Download Muscle

Muscle
• Skeletal muscle
– Unit Cell Structure
– Architecture
• Series/parallel
• Force/velocity
– Stimulation
• Summation/tetanus/rate-coding
– Muscle mechanics
• Force-length relation
• Force velocity relation
– Pre-stretch
Skeletal Muscle
• Striated and voluntary
– Cardiac muscle is striated
– Smooth muscle is unstriated and involuntary
• Attaches to skeleton via tendons
• Most abundant tissue in the body
– 45-75% of body weight
Structure of a muscle cell
A. Fascicles
– fiber bundles
B. Fibers
– muscle cell
– bundles of myofibrils
C. Myofibrils
D. Sarcomeres (series)
E. Actin & Myosin Filaments
Fascicles
• A muscle is composed of
multiple fascicles in parallel
– A sheath of connective tissue
surrounds the muscle
(epimysium)
– Each fascicle is surrounded by
connective tissue (perimysium)
– Fascicles composed of bundles
of muscle fibers
Muscle Fiber
• Long, cylindrical,
multinucleated cells
• Between fibers are blood
vessels
• Surrounded by endomysium
• Composed of myofibrils
Myofibrils
•
•
•
•
•
Literally (muscle thread)
Contractile element of muscle
Made up of filaments
Aligned in parallel
filaments make striations
– Banding pattern
• One repeating unit is called a
sarcomere
• string of sarcomeres in series
Sarcomeres
• Functional unit of muscle
contraction
• Literally ‘muscle segment’
• Number of sarcomeres in a
fiber is very important to
muscle function
• When each sarcomere
shortens the same amount,
the fiber with more
sarcomeres will shorten
more.
• Made up of myofilaments
– Thick and thin filaments
Myofilaments
– Myosin(thick)
– In central
region
– Dark bands
– Globular heads
– Arranged in
both directions
– Actin(thin)
Banding Pattern
• Based on
myofilaments:
– Z-Disc
– I-Band
– A-Band
– H-zone
– M-line
Sarcomere:
<--I-Band---><--------------------A-Band---------------> <--I-Band--->
Z-Disc
<-H-Zone->
M-line
Muscle contraction
• Sliding filament theory
– AF Huxley and HE Huxley
– Light and Electron microscopy
– Both published results same time in Nature
– Does not explain lengthening contractions
Sliding Filament Theory
• The exertion of force by muscle is
accompanied by the sliding of thick and thin
filaments past one another
• Commonly explained by cross-bridges
• cross-bridge theory:
• muscle force is
proportional to the
number of cross
bridges attached
Sliding filament
theory
• A band stay the same
• I band shorten
A single functional unit in a muscle
contraction is a
A)
B)
C)
D)
fascicle
fiber
myofibril
sarcomere
According to sliding filament theory,
during a contraction the distance
between the M and Z lines
A)
B)
C)
D)
increases
decreases
stays the same
need more information
Muscle
• Skeletal muscle
– Unit Cell Structure
– Architecture
• Series/parallel
• Force/velocity
– Stimulation
• Summation/tetanus/rate-coding
– Muscle mechanics
• Force-length relation
• Force velocity relation
– Pre-stretch
Muscle architecture
• Organization of muscle fibers
– Muscle also organized at macro level
– Architecture is the arrangement of muscle fibers
relative to the axis of force generation
• Muscle fibers have fairly consistent diameters among
muscle of different size, but arrangement can be very
different
• So cannot tell force capacity of a muscle from a biopsy
– Need number of fibers and how arranged
3 types of arrangements
• Longitudinal (parallel)
– Fibers run parallel to force generating axis
• Pennate
– Fibers at a single angle
– shallow
• Multipennate
– several angles
What are advantages/disadvantages of
a) longitudinal arrangement?
b) pennate arrangement?
Muscle architecture
• Determines
– Max muscle force
• Fibers in parallel
• Pennation angle
– Max muscle shortening velocity
• no of sarcomeres in series
Hill Muscle Model
CE: Contractile Element (active force generation)
SE: Series Elastic Element
represents elasticity in:
cross-bridges and myofilaments
tendon and aponeuroses
PE: Parallel Elastic Element
connective tissue surrounding muscle fibers
• Can use Hill muscle model to illustrate effects
of muscle length and width on muscle’s
– maximum force
– maximum shortening velocity
f, Dl
f, Dl
f, Dl
Series
f, Dl
f, Dl
f, Dl
f, Dl
Parallel
F=?
