Chemical Control of the Animal Body: The Endocrine System Chapter 37 pages 713-732 Chemical Control of the Animal Body: The Endocrine System

How Do Animal Cells Communicate? Individual cells communicate with one another to ensure the proper functioning of the whole organism Methods of communication between cells fall into four categories: Direct Synaptic Paracrine Endocrine

Direct Tissues such as heart muscle have gap junctions that directly link the insides of adjacent cells, allowing ions and electrical signals to flow between them This type of communication is very fast, but also has a very short range

Non-direct Communication In the other three types of communication, “sending” cells release messenger chemicals through their plasma membranes chemicals move to “receiving” cells and alter their physiology by binding to receptors, specialized proteins on the surface or inside the receiving cells When the messenger binds to a receptor, the recipient cell responds in a way that is determined by the messenger, receptor, and type of cell

Target Cells Every cell has dozens of receptors, each capable of binding a specific messenger and stimulating a particular response Cells with receptors that bind a messenger molecule and respond to it are target cells for that message Cells without the correct receptors cannot respond to the messenger and are not target cells Therefore, a given cell can be a target cell for some messenger molecules but not others, depending it’s receptors

Synaptic Synaptic communication is used in the nervous system Electrical signals within individual nerve cells send information to the farthest reaches of the body in a fraction of a second Then, the nerve cell communicates with a small number of other cells at junctions called synapses At a synapse, a nerve cell elicits responses from a target cell by releasing neurotransmitters across a space between the nerve cell and its target

Paracrine Cells release chemicals that diffuse through the extracellular fluid to other cells in the immediate vicinity They influence only a small group of cells, but do so quickly because the distances are very short

Endocrine Endocrine hormones are released into the bloodstream and move throughout the body in a few seconds. They trigger responses that may last from a few seconds to a lifetime

Local Hormones Local hormones diffuse to nearby target cells Many cells engage in paracrine communication, secreting local hormones into the extracellular fluid Histamine, released as part of the allergic and inflammatory responses Cytokines, by which cells of the immune system communicate with one another Local hormones have only short range actions because they are either degraded rapidly or taken up by nearby cells and cannot get far from the cells that secrete them

Prostaglandins Prostaglandins - modified fatty acids - important local hormones secreted by cells throughout the body They have diverse roles During childbirth they cause the cervix to dilate and help stimulate the muscles of the uterus to contract Prostaglandins contribute to inflammation and pain sensations Drugs such as aspirin, acetaminaphen, and ibuprofen provide relief from these symptoms by blocking the enzymes that synthesize prostaglandins

Endocrine Hormones Endocrine hormones are messenger molecules produced by the endocrine glands The secretory cells of an endocrine gland are embedded within a network of capillaries, and the cells secrete their hormones into the extracellular fluid surrounding the capillaries The hormones diffuse into the capillaries and are carried throughout the body by the bloodstream

Oxytocin Stimulates the contraction of uterine muscles during childbirth because the muscle cells have receptors that bind oxytocin Oxytocin does not cause other muscles of the body to contract because their cells do not have the necessary receptors Uterine muscles contain target cells for oxytocin, whereas other muscles do not

Hormone Release, Distribution, and Reception Endocrine cells release hormone 1 The hormone enters the blood and is carried throughout the body 2 (extracellular fluid) The hormone leaves the capillaries and diffuses to all tissues through the extracellular fluid 3 capillary biceps uterus The hormone affects cells bearing receptors to which the hormone can bind 4 The hormone cannot affect cells that only bear receptors to which the hormone cannot bind 5

Results The changes induced by hormones may be prolonged and irreversible The onset of puberty or the transformation of a caterpillar into a butterfly The changes are temporary and reversible, and help to regulate the physiological systems of the animal body within a course of time of seconds to hours

Vertebrate Endocrine Hormones Are evolutionarily ancient Insulin is found not only in vertebrates, but also in protists, fungi, and bacteria, although the function of insulin in these organisms is unknown Thyroid hormones have been found in invertebrates such as worms, insects, and mollusks, which do not have thyroid glands Hormones appear to work similarly in the cells of invertebrates

