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

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

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. 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Steroid Hormone (con’t)

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

Peptide Hormone (con’t)

 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).  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. Nature of Hormones (continued)  Steroid hormones pass through cell membranes and bind to receptors within the cell. They synthesize protein via a process called direct gene activation.

 Nonsteroid hormones bind to receptors on the cell membrane, which triggers a second messenger within the cell, which in turn triggers numerous cellular processes.  A negative feedback system regulates the release of most hormones.  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.

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)

Acute Exercise Response

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

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.

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

RENIN-ANGIOTENSIN MECHANISM

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. Fluid Regulation

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

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