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Chapter 9 Notes
Muscles
I. Muscle Twitch
A. Forms of Muscle Twitch
(1). As stimulus is applied to a muscle, contractile activity can be recorded during this
contractive activity. The readout of this activity is called a myogram.
(2). There are three distinct phases that occur that can be read from the graph. The
Latent period occurs during the first milliseconds and no evidence of contraction can
be seen. Why? Excitation and Contraction coupling is occurring at this point.
During the period of contraction you begin to see evidence of a muscular
contraction. Cross-bridges are set and active. This period lasts for 10-100 ms.
During relaxation calcium ions are being reabsorbed back into the SR and the crossbridges are broken. See figure 9.14.
 Make sure you relate to what is occurring during excitation and contraction
events to the lines found on the graph.
B. Graded Responses
(1). Our muscles do not work in the method shown on the twitch graphs (myograms).
Contractions are smooth and may vary in intensity and strength. Graded muscle
responses are variations in muscular contractions. This is referred to as graded or
application of changing the frequency of stimulation and by varying the strength of
the stimulus applied.
(2). Changing Stimulation Frequencies. If you apply two identical stimuli to a muscle
the second contraction on the myogram will be greater than the first. Why? As the
second stimuli is applied the first contraction has not been completed and has not
fully relaxed. What is the physiological reasoning behind this? Because the first
muscle contraction is still in progress, more calcium ions are released causing an
even stronger second contraction. This results in further shortening of the muscle.
This is called wave summation. See figure 9.15.
(3). What happens if the same stimulation is applied more frequently (assuming the
stimulus strength is held constant). The faster the stimulation is applied, the less
relaxation time between twitches. Calcium ion concentration increases which
increases the degree of summation! So each twitch becomes stronger than the
previous twitch. See figure 9.15 number 3. You would see quivering of the muscle
or incomplete (unfused) tetanus. If the stimulations are applied faster and faster all
muscle relaxation finally disappears and we see a smooth continuous contraction.
This is called complete or fused tetanus.
(4). What eventually happens? Muscle fatigue occurs and the muscle no longer
contracts. See number 4.
(5). Increasing stimuli strength.
C. Treppe
(1). As a muscle contracts after a long rest and continues to contract, the initial
contractions are not as powerful as the latter contractions. This is due to an
increased availability of Ca ions to expose the maximum amount of active binding
sites. The muscle continues to warm and produces heat and the enzymes become
more efficient and make stronger contractions. The muscle becomes more pliable
or flexible.
(2). Tone. Muscle tone refers to the slight contraction of muscles at all times. This is
under spinal reflex control and is normal. This keeps muscles healthy, strong, and on
the ready to respond to stimuli. Also stabilizes joints and maintains posture.
(3). Isotonic contractions refers to muscle length changes and moves the load. One
type is called concentric contraction in which the muscle shortens and does the
work. (lifting a stone, kicking a ball) The other type is called eccentric contractions
where the muscle generates force when it lengthens (coordination and controlled
movements such as your calf muscle when walking up a hill).
(4). Isometric contractions refers to when a muscle builds tension but neither shortens
nor lengthens. You experience an isometric contraction when you try to move a
load much heavier than you are able to.
II. Metabolism (Plus your board notes)
(1). ATP drives the work of a cell. Muscles need ATP for contraction purposes as we
have talked about in class.
(2). Where does this ATP come from? Actually three sources:
(a). Creatine Phosphate. This compound is stored in muscles. During times of extensive
muscular work, the phosphate group is removed and attached to ADP to form ATP.
Any time a phosphate group is attached to another substance this is called
phosphorylation. Muscles store 5x more CP than ATP, so there is always a quick
source of ATP synthesis available to the muscles. An enzyme called creatine kinase
mediates this reaction.
(b). Aerobic Respiration. When cells are in aerobic situations or have plenty of oxygen
they can metabolize glucose (oxidize and break down) and produce a tremendous
amount of ATP. This occurs in the mitochondria of the cell. First glucose is oxidized
and converted to a 3-C compound called pyruvate. If oxygen is present in the cell,
then pyruvate enters the Kreb’s cycle and aerobic respiration pathways in the cells.
One glucose molecule can produce up to 38 ATP.
(c). Anaerobic Respiration. What happens if the muscle cells are depleted of oxygen?
