The Endocrine System PDF

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17 The Endocrine System Dragoljub Bankovic/Shutterstock.com Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 651 Chapter Introduction In this chapter you will learn… Endocrine System Hypothalamus Pineal gland the functions of the endocrine system. Pituitary gland the methods and Thyroid and regulation of hormone parathyroid synthesis, secretion, glands activity and inactivation. the major endocrine Thymus glands and hormones of the body. Adreal glands the mechanisms of action of steroid, peptide Pancreas or protein, and amine hormones. the site of synthesis, target(s), and actions Ovary of each major hormone. Testes in a biological male Placenta (during pregnancy) the regulation of hormone secretion by the hypothalamus and pituitary gland. Your body is made up of around 40 trillion cells. That’s a much higher number than the number of people that live on this planet. Imagine trying to create a signal that reaches a majority of the planet’s people and coordinates their behavior within a few seconds. It may seem impossible and yet, within your own body, widespread communication and coordination is made possible all the time. Efficient, widespread, and rapid systemic communication is made possible by the endocrine system. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 652 Unit 3 Regulation, Integration, Control 17.1 An Overview of the Endocrine System Learning Objectives: By the end of this section, you will be able to: 17.1.1 Describe the major functions of the functions, the anatomical pathways by endocrine system. which the signals reach their targets, what determines the target of the pathway, 17.1.2 Define the terms hormone, endocrine gland, the speed of the target response(s), the endocrine tissue (organ), and target cell. duration of the response, and how signal 17.1.3 Compare and contrast how the nervous intensity is coded. and endocrine systems control body Communication is a process in which a sender transmits signals to one or more receiv- ers to control and coordinate actions. In the human body, two major organ systems participate in relatively “long-distance” communication: the nervous system and the endocrine system. The endocrine system uses secreted chemical signals, called hor- mones, as its means of communication. Hormones are signaling molecules released into the bloodstream; they can reach most cells of the body and have widespread LO 17.1.1 effects. The nervous system uses electrical signals that traverse down the long axons of neurons. Chemical signals are utilized by the nervous system just for communication between one cell and another. The communication and coordination these two systems Student Study Tip achieve together maintains homeostasis in the body. The endocrine system is respon- You can remember the function of the sible for regulating the body’s use of calories and nutrients, the secretion of wastes, the endocrine system by its name: the maintenance of blood pressure and osmolarity, growth, fertility, sex drive, lactation, prefix endo means “in” and crine means and sleep. The hormones of the endocrine system also impact mood and emotion. As “secretion,” which tells us how hormones discussed in Chapter 16, our ability to respond to stress—a real or perceived threat to are secreted into the bloodstream! our homeostasis—involves both the nervous and endocrine systems. 17.1a Chemical Signaling Universal chemical signaling involves exocytosis, which releases molecules that diffuse throughout body fluids and are bound by a molecular receptor on or in a target cell. There are four types of chemical signaling in the body: Local intercellular communication, in which a chemical is released by one cell and induces a response in neighboring cells, is paracrine signaling. Local communication can also affect the cell that releases the signal; this is known as autocrine signaling. Chemicals released by neurons to communicate with a local cell or cells are neurotransmitters. Releasing chemical signals into the bloodstream that induce responses in specific target cells throughout the body is endocrine signaling. In endocrine signaling, hormones secreted into the extracellular fluid diffuse into the blood or lymph, and can then travel great distances throughout the body. In contrast, autocrine signaling affects the same cell that released the signal. Paracrine signaling is limited to the local environment. Although paracrine signals may enter the bloodstream, their concentration is generally too low to elicit a response from distant The Human Anatomy and Physiology Society includes more than 1,700 educators who work together to promote excellence in the teaching of this subject area. The HAPS A&P Learning Outcomes measure student mastery of the content typically covered in a two-semester Human A&P curriculum at the undergraduate level. The full Learning Outcomes are available at https://www.hapsweb.org. