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SKELETAL MUSCLES: Introduction - Montefiore · PDF file1 SKELETAL MUSCLES: Introduction 3...

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1 SKELETAL MUSCLES: Introduction 3 kinds of muscles Skeletal muscles Heart muscle Smooth 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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Page 1: SKELETAL MUSCLES: Introduction - Montefiore · PDF file1 SKELETAL MUSCLES: Introduction 3 kinds of muscles ¾Skeletal muscles ¾Heart muscle ¾Smooth muscles SKELETAL MUSCLES: Introduction

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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


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