DL=?
f, Dl
f, Dl
f, Dl
Series
A)
B)
C)
D)
E)
F = f ; DL = Dl
F = 3f ; DL = 3Dl
F = 3f ; DL = Dl
F = f ; DL = 3Dl
don’t understand
f, Dl
f, DL
DL=nDl
f, Dl
f, Dl
f, Dl
Series
f, Dl
F,Dl
F=nf
f, Dl
f, Dl
Parallel
f, Dl
A)
B)
C)
D)
E)
F = f ; DL = Dl
F = 3f ; DL = 3Dl
F = 3f ; DL = Dl
F = f ; DL = 3Dl
don’t understand
Pennation Angle
Pennation Angle
• Pennation angle is a space saving strategy
• Allows you to pack more fibers into a smaller space
• Doesn’t hurt b/c cos0=1, cos 30=0.87 (13% force loss)
Muscle architecture
• Determines
– Max muscle force
• Fibers in parallel
• Pennation angle
– Max muscle shortening velocity
• no of sarcomeres in series
Physiological Cross-Sectional Area
• PCSA ~ max muscle force
•
•
•
•
M=muscle mass (g)
r=muscle density (g/cm3) = 1.056 g/cm3
l=fiber length (cm)
V= Muscle volume = M/r
M cosq V cosq
PCSA(cm ) =
=
rl
l
2
How do we measure PCSA?
More on PCSA
• Not proportional to muscle mass
• Not proportional to anatomical cross-sectional
area
Muscle architecture
• Determines
– Max muscle force (~PCSA)
• Fibers in parallel
• Pennation angle
– Max muscle shortening velocity
• no of sarcomeres in series
Muscle fiber length
• Assumed that fiber length ~fiber velocity
• Fiber length ~ no. of sarcomeres in series
Muscle architecture
• Determines
– Max muscle force (~PCSA)
• Fibers in parallel
• Pennation angle
– Max muscle shortening velocity (~Fiber length)
• no of sarcomeres in series
What are advantages/disadvantages of
a) longitudinal arrangement?
b) pennate arrangement?
Significance of Architecture
• Clever design
– Same functional component can yield so many
different motors
• Muscles designed for a purpose
– Perhaps this simplifies the control
Problem
Imagine you have 10 sarcomeres; each generates a maximum of 1 unit
of force, and shortens with a maximum velocity of 1 unit/s.
Diagram an arrangement of sarcomeres that will create a muscle
fiber with the following force and velocity characteristics. Use I to
represent individual sarcomeres, and draw ellipses around
sarcomeres to specify fibers.
i) Fmax= 5 units; Vmax= 2 units/s
ii) Fmax= 2 units; Vmax=5 units/s
iii) Fmax=5cos10o units; Vmax=2cos10o units/s
Net muscle force
Vector math can illustrate the effect of
coactivating different parts of the
pectoralis major muscle.
Suppose clavicular component exerted
a force of 224N at 0.55 rad above
horizontal, and the sternal portions has
a magnitude of 251N at 0.35 rad below
horizontal.
What is the resultant force?
A)
B)
C)
D)
E)
F = 472 N, angle = 64.5 deg
F = 472 N, angle = 25.4 deg
F = 428 N, angle = 4.17 deg
F = 428 N, angle = 85.82
I don’t understand
Enoka Fig 1.6
Enoka Fig 1.6
Muscle
• Skeletal muscle
– Unit Cell Structure
– Architecture
• Series/parallel
• Force/velocity
– Stimulation
• Summation/tetanus/rate-coding
– Muscle mechanics
• Force-length relation
• Force velocity relation
– Pre-stretch
Temporal Summation
• Excitation fast (~1-2ms)
• Contraction/relaxation slow (100ms)
– Muscle twitch lags because slack in the elastic components must be
taken up.