Three Classes of Vertebrate Endocrine Hormones Peptide hormones - chains of amino acids Amino acid–derived hormones - composed of one or two modified amino acids Steroid hormones - synthesized from cholesterol

How Do Animal Hormones Work? Hormones act by binding to receptors on or in target cells Receptors for hormones are found in two locations on target cells: On the plasma membrane Inside the cell, within the cytoplasm or the nucleus

Peptide and Amino Acid Hormones Peptide and amino acid hormones bind to receptors on the surfaces of target cells Cannot diffuse through the phospholipid bilayer of the plasma membrane and must bind to receptors on the surface of the target cell’s plasma membrane Hormone–receptor binding activates an enzyme that synthesizes a molecule, called a second messenger, inside the cell An example is cyclic adenosine monophosphate (cyclic AMP), which regulates many cellular activities

Second Messengers Transfers the signal from the first messenger—the hormone—to other molecules within the cell, often activating specific intracellular enzymes These activated enzymes initiate a chain of biochemical reactions that vary depending on the hormone, the second messenger, and the target cell Epinephrine stimulates the synthesis of cyclic AMP in both heart muscle and liver cells, but the result is different in the two cell types Cyclic AMP causes heart muscle cells to contract more strongly. In liver cells, it activates enzymes that breakdown glycogen to glucose

Animation: The Action of Nonsteroid Hormones

Actions of Peptide and Amino Acid-Derived Hormones peptide or amino acid-derived hormone (first messenger) Hormone–receptor binding activates an enzyme that catalyzes the synthesis of a second messenger, such as cyclic AMP 2 The hormone binds to a receptor on the plasma membrane of a target cell 1 cyclic AMP- synthesizing enzyme (cytoplasm) (extracellular fluid) ATP active enzyme receptor product cyclic AMP (second messenger) 4 The activated enzymes catalyze specific reactions plasma membrane inactive enzyme reactant The second messenger activates other enzymes 3 nuclear envelope (nucleus)

Steroid hormones bind to receptors inside target cells Steroid hormones are lipid soluble and diffuse through the plasma membrane of target cells Bind to receptors inside target cells which are in the nucleus or move into the nucleus after hormone binding The hormone–receptor complex then binds to the DNA of the promoter region of specific genes and stimulates transcription of messenger RNA The mRNA travels to the cytoplasm and directs protein synthesis

Steroid Hormone Action on Target Cells (extracellular fluid) The hormone binds to a receptor in the nucleus or to a receptor in the cytoplasm that carries it into the nucleus 2 The hormone–receptor complex binds to DNA and causes RNA polymerase to bind to a nearby promoter site for a specific gene 3 A steroid hormone diffuses through the plasma membrane 1 DNA plasma membrane hormone receptor ribosome RNA polymerase The mRNA leaves the nucleus, then attaches to a ribosome and directs the synthesis of a specific protein product 5 RNA polymerase catalyzes the transcription of DNA into messenger RNA (mRNA) 4 mRNA gene new protein nuclear envelope (cytoplasm) (nucleus)

Animation: The Action of Steroid Hormones

Thyroid Hormone Although it is not a steroid, thyroid hormone acts intracellularly Actively transported into many cell types Once inside, thyroid hormone binds to intracellular receptors and activates transcription of specific genes Hormones that bind to intracellular receptors may take several minutes or even days to exert their full effects

Negative Feedback Hormone release is regulated by negative feedback mechanisms A response to a change that counteracts a change and restore the system to its original condition Example, after jogging on a hot, sunny day, you have lost a quart of water through perspiration Your pituitary releases antidiuretic hormone (ADH), which causes increased water reabsorption by your kidneys, concentrating your urine If you drink two quarts of water, your would have excess blood volume Negative feedback acts to restore the original condition by turning off ADH secretion, and your kidneys eliminates the excess water

Positive Feedback In a few cases, hormone release is temporarily controlled by positive feedback In this case, the response to a change enhances the change Contractions of the uterus early in childbirth push the baby’s head against the cervix, which causes the cervix to stretch Stretching the cervix sends nervous signals to the mother’s brain, which in turn causes the release of oxytocin Oxytocin stimulates continued contractions of the uterus, pushing the baby harder against the cervix until delivery is complete

