Essentials of the Living World Second Edition George B. Johnson Jonathan B. Losos Chapter 31 Chemical Signaling Within the Animal Body Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

31.1 Hormones A hormone is a chemical signal produced in the body it is stable enough to be transported in active form from where it is produced it typically acts a distant site hormones are produced in glands that are completely enclosed in tissue these glands are called endocrine glands

Figure 31.1 Majors glands of the human endocrine system

31.1 Hormones There are three big advantages to using chemical hormones as messengers rather than speedy electrical signals (like nerve signals) chemical molecules can spread to all tissues via the blood chemical signals can persist much longer than electrical ones many different kinds of chemicals can act as hormones

31.1 Hormones The glands that produce hormones are generally controlled by the nervous system the endocrine system and the motor nervous system are the two main routes the CNS uses to issue commands to the organs of the body the two are so closely linked that they are often considered a single system—neuroendocrine system the hypothalamus is the main switchboard of the neuroendocrine system

31.1 Hormones The CNS regulates the body’s hormones through a chain of command for example, the hypothalamus controls the pituitary gland with thyrotropic-releasing hormone (TRH) this causes the pituitary to release thyrotropin or thyroid-stimulating hormone (TSH) the hypothalamus also secretes inhibiting hormones that keep the pituitary from secreting specific hormones

31.1 Hormones Hormones are effective messengers within the body because a particular hormone can influence a specific target cell cells that the body has targeted to respond to a particular hormone have receptor proteins shaped to fit that hormone and no other

31.1 Hormones Hormones secreted by endocrine glands belong to four different chemical categories polypeptides glycoproteins amines steroids

31.1 Hormones The path of communication taken by a hormonal signal is a series of simple steps issuing the command the hypothalamus controls the release of many hormones transporting the signal most are transported throughout the body by the bloodstream hitting the target the hormone binds to a receptor on the target cell having an effect when the hormone binds to the receptor protein, the protein changes shape and triggers a change in cell activity

Figure 31.2 How hormonal communication works

31.2 How Hormones Target Cells The steroid hormones are recognize by protein receptors located in the cytoplasm or nucleus of the target cell steroids are lipid-soluble because they are manufactured from cholesterol steroid hormones can pass across the lipid bilayer of the cell plasma membrane

31.2 How Hormones Target Cells The complex of a steroid hormone and its receptor inside the target cell bind to DNA in the nucleus this activates the transcription of a specific gene and a protein is subsequently synthesized

Figure 31.3 How steroid hormones work

31.2 How Hormones Target Cells Anabolic steroids are synthetic compounds that resemble the male sex hormone, testosterone their injection into muscle cells activates growth genes and cause the muscle cells to produce more protein this results in bigger muscle and increased strength anabolic steroids have many dangerous side effects in both men and women liver damage, heart disease, high blood pressure, acne, balding, and psychological disorders men can also experience feminization while females can undergo masculination

31.2 How Hormones Target Cells The receptor for peptide hormones embedded within the plasma membrane the binding of the hormone to the receptor triggers changes in the cytoplasmic end of the receptor protein using second messengers, this change is amplified and causes changes in the cell second messengers activate enzymes one of the most common is cyclic AMP (cAMP)

Figure 31.4 How peptide hormones work

31.2 How Hormones Target Cells A single hormone binding to a receptor in the plasma membrane can result in the formation of many second messengers in the cytoplasm cyclic AMP is made from ATP by an enzyme that removes two phosphate units each second messenger can activate many molecules of a certain enzyme

Figure 31.5 How second messengers work

31.3 The Hypothalamus and the Pituitary The pituitary gland is located beneath the hypothalamus and is the location where nine hormones are produced these hormones act principally to influence other endocrine glands the pituitary is actually two glands posterior lobe regulates water conservation and, in women, milk letdown and uterine contraction anterior lobe regulates other endocrine glands

31.3 The Hypothalamus and the Pituitary The hypothalamus and the posterior pituitary are connected by a tract of neurons hormones are produced by cell bodies in the hypothalamus and transported to the posterior pituitary vasopressin or antidiuretic hormone (ADH) regulates the kidney’s retention of water oxytocin initiates uterine contractions during childbirth and milk release in mothers

Figure 31.6 The posterior pituitary contains cells that originate in the hypothalamus

31.3 The Hypothalamus and the Pituitary The anterior pituitary is a complete gland that produces the hormones that it secretes thyroid-stimulating hormone (TSH) stimulates the thyroid gland to produce thyroxine, which in turn stimulates oxidative respiration adrenocorticotropic hormone (ACTH) stimulates the adrenal gland to produce hormones growth hormone (GH) simulates the growth of muscle and bone throughout the body

