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Weekly Thoughts Module 13.doc

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Physiology 2130

WEEKLY THOUGHTS MODULE 13- The Endocrine System The endocrine system is responsible for the release and regulation of the hormones that circulate throughout your body. As you may have noticed, it is essential to control the levels of hormones circulating to maintain normal health and homeostasis. Normally, in a healthy person, these levels do not fluctuate too much but are tightly regulated via negative feedback, with a few exceptions. When hormone levels are altered, physiological consequences are inevitable (such as too much thyroid hormone or too much cortisol). Remember that the nervous system and endocrine system act together to “signal” and maintain homeostasis…with the nervous system being very quick, producing almost immediate results, while the endocrine system is much slower, but its effects can last much longer. The reason the endocrine system is slower is because it is like driving the country roads to a destination through the winding, scenic routes of the bloodstream rather than the direct 401, no fooling around pathway down a neuron. But, since the hormones circulate around and around and around until they are either eliminated by the kidney or destroyed by enzymes in the body, they can have longer lasting effects. 1) What is a Hormone? A hormone is a substance that makes its journey through the BLOODSTREAM and produces a “signal” at a distant site. It is released by endocrine tissue such as the kidney, pancreas, pituitary, gonads, hypothalamus, thyroid, and adrenal glands. Several classes of hormones exist, which differ in their location of the receptor and properties of “transportation”. 2) If hormones travel through the bloodstream, why don’t they affect every single organ in the body? For example, why does thyroid stimulating hormone affect the thyroid and not the adrenal glands? In order to “respond” to a hormone, you must have a receptor that can bind the hormone. No receptor means no action! Since the thyroid has receptors that bind thyroid stimulating hormone (TSH) it will produce a response to TSH (and secrete T3 and T4). However, the adrenal gland does not have receptors for TSH and therefore, nothing happens when TSH is present in the bloodstream of the adrenal glands. 3) Classes of Hormones These are subdivided mainly based on their SOLUBILITY. If you remember back in Modules 2 and 3, lipophilic (or hydrophobic) substances can dissolve right through cell membranes while hydrophilic substances do not. So, you have lipophilic (lipid- loving/hydrophobic) and hydrophilic hormones. Just based on that tidbit of information, it is probably easy for you to determine each of their properties. A) If a hormone is hydrophilic then...they dissolve in the bloodstream easily (because they like the watery environment) and can’t cross the cell membrane. So where would your receptor have to be? ON THE OUTSIDE OF THE CELL. Examples of this class of hormones are all protein hormones, including insulin, glucagons (pancreatic hormones), pituitary hormones (GH, prolactin, TSH, LH, FSH etc), parathyroid hormone, and most of the hypothalamic hormones (for this course, consider all the hypothalamic hormones in this category). These hormones are derived from amino acids. B) If a hormone is lipophilic/hydrophobic then…they easily dissolve into cells but do not like to float in the ciruculation because it is a watery environment. Because of this, these hormones need to circulate bound to a protein carrier (these protein are hydrophilic and help the lipophillic hormones dissolve in the blood). Receptors for these hormones are located INSIDE THE CELL (they don’t need a receptor on the cell membrane surface to carry them into the cell). So the lipophillic hormones diffuse right inside and either binds to receptors in the cell’s cytoplasm or on the nucleus of the cell. Examples of these types of hormones are steroid hormones including those of the adrenal cortex, as well as sex steroids testosterone, estrogen, progesterone. These hormones are all derived from cholesterol. Interestingly enough, thyroid hormones (T3 and T4) also fall into this category even though they are made up of the amino acid tyrosine. Rather than behaving like a protein hormone, these are lipophilic. 