muscle contraction 09
TRANSCRIPT
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Skeletal Muscle BasicsContraction and Basic mechanical
properties
Taken from:Professor Bruce Lynn
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Skeletal Muscle Basics
3 Lectures:
Basic structure of muscle
Muscle activation & relaxation
Basic mechanical properties
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Outline
Sliding filaments & the crossbridge cycle
Force & Power
Antagonistic muscles
Series & Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening & stretch
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Dependence of isometric force on sarcomere length
Sarcomere length (% of optimum)
Force (%
of max)
Force is proportional to filament overlap:
important evidence for sliding filaments
The Tension Length Curve
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What causes the filament sliding?
Myosin heads bind to actin, then gothrough a cycle of events the cross
bridge cycle
Overall effect is force generation and ATP
hydrolysis
As all myosin molecules are identical, can
reduce problem to considering just a
single myosin head interacting with actin
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Z
ADP Pi
1
Z
ADP Pi
attach
2
Z
ADP
Pi release &
weak to strong
Pi
3
Z
ADP release
& filamentsliding
ADP4ATP
binding
Z
ATP
ATP
5
detach &
ATP
hydrolysis
Does not occur
when [Ca] low
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Force in Isometric contraction: no sliding
Thin filament
Thick filament
Attached crossbridge, no
force (spring not stretched)
Attached crossbridge has
changed shape to stretch
spring, force but no sliding
2
3Direction of isometric force: toward M line
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Cross-bridge Cycle: Key Features
1)ATP is used in each cycle to provide the energyRigor mortis occurs if ATP concentration = 0
2) Direction of filament force and sliding (if sliding
occurs) is one-way (thin filament moves toward M-line
at the centre of the sarcomere)
3) Step size is small: sliding produced by one cycle is
only about 1% of the sarcomere lengthMany cycles occur in succession to cause largemovements (as in running, walking, etc)
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Why so complicated?
Some constraints due
to muscle properties
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Outline
Sliding filaments & the crossbridge cycle
Force & Power
Antagonistic muscles
Series & Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening & stretch
What are they? How are they
different?
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Isometric
Force
Power
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What is isometric contraction?
Muscles are active (=contracting)
producing isometric force
The muscle force resists gravity and
prevents the arm and book falling
Isometric means the muscle length is
constant
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Contraction with shortening (concentric)
Biceps contracts and its shortening
flexes the elbow
Biceps does work lifting the book
POWER is the rate at which work
is done
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Outline
Sliding filaments & the crossbridge cycle
Force & Power
Antagonistic muscles
Series & Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening & stretch
Required due to crossbridge cycle
& sliding filament arrangement
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Tendon
Tendon
Biceps
Tendon
Triceps
Tendon
Example: Rotation around the elbow
Active (contracting) muscle can shorten (pull towards its
center)
Therefore, antagonistic muscles are required
BUT it cannot elongate (push away from its center)
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Rotation around the elbow: Flexion
Biceps contracts & shortens
Triceps is lengthened
(not contracting)
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Rotation around the elbow: Extension
Biceps is lengthened
(not contracting)
Triceps contracts & shortens
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Outline
Sliding filaments & the crossbridge cycle
Force & Power
Antagonistic muscles
Series & Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening & stretch
How the arrangement ofstructures affect force and
length change
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Springs connected in Parallel
Fixed position
Springs connected in Series
Fixed position
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Structures in Series:
Force at A and B are equal.
For structures in series, forces do NOT add up
Fixed position
A
Fixed position
A B
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A
Fixed position
Force = A
Fixed position
A
B
Force = A+B
For structures in parallel, forces add up
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Length changes
Length 2.0 m
Length 1.5 m
= 0.5 m
Length 3.0 m
= 1.0 m
Length 4.0 m
Connect in series
For structures in series, length changes add up
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Length changes
Length 2.0 m
Length 1.5 m
= 0.5 m
Length 2.0 m
Connect in parallel
= 0.5 mLength 1.5 m
For structures in parallel, length changes do NOT add up
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Contractile and Elastic Structures
In series and in parallel
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A muscle-tendon complex (MTC)
Bone
Bone
Muscle fibresor Contractile Component (CC)
Tendon
or Series elastic component (SEC)
Parallel elastic
component
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A muscle-tendon complex (MTC)
Because muscle and tendon are in series:
Both experience the same force at each
moment. An observed length change of MTC could
be due to either component
Tendon can only be stretched when
muscle is active
Muscle cannot move bones without first
stretching tendon
Bone
Bone
Muscle
or CC
Tendon
or SEC
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Elasticity also in parallel
The parallel element:
Can exert force when CC
is relaxed.