Vertebrate and Invertebrate Endocrine Hormones Vertebrate and invertebrate endocrine hormones have similar mechanisms of action Insects molt in order to grow - controlled by the steroid hormone ecdysone, or molting hormone Ecdysone acts on receptors located within the nucleus and affects gene transcription, initiating a complex process in which the epithelial cells detach from the old cuticle and secrete a soft new cuticle beneath it The insect expands its body by pumping itself full of air This splits open the old cuticle and stretches out the new one to accommodate future growth As the insect emerges, it leaves an insect-shaped cuticle behind

Insect Molting Emerging cicada Old cuticle

Structures and Functions of the Mammalian Endocrine System The mammalian endocrine system consists of the endocrine hormones and the glands that produce them The major endocrine glands and organs are: The hypothalamus–pituitary complex The thyroid gland The pancreas The sex organs The adrenal glands

The Major Mammalian Endocrine Glands and Their Hormones Hypothalamus ADH, oxytocin, and regulatory hormones for the anterior pituitary Pineal gland melatonin Pituitary gland anterior pituitary: ACTH, TSH, GH, PRL, FSH, LH posterior pituitary: oxytocin and ADH Parathyroid glands (on the posterior surface of the thyroid gland) parathyroid hormone Heart atrial natriuretic peptide Thyroid gland thyroxine, calcitonin Kidneys erythropoietin Thymus gland thymosins Digestive tract several hormones (see Chapter 34) Adrenal glands (one on each kidney) medulla: epinephrine, norepinephrine cortex: glucocorticoids (cortisol), mineralocorticoids (aldosterone), testosterone Fat leptin Gonads testes (male): androgens, especially testosterone ovaries (female): estrogens, progesterone Pancreas islet cells insulin, glucagon testis ovary

Hypothalamus and Pituitary Hormones of the hypothalamus and pituitary gland regulate functions throughout the body The hypothalamus and pituitary gland coordinate the action of many key hormonal systems The hypothalamus contains clusters of specialized nerve cells called neurosecretory cells Neurosecretory cells synthesize peptide hormones, store and release when stimulated

Pituitary Gland Pea-sized gland connected to the hypothalamus by a stalk Two distinct parts: The anterior pituitary, a true endocrine gland, composed of several types of hormone-secreting cells in a network of capillaries The posterior pituitary, mainly a capillary bed and the endings of neurosecretory cells whose cell bodies are in the hypothalamus The hypothalamus controls the release of hormones from both parts of the pituitary

Pituitary & Hypothalamus Hypothalamic hormones control hormone release in the anterior pituitary Neurosecretory cells of the hypothalamus produce +/- seven hormones that regulate release of hormones from anterior pituitary These hypothalamic hormones are releasing hormones or inhibiting hormones, depending on whether they stimulate or inhibit the release of a particular pituitary hormone

How it works… 1. Neurosecretory cells of the hypothalamus produce releasing and inhibiting hormones (green dots, page 721) 2. Release and inhibiting hormones are secreted into a capillary bed feeding the anterior lobe of the pituitary 3. Endocrine cells of the anterior pituitary secrete hormones in response to releasing hormones, pituitary hormones enter bloodstream Some hypothalamic hormones (growth hormone-releasing hormone) stimulate the release of pituitary hormones, others inhibit the release of pituitary hormones

The Hypothalamus–Pituitary System Neurosecretory cells of the hypothalamus produce oxytocin and ADH 1 Neurosecretory cells of the hypothalamus produce releasing and inhibiting hormones 1 Releasing or inhibiting hormones (green circles) are secreted into capillaries feeding the anterior lobe of the pituitary 2 Oxytocin and ADH (blue triangles) are secreted into the blood via capillaries in the posterior pituitary 2 blood flow pituitary (anterior lobe) endocrine cell capillary bed pituitary (posterior lobe) Endocrine cells of the anterior pituitary secrete hormones (red squares) in response to releasing hormones; the pituitary hormones enter the bloodstream 3 capillary bed blood flow

Anterior Pituitary These regulate hormone production in other endocrine glands: Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the production of sperm and testosterone in males, production of eggs, estrogen, and progesterone in females Thyroid-stimulating hormone (TSH) stimulates the thyroid gland to release its hormones Adrenocorticotropic hormone (ACTH) causes the release of the hormone cortisol from the adrenal cortex