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 653 Figure 17.1 A Comparison of Endocrine and Exocrine Glands Secretions from both endocrine and exocrine glands are released through exocytosis. In endo- crine secretion, exocytosis releases the secretion into the bloodstream or surrounding extracel- lular fluid. In exocrine secretion exocytosis releases the secretion into a duct that carries the secretions to a surface. Endocrine Glands Exocrine Glands Blood Blood Acinus Endocrine tissue Vesicles Exocytosis Secretion Exocrine ˜uids (ions + water) Duct Exocytosis Vesicles Vesicle Secretory tissue tissues. The neurotransmitters of the nervous system are a specific example of a para- crine signal that acts only locally within the synaptic cleft. Learning Connection The endocrine system consists of cells, tissues, and organs that secrete hormones. Chunking Endocrine glands are the major players in this system. The primary function of endo- Which of these systems are local? Which crine glands is to secrete their hormones directly into the surrounding fluid; unlike exo- achieve long-distance communication? crine glands, they do not have ducts that carry their secretions away (Figure 17.1). The Which system(s) depend on receptors? hormones are carried throughout the body in the blood. The endocrine system includes Which system(s) use electrical signals? the pituitary, thyroid, parathyroid, adrenal, and pineal glands (Figure 17.2). Some of these glands have both endocrine and non-endocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete hormones. The hypo- thalamus, thymus, heart, kidneys, stomach, small intestine, liver, adipose tissue, ovaries, LO 17.1.2 and testes are other organs that contain cells with endocrine function (Figure 17.3). Let’s track a hormone as it travels throughout the body in the bloodstream. Depending on the rate that the heart is beating, it will take somewhere around a min- ute for a hormone molecule to make a loop throughout the circulatory system. Given enough loops, it has the opportunity to make its way to just about any of the 40 trillion cells of the body. Which cell (or cells) will it affect? A given hormone only impacts the activity only of its target cells—that is, cells with receptors for that particular hor- Student Study Tip mone. Once the hormone binds to the receptor, a chain of events is initiated that leads A hormone reaches everywhere blood to the target cell’s response. Hormones play a critical role in the regulation of physi- flows in the body, but only where there ological processes because of the target cell responses they regulate. These responses is a lock (target cell’s receptor) that fits contribute to human reproduction, growth and development of body tissues, metabo- the hormone key will the hormone effect lism, fluid, and electrolyte balance, sleep, and many other body functions. change. This is like how your house key doesn’t open every door in your neighborhood. 17.1b Neural and Endocrine Long-Distance Signaling The body is a vast galaxy of cells, and two different systems—the nervous system and the endocrine system—are capable of long-distance communication. The nervous system LO 17.1.3 uses two forms of communication—electrical and chemical signaling. Neurotransmitters, Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 654 Unit 3 Regulation, Integration, Control Figure 17.2 The Primary Endocrine Glands of the Human Body The primary function of the pineal, pituitary, thyroid, and adrenal glands is to produce hormones. The pancreas is an organ that functions equally in endocrine and exocrine secretions. Pituitary gland Pineal gland Thyroid Thyroid (posterior view) Adrenal glands Kidney Parathyroid glands Pancreas the chemical signals of the nervous system, act locally and rapidly. Once the neurotrans- mitters bind to receptors on the postsynaptic cell, the receptor stimulation triggers a cellular response. The target cell responds within milliseconds of receiving the chemical stimulation and the response then ends very quickly. In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition. In contrast, the endocrine system uses just one method of communica- tion: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals—the hormones—into the extracellular fluid. Hormones are transported throughout the body by the blood and circulatory system, where they bind to receptors on target cells and induce a response. There are two consequences to these differences in structure. One is that endocrine signaling can be more widespread, affecting many target cells at once. Another is that endocrine signaling requires more time than neural signal- ing to prompt a response. Some target cell responses are rapid, while other responses can take longer. For example, the emergency stress hormones released when you are Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 655 Figure 17.3 Organs That Have Endocrine Roles The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, adipose tissue, ovaries, and testes all have other functions in the body, but each of these organs secretes one or more hormones and is therefore also considered an endocrine gland. Hypothalamus Thymus Heart Liver Stomach Kidney Adipose tissue Small