotyrosine. If you attach two iodine to tyrosine it will produce diiodotyrosine. When monoiodotyrosine and diiodotyrosine join together it will make triiodothyronine. Theses thyroid hormones "stimulate protein synthesis, promote maturation of the nurvous system, and increase the rate of cell respiration in most tissues of the body." Fox, Stuart. Embedded in the posterior side of the thyroid gland is where the parathyroids are located. The parathyroid gland secreates only one hormone and that is called the parathyroid hormone. This hormone is very important to the body since it is the one that controls the calcium levels in the blood.


thyroid-gland.jpgparathyroid_glands.jpg




Essential Questions:

-Describe how the hypothalamus regulates the action of the posterior pituitary and the anterior pituitary.

Between the hypothalamus and anterior pituitary is a link called hypothalamo-hypophyseal portal system. The regulatory hormones are transferred into and through the hypothalamo-hypophyseal portal systems by neurons that come from the hypothalamus. This is how the hypothalamus regulates the action of the anterior pituitary. The hypothalamus controls the posterior pituitary by means of sending neurons through axons in the hypothalamo-hypophyseal tract.

-Describe one hormone that the posterior pituitary and anterior pituitary makes and describe the act of the hormone.

In the Anterior Pituitary, one hormone is the Follicle-stimulating hormone (FSH) that targets the gonads that help that gland produce growth hormone in the reproductive system.

In the Posterior Pituitary, one hormone is antidiuretic hormone (ADH) that targets the kidneys that stimulates water retention, raises the blood pressure by contracting arterioles.

-What are the trophic hormones and their role in hormone communication.

Trophic hormones, which control other endocrine glands. Trophic hormones are secreted by the anterior pituitary to stimulate target endocrine glands to secrete their own hormones.

Examples of trophic hormones are: adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete cortisol. The other trophic hormones secreted by the anterior pituitary are thyrotropin (thyroid stimulating hormone - TSH) and the gonadotropins, which are follicle stimulating hormone (FSH), and luteinizing hormone (LH).

The hypothalamus controls the release of trophic hormones by secreting a class of hypothalamic neurohormones called releasing and release-inhibiting hormones—which are released to the hypothalamo-hypophyseal portal system and act on the anterior pituitary.

How does this pertain to PTA.
Endocrinology is very important for our field of study. It is important for us to know all the endocrine glands and what they do so we can help our patients more efficiently. For example if they had something wrong with their hypothalamus you know they might have trouble with their balance, body temperature, and autonomic nervous system. So in that case we would have to make sure we are always there so they don't fall down and hurt themselves or work them to hard so their body can't cool itself back down.


1. Steroid hormone picture retrieved 1/28/11 at http://www.google.com/images?hl=en&gbv=2&biw=1345&bih=583&tbs=isch%3A1&sa=1&q=mechanism+of+steroid+hormone+action&aq=0&aqi=g1&aql=&oq=mechanism+of+steroid+hormone
2. http://highered.mcgraw-hill.com/sites/dl/free/0073378119/873587/Ch11_Images.ppt
3. http://www.google.com/images?hl=en&gbv=2&biw=1345&bih=583&tbs=isch%3A1&sa=1&q=adrenal+gland&aq=0&aqi=g10&aql=&oq=adrenal+g
4. http://www.google.com/images?hl=en&gbv=2&biw=1345&bih=583&tbs=isch%3A1&sa=1&q=structure+of+adrenal+gland&aq=f&aqi=g1&aql=&oq=
5. http://www.google.com/images?hl=en&gbv=2&biw=1345&bih=583&tbs=isch%3A1&sa=1&q=thyroid+gland&aq=0&aqi=g10&aql=&oq=thyroid+gl
6. http://www.google.com/images?hl=en&gbv=2&biw=1345&bih=583&tbs=isch%3A1&sa=1&q=parathyroid+glands&aq=f&aqi=g2g-m2&aql=&oq=
7. Fox, Stuart I. "Human Physiology." New York: McGraw-Hill, 2011. Print.
8. http://www.merckmanuals.com/home/sec13/ch161/ch161b.html