Chapter 45

1. Chapter 45 Hormones and the Endocrine System

2. Internal Communication • Animals have 2 systems of internal communication and regulation: • 1. The nervous system. • 2. The endocrine system.

3. 1. The Nervous System • The nervous system is the pathway of communication involving high speed electrical signals.

4. 2. The Endocrine System • The endocrine system is all of the animal’s hormone secreting cells. • The endocrine system coordinates a slow, long-lasting response.

5. Endocrine Glands • Endocrine glands are hormone secreting organs. • They are ductless glands. • Their product is secreted into extracellular fluid and diffuses into circulation.

6. Endocrine and Nervous Systems • It is convenient to think of the nervous system and the endocrine as separate. • They are actually very closely linked. • Neurosecretory cells are specialized nerve cells that release hormones into the blood. • They have characterisitics of both nerves and endocrine cells.

7. Neurosecretory Cells • The hypothalamus and the posterior pituitary gland contains neurosecretory cells. • These produce neurohormones which are distinguishable from endocrine hormones. • Some hormones serve as both endocrine hormones and neurotransmitters.

8. Neurosecretory Cells • They can stimulate a response, or they can induce a target cell to elicit a response. • For example, a suckling infant and oxytocin release is an example.

9. Biological Control Systems • Recall, • These are comprised of a receptor/sensor which detects a stimulus and sends information to a control center that controls an effector. • The control center processes the information and compares it to a set point. • The control center sends out processed information and directs the response of the effector.

10. 3 General Hormonal Pathways • 1. A simple endocrine pathway. • 2. A simple neurohormone pathway. • 3. A simple neuroendocrine pathway.

11. 1. A Simple Endocrine Pathway • A stimulus elicits a response on an endocrine cell causing a hormone release. • The hormone diffuses into the blood where it reaches a target effector eliciting a response.

12. 1. A Simple Endocrine Pathway • For example: • A low glucose level in the blood stimulates the pancreas to release glucagon. • Glucagon acts on liver cells to release glycogen. • Glycogen breaks down into glucose and gets into the blood.

13. 2. Simple Neurohormone Pathway • In the simple neurohormone pathway, a stimulus travels via a sensory neuron to the hypothalamus/posterior pituitary gland. • Neurosecretory cells here release hormones into the blood. • These hormones travel to the target cells and elicit a response.

14. 2. Simple Neurohormone Pathway • For example: • A suckling infant’s stimulation is sent via a sensory neuron to the hypothalamus/posterior pituitary where oxytocin is made and released into the blood. • The hormones travel to the smooth muscle in the breast which responds by contracting and releasing milk.

15. 3. A Simple Neuroendocrine Pathway • A stimulus sends the signal to the hypothalamus via a sensory neuron. • The neurosecretory cells of the hypothalamus release hormones into the blood. • These act on endocrine cells to release hormones into the blood. • These hormones have an effect on target cells and elicit a response.

16. 3. A Simple Neuroendocrine Pathway • For example: • Neural and hormonal signals tell the hypothalamus to secrete prolactin releasing hormone. • The hormone travels through the blood to the anterior pituitary which releases prolactin. • Prolactin travels through the blood to the mammary glands stimulating milk production.

17. Positive and Negative Feedback • Recall, • Positive feedback acts to reinforce the stimulus. It leads to a greater response. • Negative feedback acts to reduce the response of the stimulus.

18. Molecules Functioning as Hormones • There are 3 major classes of molecules that function as hormones: • 1. Proteins/peptides-water soluble. • 2. Amines-water soluble. • 3. Steroids-not water soluble.

19. Key Events • There are 3 key events involved in signaling: • 1. Reception-is when the signal binds to the receptor protein in or on the target cell. • Receptors can be inside or outside the cell. • 2. Signal transduction-signal binds and triggers events within the cell (cascade events). • 3. Response-changes a cell’s behavior.

20. Signal Transduction • Receptors for most water soluble proteins are embedded in the plasma membrane. • Binding of a hormone initiates a signal transduction pathway.

21. Signal Transduction • The pathway is a series of changes where cellular proteins convert an extracellular chemical signal into an intracellular response. • Examples: • Activation of an enzyme • Uptake or secretion of a specific molecule • Rearrangement of a cytoskeleton

22. Signal Transduction • The signals can activate proteins that can act to directly or indirectly regulate transcription of certain genes. • Hormones can cause a variety of responses in target cells with different receptors. • These responses are types of signal transductions.

24. Water Soluble Hormones • Most water soluble hormones have receptors embedded in the membrane. • Surface receptor proteins activate proteins in the cytoplasm which then move into the nucleus and regulate transcription.

25. Epinephrine Example-Water Soluble Hormone • Liver cells and smooth muscle of blood vessels supplying skeletal muscle contain b-type epinephrine receptors.

26. Epinephrine Example-Water Soluble Hormone • Smooth muscle of intestinal blood vessels contain a-type receptors. • The tissues respond differently to epinephrine. • Increased blood flow and glucose to the skeletal muscles. • Decreased blood flow to the digestive tract.

27. Lipid Soluble Hormone • Lipid soluble hormones have their receptors located inside of the cell. Either in the cytoplasm or the nucleus. • Entrance of the signal and binding of the signal to the receptor initiates the signal transduction pathway. • Binding to DNA stimulates transcription of genes. • mRNA produced is translated into protein within the cytoplasm.

