10 Muscular
Contraction
Taft College
Human
Physiology
Muscular Contraction
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Sliding filament theory (Hanson
and Huxley, 1954)
These 2 investigators proposed
that skeletal muscle shortens
during contraction because the
thick (myosin) and thin (actin)
filaments slide past one another.
The previous idea was that the
filaments change in length.
The myosin heads pull the thin
filaments toward the center of
the sarcomere which shortens the
sarcomere.
Remember, the I band and the H
zones disappear as the thin
filaments move to the center. The
A band stays the same length.
Only the length of the sarcomere
changes, not the length of the
filaments!
2 Sarcomeres
Sarcomere Sarcomere
Closer Look at the Events During
Contraction
• We will now take a look at contraction in a
step-by-step fashion.
• We will discuss 17 steps during a muscle
contraction event.
• Figure 10.12, explains these events in 9
steps. It is not important that you can
name a step, but that you can explain how
muscle contraction works.
Events During
Muscle Contraction
Diagram
In Text
Events During Muscle Contraction
• (Steps 1- 4 represent nerve impulse)
• 1. Nerve impulse arrives at neural muscular junction.
• 2. ACh release from axon terminals.
• 3. ACh binds to active sites (receptors) on motor end
plate.
• 4a. ACh receptor protein channel opens and increases
permeability of Na+ into sarcoplasm.
• 4b. If there is enough ACh, an action potential (AP)will
occur.
• 4c. Acetylcholinesterase degrades ACh.
Events During Muscle Contraction
• (Steps 5-7 represent depolarization.)
• 5. Na+ enters muscle fiber, rapid depolarization of
sarcolemma occurs = action potential.
• Voltage changes to a less negative charge.
• 6. The action potential spreads away from the end plate
in all directions and depolarizes the T tubules.
• 7. The action potential continues down the T tubules
into the sarcoplasm where it depolarizes the
sarcoplasmic reticulum (SR) membranes.
Events During Muscle Contraction
• 8. The SR responds to the action potential by
opening Ca++ release channels which floods the
surrounding sarcoplasm located between the
thick and thin filaments with Ca++.
• 9. Ca++ combines with regulatory protein
troponin, associated with actin filaments.
• 10. Troponin changes shape, and
exposes the myosin binding sites on actin.
• Steps 5-10 are all a part of the latent period =
lag time between stimulation and contraction.
The Role of Ca++ in Contraction
Events During Muscle Contraction
• 11. Myosin heads (cross bridges) attach to actin binding
sites on thin filament.
• 12. Myosin head flexes (tilts, shifts), drawing actin
filaments of sarcomere toward each other.
• 13. Once myosin head is flexed, ATP binding site is
exposed and ATP binds to the head.
• 14. Myosin head detaches from actin binding site under
the influence of ATP binding. Energy from ATP returns
the myosin head to the cocked forward position. Myosin
head attaches to a new binding site on actin.
• (Steps 11-14 = Contraction and are repeated over
and over during a single contraction event as long
as ATP and Ca++ are available.)
actin
Z
myosin
The Contraction Cycle
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11
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Rigor Mortis = Stiff Death
• Rigor mortis- Notice that ATP is responsible for myosin
heads detaching from actin, which leads to muscle
relaxation.
• This is illustrated by rigor mortis = stiff death. When a
person dies, no more ATP is synthesized as no more 02
and glucose are supplied to the tissues.
• The myosin heads cannot detach themselves from actin
resulting in a condition in which muscles are in a state of
rigidity called rigor mortis. The muscles contract as Ca++
diffuses out of sarcoplasmic reticulum (the Ca++ pump
energized by ATP has quit working).
• This state lasts about 24 hours and disappears as the
tissues undergo autolysis.
Events During Muscle Contraction
• Steps 15-17 = Relaxation
• 15. Ca++ is returned to the SR by Ca++ active transport
pump (requires ATP). Sarcoplasm is now Ca++ poor.
