Endocrine System and Exercise

1. Endocrine System and Exercise EXS 558 Lecture #3 September 14, 2005

2. Review Question #1 • By what two methods does the central nervous system regulate muscular force production? • RATE CODING and RECRUITMENT

3. Review Question #2 • TRUE/FALSE: With small homogenous muscle, first see ↑ rate coding of low threshold MU’s, then see ↑ recruitment until reach 90% MVC, then ↑ rate coding to reach 100% MVC.

4. Review Question #3 • According to the size principle (Henneman et al., 1965) to following muscle fibers are recruited first during prolonged activity? a.) Type I b.) Type IIa c.) Type IIb d.) Type III

5. Review Question #4 • Why are slow-twitch muscle fibers selected first according to the size principle in exercises of long duration? • smaller motor neurons • Easier to stimulate an AP (action potential) • lower threshold

6. Review Question #5 • Does the size principle apply to activities of shorter, higher intensity activities? • According to Yamano et al. (2005) the size principle does not apply in thoroughbred horses

7. Review Question #6 • Which of the following is NOT TRUE regarding fast twitch (type II) motor neurons as compared to slow twitch (type I) motor neurons? a.) AP propagation is quicker b.) Myofiber CSA is larger c.) Increased calcium concentration d.) Have greater # of myofibers associated with each motor neuron

8. Review Question #7 • What three neural adaptations occur as a result of resistance training? 1.) Synchronization and recruitment of additional motor units 2.) Coactivation of agonist and antagonist muscles 3.) Rate coding—the firing frequency of motor units

9. Review Question #8 • EMG activity increases OR decreases when activating both limbs as opposed to one limb singly. • This is known as ________________ BILATERAL DEFICIT

10. Review Question #9 • TRUE/FALSE: Early strength gains are a result of muscle hypertrophy.

11. Review Question #10 • What three neural adaptations occur as a result of endurance training? 1.) Increase in MU activation 2.) Rotation of activity among synergists and among MU of prime mover 3.) Training ↑ consistency of firing rates of motor neurons

12. Endocrine and Exocrine Functions • Endocrine system composed of endocrine glands—ductless glands that secrete hormones directly into the blood • Exocrine glands secrete their products into ducts (e.g. sweat glands) • Pancreas has both functions: exocrine—digestive enzymes; endocrine—insulin and glucagon

13. Basic Hormone Introduction • Hormones: chemical messengers that circulate in blood and interact with organs to help combat various stresses • Primary role is maintain homeostasis • Most hormones are synthesized in the endocrine glands • Receptors are specific to hormones such that only the correct hormone will “fit” the correct receptor—each cell has 2,000 to 10,000 specific receptors

14. Hormone Regulation • Negative Feedback: secretion acts to inhibit further secretion (either direct or indirect) • Self-limiting • Positive Feedback: secretion acts to stimulate further secretion • Rare • Ex: oxytocin released by pituitary gland stimulats cervix dilation

15. Changes in Hormone Concentrations • Increases [ ] affected by physiological mechanisms 1.) Exercise 2.) Physical Stress 3.) Psychological Stress 4.) Fluid volume shifts 5.) Venous pooling of blood • ↑ potential for receptor interaction • Receptors are specific to hormones such that only the correct hormone will “fit” the correct receptor—each cell has 2,000 to 10,000 specific receptors

16. Lock-and-Key Theory • Hormone-receptor interaction • Hormone = key • Receptor = lock • Cross-reactivity: more than one hormone can bind with a receptor • This changes subsequent biological rxns • Signal sent to nucleus for inhibition or facilitation of protein synthesis

17. Alteration of Receptors Down-regulation—Decrease in number of cell receptors; less hormone can bind to the cell and higher concentrations of the hormone remain in the blood plasma Up-regulation—Increase in number of cell receptors; more hormone can bind to the cell and lower concentrations of the hormone remain in the blood plasma

18. Types of Hormones 1.) Steroids • Ex: testosterone, cortisol 2.) Peptides • Ex: HGH (human growth hormone), insulin

19. Steroid Hormones w Lipid soluble w Diffuse easily through cell membranes; receptors located within cell w Chemical structure is derived from or is similar to cholesterol w Secreted by adrenal cortex (e.g., cortisol), ovaries (e.g., estrogen), testes (e.g., testosterone), placenta (e.g., estrogen)

20. Steroid Hormone (con’t)

21. Peptide Hormones w Lipid soluble w Diffuse easily through cell membranes; receptors located within cell w Chemical structure is derived from or is similar to cholesterol w Secreted by adrenal cortex (e.g., cortisol), ovaries (e.g., estrogen), testes (e.g., testosterone), placenta (e.g., estrogen)

22. Peptide Hormone (con’t)

23. w Steroid hormones pass through cell membranes and bind to receptors within the cell. They synthesize protein via a process called direct gene activation. (continued) Nature of Hormones w Hormones are classified into steroidal types (lipid soluble and formed from cholesterol) or nonsteroidal types (nonlipid soluble and formed from amino acids, peptides, or proteins). w Hormones are secreted in the blood and travel to sites where they exert an effect on only those target cells that have receptors specific to that hormone.

24. w Nonsteroid hormones bind to receptors on the cell membrane, which triggers a second messenger within the cell, which in turn triggers numerous cellular processes. w A negative feedback system regulates the release of most hormones. w The number of receptors on a cell can change the cell's sensitivity to hormones. Up-regulation is the increase of receptors and down-regulation is the decrease in receptors.

25. Testosterone (steroid hormone) • Androgen  masculinizing effects • Anabolic  maintains and aids in growth of muscle and bone tissue • Produced in the testes under stimulus from luteinizing hormone (LH)

26. Acute Exercise Response

27. Fluid Regulatory Hormones • Job: maintain electrolyte balance • Important in prolonged exercise • Hormonal action effects both renal and circulatory system to prevent dehydration

28. Fluid Regulatory Hormones (con’t) Aldosterone—Released by the adrenal cortex in response to decreased blood pressure; promotes sodium reabsorption in kidneys and increases plasma volume. Anitdiuretic hormone (ADH)—Released by the posterior pituitary in response to increased blood osmolarity; promotes water conservation by increasing plasma volume.

29. How ADH conserves body water ADH = antidiuretic hormone (arginine vasopressin)

30. RENIN-ANGIOTENSIN MECHANISM

32. Fluid Regulation Following the initial drop, plasma volume remains relatively constant throughout exercise due to 1. The actions of aldosterone and ADH, 2. Water returning from the exercising muscles to the blood, and 3. The increase in amount of water produced by metabolic oxidation within muscles.

33. w Angiotensin II increases peripheral arterial resistance, which increases blood pressure and triggers the release of aldosterone. (continued) Hormones and Fluid Balance w Aldosterone and ADH are the two primary hormones involved in regulating fluid balance. w When plasma volume or blood pressure decrease, the kidneys produce renin that eventually converts to angiotensin II.

34. Hormones and Fluid Balance w Aldosterone promotes sodium reabsorption in the kidneys, which in turn causes water retention, thus increasing the plasma volume. w ADH is released in response to increased plasma osmolarity and acts on the kidneys to promote water conservation. w Plasma volume increases, which results in dilution of the plasma solutes and blood osmolarity decreases.