4) How Do Hormone Receptors Work? How Can They Tell the Cells What To Do? This is a bit complicated. Obviously if you are a receptor on the outside of a cell, it is harder to get the message into the cell than if you are already inside. If you are “on the outside” you must use a “messenger” to get the message to the inside of the cell. Most receptors have a “messenger” built into them, such as G proteins that can produce cAMP or open ion channels, or tyrosine kinases. When the hormone binds to its receptor outside the cell, the “messenger” causes a change in the activity of the cell and ALTERS proteins that are already present within the cell. But if you are “on the inside”, there is no need for a messenger. Once the hormone diffuses into the cell and binds to its receptor, this complex of hormone+receptor can bind to DNA (the genetic material in a cell) and change the cell’s activity and increase or decrease the production of new proteins. Don’t worry, I won’t ask the exact mechanisms of how cAMP or tyrosine kinase works but know what I’ve mentioned above. 5) The Main Structures in Hormone Regulation and Secretion, the Hypothalamus You’ve probably noticed that the hypothalamus plays a big part in almost every system, from temperature control to osmolarity regulation, to hunger and thirst, to total body volume regulation and now hormone regulation. The hypothalamus is often been referred to as the “little brain” which surprisingly is less than 1% of the weight of the entire brain itself. In this module, you learn about how important it is in controlling the anterior and posterior pituitary. The hypothalamus secretes many different hormones. These include the releasing factors that control the anterior pituitary. However, the hypothalamus also synthesizes the hormones oxytocin and ADH (also known as vasopressin) that are released from the posterior pituitary. 6) Are the Posterior and Anterior Pituitary Different? The pituitary gland is made up of two different “tissues”. Seems funny for a gland that is so small, but both regions are very different. A) The Anterior Pituitary- The anterior pituitary is made up of endocrine tissue and secretes a lot of different hormones in response to the hypothalamic releasing/inhibiting hormones. It “receives” the hypothalamic hormones via a special circulatory system that comes from the hypothalamus and circulates directly to the anterior pituitary. This special circulation is called the hypothalamic-hypophyseal portal system. So the hypothalamic releasing/inhibiting hormones hop into the hypothalamic-hypophyseal portal system and are delivered to the anterior pituitary. The anterior pituitary then responds by secreting hormones in response to a particular hypothalamic hormone. The hormone from the anterior pituitary then hops into the general systemic circulation directly to circulate throughout the body. B) The Posterior Pituitary The posterior pituitary is actually considered to be neural tissue. There are neurons that have their cell bodies in the hypothalamus BUT their axons are sent down into the posterior pituitary (since the hypothalamus and pituitary are so close together). The posterior pituitary releases the hormones oxytocin and ADH (that are made in the hypothalamus) out of the axon terminals found in the posterior pituitary into general systemic circulation. So if action potentials occur in this region, you will have hormones released. You already know the actions of ADH from the kidney section, but oxytocin is responsible for uterine contractions during labour and the release of milk during nursing (this hormone is not yet known to function in males or in non-pregnant females). 7) Anterior Pituitary hormones: As I said before, the anterior pituitary releases certain hormones depending on what hypothalamic hormones are “delivered”. Here is the actual list (see 13.21). If the anterior pituitary receives___(a)___ from the hypothalamus, it secretes ___(b)______. (a) Thyroid releasing hormone (TRH) ----- (b) Thyroid stimulating hormone (TSH) (a) Corticotropin releasing hormone (CRH) ----- (b) Adrenocorticotropic hormone (ACTH) (a) Prolactin releasing hormone (PRH) ----- (b) Prolactin (a) Prolactin inhibiting hormone (PIH) ---- also affects prolactin (a) Growth hormone releasing hormone (GHRH) ---- (b) growth hormone (GH) (a) Growth hormone inhibiting hormone (GHIH) ----also affects growth hormone (a) Gonadotropin releasing hormone (GnRH) ---- (b) Luteinizing hormone (LH) & Follicle stimulating hormone (FSH) Note: I don’t want to give you the impression that only 1 hypothalamic hormone is “received” by the pituitary at one particular time. Rather several hypothalamic hormones can be delivered continuously or in pulses depending on the requirements by the body for homeostasis. Seems easy, right? However, it is a bit more complicated than that. To maintain appropriate levels of hormones for homeostasis, there is some feedback of (b) on (a) in some cases. This means if there is too much of hormone (b) present, it will shut off the release of hormone (a) through short loop feedback. This generally occurs for the hormones that do not go on to secrete a third hormone (so prolactin and growth h
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