• 16. Troponin again covers actin binding sites. Therefore
no myosin actin interaction can occur.
• 17. Muscle fiber relaxes. Movement of relaxation is due
to:
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A. "Elastic effect" of coiled elastic fiber (titan)
molecules. And/or,
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B. Due to pull of C.T. within muscle.
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Nerve impulse
Ach released
Ach binds to motor end plate
Increased permeability of Na+
into sarcolemma
Depolarization of sarcolemma,
action potential
Depolarization of T-tubule
Depolization of SR membranes
SR releases Ca++ between thin
& thick filaments
Ca++ combines w/ troponin
Troponin changes shape and
exposes myosin binding site
Myosin heads attach to actin
binding site
Myosin heads tilts/shifts
drawing actins of sarcomere
toward each other
Tilting of myosin head exposes
ATP binding site- ATP Binds
Myosin head detaches, ATP
repositions myosin head,
myosin bind to new site
Ca++ is returned to S-R
Troponin again covers actinmyosin binding sites
Muscle relaxes
1
2
6-8
3-5
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16
9-10
15
11-14 = Contraction
ATP and Contraction
• We see that ATP is required for 3 major roles
in contraction:
• 1. Repositions (cocks) the myosin heads.
• 2. Detachment of myosin heads from actin
once the power stroke is complete.
• 3. Powers the Ca++ active transport pumps
that rapidly remove Ca++ from the sarcoplasm
back into the sarcoplasmic reticulum (reservoir).
• The concentration of Ca++ is 10,000 times lower
in the sarcoplasm of a relaxed muscle fiber than
inside the SR.
ATP Produced in 3 Ways
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ATP is very important but muscle only stores enough for about
4-6 seconds of activity
• ATP is produced in 3 ways
• 1. Phosphagen System = ATP Creatine Phosphate System
• Product = 1 ATP + 1 creatine phosphate + 1 creatine
• Duration of energy = 15 seconds
• Function = Quick Power , 100 meter sprint
ATP Produced in 3 Ways
With out Oxygen
2. Anaerobic System =
Glycogen Lactic Acid System
Product = 2 ATP/ Glucose
Duration = 30- 40 Seconds
Function = 300 meter sprint
•Lactic acid is produced as a
waste product that cause
burning sensation and pain.
•Together creatine phosphate (1)
and anaerobic system (2) can
provide enough ATP for a 400
meter sprint.
ATP Produced in 3 Ways
3. Aerobic System = Aerobic Respiration With Oxygen
Product = 36 ATP / Glucose
Duration = Hours
Function = Aerobic work , long distance running or
swimming
Occurs in Mitochondria
36
Oxygen Consumption after
Exercise
• oxygen debt = recovery oxygen uptake
(Older term)
(New term)
• = the amount of oxygen that must be paid back
to the body following exercise
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Oxygen does 3 things:
• 1. Converts the lactic acid back into glycogen
(fuel) stores in the liver
• 2. Allows production of creatinine phosphate
and ATP.
• 3. Replaces oxygen removed from storage in
muscle tissue (myoglobin).
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Physiological Properties of Muscles
Stimulus = an impulse.
An impulse may travel along a motor neuron that is not strong
enough to cause a contraction.
• A stimulus that does not cause a response by the muscle is called
subliminal or subthreshold stimulus. (Ex. -70 mv to -60 mv)
• By increasing the stimulus, a barely perceptible response may be
obtained = liminal or threshold stimulus. (Ex -70 to -55 mv).
• A liminal stimulus is just strong enough to cause a depolarization
and production of an action potential.
• All or none –if threshold is reached all muscle cells of a motor
unit will contract maximally, if not reached, none will contract.
“1st Domino”
When threshold is reached,
a rapid depolarization
occurs called an action
potential that leads to a
muscle contraction.
= Polarized at Rest =-70 mV
Grading of the Strength of a
Muscle Contraction
• How can we control how much strength a muscle (like
the biceps or triceps brachii) produces?