intestines Ovary Testes in a biological male confronted with a dangerous or frightening situation—epinephrine and norepineph- rine—will trigger responses in their target tissues within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones. One note about the efficiency of chemical signals in the body: The body produces only a few dozen individual signaling molecules; however, these molecules can have dif- ferent effects in different tissues. One hormone may have multiple receptors and therefore cause different types of changes in its different target cells. A single signaling molecule may be a paracrine signal in some instances or a hormone in others, depending on where it is released. Because of this redundancy in the body, drugs that target the production, release, or reception of different signaling molecules can have widespread effects. In broad strokes we can generalize that the nervous system involves quick responses to changes in the internal or external environment, and the endocrine system is usually slower acting—making changes to maintain homeostasis and working toward longer-term goals such as nutrient storage, growth, and reproduction. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 656 Unit 3 Regulation, Integration, Control Learning Check 1. Which of the following is the chemical signal used in the endocrine system? a. Hormones c. Sodium b. Neurotransmitters d. Calcium 2. The femoral nerve synapses on the quadriceps muscles. In order to make the muscles contract, the neurotransmitter is released from the axon terminal into the synapse, which binds to the receptors of the neighboring muscle cell. What type of chemical signaling is this an example of? a. Paracrine signaling c. Synaptic signaling b. Autocrine signaling d. Endocrine signaling 3. The adrenal gland releases a group of hormones called corticosteroids. These hormones act as anti-inflammatory agents, maintain blood sugar levels, maintain blood pressure, and regulate water balance. Which of the following explains how these hormones can affect many organs in the body? a. These hormones bind to all the organs in the body. b. These hormones can bind to several different receptors. c. The body has one receptor throughout all organs. d. Hormones travel in the blood and do not target specific organs. 17.2 Hormones Learning Objectives: By the end of this section, you will be able to: 17.2.1 List the three major chemical classes of 17.2.4 Compare and contrast the mechanisms hormones (i.e., steroid, peptide, amino acid- of action of plasma membrane hormone derived [amine]) found in the human body. receptors and intracellular hormone receptors, including the speed of the 17.2.2 Compare and contrast how steroid and response. peptide hormones are produced and stored in the endocrine cell, released from the 17.2.5 Describe the various signals that initiate endocrine cell, and transported in the blood. hormone production and secretion (e.g., monitored variables, direct innervation, 17.2.3 Compare and contrast the locations of neurohormones, other hormones). target cell receptors for steroid and peptide hormones. 17.2a Types of Hormones The hormones of the human body can be divided into three main groups on the basis of their chemical structure. The chemical structure will impact the distribution and LO 17.2.1 location of reception for the hormones. Some hormones are derived from lipids. These so-called steroid hormones are lipid-based and lipophilic (literally, “fat-loving”), so they cross membranes easily. This means they cannot be stored or excluded from a cell (see the “Anatomy of a Steroid Hormone” feature). The two other groups of hormones are both based on amino acids, the monomers of proteins (see the “Anatomy of a Pro- tein Hormone” feature). Amine hormones are modified versions of single amino acids. As such they are very small and transport easily. Their precise characteristics are based on the chemical makeup of their functional groups. Peptide hormones are produced when multiple amino acids are joined together. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 657 Anatomy of... A Steroid Hormone The DNA contains Steroid hormones Because they are Steroid hormones Steroid hormones Steroid hormone instructions to are made in the hydrophobic, steroid diffuse across the bind to carrier receptors are found make enzymes smooth hormones are transported cellular plasma proteins to travel within the target that modify endoplasmic bound to the membrane membrane and also in the blood. cell and not on the cholesterol to reticulum (ER). of vesicles, rather than diffuse in and out of plasma membrane. produce steroid in the vesicle itself. the bloodstream. hormones. Target Cell Vesicle DNA Hormone Rough Smooth Steroid hormone receptor ER ER Carrier protein Endocrine Cell Anatomy of... A Protein Hormone The DNA contains the Protein hormones are made Protein hormones Protein hormones The receptors for protein hormones genes that code for in the rough endoplasmic are released from travel in the blood. are located on the surface of protein hormones. reticulum (ER) or by ribosomes the cell through the target cell because the protein and packaged in vesicles. exocytosis. hormone cannot readily diffuse through the membrane. Vesicle DNA Target Rough Smooth Cell Protein hormone ER ER Endocrine Cell Steroid Hormones The steroids, or steroid hormones, are produced by modifying cholesterol (Figure 17.4). Like cholesterol, steroid hormones are not soluble in water (they are hydrophobic). Two famous steroid hormones, testosterone and the estrogens, are produced by the gonads. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 658 Unit 3 Regulation, Integration, Control Figure 17.4 Steroid Hormones Steroid hormones are derived from cholesterol and maintain the characteristic ring shape. Because they are lipids, they are lipophilic and diffuse through membranes easily. CH2OH C O HO O Corticosterone H H OH H H HO Cholesterol O Testosterone Amine Hormones Amine hormones are produced by modifying a single amino acid (Figure 17.5). There are amine hormones produced from the modification of either tryptophan or tyrosine. An example of a hormone derived from tryptophan is mela- tonin, which is secreted by the pineal gland and helps regulate circadian rhythm. Tyrosine derivatives include epinephrine and norepinephrine. Epinephrine and nor- epinephrine are secreted by the adrenal medulla and play a role in the fight-or-flight response. Peptide and Protein Hormones Whereas the amine hormones are derived from a single amino acid, peptide and protein hormones consist of multiple amino acids that link to form an amino acid chain (Figure 17.6). By convention, a chain of up to 50 amino acids is called a peptide, while a chain of greater than 50 amino acids is called a protein. An example of a peptide hormone is antidiuretic hormone (ADH), a pituitary hormone consisting of nine amino acids, which is important in fluid balance. An example of a protein hormone is insulin, a pancreatic hormone consisting of 51 amino acids, which promotes glucose uptake into many body cells. Figure 17.5 Amine Hormones Tyrosine, an amino acid, is the basis for many amine hormones, which are modified versions of this single amino acid. HO H H H H H H HO C C N HO C C N H C H H H H O OH Tyrosine Dopamine Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 659 17.2b Pathways of Hormone Action Production All hormones are produced within the cells of endocrine glands. Humans do not carry genes in their DNA that directly code for steroid hormones. Rather, our genomes contain the instructions to produce enzymes that are capable LO 17.2.2 of modifying cholesterol into a new structure as a steroid hormone. Whereas pep- tide hormones consist of short chains of amino acids, protein hormones are longer Figure 17.6 Peptide Hormones polypeptides. Both types are synthesized like other proteins: DNA is transcribed into mRNA, which is translated into amino acids at a ribosome or rough endoplas- Peptide hormones are composed of up to 50 mic reticulum. amino acids joined by peptide bonds. Once produced, like most proteins, protein hormones may be modified by the golgi apparatus and then packaged into vesicles. Once the cell is triggered to release the hormone, the vesicles merge with the plasma membrane and the protein hormones are released through the process of exocytosis. Not steroid hormones, though; steroid hor- mones are unable to be stored in vesicles. Because they are lipophilic, they will readily diffuse out of the vesicle. Steroid hormones, therefore, cannot be kept in storage wait- ing to be released, but must be made on demand when the endocrine cell is triggered to begin production. As soon as the steroid hormone is made, it will diffuse out of the cell and into the extracellular fluid. Travel Similarly, because of the difference in their hydrophobicity, protein and steroid hormones travel differently in the blood. Because blood is water-based, lipid-derived hormones must travel to their target cell bound to a transport pro- tein. With their structure and traveling mode of being bound to proteins, steroid hormones tend to last longer in the bloodstream; they are likely to persist for 30–90 minutes after release, compared to only a few minutes for a protein-based hormone. Reception Once released, every hormone has the ability to reach just about any cell in a body. Which cells are its targets is determined by their expression of a LO 17.2.3 hormone receptor, a protein located either inside the cell or within the cell mem- brane. The receptor is specific to the hormone. Figure 17.7 shows a target cell that LO 17.2.4 expresses several receptor molecules. This target cell contains hormone receptors specific for steroid hormone A and protein hormone B. The cell is not the target of, and will not respond to, protein hormone A. You may notice that steroid hormone A is found both inside and outside the cell; because the steroid is lipophilic, it diffuses easily through the cell membrane and can be found inside of cells regard- less of whether they have receptors for the steroid. Steroid hormone receptors are usually found inside the cell. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. And there is often more than one type of receptor for a single hormone. Thus, the response trig- gered by a hormone depends not only on the hormone, but also on the presence or absence of specific hormone receptors in or on the target cell. Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, or stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 660 Unit 3 Regulation, Integration, Control Figure 17.7 Hormone Receptors Proteins are too large to diffuse through the membrane; therefore, their receptors are expressed on the cell surface. Steroid hormones and thyroid hormones are lipid soluble, so they diffuse into and out of the cell randomly. Cells express receptors for these hormones internally rather than on the surface. The hormone receptor may be in the cytosol or even within the nucleus. Once bound, the hormone-receptor complex acts as a transcription factor. The cell shown in this figure is a target cell of steroid hormone A and protein hormone B, but it is not a target cell of protein hormone A. Capillary Steroid Protein Protein hormone hormone hormone A A B Extracellular ˜uid Receptor Cytoplasm Target cell Activated protein Nucleus DNA Transcription Transcription mRNA Pathways Involving Intracellular Hormone Receptors Intracellular hormone recep- tors are located inside the cell. Hormones that bind to this type of receptor must be able to cross the cell membrane. Steroid hormones are derived from cholesterol and therefore can readily diffuse through the lipid bilayer of the cell membrane to reach the intracellular receptor. Thyroid hormones are also lipid-soluble due to their unique structure and can enter the cell. An internal hormone receptor may reside within the cytosol or within the nucleus. In either case, when it binds its hormone, the hormone and receptor move together within the cell nucleus and bind to a particular segment of the cell’s DNA. The binding of the hormone-receptor complex to DNA triggers transcription of a target gene, gen- erating an mRNA transcript, which moves to the cytosol and directs protein synthesis by ribosomes (see Figure 17.7). Pathways Involving Membrane-bound Hormone Receptors Hydrophilic, or water- soluble, hormones are unable to diffuse through the lipid bilayer of the cell mem- brane. Cells express the receptor for these hormones on their cell surface. Except for thyroid hormones, which are lipid-soluble, all amino acid–derived hormones bind to cell membrane receptors that are located on the extracellular-facing side of the cell membrane. In order to effect a change in the cell, these receptors are connected to a cascade of signaling molecules within the cell. Like a domino effect or a relay race, each signaling molecule effects a change in the next molecule in the cascade. In many Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 661 cases we call the internal signaling molecules messengers with the internal signaling molecule being called a second messenger and the hormone itself called a first messenger. The second messenger used most commonly is cyclic adenosine monophosphate (cAMP) (Figure 17.8). 1. The cAMP second messenger pathway begins when a hormone binds to its recep- tor in the cell membrane. 2. This receptor is associated with an intracellular component called a G protein, and G protein (guanine nucleotide-binding binding of the hormone activates the G-protein component. protein) 3. The activated G protein in turn activates an enzyme called adenylyl cyclase, also known as adenylate cyclase. 4. Adenylyl cyclase converts adenosine triphosphate (ATP) to cAMP. 5. As the second messenger, cAMP activates a type of enzyme called a protein kinase that is present in the cytosol. 6. Activated protein kinases initiate a phosphorylation cascade, in which multiple protein kinases phosphorylate (add a phosphate group to) numerous and various cellular proteins, including other enzymes. The phosphorylation of cellular proteins can trigger a wide variety of effects, including cellular movement, the rate of cellular respiration, or triggering the synthesis Figure 17.8 cAMP as a Second Messenger Water-soluble hormones cannot diffuse through the plasma membrane and so their receptors are expressed on the cell surface. Many of these receptors are associated with a G-protein-coupled receptor, which triggers an intracellular signaling cascade that leads to a cellular response. Capillary Water-soluble hormone Extracellular ˜uid Cell membrane receptor Adenylyl cyclase G protein (activated) ATP cAMP Protein kinase Cytoplasm ATP Activated protein kinase Nucleus ADP Inactivated + P protein DNA P Activated protein Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 662 Unit 3 Regulation, Integration, Control of new molecules. The effects vary according to the type of target cell, the G proteins and kinases involved, and the phosphorylation of proteins. Examples of hormones that use cAMP as a second messenger include calcitonin, which is important for bone construction and regulating blood calcium levels; glucagon, which plays a role in blood glucose