28. Estrogen Example-Lipid Soluble Hormone • Estrogen induces cells within the female bird’s reproductive system to make large amounts of ovalbumin.

29. Paracrine Signaling • Neighboring cells signal local regulators that convey signals between these neighboring cells. • Neurotransmitters, cytokines, and growth factors are all examples of local regulators.

30. Paracrine Signaling-Example • Nitric oxide (NO). • When blood O2 levels fall, endothelial cells in the blood vessel walls synthesize and release NO. • NO activates an enzyme that relaxes neighboring smooth muscle. • This results in the dilation of blood vessels and improves blood flow.

31. Endocrine Control • The hypothalamus integrates the vertebrates’ nervous and endocrine systems. • It is found on the underside of the brain. • It receives information from nerves throughout the body and brain. • It initiates the appropriate endocrine signals for varying conditions.

32. The Hypothalamus • Contains 2 sets of neurosecretory cells. • The secretions from these cells are stored in or regulate the activity of the pituitary gland.

33. The Pituitary • The pituitary gland has 2 parts. • The anterior and the posterior.

34. The Anterior Pituitary Gland • It is regulated by hormones produced by neurosecretory cells in the hypothalamus. • Some inhibit hormone release, others stimulate it. • The adenohypophysis consists of endocrince cells that make and secrete at least 6 different hormones. • Many of them target and stimulate endocrine glands.

35. The Anterior Pituitary Gland • FSH-stimulates production of ova and sperm. • LH-stimulates ovaries and testes. • TSH-stimulates the thyroid gland. • ACTH-stimulates production and secretion of the hormones of the adrenal cortex. • MSH-stimulates concentration of melanin in skin. • Prolactin-stimulates mammary gland growth and milk synthesis.

36. The Posterior Pituitary Gland • The neurohypophysis is an extension of the hypothalamus. • It stores and secretes 2 hormones: ADH and oxytocin. • ADH acts on the kidneys increasing H2O retention. • Oxytocin signals uterine muscle contraction and mammary gland excretion of milk.

37. The Thyroid Gland • The thyroid produces 2 hormones. • Triiodothyroxine (T3) • Thyroxin (T4) • In mammals, T4 is converted to T3 by target cells. • T3 is mostly responsible for the cellular response.

38. The Thyroid Gland • The thyroid is crucial to development. • It controls metamorphosis in frogs. • It is required for normal functioning of bone-forming cells. • It promotes branching of nerves in utero. • It helps skeletal growth and mental development. • It helps maintain muscle tone, digestion, reproductive functions, b.p., h.r.

39. The Thyroid Gland • The thyroid creates calcitonin. • It works in conjunction with the parathyroid to maintain calcium homeostasis.

40. Parathyroid Hormone • Released by the parathyroid gland in response to low blood calcium levels. • PTH induces the breakdown of osteoclasts. • Ca2+ is then released into the blood. • PTH stimulates Ca2+ uptake by the renal tubules.

41. Parathyroid Hormone • PTH also promotes the conversion of vitamin D into its active form. • The active form of vitamin D acts on the intestines stimulating the uptake of Ca2+ from food. • When Ca2+ gets above a certain setpoint, it promotes the release of calcitonin which opposes the effects of PTH lowering blood Ca2+ levels.

42. Pancreas • The pancreas is both an endocrine and a exocrine gland. • Exocrine-releases secretions into ducts. • Endocrine-secretions diffuse into bloodstream. • Islets of Langerhans are scattered throughout the exocrine portion of the pancreas.

43. Pancreas • Each islet contains a-cells and b-cells. • a-cells produce glucagon. • b-cells produce insulin. • Insulin and glucagon oppose each other and regulate the concentration of glucose in the blood.

44. Blood Glucose • Glucagon gets released when blood glucose falls below a setpoint. • Insulin gets released when blood glucose is elevated. • Insulin stimulates most cells to take up glucose from the blood. • It also acts to slow glycogen breakdown in the liver.

45. Diabetes Mellitus • Diabetes is an endocrine disorder caused by a deficiency in insulin or decreased response to insulin. • There are 2 types: • Type I-insulin dependent. • Type II-non-insulin dependent.

46. Type I Diabetes • Insulin dependent. It’s an autoimmune disease resulting in the destruction of the body’s b-cells. • The pancreas can’t produce insulin and the person requires insulin injections.

47. Type II Diabetes • Non-insulin dependent. • It is caused either by a deficiency in insulin, or usually by a reduced responsiveness by the cells to insulin.

48. Adrenal Glands • They are adjacent to the kidneys. • They are made up of 2 different cell types. • Adrenal cortex-the outer portion. • Adrenal medulla-the inner portion.

49. Adrenal Cortex • Responds to endocrine signals. • ACTH released from the anterior pituitary stimulates the release of corticosteriods. • Glucocorticoids-cortisol involved in bioenergetics. • Mineralcorticoids-aldosterone acts on salt balance. • The cortex also releases sex hormones.

50. Adrenal Medulla • The medulla responds to endocrine signals. • Produces the catecholamines epinephrine and norepinephrine. • These are involved in the ‘fight-or-flight’ response.