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?
– 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.
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
– Easier to stimulate an AP (action potential)
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.
BILATERAL DEFICIT
This is known as ________________
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.)
2.)
3.)
4.)
5.)
Exercise
Physical Stress
Psychological Stress
Fluid volume shifts
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
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)
Steroid Hormone (con’t)
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)
Peptide Hormone (con’t)
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.
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)
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.
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
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.
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.
w Angiotensin II increases peripheral arterial
resistance, which increases blood pressure
and triggers the release of aldosterone.
(continued)
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.