Muscle Physiology
CHAPTER 6
Muscle Physiology Questions to ponder…..
Can I turn fat into muscle if I start
working out?
Body fat and muscle are two completely different tissues.
They have different structures and functions, they react to
training in different ways and, simply put, one does not
have the capability to turn into the other
Can I increase the number of muscle
cells I have by working out?
While the number of muscles cells/fibers can not increase,
each individual muscle fiber has the potential to increase in
size, density and efficiency. These changes may occur
together but not necessarily to the same degree, however,
all will translate to an increase in strength.
I keep doing abdominal crunches,
why can I not get those 6-pack abs?
I
Comparison of different types of muscle tissue
A. Skeletal Muscle
1. Cells/fibers are long (some up to 1 ft), cylindrical and striated in
appearance. They have multiple nuclei in each cell.
2. The tissue is under mostly voluntary control.
3. The tissue is located in the body attached to bones and/or skin (ex.
facial muscles)
4. Fiber contractions vary from slow to fast depending on type and
location in the body.
a. Slow oxidative fibers: posture muscles in the back, typically very
red; fatigue slowly.
b. Fast oxidative fibers: sprinting and walking muscles; fatigue is
moderate.
c. Fast glycolytic fibers: short-term intense muscles, ex. hitting a
baseball; fatigue quickly.
d. Some aptitude in sports is due to the ratio of these fibers in an
individual. Ex. Marathon runners have more slow oxidative
fibers for less fatigue and more endurance.
5. Fibers have very specific arrangement to enhance
the strength and durability of the muscle tissue.
a: Each fiber is surrounded by connective
tissue – Endomysium
b. Bundle of muscle fibers are surrounded by
connective tissue – Perimysium
(called a fascicle, can be large or
small)
c. Many fascicles are bound together by
connective tissue – Epimysium (these layers
blend together to form tendon). Extremely
durable tissue for crossing joints and bony
projections so that muscles are not torn.
d. Orientation of the bundles in the muscle are
different for different muscles. Ex. fat belly
of the biceps brachii vs. the flat parallel
bundles of the latissimus dorsi.
B. Smooth Muscle
1. Cells/fibers are long spindle shaped,
not striated and have one nucleus.
2. The tissue is under involuntary
nervous control.
3. The tissue is located surrounding
hollow organs (blood vessels,urinary
bladder, esophagus, stomach,
intestine, and respiratory
passages)
4. Composed of two layers with the
fibers running perpendicular to each
other. Helps muscle prevent
muscle fatigue and change
the size and shape of the organ.
(ex. urinary bladder empty
vs. full)
5. Fatigue very slowly, contraction is
slow and steady.
c. Cardiac Muscle
1. Cells/fibers are long, cylindrical fibers
that are striated and branched.
2. Tissue is under involuntary nervous
control.
3. Fibers are connected by intercalated
discs to facilitate the passing of the
contraction impulse through the
tissue of the organ. Fibers are
arranged in a spiral arrangement to
help with coordinated heart
contractions.
4. Fibers are located only in the heart.
II
The functions of muscles
1. Producing movement – facial expressions, movement of
food stuffs through digestions and blood through the heart.
2. Maintaining posture (skeletal only), very fatigue resistant
muscles, ex.erector spinae; and stabilizing joints (holding the
skeleton together)
3. Generating heat to maintain body temperature. Muscles
make up 40% of body weight and ¾ of energy given off by
ATP is heat.
III How the muscle fiber contracts – The Sliding Filament
Theory
a. Basic structure of sarcomere.
1. Sarcomere is the functional contracting unit of the
muscle fiber.
Muscles are made of muscle fibers (cells)
 Muscle fibers (cells) are made of myofibrils
which contain the functioning units of
contraction called sarcomeres

b. Functioning of the sarcomere. What you can do in 1000th of a
second!
http://media.pearsoncmg.com/bc/bc_campbell_biology_6/cipl/ins/49/HTML/source/71.html
1. Nerve impulse reaches the synaptic cleft between the nerve
and the muscle fiber (each fiber must be individually
stimulated). This is called the neuromuscular junction.
