1. Feedback mechanisms, hormones and the endocrine system
Biology 12
Mr. C

2. What is a feedback mechanism
Feedback is (generally) information about actions.

3. In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. Often this is done intentionally, in order to control the dynamic behaviour of the system. Feedback is observed or used in various areas dealing with complex systems, such as engineering, architecture, economics, and biology.

4. Drawing a feedback loop
Lines are usually drawn, directed from input through the system and to output. The feedback is shown by another arrowed line, directed from output outside the system to an input, resulting in a loop on the diagram, called feedback loop. This notion is important; for example, the feedback loop is a convenient place for a control device.

5. In nature
In biological systems such as organisms, ecosystems, or the biosphere, most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of the parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel.

6. Positive and negative feedback
Biological systems contain many types of regulatory circuits, among which positive and negative feedbacks. Positive and negative don't imply consequences of the feedback have positive or negative final effect. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to accelerate it.

7. Useful vocab
negative feedback The stopping of the synthesis of an enzyme by the accumulation of the products of the enzyme-mediated reaction.
negative feedback control  Occurs when information produced by the feedback reverses the direction of the response; regulates the secretion of most hormones.
negative feedback loop A biochemical pathway where the products of the reaction inhibit production of the enzyme that controlled their formation.

8. Negative feedback
Feedback and regulation are self related. The negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium

9. An example of a simple negative feedback loop

10. What if you get cold?

11. What if you blood sugar changes?

12. Integrating organs with feedback

13. For example, in an organism, most positive feedbacks provide for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and play a key role in regulation of morphogenesis, growth, and development of organs, all processes which are in essence a rapid escape from the initial state.
Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level.

14. Endocrine system
The endocrine system is a control system of ductless glands that secrete chemical messengers called hormones that circulate within the body via the bloodstream to affect distant organs. Hormones act as "messengers", and are carried by the bloodstream to different cells in the body, which interpret these messages and act on them. The endocrine system does not include exocrine glands such as salivary glands, sweat glands and glands within the gastrointestinal tract.

15. What is a hormone?
hor·mone (hôr'mōn') n. A substance, usually a peptide or steroid, produced by one tissue and conveyed by the bloodstream to another to effect physiological activity, such as growth or metabolism.

16. How are hormones classified
Hormones are grouped into three classes based on their structure:
steroids
peptides
amines

17. Steriods
Steroids are lipids derived from cholesterol. Testosterone is the male sex hormone. Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.

18. Steroid structure

19. Peptides and Amines
Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys. Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla. Solubility of the various hormone classes varies.

20. The integration of body functions in humans and other higher organisms is carried out by the nervous system, the immune system, and the endocrine system.
The endocrine system is composed of a number of tissues that secrete their products, called endocrine hormones, into the circulatory system; from there they are disseminated throughout the body, regulating the function of distant tissues and maintaining homeostasis.
In a separate but related system, exocrine tissues secrete their products into ducts and then to the outside of the body or to the intestinal tract.

21. Endocrine Hormones
Classically, endocrine hormones are considered to be derived from amino acids, peptides, or sterols and to act at sites distant from their tissue of origin.
However, the latter definition has begun to blur as it is found that some secreted substances act at a distance (classical endocrines), close to the cells that secrete them (paracrines), or directly on the cell that secreted them (autocrines). Insulin-like growth factor-I (IGF-I), which behaves as an endocrine, paracrine, and autocrine, provides a prime example of this difficulty.

22. What is the amount of hormones in the blood?
Hormones are normally present in the plasma and interstitial tissue at concentrations in the range of 10-7M to 10-10M.
Because of these very low physiological concentrations, sensitive protein receptors have evolved in target tissues to sense the presence of very weak signals.
In addition, systemic feedback mechanisms have evolved to regulate the production of endocrine hormones.

