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SKELETAL MUSCLES: Introduction
3 kinds of muscles
Skeletal musclesHeart muscleSmooth muscles
SKELETAL MUSCLES: Introduction
Skeletal muscles
• Striated• Controled by voluntary nerves• Activated by electric twitches• Reach a tetanized state if stimulated by twitches at
a sufficient frequency
SKELETAL MUSCLES: Introduction
Heart muscle
• Striated• Not controled by voluntary nerves• Never tetanized in normal conditions• Function in single twitches
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SKELETAL MUSCLES: Introduction
Smooth muscles
• Not striated• Not controled by voluntary nerves• Activated by electric twitches
SKELETAL MUSCLES: Introduction
Skeletal muscles are the active part of themusculo-skeletal system →human movementsPart of a very complex system with feedback involving:
• Electric properties• Chemical properties• Mechanical properties• Thermal properties
SKELETAL MUSCLES: Introduction
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SKELETAL MUSCLES: Introduction
Functional arrangements
SKELETAL MUSCLES: Introduction
Functional arrangementExample of parallel arrangement
Volume conservation
SKELETAL MUSCLES: Introduction
Functional arrangementExample of pinnate arrangement
Volume conservation
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SKELETAL MUSCLES: Structure
Fasciculi
TendonFasciculusFasciculus
Epimysium
Perimysium
Group offibers
Bone
Muscle fibers
SKELETAL MUSCLES: Structure
Group of fibres (10 to 100 fibres)
Motor neuron
Capillary vessel
Nucleus
Muscle fiber: length: 3 to 50 cm
SKELETAL MUSCLES: StructureMyofibrils
Myofibril: 1-2 micron diameter
Muscle fiber
Myofilament
« A » bands « Z » bands« I » bands
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SKELETAL MUSCLES: StructureMyofilaments
Myofibril
Myosin
Actin
Actin
Tropomyosin
Troponin
Actin molecule
(molecular weight: 42000)
7 nm
SKELETAL MUSCLES: Structure
Myosin filament
(molecular weight of 1 myosin molecule: 500000)
60°
SKELETAL MUSCLES: Structure
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Spatial organization of a myofilament
SKELETAL MUSCLES: Structure
Actin Myosin Actin + myosin
3D structure of myofilaments
SKELETAL MUSCLES: Structure
: 2.5sarcomere mµ≈
Muscle contraction
SKELETAL MUSCLES: Structure
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SKELETAL MUSCLES: Stimulation
Motor units
Muscle fibres stimulated by motor neurons
Each motor neuron may innervate many muscle fibers
Not all the fibers are excited at the same time
The total force of contraction depends on how many fibersare stimulated
A motor unit is the set of muscle fibers stimulated by a single nerve fiber
The size of a motor unit is the number of muscle fibersstimulated by a single motor nerve fiber
SKELETAL MUSCLES: Stimulation
Response to stimuli
The precision of muscle response is governed by the size of themotor units
Small muscles that react rapidly and with precision have smallmotor units (as small as 2 or 3 fibers in some of the laryngealmuscles) and have many nerve fibers going into each muscle)
Large muscles that do not require a fine degree of precision, such as gastrocnemius muscle, may have as many as 1000 muscle fibers in each motor unit
Fibers in adjacent motor units generally overlap
Twitch frequency: 100Hz
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350 400 450
Time (msec)
Forc
e of
con
trac
tion
Twitch frequency: 25Hz
0 50 100 150 200 250 300 350 400 450
Time (msec)
Twitch frequency: 25Hz
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350 400 450
Time (msec)
Forc
e of
con
trac
tion
Twitch frequency: 5Hz
0 50 100 150 200 250 300 350 400 450
Time (msec)
SKELETAL MUSCLES: StimulationSingle twitch and wave summation
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SKELETAL MUSCLES: StimulationSingle twitch and wave summation
Wave summation and tetanization
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0 50 100 150 200 250 300 350
Time (msec)
f=100 f= 50 f= 25 f= 5
Forc
e of
cont
ract
ion
Isotonic: R=0.
Isokinetic: L=cst.
SKELETAL MUSCLES: Modelling
Muscle conditions
R
L
R
Static(isometric)
state of rest (no twitch; different lengths):.L=0 ; R=0
state of contraction (twitches applied)
L=0 ; R=0.
Dynamic
Excentric Concentric
L>0 ; R=0.
L<0 ; R=0.
SKELETAL MUSCLES: Modelling
Muscle at rest
R=0
L
R=0 Viscoelastic material
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SKELETAL MUSCLES: Modelling
Muscle in isometric conditions
Twitches at high frequency → tetanized
Slack length0L
0R
0LL =
0RR =
SKELETAL MUSCLES: Modelling
Muscle in concentric conditionsTwitches at high frequency → tetanized
)())(( 0 aRbbRav +=++○
○
○○ ○
○○ R
v
decreases todecreasessuddenly , At time 0
LRRt
⇒
)0(at =−= tdtdLv
0L L
0R R
v
SKELETAL MUSCLES: ModellingMuscle in excentric conditions
Twitches at high frequency → tetanized
???
)0(at =−= tdtdLv
0LL
0RR
v
increases toincreasessuddenly , At time 0
LRRt
⇒
concentric excentricv
R0R
)())(( 0 aRbbRav +=++
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SKELETAL MUSCLES: Modelling
Hill’s three-element model
Contractile elementNo stress at rest
Shortens if activated
Series elementIntrinsic elasticity of actin and
myosin molecules
Parallel elementConnective tissues
TT
SKELETAL MUSCLES: Modelling
Hill’s three-element model
Parallel elementIts properties can be determined fromexperiments on a non activated muscle
A more sophisticated model could be used to include viscoelasticity of connective tissues
SKELETAL MUSCLES: Modelling
Hill’s three-element model
Series elementIts properties are difficult to determine because it is in series withthe contractile element
A more sophisticated model could be used to include viscoelasticity in the series element
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SKELETAL MUSCLES: Modelling
Hill’s three-element model
Contractile element
actin
myosin
Defined to model the relative motion of actin and myosin in the sarcomeres
Series elementDefined to model the intrinsicelasticity of actin and myosin
SKELETAL MUSCLES: ModellingCritique of Hill’s three-element model
Many simplifying hypotheses
Difficult identification
Muscle activation: yes or no
no possibility to consider the muscle in a non tetanized state
no consideration of feed back
A lot of research is still needed
THE END