levels; and thyroid-stimulating hormone, which causes the release of T3 and T4 from the thyroid gland. Overall, the signaling cascade functions to amplify the efficacy of a hormone. Since each step within the cascade can affect multiple downstream molecules, a signifi- cant cellular response can emerge when triggered by even a very low concentration of hormone in the bloodstream. This amplification also makes for a very rapid response to hormone reception. However, the duration of the cellular response to the hormone is short, as cAMP is quickly deactivated by the enzyme phosphodiesterase (PDE), which is located in the cytosol. The action of PDE helps to ensure that a target cell’s response ends quickly. If new hormone molecules arrive at the cell membrane, then the cellular response will continue. Not all water-soluble hormones initiate the cAMP second messenger system. One common alternative system uses calcium ions as a second messenger. In this system, G proteins activate the enzyme phospholipase C (PLC), which functions similarly to adenylyl cyclase. Once activated, PLC cleaves a membrane-bound phospholipid into two molecules: diacylglycerol (DAG) and inositol triphosphate (IP3). Like cAMP, DAG activates protein kinases that initiate a phosphorylation cascade. At the same time, IP3 causes calcium ions to be released from storage sites within the cytosol, such as from within the smooth endoplasmic reticulum. The calcium ions then act as second mes- sengers in two ways: they can influence enzymatic and other cellular activities directly, or they can bind to calcium-binding proteins, the most common of which is calmodu- lin. Upon binding calcium, calmodulin is able to modulate protein kinase within the cell. Examples of hormones that use calcium ions as a second messenger system include angiotensin II, which helps regulate blood pressure through vasoconstriction, and growth hormone–releasing hormone (GHRH), which causes the pituitary gland to release growth hormones. 17.2c Factors Affecting Target Cell Response You will recall that the only cells that will respond to a given hormone are those that express receptors for that hormone. However, in addition to receptor expression, several other factors influence the target cell response. For example, when a hormone circulates in the bloodstream at a high concentration for a persistent length of time, target cells may decrease their number of expressed receptors for that hormone. This process is called downregulation, and it allows cells to become less reactive to the excessive hormone levels. An analogy would be that when the volume is too high on your music, you might cover your ears so that less of the sound makes its way to your inner ear. Similarly, when the level of a hormone is chronically reduced, target cells engage in upregulation to increase their number of receptors. This process allows cells to be more sensitive to the hormone that is available. Cells may also alter the availability of their downstream signal- ing molecules, which will change the magnitude of their response to bound hormone. 17.2d Regulation of Hormone Secretion LO 17.2.5 Hormones have profound impacts on homeostasis and therefore must be tightly controlled to maintain homeostasis. Ideally, to maximize control, we would want to be able to carefully regulate the release of a hormone into the bloodstream. We would also Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 663 want the hormone to only act for a short time so its effects would not go on too long; more hormone could always be released as needed. Thus, the body maintains control over hormone action by regulating hormone production and degradation. Feedback loops are one central tool in hormone regulation. Role of Feedback Loops Most hormones are regulated through negative feedback loops. Some hormones, oxytocin being the most notable example, follow positive feedback loop patterns. Negative feedback is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. In other words, negative feedback loops turn themselves off through their own actions. As a negative Student Study Tip feedback loop begins, the events leading to its end are set in motion. Figure 17.9 illus- One real-life example of a negative feed- trates a negative and positive feedback loop. In the negative feedback loop, the produc- back loop is an air conditioner. When it tion and release of the hormone calcitonin is regulated by the levels of calcium ions in runs, the room gets cold, so you turn the the blood. A rise in blood calcium ions following a meal triggers the release of calcito- AC off. The action of the cycle leads to it turning off. nin. The action of calcitonin is that calcium is removed from the blood and added to Figure 17.9 Negative and Positive Feedback Loops In a negative feedback loop, the product of the cycle resolves the stimulus, ending the cycle. In a positive feedback loop, the cycle will not stop on its own, but the products of the loop perpetuate the cycle until an external interruption occurs. Negative Feedback Loop VS Positive Feedback Loop Bloodstream The level of Ca2+ in the Ca2+ That stimulation triggers the blood rises following a release of oxytocin from the meal of calcium-rich foods. posterior pituitary gland. Oxytocin Cells within the thyroid Thyroid gland monitor blood Ca2+ levels. When blood Ca2+ is elevated, the thyroid Calcitonin gland secretes calcitonin An infant suckles Posterior into the blood. at its mother’s pituitary Ca2+ breast to begin nursing. Calcitonin encourages Calcitonin osteoblasts to remove Ca2+ from blood and deposit it Oxytocin causes the Bone release of breast milk. into bone matrix. The infant matrix suckles harder. Ca2+ Osteoblasts As blood Ca2+ levels fall back to within the Ca2+ homeostatic range, the thyroid gland stops releasing calcitonin. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 664 Unit 3 Regulation, Integration, Control the bone matrix. As this action takes place in the bone tissue, it causes the concentration of calcium ions in the blood to decrease, thereby turning off the stimulus for further calcitonin release. Hormones regulated by negative feedback allow blood levels of the hormone to be regulated within a narrow range. Positive feedback loops, on the other hand, perpetuate themselves. They will not turn off until an external event stops them. Oxytocin is a hormone whose release fol- lows a positive feedback loop pattern. One trigger for oxytocin release is sensory stimu- lation of the nipples. In the case of a lactating person, an infant suckling at the nipples will cause the release of oxytocin. One of oxytocin’s actions is to cause the release of Learning Connection breast milk. If milk is not released, the infant may stop suckling, but with milk to drink, Explain a Process the infant likely continues suckling, perhaps more vigorously. With continued stimula- Try explaining negative and positive tion of the nipples, oxytocin continues to be released, milk continues to flow, and the feedback to a person who has never feeding continues. Unlike a negative feedback loop, nothing intrinsic will turn this studied these processes. For example, cycle off. It is only by an external event—when the infant is full and stops sucking at the negative feedback can be compared to the nipple—that oxytocin secretion wanes and the cycle resolves (Figure 17.9). regulation of temperature in your house by a thermostat. Can you think of other Role of Endocrine Gland Stimuli In general, we can say there are three different types examples? of stimuli that control the release of hormones (Figure 17.10). Figure 17.10 Three Triggers of Hormone Release Three types of stimuli control the release of hormones. These include non-hormone chemicals such as calcium ions (A), tropic hormones like TRH (B), and stimulation from the nervous system (C). Bloodstream Hypothalamus Central Nervous System TRH Ca2+ TRH released Thyroid Anterior pituitary Preganglionic gland sympathetic TSH neuron TSH released Calcitonin Bloodstream Bloodstream Ca2+ TSH Adrenal gland Medulla Calcitonin of adrenal Thyroid gland gland TH Bone matrix TH released Osteoblasts Ca2+ Bloodstream Capillary Bloodstream TH Epinephrine and Ca2+ norepinephrine A B C Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 665 One trigger can be when blood levels of non-hormone chemicals, such as nutri- ents or ions, increase or decrease outside of the homeostatic range. In this example in Figure 17.10A, as well as the example of a negative feedback loop in Figure 17.9, the endocrine gland directly monitors the blood levels of a nutrient. An endocrine gland may also secrete its hormone in response to the presence of another hormone produced by a different endocrine gland. These hormones that act on endocrine glands to trigger the release of other hormones are called tropic hormones. Such hormonal stimuli often involve the hypothalamus (Figure 17.10B). In addition to these chemical signals, hormones can also be released in response to stimulation from the nervous system. A common example is the adrenal medulla, which secretes both the hormones norepinephrine and epinephrine in response to stimulation by the sympathetic nervous system (Figure 17.10C). Learning Check 1. How are steroid hormones transported across a cell membrane? a. Steroid hormones are actively transported through protein channels. b. Steroid hormones are passively transported through protein channels. c. Steroid hormones are actively transported through the cell membrane. d. Steroid hormones are passively transported through the cell membrane. 2. Someone who has Type 1 diabetes will have less insulin in their body. People with Type 1 diabetes often take an oral form of insulin as medication. This is an agonist and binds to the same receptors as insulin. When it binds to the hormone receptor, it activates a G protein. What is the next step after the G protein is activated? a. Activation of adenylyl cyclase c. Activation of protein kinase b. Conversion of ATP to cAMP d. Initiation of phosphorylation cascade 3. Which of the following are true about the production of protein hormones? Please select all that apply. a. Cholesterol is modified to produce the hormone. c. The hormone will be made upon demand. b. DNA and mRNA synthesize the hormone. d. Vesicles store the hormones until needed. 