More Endocrine Hormones These hormones of the anterior pituitary do not act on other endocrine glands Prolactin (with help) stimulates development of milk-producing mammary glands in the breasts during pregnancy Growth hormone acts on nearly all the body’s cells by increasing protein synthesis, promoting the use of fats for energy, and regulating carbohydrate metabolism During childhood growth hormone stimulates bone growth, which influences human height; too little growth hormone results in dwarfism, and too much results in gigantism

Anterior Pituitary Malfunctions

Posterior Pituitary Releases hormones synthesized by cells in the hypothalamus The hypothalamus contains two types of neurosecretory cells that send axons into the posterior pituitary These axons end in a capillary bed into which they release hormones that are then carried by the bloodstream to the rest of the body. 1. Neurosecretory cells synthesize antidiuretic hormone (ADH) or oxytocin. (blue) 2. Secreted into blood via capillaries of posterior pituitary

The Hypothalamus–Pituitary System Neurosecretory cells of the hypothalamus produce oxytocin and ADH 1 Neurosecretory cells of the hypothalamus produce releasing and inhibiting hormones 1 Releasing or inhibiting hormones (green circles) are secreted into capillaries feeding the anterior lobe of the pituitary 2 Oxytocin and ADH (blue triangles) are secreted into the blood via capillaries in the posterior pituitary 2 blood flow pituitary (anterior lobe) endocrine cell capillary bed pituitary (posterior lobe) Endocrine cells of the anterior pituitary secrete hormones (red squares) in response to releasing hormones; the pituitary hormones enter the bloodstream 3 capillary bed blood flow

ADH and Oxytocin Antidiuretic hormone (ADH) helps prevent dehydration by causing more water to be reabsorbed from the urine by the kidneys and returned to the bloodstream Alcohol inhibits the release of ADH and increases urination, resulting in the loss of more water than is consumed with dehydration resulting Oxytocin causes contractions of uterine muscles during childbirth and triggers “milk letdown” in nursing mothers by causing muscle tissue within the mammary glands to contract in response to the suckling infant In humans, oxytocin may play a role in emotions, including trust and both romantic and maternal love

Animation: Hypothalamic Control of the Pituitary

Thyroid and Parathyroid Glands Influence metabolism and calcium levels Front of the neck below the larynx, the thyroid gland produces two hormones: thyroxine and calcitonin The parathyroid gland - two pairs of small disks of cells on each side of the thyroid, releases parathyroid hormone

The Thyroid and Parathyroid Glands larynx thyroid gland esophagus parathyroid glands trachea

Thyroxine Influences energy metabolism Thyroxine or thyroid hormone, is an iodine-containing amino acid derivative that works by binding to intracellular receptors that regulate gene activity By stimulating glucose breakdown and providing the resulting energy from it, thyroid hormone elevates the metabolic rate of many body cells In juvenile animals, including humans, thyroxine helps regulate growth by stimulating both metabolic rate and nervous system development Undersecretion of thyroid hormone leads to cretinism, a condition characterized by retardation

Iodine Deficiency An iodine-deficient diet can reduce the production of thyroxine and trigger a feedback that attempts to restore normal hormone levels by increasing the number of thyroxine-producing cells The thyroid gland becomes enlarged, forming a goiter Iodine deficiency in pregnant women and young children is the leading preventable cause of mental retardation Iodized salt is a simple, and cheap, solution to iodine deficiency

Goiter

Thyroxine Release is controlled by the hypothalamus and anterior pituitary 1. Thyroid stimulating hormone-releasing hormone is produced by neurosecretory cells in hypothalamus, travels to anterior pituitary 2. TSH releasing hormone causes anterior pituitary to secrete TSH – thyroid stimulating hormone 3. TSH travels in blood to thyroid and stimulates release of thyroxine 4. Secretion of TSH-releasing hormone and TSH are regulated by negative feedback Adequate levels of thyroxine inhibit the secretion of both TSH-releasing hormone from the hypothalamus and TSH from the anterior pituitary

