Chemical Signals in Animals

1. Chemical Signals in Animals Chapter 47

2. Chemical Signals in Animals • Hormone - chemical signal that circulates through the blood or other bodily fluids and affects distant target cells. • The endocrinesystem is responsible for the production and secretion of hormones. • There are six major categories of chemical signals in animals • Six chemical messenger classes do not coincide with six structurally distinct classes of molecules

3. Hormone Structure and Function

4. Autocrine Signals • Autocrine signals act on the same cell that secretes them. • An example is cytokines; most cytokines amplify the response of a cell to a stimulus • Like in bone marrow cells for defense • IL-1 is one of the most important immune responses, enhances the activation of T-cells in response to antigen

5. Paracrine signals • Paracrine signals diffuse locally and act on neighboring cells • May be due to degradation • Growth factors that are important in coordination of development • Responses to allergens • Has been linked to tumor growth and cancer

6. Endocrine Signals • Endocrine signals are produced by cells that may be organized into discrete organs called glands or may be interspersed among the cells of other organs • May be carried by blood or interstitial fluid • Act on target cells throughout the body • Hormones such as testosterone and estrogen

7. Neural Signals • Neural signals are the chemical messengers called neurotransmitters • Cause an action potential to be created in a neuron and create a neural signal • Held in vacuoles within the neuron • Acetylcholine stimulates muscle neurons • Norepinephrine stimulates wakefulness

8. Neuroendocrine Signals • Neuroendocrine signals are released from neurons but act on distant cells instead of acting at the adjacent synapse • Signals from stomach stimulate pituitary and brain cells to tell the brain and pituitary gland that it is full or it is empty

9. Chemical Characteristicsof Hormones • The three types of chemical messengers: polypeptides, amino acid derivatives, and steroids • All are similar in that they are • Organic compounds • Synthesized and secreted from the cells • Act on target cells remote from their point of origin

10. Chemical Characteristicsof Hormones • Similarities: • Present in extremely small concentrations yet have large effects • Key differencein these hormone types is that steroids are lipid soluble but polypeptides and amino acid derivatives are not. • Steroids cross cell membranes much more readily than do other types of hormones.

12. The Human Endocrine System—an Overview • Organs that secrete hormones into the bloodstream are called endocrine glands • The major human endocrine glands are:

13. Hypothalamus • The hypothalamus, a region of the lower brain • Contains different sets of neurosecretory cells • Receive nerve signals from throughout the body

15. Hypothalamus Neurosecretory cells of the hypothalamus Axon Posterior pituitary Anterior pituitary ADH HORMONE Oxytocin Mammary glands, uterine muscles TARGET Kidney tubules Pituitary Gland • The pituitary gland, which sits just below the hypothalamus and has distinct anterior and posterior regions

16. Posterior Pituitary Hormones • The two hormones released from the posterior pituitary • Act directly on nonendocrine tissues • Oxytocin • Induces uterine contractions and milk ejection • Antidiuretic hormone (ADH) • Enhances water reabsorption in the kidneys

17. Anterior Pituitary Glands • The anterior pituitary • Produces both tropic and nontropic hormones • The four strictly tropic hormones are • Follicle-stimulating hormone (FSH) • Luteinizing hormone (LH) • Thyroid-stimulating hormone (TSH) • Adrenocorticotropic hormone (ACTH)

19. Thyroid Gland • The thyroid gland • Situated in the Neck • Consists of two lobes located on the ventral surface of the trachea • Produces two iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4)

20. Thyroid Hormones • The thyroid hormones • Play crucial roles in stimulating metabolism and influencing development and maturation • Also important in maintaining homeostatic functions • Hyperthyroidism, excessive secretion of thyroid hormones • Can cause Graves’ disease in humans • Hypothyroidism can cause weight gain

22. Thyroid and Parathyroid Hormones • The four parathyroid glands are embedded in the thyroid gland • Act in opposition to thyroid • Calcitonin, secreted by the thyroid gland • Stimulates Ca2+ deposition in the bones and secretion by the kidneys, thus lowering blood Ca2+ levels • PTH, secreted by the parathyroid glands • Has the opposite effects on the bones and kidneys, and therefore raises Ca2+ levels • Also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food

24. Kidneys and Adrenal Glands • The two kidneys, which lie in the posterior part of the abdominal cavity • The two adrenal glands, which sit atop the kidneys and have an outer cortex and a central medulla • The adrenal medulla secretes epinephrine and norepinephrine • In response to stress-activated impulses from the nervous system

26. Pancreas • Located in the anterior part of the abdominal cavity • Two types of cells in the pancreas • Secrete insulin and glucagon, antagonistic hormones that help maintain glucose homeostasis and are found in clusters in the islets of Langerhans

28. Gonadal Sex Hormones • Suspended below the pelvic cavity, respectively • Produce most of the body’s sex hormones: androgens, estrogens, and progestins