23. How do hormones travel in the blood!
Once a hormone is secreted by an endocrine tissue, it generally binds to a specific plasma protein carrier, with the complex being disseminated to distant tissues.
Plasma carrier proteins exist for all classes of endocrine hormones. Carrier proteins for peptide hormones prevent hormone destruction by plasma proteases.
Carriers for steroid and thyroid hormones allow these very hydrophobic substances to be present in the plasma at concentrations several hundred-fold greater than their solubility in water would permit.
Carriers for small, hydrophilic amino acid--derived hormones prevent their filtration through the renal glomerulus, greatly prolonging their circulating half-life.

24. Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response.

25. Action of nonsteroid hormones

26. Step two

27. Step Three

28. Action of steroid hormone
The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process.

29. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex.

30. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins.

31. How do tissue react to hormones?
Tissues capable of responding to endocrines have 2 properties in common: they posses a receptor having very high affinity for hormone, and the receptor is coupled to a process that regulates metabolism of the target cells.
Receptors for most amino acid--derived hormones and all peptide hormones are located on the plasma membrane. Activation of these receptors by hormones (the first messenger) leads to the intracellular production of a second messenger, such as cAMP, which is responsible for initiating the intracellular biological response.
Steroid and thyroid hormones are hydrophobic and diffuse from their binding proteins in the plasma, across the plasma membrane to intracellularly localized receptors. The resultant complex of steroid and receptor bind to response elements of nuclear DNA, regulating the production of mRNA for specific proteins.

32. What we have seen so far.. Stomach and intestines Gastrin Secretin
Cholecystokinin (CCK)
Somatostatin
Neuropeptide Y

33. Getting a head start on hormones

34. List of hormones and organs related to hormones
Hypothalamus
Thyrotropin-releasing hormone (TRH)
Gonadotropin-releasing hormone (GnRH)
Growth hormone-releasing hormone (GHRH)
Corticotropin-releasing hormone (CRH)
Somatostatin
Dopamine

35. Hypothalamus to Pituitary

36. Pituitary gland Anterior lobe (adenohypophysis)
GH (human growth hormone)
PRL (prolactin)
ACTH (adrenocorticotropic hormone)
TSH (thyroid-stimulating hormone)
FSH (follicle-stimulating hormone)
LH (luteinizing hormone)
Posterior lobe (neurohypophysis)
Oxytocin
ADH (antidiuretic hormone)

37. Integration of blood and hormones

38. A division of labour

39. Pineal gland Thyroid gland Parathyroid gland Heart Melatonin
Thyroxine (T4), a form of thyroid hormone
Triiodothyronine (T3), a form of thyroid hormone
Calcitonin
Parathyroid gland
Parathyroid hormone (PTH)
Heart
Atrial-natriuretic peptide (ANP)

40. Adrenal glands
Adrenal cortex
Glucocorticoids - cortisol
Mineralocorticoids - aldosterone
Androgens (including testosterone)
Adrenal medulla
Adrenaline (epinephrine)
Noradrenaline (norepinephrine)

41. Adrenal gland and kidney
Renin
Erythropoietin (EPO)
Calcitriol

42. Islets of Langerhans in the pancreas
Liver
Insulin-like growth factor
Angiotensinogen
Thrombopoietin
Islets of Langerhans in the pancreas
Insulin
Glucagon
Somatostatin

43. Skin
Calciferol (vitamin D3)
Adipose tissue
Leptin

44. In males only
Testes
Androgens (testosterone)

45. Placenta (when pregnant)
In females only
Ovarian follicle
Oestrogens
Testosterone
Corpus luteum
Progesterone
Placenta (when pregnant)
Human chorionic gonadotrophin (HCG)
Human placental lactogen (HPL)

46. Your challenge…. Most hormones turn on and off a response
You should be able to make feed back loops for regulating levels of major chemical groups in the body.
What are feedback loops for regulating:
Oxygen and Carbon Dioxide
Blood sugars ( you have seen it!)
Blood salts
Sexual and growth development

47. More challenges… Can you link feedback mechanisms to specific systems?
Could you make a comparative table pairing hormones with organ systems?
What disorders are associated with your adrenal gland, thyroid gland, pancreas, bones and blood sugar imbalance. (see on-line references)

48. A possible end..or beginning

49. List of on-line references