Negative Feedback in Thyroid Gland Function Neurosecretory cells of the hypothalamus secrete TSH-releasing hormone 1 Thyroxine inhibits TSH-releasing hormone and TSH release by negative feedback 4 releasing hormone The releasing hormone causes the anterior pituitary to secrete thyroid-stimulating hormone (TSH) 2 TSH endocrine cells of the anterior pituitary thyroid gland thyroxine hormone- producing cells of the thyroid TSH causes the thyroid to secrete thyroxine, which increases cellular metabolism throughout the body 3

Thyroxine Effects Varied effects in different vertebrates In amphibians - has the effect of triggering metamorphosis In 1912, tadpoles were fed minced horse thyroid and metamorphosed prematurely into miniature adult frogs Thyroxine also regulates the seasonal molting of most vertebrates from snakes to birds to the family dog. Surges of thyroxine stimulate the shedding of skin, feathers, and hair

Parathyroid Hormone and Calcitonin Regulate calcium metabolism The proper concentration of calcium is essential to nerve and muscle function, the parathyroid hormone and calcitonin work together to maintain constant calcium levels in the blood If blood calcium levels drop, parathyroid hormone causes the bones to release calcium and the kidneys to reabsorb more calcium from urine If blood calcium gets too high, calcitonin inhibits the release of calcium from bone

The Pancreas Both digestive and endocrine functions The pancreas produces bicarbonate and enzymes that are released into the small intestine, promoting the digestion of food The endocrine portion of the pancreas consists of clusters of islet cells that produce one of two peptide hormones: insulin or glucagon

Insulin and Glucagon Control Glucose Levels in the Blood Insulin and glucagon work in opposition to regulate carbohydrate and fat metabolism Insulin reduces the blood glucose level Glucagon increases it 1. eating raises blood sugar 2. high glucose stimulates insulin release and inhibits glucagon release 3. Insulin stimulates glucose uptake by body cells, liver converts glucose glycogen

Maintaining a Glucose Balance 4. glucose uptake into cells and conversion of glucose glycogen reduces blood glucose 5. exercise and fasting also reduce blood glucose 6. low blood glucose stimulates glucagon release and inhibits insulin release 7. glucagon stimulates cells to burn fat instead of glucose, liver converts glycogen to glucose 8. blood glucose is increased

The Pancreas Controls Blood Glucose Levels glucose is increased 8 Eating raises blood glucose 1 Glucagon stimulates cells to burn fat instead of glucose; the liver converts glycogen to glucose 7 high blood glucose High blood glucose stimulates insulin release and inhibits glucagon release 2 pancreas glucagon Insulin stimulates glucose uptake by body cells; the liver converts glucose to glycogen 3 insulin liver Low blood glucose stimulates glucagon release and inhibits insulin release 6 low blood glucose muscle Exercise and fasting also reduce blood glucose 5 Glucose uptake into cells and conversion of glucose to glycogen reduce blood glucose 4

Diabetes Results from malfunctioning insulin control system Lack of insulin production or failure of target cells to respond to insulin results in diabetes mellitus In either case, blood glucose levels are high because cells cannot take up glucose unless they are stimulated by insulin, and they rely on fats as an energy source, which leads to high levels of blood lipids Many diabetics suffer from heart and blood vessel disease caused by fat deposition Insulin replacement therapy

Sex organs Produce gametes and sex hormones Besides producing sperm or eggs, the testes in males and ovaries in females are also important endocrine organs The testes secrete several steroid hormones, collectively called androgens, the most important being testosterone The ovaries secrete two types of steroid hormones: estrogen and progesterone

Sex Hormones Levels increase during puberty Puberty is the phase of life during which the reproductive systems become mature and functional It begins when the hypothalamus secretes increasing amounts of releasing hormones, which stimulate the anterior pituitary to secrete more (LH) and (FSH) LH and FSH stimulate target cells in the testes and ovaries to produce higher levels of sex hormones

Steroid Hormones Testosterone, secreted by the testes, promotes sperm production and stimulates the development of male secondary sexual characteristics, ie. body and facial hair, broad shoulders, and muscle growth Estrogen from the ovaries stimulates breast development and the maturation of the female reproductive system, including egg production Progesterone prepares the reproductive tract to receive and nourish the fertilized egg