30. What Do Hormones Do? • Hormones coordinate the activities of diverse groups of target cells • Change according to environment or signals • The stimuli to which hormones respond can be simple or complex • Coordinate responses to environmental change • Direct developmental processes

31. Digestive Hormones • Digestive hormones function in simple stimulus-and-response circuits • When acidic food passes from the stomach to the upper part of the small intestine, the food triggers intestinal cells to release secretin and cholecystokinin into the bloodstream • Secretin induces the pancreas to secrete a solution that neutralizes acid • Cholecystokinin causes the pancreas to secrete digestive enzymes into the small intestine and the gallbladder to eject bile salts into the intestine to emulsify fats

32. Responses to Stress • When a person is in danger, hormones regulate both the short-term and long-term responses • The short-term reaction—the fight-or-flight response—occurs through the activation of the sympathetic nervous system • Long-term stress involves glucocorticoids produced in the adrenal cortex. • Ensures the continued availability of fuel molecules to support important body functions

33. Stress Nerve signals Hypothalamus Spinal cord (cross section) Releasing hormone Nerve cell Anterior pituitary Blood vessel Adrenal medulla secretes epinephrine and norepinephrine. Nerve cell Adrenal cortex secretes mineralocorticoids and glucocorticoids. ACTH Adrenal gland Kidney (a) Short-term stress response (b) Long-term stress response Effects of epinephrine and norepinephrine: Effects of mineralocorticoids: Effects of glucocorticoids: 1. Glycogen broken down to glucose; increasedblood glucose 1. Retention of sodiumions and water bykidneys 1. Proteins and fatsbroken down andconverted to glucose,leading to increasedblood glucose 2. Increased blood pressure 3. Increased breathing rate 2. Increased bloodvolume and bloodpressure 4. Increased metabolic rate 5. Change in blood flow patterns, leading to increased alertness and decreased digestive and kidney activity 2. Immune system may be suppressed Stress and the Adrenal Gland

34. How Do Hormones Direct Developmental Processes? • Growth hormones and sex hormones promote cell division, increase overall body size, and promote sexual differentiation as an individual matures. • Certain hormones direct the development of particular cells and tissues at critical junctures in an individual’s life • The major hormonal effects on development are:

35. Primary Sex Determination • Events early in development that dictate whether the sex organs become male (testes) or female (ovaries) • Once they develop, they begin producing male-specific hormones (testosterone) or female-specific hormones (estradiol, a member of the estrogen family of hormones).

36. Puberty • At puberty, surges of sex hormones lead to the physical and emotional changes associated with adolescence • These developmental changes create the adult phenotype and the ability to produce offspring.

37. Full Growth and Development • In humans and other mammals, the attainment of full adult stature is mediated by growth factors that are regulated by growth hormone produced in the pituitary gland.

38. Reproduction • Most long-lived animals reproduce seasonally • In many species, environmental cues trigger the release of sex hormones • Although humans do not breed seasonally, sex hormones regulate sperm production and the menstrual cycle

39. Hormones and Homeostasis • Hormones that act as messengers in homeostatic systems include antidiuretic hormone (ADH), aldosterone, and erythropoietin (EPO). • Calcitonin and parathyroid hormone work together to keep Ca2+ levels in the blood close to a set point

41. How are Hormones Regulated? • In many cases, hormone production is directly or indirectly controlled by the nervous system. • Adrenocorticotropic hormone (ACTH) is a regulatory hormone that controls release of glucocorticoids from the adrenal cortex • These hormones act as regulators and all are involved in negative feedback, or feedback inhibition

43. 47.4 How Do HormonesAct on Target Cells? • Differences in lipid solubility influence where a target cell receives the chemical message. • Steroids often act inside the cell, whereas most amino acid derivatives and all polypeptides act at the cell surface. • Steroid hormone-receptor complexes bind to specific sites in DNA called hormone-response elements

45. Hormones That Bind to Cell-Surface Receptors • Epinephrine and the peptide hormones are not lipid soluble and cannot enter the target cell • They bind to receptors on the cell surface • Activate the receptor on the cell surface by signal transduction • Epinephrine produces two distinct patterns of responses because there are two types of receptor, alpha and beta

46. Identifying theEpinephrine Receptor Epinephrine and its agonists (molecules that bind to the same receptors as the hormone itself) produce two distinct patterns of responses because there are two types of receptor, alpha and beta. In turn, there are two types of alpha and two types of beta receptor.

47. Signal Transduction and the Role of Second Messengers Epinephrine activates phosphorylase, the enzyme that catalyzes the formation of glucose from glycogen (Figure 47.15).

49. Epinephrine Signal Transduction • Epinephrine triggers a signal transduction cascade that includes cyclic adenosine monophosphate (cAMP) as a second messenger to amplify the signal