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PHGY 210
Michael Guevara

Sarah Margareta Ibrahim▯ Monday, January 7th 2012 PHGY 210 - Mammalian Physiology II Lecture 1 - Endocrinology (Part 1 of 6) Introduction to Endocrinology Endocrinology - a molecular signal (a hormone) that is detected by a molecular detector (a receptor) that leads to a metabolic change in a cell and taken at an organismal level, this leads to a physiological change. • Classically endocrinology is one of the primary means that the body uses to coordinate physiological processes over long distances. • So you can have an endocrine signal in the form of a hormone released by the brain for example which travels to the far extremities of the body to illicit itʼs physiological response • It can also take place over very short distances or within the same cell talking to itself as we will see. • So the endocrine system along with the nervous system is one of the primary systems for long distance coordination of physiological events within the body. Coordination of Physiological Processes: 1) In a living organism there must be coordination of number of physiological activities taking place simultaneously such as: movement, respiration, circulation, digestion, excretion and metabolism. 2) The central nervous system and the endocrine system represent the two major means by which these functions are coordinated. Long Distance Communication: 1) Communication between cells that are not in contact is achieved through a number of chemical substances, which are secreted by releasing cells and interact with specific receptors on distant target cells. 2) Signaling through these receptors leads to a specific physiological effect. Endocrine Signaling Endocrine signaling always involves a hormone that is secreted into the blood stream by an endocrine gland. As we will see, there are glands in the body that are really dedicated endocrine structures. However, essentially any cell in the body can send out a signal that effectively, is hormonal in nature. So you have a hormonal signal that is released and is transported through the bloodstream to itʼs target (distant target site). Itʼs target is whatever cell happens to have the receptor for that particular hormone. Figure on the right: The anterior pituitary is like a central clearing house for hormonal signals (weʼll see it many times) but in this case the APG is giving rise to luteinizing hormone (LH) or follicle stimulating hormone (FSH) going to the gonads and then the gonads release steroid hormones. This figure also shows cascades of hormonal signalling - ie. youʼll have a primary endocrine organ which signals to a secondary endocrine organ which will sometimes even signal to a tertiary endocrine organ. ▯ 1 Sarah Margareta Ibrahim▯ Monday, January 7th 2012 ➡ Neuroendocrine Signaling - a variation on any type of endocrine signal except the hormone releasing cell is a neuron. The hormone released is called a neurohormone. In the case of the figure to the right, the hypothalamus is talking to the pituitary. The blood system here is a portal blood system that connects the hypothalamus directly to the pituitary so there is travel at a distance but itʼs a relatively short distance in this case. ➡ Two types of (short-distance) signaling: ▯ (see figure below) 1) Paracrine signaling - when you have two cells in close juxtaposition, a hormone is released from one cell and it detects a receptor on an adjacent cell. The adjacent cell can be a different cell type or it can be a different cell (can have epithelial cells talking to epithelial cells for example). 2) Autocrine signaling - cell actually talks to itself. ➡ Communication by hormones (or neurohormones) can involve six steps: *Each of these steps is a potential point of regulation! (Very important!!) 1. Synthesis of the hormone by endocrine cells (or neurons in case of neurohormone). • Example. To regulate synthesis of insulin - control the rate of transcription of the gene that produces insulin (so the rate of production of a mature insulin molecule) 2. Release of the hormone by the endocrine cells (or the neurohormones by the neurons). • Example. What happens during reproductive endocrinology - LH levels build up in the anterior pituiary without being released and all of a sudden there is a tipping point and thereʼs all this LH hormone that is released and LH levels in the blood spike and when they get to the developing follicle - you have ovulation! 3. Transport of the hormone or neurohormone to the target site by the blood stream. • Most hormones donʼt travel through the bloodstream naked - they travel through the blood stream bound to various carrier proteins 4. Detection of the hormone or neurohormone by a specific receptor protein on the target cells. • The essence of endocrinology is that although lots of proteins travel through the blood stream (albumin for example), the hormone has a specific receptor on a specific cell type which is capable of detecting the presence of that hormone in the blood stream ▯ 2 Sarah Margareta Ibrahim▯ Monday, January 7th 2012 5. A change in cellular metabolism triggered by the hormone‐ receptor interactions • Cells have lots of proteins on their surface which detect specific molecules - for example youʼll have proteins that act as transporters for amino acids but thatʼs all that they do, they increase uptake of the amino acid, cʼest tout. The important thing in the hormone receptor relationship is that the binding of the hormone to the receptor illicits a series of biochemical responses within the cell which bears the receptor. These biochemical responses, taken at the level of the tissue or the organism correspond to the physiological response to that particular hormonal signal. 6. Removal of the hormone, which often terminates the cellular response • The circulating half life of a number of hormones in the blood stream is extremely short. Why so short? Because you want the signal to be around when you want it around but you donʼt want it to hang around for hours after youʼve finished needing the signal. Itʼs an excellent way to regulate by having a tight regulation on how long the hormone hangs around in the circulation. Classic Endocrine Organs (Think: HAPpy BOT) 1. Brain: in particular the hypothalamus and the pituitary - sort of one continuous structure which is part neuronal, part endocrine) 2. Thyroid and parathyroid glands: basically what you see in the figure - the upside-down bowtie - those are your thyroid glands connected by the isthmus and the parathyroid are four tiny little guys which you canʼt see here because theyʼre like little dots stuck two on each side of the thyroid gland. *Note: DONʼT GET CONFUSED by the names- the parathyroid hormone is produced by the parathyroid glands and is called that way because the parathyroid glands are stuck to the thyroid. Parathyroid physiology is a distinct signaling system from thyroid physiology so itʼs not like the parathyroid is a cofactor or the thyroid or something. 3. Heart: a neoclassical endocrine organ in the sense that atrial natriuretic peptides (ANPs) were discovered in the 1950s. Wonʼt really talk about this. 4. Adrenal glands: sit on top of your kidneys and are a major source of a number of hormones - particularly the non-sex steroid hormones and to a lesser degree the sex steroid hormones 5. Pancreas: Not that small of an organ but less than 1% of it is endocrine (most of it is a digestive organ). Is famous for producing insulin from the Islets of Langherans) 6. Ovaries/Testes: will deal with these in detail when we look at reproductive physiology. ▯ 3 Sarah Margareta Ibrahim▯ Monday, January 7th 2012 Hypothalamic-Pit
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