Adds its force to that of
muscle when CC is active.
More complicatedconnections can switch
elasticity between series
and parallel.
Bone
Bone
CC
SEC
PEC
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Where and what are the SEC and PEC
relative to the crossbridges?
Tendon (collagen) series
Aponeuroses (collagen) series
Epimysium (collagen) parallel
Filaments (titin) parallel
Filaments (myosin, actin) series
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Outline
Sliding filaments & the crossbridge cycle
Force & Power
Antagonistic muscles
Series & Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening & stretch
How the arrangement ofstructures affect force and
length change
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Arrangement of muscle fibres: some examples
parallel
fusiform
triangular
unipennate
bipennate
multipennate
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Volumes equal and line of muscle force is the same
Each fibre in pennate muscle is half the length
of the fibres in the parallel muscle
and at angle to the line of muscle force;
force along line of muscle (F) = cos * force
along line of fibre (f)
For = 30o, cos = 0.87
But there are twice as many fibres in thepennate muscle as in the parallel muscle
Net effect: pennate muscle produces 2 * 0.87 =
1.74 times more force than the parallel muscle
Arrangement of fibres within muscle:
Pennation increases muscle force
parallel pennate
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Net effect: pennate muscle shortening is only 0.5 * 0.87
= 0.41 times as much as the parallel muscle per unit
time
the cos rule means that muscle shortening
is cos * fibre shortening.
Pennation reduces muscle shortening velocity
also each fibre in the pennate muscle only
shortens half as far as each fibre in the
parallel muscle.
In each unit of time
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Outline
Sliding filaments & the crossbridge cycle
Force & Power
Antagonistic muscles
Series & Parallel structures
Arrangement of fibres with muscle
Force-velocity relation, also power
For shortening & stretch
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Contraction with shortening (concentric)
Biceps contracts and its shortening
flexes the elbow
Biceps does work lifting the book
POWER is the rate at which work
is done
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Muscle Force
Muscle length
time(Lever movement)
Before stimulation of the muscle
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start stimulation of the muscle
Isometric phasemuscle force toosmall to lift weight
Muscle Force
Muscle length
time
(Lever movement)
Stim
I i h i
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During stimulation, muscle force
enough to lift weight
Muscle Force
Muscle length
time
(Lever movement)
Stim
Isotonic shortening:constant forceduring shortening
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Muscle Force
Muscle length
time(Lever movement)
Before stimulation of the muscle
Larger weight
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During stimulation of the muscle
time
Muscle Force
Muscle length
(Lever movement)
Stim
Isometric phasemuscle force toosmall to lift weight
I t i h t i
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During stimulation of the muscle
Muscle Force
Muscle length
time
(Lever movement)
Stim
Isotonic shortening:constant forceduring shortening
Larger force &
slower velocity
P k t
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Power = work rate
Velocity
0.0 0.5 1.0 1.5
Power
0.0
0.1
0.2
Velocity
0.0 0.5 1.0 1.5
Force
0.0
0.5
1.0
Inverse relation between
force and velocity of
shortening
The Force Velocity Curve
= (force x length ) / time
= force x (length / time)
= force x velocity
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Contraction with lengthening (eccentric)
Biceps is acting as a brake.
Biceps is producing force, EMG, etc,(=contracting)
The book is lowered in a slow,controlled movement.
The elbow extends as the length of
biceps increases due to the book &
gravity. Work is done on biceps.
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Force during isovelocity stretch of active
muscle
stim
stim
stim
Force-Velocity relation for Stretch
Velocity
stretch shorten
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Stretch of active muscle
Occurs during normal every-day activities
Contracting muscle fibres act as a brake
Large forces can be produced
But not much fuel (ATP) is used
Forces can be large enough to cause
damage
Not covered in many standard textbooks
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Skeletal Muscle BasicsContraction. Basic mechanical properties
Summary
Tension-length curve, max force at max filament overlap
Cross bridge cycle, myosin head binds to actin, ATP splitting,
repetitiveMuscle morphology:
short fat muscles, high force, low speed;
long thin muscles, low force, high speed
Inverse force-velocity relationPower = Force*velocity; max power at ca 1/3 max force or velocity
Eccentric contractions, high force.
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Skeletal Muscle Basics
Contraction. Basic mechanical properties
Good source of information
Jones et al., Skeletal Muscle from Molecules to Movement, 2004,
Churchill Livingstone.
Acknowledgements
Thanks for Nancy Curtin, Imperial College, for use of many of her slides.