4. What is the role of a negative feedback loop? a. To promote further secretion of hormones b. To inhibit further secretion of hormones c. To initiate hormone breakdown 17.3 Endocrine Control by the Hypothalamus and Pituitary Gland Learning Objectives: By the end of this section, you will be able to: 17.3.1 Describe the locations and the anatomical posterior pituitary, and the hormones’ relationships of the hypothalamus, anterior primary targets and effects. pituitary, and posterior pituitary, including the hypothalamic-hypophyseal portal system. 17.3.4 Explain the role of hypothalamic neurohormones (regulatory hormones) in 17.3.2 Explain the role of the hypothalamus in the release of anterior pituitary hormones. the release of hormones from the posterior pituitary. 17.3.5 Describe major hormones secreted by the anterior pituitary, their control pathways, 17.3.3 Name the two hormones produced by and their primary target(s) and effects. the hypothalamus that are stored in the Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 666 Unit 3 Regulation, Integration, Control The hypothalamus–pituitary complex is often thought of as the “command center” of the endocrine system because it plays a central role in the release of many different hormones. The hypothalamus and the pituitary glands secrete several hormones that directly produce responses in target tissues, as well as many tropic hormones that regu- late the synthesis and secretion of hormones of other glands. LO 17.3.1 The hypothalamus is a structure of the diencephalon of the brain located anterior and inferior to the thalamus (Figure 17.11). It has both neural and endocrine func- tions, produces and secretes many hormones, and integrates the nervous system and Figure 17.11 The Hypothalamus and the Pituitary Gland The hypothalamus is part of the diencephalon. The pituitary gland, which is suspended beneath it, really consists of two glands. The anterior pituitary is made of glandular tissue and secretes six different hormones. The posterior pituitary is an extension of the hypothalamus and con- tains neurons that originate in the hypothalamus. Hypothalamus Infundibulum Anterior Posterior pituitary pituitary Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Endocrine System Chapter 17 667 the endocrine system. In addition, the hypothalamus is anatomically and functionally linked to the pituitary glands, which can be seen directly inferior to the hypothala- mus. While the pituitary has long been described as a single gland, it is in fact two separate structures, the anterior and posterior pituitary glands. The anterior pituitary (Figure 17.12A) is an anatomically distinct gland that is physically attached to the posterior pituitary. The posterior pituitary is connected to the hypothalamus by a stem called the infundibulum. The infundibulum is similar to a tract in that it is filled with infundibulum (pituitary stalk) neuron axons. These neurons have their cell bodies in the hypothalamus and their axon terminals in the posterior pituitary (Figure 17.12B). Two hormones are made in these neurons; their production occurs in the cell bodies in the hypothalamus. The hormones are transported down the length of the axons and released from the axon terminals and into the blood of the posterior pituitary. The anterior pituitary is made of glandular tissue that has a different (non-neural) embryonic origin. It hangs onto the infundibulum like a child getting a piggyback ride, but it is not an extension of the hypothalamus the way that the posterior pituitary is. The anterior pituitary is intimately connected to the hypothalamus in a different way though. A portal system is a system of blood vessels that connects two locations. In a portal system, one tissue or region has a capillary bed (a system of small blood vessels) through which it receives nutrients and releases any products or wastes; a second tissue or region has another capillary bed that serves the same function. These two tissues are connected because their capillary beds are connected to each other and blood flows directly from the first tissue to the second without circulating elsewhere or returning to Figure 17.12 The Anterior and Posterior Pituitary (A) The anterior pituitary is anatomically and functionally separate from the hypothalamus. They are connected by the hypothalamic-hypophyseal portal system. (B) The posterior pituitary contains the axons and axon terminals of neurons that originate in the hypothalamus. Hormones are produced by these neurons and released from the posterior pituitary into the bloodstream. Neurosecetory Neurosecretory cells cells Hypothalamus Superior hypophyseal artery First capillary bed Blood ˜ow of hypothalamic- from heart hypophyseal portal Axons descend Infundibulum system through infundibulum Hypophyseal portal veins Secondary capillary bed of hypothalamic- hypophyseal portal Capillary plexus system Pituitary gland Anterior Posterior Anterior