31.3 The Hypothalamus and the Pituitary Follicle-stimulating hormone (FSH) in females, it triggers the maturation of egg cells and stimulates the release of estrogen in males, it regulates sperm development Luteinizing hormone (LH) in females, it triggers ovulation of a mature egg in males, it stimulates the gonads to produce testosterone

31.3 The Hypothalamus and the Pituitary Prolactin (PRL) stimulates the breasts to produce milk Melanocyte-stimulating hormone (MSH) stimulates, in reptiles and amphibians, color changes in the epidermis its function in humans is poorly understood

Figure 31.7 The role of the pituitary

31.3 The Hypothalamus and the Pituitary The hypothalamus controls production and secretion of the anterior pituitary hormones by means of a family of special hormones neurons in the hypothalamus secrete both releasing and inhibiting hormones they travel to the anterior pituitary through a special system, called the hypothalamohypohyseal portal system a portal system has a second capillary bed downstream from the front

Figure 31.8 Hormonal control of the anterior pituitary gland by the hypothalamus

31.3 The Hypothalamus and the Pituitary Negative feedback (feedback inhibition) controls how target gland hormones in the anterior pituitary are produced when enough of the target hormone has been produced, the hormone then feeds back to the hypothalamus and inhibits the release of stimulating hormones from the hypothalamus and the anterior pituitary

Figure 31.9 Negative feedback

31.4 The Pancreas The pancreas has both exocrine and endocrine functions it secretes digestive enzymes and hormones the hormones, produced in the islets of Langerhans, are insulin and glucagon insulin promotes the accumulation of glycogen in the liver and triglycerides in fat cells glucagon causes liver cells to release stored glucose and to break down triglycerides

Figure 31.10 Insulin and glucagon secreted by the pancreas regulate blood glucose levels

31.4 The Pancreas Diabetes mellitus is a serious disorder in which affected individuals’ are unable to take up glucose from the blood it is the seventh leading cause of death in the US there are two kind of diabetes mellitus Type I is a hereditary autoimmune disease that attacks the islets of Langerhans, resulting in abnormally low insulin secretion Type II is an abnormally low number of insulin receptors in the target tissue

31.5 The Thyroid, Parathyroid, and Adrenal Glands The thyroid gland makes several hormones throxine increases metabolic rate and promotes growth thyroxine contains iodine and if the amount of iodine in the diet is too low, the thyroid cannot make enough thyroxine to inhibit the hypothalamus as a result, the hypothalamus stimulate the thyroid to grow bigger, which produces a condition in the neck called goiter calcitonin inhibits the release of calcium from bones

Figure 31.11 The thyroid gland secretes thyroxine

31.5 The Thyroid, Parathyroid, and Adrenal Glands The parathyroid glands are four small glands attached to the thyroid these glands produce parathytoid hormone (PTH), a hormone that is absolutely essential for survival because it acts on calcium uptake into and out of the bones it also regulates calcium levels in the blood, which are necessary for muscle contractions

Figure 31.12 Maintenance of proper calcium levels in the blood

31.5 The Thyroid, Parathyroid, and Adrenal Glands The adrenal glands are located just above the kidney and each is comprised of two parts medulla is the inner core and produces epinephrine and norepinephrine cortex is the outer region and produces the steroid hormones cortisol and aldosterone

31.5 The Thyroid, Parathyroid, and Adrenal Glands The medulla releases epinephrine (adrenaline) and norepinephrine in times of stress these hormones act as emergency signals that stimulate rapid deployment of body fuel The cortex secretes cortisol, which acts to maintain nutritional well-being it is also released in times of stress but can become a chronic problem if stress continues Aldosterone is another product of the cortex it affects water reabsorption in the kidney and affects both blood volume and blood pressure

31.6 A Host of Other Hormones There are a variety of hormones secreted by organs whose primary functions are not endocrine in nature the thymus secretes hormones that regulate the immune system the right atrium of the heart secretes a hormone that stimulates the kidney to excrete water and salts in the urine the kidneys secrete erythropoeitin, which stimulates the bone marrow to make red blood cells the skin secretes vitamin D

31.6 A Host of Other Hormones In insects, hormonal secretion influence both metamorphosis and molting prior to molting, neurosecretory cells on the surface of the brain secrete brain hormone brain hormone then stimulates a gland in the thorax to produce molting hormone (ecdysone) juvenile hormone is produced in the corpora allata area of the brain and determines the result of a particular molt when juvenile hormone levels are high, the molt produces another larva

Figure 31.13 The hormonal control of metamorphosis in the silkworm, Bombyx mori

Inquiry & Analysis Is hCG detectable on the graph when a woman is not pregnant? Would a test for the presence of hCG in a woman’s urine, if sensitive, provide a reliable test of pregnancy? Graph of Levels of hCG in a Pregnant Woman