Adrenal Glands Secrete hormones that regulate metabolism and responses to stress The adrenal glands consist of two parts: The adrenal cortex The adrenal medulla

The Adrenal Glands The adrenal medulla secretes epinephrine and norepinephrine The adrenal cortex secretes glucocorticoids, mineralocorticoids, and testosterone kidney

Adrenal Cortex Produces steroid hormones The outer layer forms the adrenal cortex, which secretes three steroid hormones: Glucocorticoids, which help control glucose metabolism Mineralocorticoids, which regulate salt metabolism Small amounts of testosterone

Cortisol, a Glucocorticoid Release Glucocorticoid release is stimulated by adrenocorticotropic hormone (ACTH) from the anterior pituitary, which is stimulated by releasing hormones from the hypothalamus Glucocorticoids are released in response to stimuli such as stress, trauma, or exposure to temperature extremes Cortisol – the most abundant glucocorticoid Increases blood glucose levels by stimulating glucose production, inhibiting the uptake of glucose by muscle cells, and promoting the use of fat for energy

Glucose Metabolism Many different hormones are involved in glucose metabolism: thyroxine, insulin, glucagon, epinephrine, and the glucocorticoids The reason for so many different hormones in the regulation of glucose can be traced to the metabolic requirements of the brain Most body cells can produce energy from fats and proteins as well as from carbohydrates Brain cells only metabolize glucose, so glucose levels in the blood cannot be allowed to fall too far or brain cells rapidly die

Mineralocorticoid Hormones Regulate the mineral (salt) content of the blood Most important is aldosterone (ADS) A constant blood sodium concentration is crucial for cellular events, including production of electrical signals by nerve cells If blood sodium falls, adrenal cortex releases ADS, which causes the kidneys and sweat glands to retain sodium When blood sodium returns to normal, ADS secretion is turned off

Testasterone and the Adrenal Cortex In women and men the adrenal cortex also produces testosterone, although in smaller quantities than produced by the testes Tumors of the adrenal cortex can lead to excessive testosterone release, causing masculinization of women

Adrenal Medulla Produces amino acid-derived hormones Located in the center of each adrenal gland Produces two hormones in response to stress or exercise: epinephrine and norepinephrine Prepare the body for emergency action by increasing heart and respiratory rates, blood pressure, causing blood glucose levels to rise, and directing blood flow away from the digestive tract and toward the brain and muscles

Hormones also produced in… Hormones are also produced by the pineal gland, thymus, kidneys, heart, digestive tract, and fat cells The pineal gland produces the hormone melatonin Secreted in a daily rhythm, which is regulated by light entering the eyes The pineal appears to regulate the seasonal reproductive cycles of many animals The function of melatonin and the pineal gland in humans is not known

Thymus Gland Located in the chest cavity behind the breastbone Produces thymosin, stimulates the development of T-cells Large in infants, but under the influence of sex hormones, decreases in size after puberty Elderly produce fewer new T cells than adolescents and are more susceptible to new diseases

Kidneys Produce erythropoietin, a peptide hormone that is released when oxygen content of the blood is low Stimulates the bone marrow to increase red blood cell production The kidneys also produce an enzyme called renin in response to low blood pressure, which catalyzes the production of the hormone angiotensin from proteins in the blood Angiotensin raises blood pressure by constricting arterioles and stimulating the release of aldosterone by the adrenal cortex, which leads to increased sodium and water reabsorption by the kidney and increased blood volume and pressure

More Hormones Stomach and small intestines produce a number of peptide hormones that help regulate digestion Include gastrin, ghrelin, secretin, and cholecytokinin The heart releases atrial natriuretic peptide (ANP), which inhibits the release of ADH and aldosterone and increases the excretion of sodium The actions of ANP lead to a drop in blood volume by reducing reabsorption of water and salt by the kidneys

Fat can act as an endocrine organ In 1995, Jeffrey Friedman at Rockefeller University discovered the peptide hormone leptin, which is released by fat cells Mice genetically engineered to lack the gene for leptin became obese, leptin injections caused them to lose weight Researchers hypothesized that by releasing leptin, fat tissue “tells” the body how much fat it has stored and how much to eat Leptin has other roles that include stimulating the growth of capillaries and speeding wound healing