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The microstructure of muscle
Muscle Terminology
• myofiber (muscle fiber)- a single muscle
cell
• sarcolemma- muscle cell membrane
• sarcoplasm- muscle cell cytoplasm
• myofibril- long contractile protein structure
– actin and myosin
• sarcomere- the contractile unit between two
z-lines
Connective tissue surrounding muscle
The sarcoplasm with sarcoplasmic reticulum
and transverse tubules
More Terms
• sarcoplasmic reticulum- storage and release
site of calcium
• transverse tubule- also involved in calcium
flux
The Neuromuscular Junction
Neuromuscular Junction
1) Impulse travels down motor neuron
2) at end of neuron, acetylcholine released
3) Acetylcholine diffuses across synaptic cleft
4) acetylcholine binds to receptors on
sarcolemma causing permeabilty
5) sodium enters cell causing depolarization
and muscle contraction
Muscular Contraction
• functions to produce force for locomotion
• force for breathing
• force for postural support
• heat production in cold (no force)
How do skeletal muscles
contract?
• Sliding filament model of contraction
• the interaction of actin and myosin
The sliding filament theory of
contraction
Sliding Filament Animation
• -- sliding filament animation.htm
•http://intro.bio.umb.edu/111112/112s99Lect/muscle/contract.html
How do the Actin and Myosin Interact?
• The myosin head binds to the actin filament
in a weak state initially (or unbound)
• the signal to contract initiates a strong
binding state
– Binding of calcium to troponin regulates this
strong-weak state
The Contraction Itself
• during the strong binding the myosin pulls
the actin past
• this effectively shortens or contracts the
muscle
Relationship between myosin cross-bridges
and Ca++ binding
Where does the energy for
contraction come from?
• ATP is necessary for each contraction cycle
to occur
• each contraction cycle results in a
shortening of the muscle by 1%
• some muscles can shorten by up to 60 % of
their resting length
• therefore many shortening cycles must
occur for a single contraction
Sources of ATP for Muscle
Contraction
Fig 8.7
ExcitationContraction
Coupling
Fig 8.9
Crossbridge Animation
• Quicktime - Actin Myosin Crossbridge 3D
Animation.htm
•http://www.sci.sdsu.edu/movies/actin_myosin.html
Summary of excitation contraction-coupling
Steps in Excitation - Contraction
coupling
• at rest actin and myosin are weakly bound
(or unbound)
• an excitation impulse from the a motor
nerve causes an end-plate potential
• the potential depolarizes the muscle cell
beginning at the sarcolemma
The Neuromuscular Junction
Excitation- Contraction cont’d
• depolarization travels down the T-tubules to
the sarcoplasmic reticulum
• the impulse reaches the SR and calcium is
released
• calcium binds to troponin and causes the
strong binding state
Excitation- Contraction (one
more)
• during strong binding, myosin head cocks
• this action moves actin filament along
myosin
• Binding of ATP causes the weak binding (or
release) again enabling another contraction
Summary of excitation contraction-coupling
Important Points
• depolarization causes release of calcium by
SR
• calcium enables the strong binding state
• ATP provides energy for cocking of myosin
head, BUT
• binding of ATP causes the weak binding
state (or release) of actin and myosin
A couple more important points
• contraction can continue as long as calcium
is available to enable strong binding AND
• ATP is available for energy of cocking and
release of strong binding
• the signal to stop contraction is the loss of
an impulse and uptake of calcium
Muscle fatigue is characterized by a reduced
ability to generate force
Properties of
Muscle Fiber Types
• Biochemical properties
– Oxidative capacity
– Type of ATPase
• Contractile properties
– Maximal force production
– Speed of contraction
– Muscle fiber efficiency
Individual Fiber Types
Fast fibers
• Type IIx fibers
– Fast-twitch fibers
– Fast-glycolytic fibers
• Type IIa fibers
– Intermediate fibers
– Fast-oxidative
glycolytic fibers
Slow fibers
• Type I fibers
– Slow-twitch fibers
– Slow-oxidative
fibers
Muscle Fiber Types
Fast Fibers
Slow fibers
Characteristic
Type IIx
Type IIa
Type I
Number of mitochondria
Low
High/mod
High
Resistance to fatigue
Low
High/mod
High
Predominant energy system
Anaerobic
Combination
Aerobic
ATPase
Highest
High
Low
Vmax (speed of shortening)
Highest
Intermediate
Low
Efficiency
Low
Moderate
High
Specific tension
High
High
Moderate
Comparison of maximal shortening velocities
between fiber types
Type I vs Type II (velocity)
• type II are fast twitch muscles
– type IIa are sort of like slow twitch but faster
• type I are slow twitch muscles
• therefore IIb will have the fastest shortening
velocity and type I will have the slowest
Endurance exercise training induced changes
in fiber type in skeletal muscle
Training-Induced Changes in
Muscle Fiber Type
Fig 8.13
Isotonic vs. Isometric Actions
Isometric Muscle Action
• an isometric contraction is occurs when
there is no change in muscle length when
force is being produced
• trying to push a car out of the snow
• holding up a table so it can be leveled
Isotonic Muscle Action
• an isotonic contraction occurs when there is
a change in muscle length
• concentric when muscle shortens
– bicep curl, lifting
• eccentric when muscle lengthens
– tug o war, negatives in weights, putting down a
beer
Recording of a simple twitch
Relationship between stimulus strength and
force of contraction
Stimulus Strength vs Force of
Contraction
• Weak stimulus does not recruit many motor
units
• Stronger stimulus recruits more motor units
• When all motor units are recruited, no more
force can be applied regardless of stimulus
strength
Length-tension relationship in skeletal muscle
Length Tension Relationship
• There exists an optimal length of muscle at
which it produces the greatest force
– Typically between 100-120 % resting length
• Maximal tensions at lengths longer or
shorter than the optimal length will be less
Progression of simple twitches, summation
and tetanus
Tetanus
• If twitches become more frequent, greater
force can be developed during summation
than for a single twitch
• If twitches become to frequent, tetanus will
develop and the muscle will not relax
• Typically results only from electrical
stimulation
Muscle force-velocity relationships
Muscle power-velocity relationships
The Golgi tendon organ
GTO
• Provides info to the CNS about tension
development in the muscle
• Acts like a governor to prevent damaging
tension from being generated
• Can be overridden to a certain extent by
training
– Supraphysiological strength in crisis
Muscle spindles structure and location
Spindle
• Provides info to the CNS about muscle
length or stretch
• Excessive muscle stretch, especially during
contraction is damaging
• Helps prevent damaging stretch during
contraction