• An individual motor unit fires all muscle fibers in that unit
in an ‘all or none’ fashion. All fibers contract to their
fullest extent or not at all.
• However, the tension or force produced by an entire
muscle can be adjusted.
• There are 2 ways to control strength of a muscle
contraction:
• 1. Recruitment of motor units.
• 2. Altering the contractility of individual muscle
fibers.
Grading of the Strength of a
Muscle Contraction
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1. Recruitment of motor units.
An individual neuron branches to many different muscle fibers (cells).
The neuron and the muscle fibers it innervates are called = motor unit.
Motor units vary in size- A small motor unit may consist of as few as 10
fibers, while a large one may consist of several 100 (or even 2000).
Example: Fingers contain very small motor units so they can carry our fine
movement.
Simply speaking: If a muscle needs more force, it will recruit (activate) more
motor units. The strength of the electrical stimulus determines the number of
motor units recruited.
If less force is necessary, less motor units are recruited.
Experience is important to in knowing how many motor units to recruit.
More motor units
= greater force
Grading of the Strength of a
Muscle Contraction
• 2. Altering the contractility of individual
muscle fibers (cells).
• This means changing the properties of muscle
fibers irrespective of how many fibers are
involved.
• There are 2 ways to change the contractility
of fibers.
• a. Increase the frequency of stimulation to
individual fibers.
• b. Vary the length of the fiber (length-tension
relationships).
Grading of the Strength of a
Muscle Contraction
2. Altering the contractility of individual muscle fibers
by a. Increasing frequency of stimulation.
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If we were to stimulate a muscle with a single liminal
stimulus S1, the muscle will exhibit a single quick
contraction of minimal force called a twitch.
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If we follow 1 stimulation S1 quickly by another S2,
before the muscle has a chance to relax, we see what
is called the summation effect or wave summation =
the tension produced by the second stimulation will
be added to the first:
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The increased tension is due to increased Ca++ in
the sarcoplasm produced by additional stimuli.
It takes a little more time for the Ca++ to leave and for
the muscle to relax.
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The increased tension of summation is not an infinite
effect.
S1
S1 S2
2. Altering the contractility of individual muscle fibers by :
a. Increasing frequency of stimulation.
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If there is repeated stimulation, tension will reach a certain plateau and stay there.
The sustained contraction of a muscle is known as tetanus.
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Unfused (incomplete) tetanus is observed at 20-30 stimuli/sec, the muscle shows
some relaxation between stimuli.
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Fused (complete) tetanus is observed at 80+ stimuli per second. Note, there is no
sign of relaxation in force between stimuli.
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The tension (strength) produced in tetanus is 2-4 times the tension of a single twitch.
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Tetanus represents a normal muscle contraction!!
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If you continue to stimulate the muscle, it will run out of ATP and will fatigue.
Fatigue
Tetanus =
2-4 x force
of twitch
Stimuli
2. Altering the contractility of individual muscle fibers by :
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b. Vary the length of the fiber (length-tension relationship)
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Using the same device as above, we can vary the length of the muscle and
measure the amount of tension each length could produce we would get the
following kind of plot see below.
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Maximum tension occurs at 2.2 um. This is the ‘sweet spot’ for fiber overlap and
strength.
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What is the reason for this? Let's look at a sarcomere at the different lengths.
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We can see that, the more contracted, the greater the interaction between thick and
thin filaments (as long as the actin does not overlap and interfere with interaction).
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The more they overlap (without interference), the more cross bridges which can
connect and hence, more tension can be produced.
Length - Tension Relationship of
Skeletal Muscle
Here, greatest number of myosin
heads can pull on actin.
Summary
• How do you get more tension (strength)
out of a muscle? 2 ways
• 1. recruit more motor units
• 2. alter the contractility of each muscle
fiber or muscle cell by :
• 2A. Altering frequency of stimulation.
• 2B. Altering the length of the cells.