Cells have a method to continue to produce ATP under these conditions. This is
called anaerobic respiration or lactic acid fermentation. During this process glucose
is still broken down in glycolysis but when oxygen is not present in the cells,
pyruvate is converted to lactic acid and only 2 ATP molecules are produced per one
glucose molecule. Lactic acid is picked up by the liver and can reconvert it back to
pyruvic acid or glucose.
III. Muscle Fatigue
(1). As muscle cells oxygen levels drop and the muscles themselves contract less fatigue
sets in. The reason for muscles inability to contract may be caused by a problem in
excitation-contraction coupling, lack of ATP, etc. Intense exercise usually leads to
muscle failure.
(2). Oxygen debt refers to the amount of extra oxygen the body cells must take in to
fully recover. Short very intense exercise usually has a shorter recovery time, thus
lower oxygen debt. In comparison, prolonged low-intensity exercise requires several
hours of recovery.
IV. Force and Contractions
(1). Force of a muscle contraction depends on several factors:
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Number of fibers stimulated (more units fired up stronger the contraction).
Size of muscle fibers stimulated (bigger the muscle more tension developed,
greater the strength)
Frequency of stimulation
Degree of stretch (muscles to contract with an appropriate force the lengthtension relationship must be correct. Optimal operation length is from about 80120% of normal resting length. Overstretching or under-stretching of the muscle
fibers does not allow for correct overlap of the actin and myosin filaments.
Muscles maintain this correct overlapping by the way they are attached to bones
and the joints do not allow for muscles to overstretch.
V. Velocity and Duration of Contraction
(1). Why do some muscles contract faster than others and longer than others? This
depends on what type of muscle fiber it contains.
(a). Speed of contraction
 Slow oxidative fibers contacts slow because of slow ATPases enzymes are slow;
depends on oxygen delivery and aerobic mechanisms; glucose is delivered via
blood; they are fatigue resistance and have high endurance capabilities; little
power; rich blood supply and they are red due to abundance of myoglobin.
Endurance athletes contain a high level of these type fibers.
 Fast glycolytic fibers: contract fast; gets glucose from stored glucose in glycogen;
fewer mitochondria and myoglobin; capillaries are few; Larger cells than slow
fibers; When glycogen stores of glucose runs out, the ability of the fiber to
contract will be diminished. Sprinters, weight lifters in completion, etc.
 Important to note that muscles may have more of one type of these fibers, but
muscles have a mixture of both types. Differences can be chalked up to genetics.
Marathon runners have about 80% slow oxidative fibers whereas sprinters have
60% of fast oxidative fibers
VI. Smooth Muscle – Comparisons to skeletal muscle
Skeletal Muscle
1. Voluntary
2. Attached to bones
3. Contain striations
4. Multinucleated
5. Contains all three connective
tissues (epi, endo and perimysium)
6. Presence of myofibrils
7. T-Tubules present
8. No gap junctions
9. Calcium regulation controlled by
troponin on actin
10. Speed of contractions are slow to
fast
11. No rhythmic contractions
Smooth Muscle
1. Involuntary
2. Found in visceral organs such as GI
tract.
3. No striations
4. Uninucleated
5. Contains only endomysium
6. No myofibrils, but still contains
actin and myosin which are
anchored by dense bodies.
7. No T-tubules but contain caveolae
8. Have gap junctions
9. Does not have have troponin but
instead calmodulin
10. Very slow
11. Yes, peristaltic movements in the
12. Aerobic or anaerobic respiration
13. Highly structured Neuromuscular
Junctions
14. Wide synaptic cleft in NM junction
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GI tract
12. Mostly aerobic
13. Not highly structured; contain
varicosities and diffuse junctions
14. Narrow synaptic clefts
Role of Varicosities: Smooth muscle cells are innervated by the autonomic
nervous system fibers which release neurotransmitter from varicosities into a
very wide synaptic cleft called diffuse junctions. (figure 9.25) Varicosities are
found at the ends of the nerve fibers (similar to axonal endings).
Role of Caveolae: Vesicles (v) called caveolae are numerous at the periphery of
smooth muscle cells. These work like the T tubules, increasing the surface area for
transfer of calcium into the cytoplasm
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Role of Calmodulin:
Ca2+/calmodulin (CaCM) binding
protein known as caldesmon was
involved in regulating the movement
of smooth-muscle tropomyosin on
and off the myosin binding sites of
thin filaments. This allows for
myosin to bind to the tropomyosin