2. Signal causes Calcium (Ca) to be released from the
sarcoplasmic
reticulum. (as soon as nerve impulse is over Ca
is reabsorbed)
3. Calcium causes the protein complex on actin to move away
from the binding site.
4. ATP activates the myosin head to form the cross bridge
(myosin head
attached to actin and pivots toward the Hzone. Causing a shortening
of the sarcomere.
5. This action occurs simultaneously through out the entire
muscle fiber
and through out the entire muscle causing the
muscle itself to
contract/shorten. In order to maintain a
contraction (ex. hold your arm out in front of you for several
seconds) the nerve must constantly restimulate the muscle
fiber.
c. Graded Responses and Various Muscle Contractions
1. Strength of contraction: (Picking up a feather vs.
picking up a large book): Dependent on the number of
motor units stimulated within the muscle. Only a few
motor units would be stimulated for the feather vs. many
motor units for the book. Have you ever gone to pick
something up that you thought would be heavy, when you
lifted the object it was with great force and speed?
2. Length of contraction: Dependent on the frequency
of stimulation, nerves stimulate the muscle cell so quickly it
does not have a chance to completely relax and can sustain
a contraction, this is called tetanus.
3. Muscle Twitch: Single, brief, jerky muscle
contraction. Not normally how the nervous system
stimulates a muscle.
4. Muscle paralysis: Can be due to numerous factors
such as: 1) inadequate nervous stimulation (severed
nerve); 2) no ATP in the cell; 3) blocked receptor site on
muscle cell or the actin filament (ex. poison dart frog
poison); or 4) cross bridges are not broken
5. Muscle Atrophy: Loss of muscle mass due to lack
of activity either from lack of nervous stimulation (ex.
spinal cord injuries) or extreme inactivity (ex. leg in
a cast)
6. Muscle Fatigue: Caused by oxygen debt, unable
to contract even with stimulation. Lactic acid build up
also alters the pH of the cell which makes the muscle
contraction less efficient. Recovery occurs when you
continue to breath deeply after exercise has stopped.
(new explanation deals with athletes that experience fatigue after long intense
training)
7. Muscle Tone: Even when muscles are relaxed
voluntarily, some of the motor units have been
stimulated to contract in a systematic way though out
the muscle. This keeps the muscle firm and ready for
action.
8. Isotonic Contractions: Myofilaments contract
when actin fibers slide towards each other shortening
the sarcomere and therefore the muscle. The muscle
itself shortens and movement occurs. Ex. bending
your knee.
9. Isometric Contractions: Contractions where the
muscle does not shorten. Fibers do not slide over
each other because there is an immovable object (ex.
a wall) that prevents the muscle from contracting.
Tension builds in the muscle but no movement
occurs. Ex. pushing against a wall with your foot.
10. Muscle Cramp: Involuntarily and forcibly contracted muscle that
will not relax. Cramps have multiple causes: 1)hyperexciteability of
nerves caused by vigorous activity, injury, periods of inactivity,
dehydration, body fluid changes, low blood calcium, potassium, or
magnesium; 2) depletion of ATP in the cell and cross bridges can’t break
(contractures); 3)Unintentional stimulation of muscles not necessary for
the movement, ex. writers cramp.
d. Energy for muscle contractions
1. Muscle fibers typically keep 4 to 6 seconds worth of ATP stored.
Therefore the cell must very quickly begin generating more ATP.
2. First, use of creatine phosphate to make more ATP. (about 20
seconds) adding the phosphate to ADP to create ATP
3. Aerobic respiration. Used during light exercise and rest. 95% of
all muscle energy is generated this way. Requires oxygen.
4. Anaerobic respiration. During more intense exercise (30 to 60
seconds) but results in the build up of lactic acid (byproduct) in
the muscles which promotes muscle fatigue and soreness.