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Home , Endocrine gland

See Tietz, Chap. ___

This system in the body provides and maintains coordination between different systems. Works in close harmony with the but does have similarities and differences.

• Endocrine Nervous •1) Latent Period: long, up to days <1 sec, short • •2) Message-delivery: deliv. by circulation via electrical impulses • •3) Duration of • target tissue response: long response time; up to days (postal) short-lived, millisecs • (phone) Labt 4250 • •4) Messenger: uses : protein, steroid, amino neurotransmitter • Spring 2004 •5) Anatomical arrange:wireless; acts on receptors on target tissue wired between neurons • and target tissue • •6) Main function: controls activities with longer duration coordinates rapid and • precise movements •

Exocrine( ) vs Endocrine (ductless) can be divided into three chemical classes. They are not all the same chemically so type they are affects: synthesis, transport and mechanism of action. Other Characteristics of (adds complexity): 1) Protein hormones (polypeptides) – chains of amino acids 1) A single endocrine produces multiple hormones. 2) A single hormone is produced by more than one endocrine gland. 3) A single hormone may have more than one function. 2) Steroids – cholesterol is the precursor 4) A single target response may happen for more than one hormone. 5) The same messenger can be either a hormone or a neurotransmitter. 3) Aromatic amines – all are derived from the amino acid 6) Some endocrine organs may have more than one function. tyrosine; secreted by the gland and

Hormone - from Greek word hormon “to excite” or “set in motion”; The major endocrine organs of the body include: 1) chemical messenger that is secreted into internal body fluids by one cell or 2) group of cells and has a physiological control effect on another group of 3) Thyroid gland cells in the body. 4) Parathyroid glands 5) ¾ Criteria to be a hormone: 6) Adrenal glands 7) •1)Manufactured in a specific tissue or 8) (female) •2)Must be secreted into circulation or 9) Testis (male) transported •3)Most of the time must have a specific The two most important endocrine glands are: target tissue of action. A) Hypothalamus B) Adenohypophysis (portion of )

The hypothalamus releases what is known as “releasing hormones” which then act on the adenohypophysis which stimulates the release of hormones that influence the rest of the endocrine system. These hormones are called “tropic”( means turning and changing) hormones (released from the adenohypophysis). Don’t confuse with “trophic” which means nurturing or cause growth.

1 II. Steroid hormones ¾LdL is internalized into a steroidigenic cell and broken down by enzymes Synthesis: to liberate “free cholesterol”(precursor). I.peptide or protein hormones ¾Only occurs as there is direct need for hormone. Can store cholesterol until more (ex.- epinephrine ()); made similar to any protein a cell makes hormones are needed but does not store steroid hormones. to be exported. ¾Stores precursor not finished product. Made by ribosomes (RER) ↓ ¾Specific enzymes convert cholesterol to steroid hormone by varying type and position of preprohormone (large precursor) side groups of cholesterol. ↓ to Golgi complex(packaged) ¾Each organ only has the enzymes to make specific hormone necessary by it. and cleaved to form ¾Characteristics: not as water soluble (hydrophobic) and therefore transported ↓ bound to protein carriers with only a small percentage that is free (usually prohormone(cleaved again) biologically active). ↓ Pinched off into secretory vesicle from the cell ↓ Stored and waits til signal to release it

Characteristics: water soluble( hydrophilic), circulate free (not protein-

bound) and a shorter half-life than those that are bound (T1/2 = 5 – 60 minutes.) Protein bound hormones have T1/2= 60 –100 minutes.

Mechanism of Action A. Protein hormones and amino acid-based hormones. Share a similar mechanism of action. It is based on the 2nd messenger system, and usually related to the hormone’s water solubility characteristics.

Since they are water soluble, they cannot pass through the membrane of cells Epinephrine is the 1st messenger which to cause a function. So to exert their action, they bind to the cell surface and this in turn activates adenylate cyclase to causes a chain reaction of events. convert ATP to cAMP, the 2nd messenger, which is ultimately what causes the An example is epinephrine, a hormone made from tyrosine which binds to a receptor on the surface of reaction to occur in the cell. the membrane which in turn activates another “messenger” within the cell to cause the action at that cell. This is sometimes called the adenylate cyclase second messenger system.

So Epi acts as the 1st messenger , while cAMP acts as the 2nd messenger. If at the cell, glycogen is ultimately converted to as a result.

B. Steroid hormones and . These are lipid soluble hormones and therefore can penetrate the cell membrane directly. It will then find a receptor to bind to within the cell and together the S and R can cross into the nucleus and acti- vate genes to turn on the activities of the cell by forming messenger RNA which will in turn direct protein synthesis of proteins and enzymes to cause certain metabolic functions. Regulation

Regulation of release or making of hormones has to be monitored in the body to maintain , The status quo of all systems in the body. If ever homeostasis is not maintained, then the need to release more hormones has to be established. In other words, it works via a feedback mechanism.

Most feedback mechanisms in the body are of the negative feed back type. The most classic example of this feedback is the Thyroid model.

(-)Hypothalamus releases TRF (thyrotropin releasing factor) ↓ Anterior Pituitary(adenohypophysis)(-) ↓ TSH (Thyroid stimulating factor) ↓

Thyroid Gland (+ to release and make T4, T3) ↓

2 Hypothalamic-pituitary Axis

Anterior pituitary = Adenohypophysis = Neurohypophysis

A connection between the hypothalamus and the anterior pituitary(AP) exists that is neural plus circulation so substances travel in the from the hypothalamus to the AP to Activate cells in that area.

A connection between the hypothalamus and the posterior pituitary exists that is neural only such that when an impulse or Action potential is sent down the neuron, the posterior pituitary will release its specific contents.

Within the hypothalamus, there are 2 nuclei that produce hormones and then send them down the neurons to be stored in the posterior pituitary. The paraventricular nucleus (PVN) synthesizes and then sends it to the post. Pit. to be stored until the proper stimulus is send to the posterior pituitary. Likewise the supraoptic nucleus(SON) synthesizes antidiuretic hormone (ADH), it is transported down to the posterior pit. to be stored. These 2 substances are not made in the post. Pit. But are stored there and released from there to the bloodstream to be used elsewhere in the body.

Special areas within the hypothalamus synthesize also releasing factors or hormones that will contro secretion of the anterior pituitary. Upon the proper stimulus, these factors will be released down arterioles to the AP to specific cells, attach to the surface and cause them to release AP hormones. Within the AP, there are 6 cell types that will also produce hormones to be used in the rest of the body.

Hypothalamic hormones or releasing factors

1) TRH(TRF) – thyrotropin-releasing hormone (factor) – causes the release of TSH 2) CRH(CRF) – corticotropin-releasing hormone – causes the release of ACTH 3) GHRH(GHRF) – releasing hormone – “somatotropin” – causes the release of growth hormone 4) GHIH(GHIF) – growth hormone inhibiting hormone – “” – causes the inhibition of GH release. Also made in pancreas.

5) GnRH – releasing hormone – stimulates the release of 2 hormones called : FSH and LH 6) PRH – releasing hormone – causes release of prolactin 7) PIH – prolactin inhibiting hormone – inhibits prolactin release

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