– Contraction time:
– Relaxation time:
• Summation
– If second impulse comes along before the first one has relaxed, they
sum
– Get more force with multiple impulses then alone
• Tetanic Summation
– maximum tension is sustained because rapidity of stimulation
outstrips the contraction-relaxation time of the muscle
Force
Neural Stimulation
Unfused
Tetanus
Fused
Tetanus
Twitch
Single
Stimulation
Low frequency
(Action potentials)
Time
High frequency
If the contraction-relaxation time for a muscle
twitch is 100 ms, at what stimulation frequency
will we begin to see summation?
NB: 1 Hz corresponds to 1 stimulus/second
A)100 Hz and greater
B)5 Hz and greater
C)10 Hz and greater
D)I don’t understand
Max Force
• PCSA
– No. sarcomeres in parallel
– Pennation angle
• Stimulation
Max Shortening Velocity
• No. of sarcomeres in series
– Muscle fiber length
Muscle
• Skeletal muscle
– Unit Cell Structure
– Architecture
• Series/parallel
• Force/velocity
– Stimulation
• Summation/tetanus/rate-coding
– Muscle mechanics
• Force-length relation
• Force velocity relation
– Pre-stretch
– WorkLoops
Muscle Mechanics
• Force-length
• Force-velocity
Force-Length
• Isometric force varies with muscle length
– Forces generation in muscle is a direct function of
the amount of overlap between actin and myosin
filaments
– Po is maximum tetanic force
– Length of muscle at Po is muscle’s optimal length
Force-Length Relationship
1.0
0.8
Relative
force
0.6
0.4
0.2
0
60
80
100
120
Rest length (%)
140
160
Force-Length Relationship
1.0
0.8
Relative
force
0.6
0.4
0.2
0
60
80
100
120
Rest length (%)
140
160
Force-Length Relationship
1.0
0.8
Relative
force
0.6
0.4
0.2
0
60
80
100
120
Rest length (%)
140
160
Force-Length Relationship
1.0
0.8
Relative
force
0.6
0.4
0.2
0
60
80
100
120
Rest length (%)
140
160
Force-Length Relationship
1.0
0.8
Relative
force
0.6
0.4
0.2
0
60
80
100
120
Rest length (%)
140
160
Passive force production
Titin
• Cross-bridge not
responsible, so what it?
• Origin of passive muscle
tension within myofibrils
– Researchers compared
whole muscle, single
fibers, and single fibers
w/membranes removed
(1986)
– Huge protein responsible
- titin
Force-Velocity
Muscle Actions
1. Shortening
2. Isometric
3. Lengthening
Force-Velocity
Relative ForceVelocity
100% Po
0% Vmax
95% Po
1% Vmax
90% P
2.2% Vmax
75% Po
6.3% Vmax
50% Po
16.6% Vmax
25% Po
37.5% Vmax
10% Po
64.3% Vmax
5% Po
79.1% Vmax
0% Po
100% Vmax
Shortening Contractions
• Force decreases with velocity
Knee extensor muscles in shortening contraction
during knee extension
Thigh
Knee
Shank
Isometric Contractions
Thigh
Knee
Shank
Isometric
Active and Lengthening)
Thigh
Knee
Shank
Lengthening Contractions
•
•
•
•
Higher force (160%!)
Velocity-independent
Don’t know why
Important
– Common
– Selective for soreness and
injury
– Muscle strengthening greatest
Force
How will the force-angle curves change for
different muscle actions?
Isometric
Knee Angle
Force
• PCSA
– No. sarcomeres in parallel
– Pennation angle
• Stimulation
• Sarcomere Length
– Filament overlap
• Velocity
Shortening Velocity
• No. of sarcomeres in series
– Muscle fiber length
• Force
Summary
• Force and velocity
– Structure of the unit cell
– Sliding Filament Theory
– Architecture
– Stimulation
– F-L
– F-V
Put it all together
• Compare muscles w/two different pcsas
– Draw F-L
– Draw F-V for same fiber length
• Compare muscle w/different fiber lengths
– Draw F-L, for same pcsa
– Draw F-V
Muscle
• Skeletal muscle
– Unit Cell Structure
– Architecture
• Series/parallel
• Force/velocity
– Stimulation
• Summation/tetanus/rate-coding
– Muscle mechanics
• Force-length relation
• Force velocity relation
– Pre-stretch
Prestretch: muscle is active and stretched
before beginning to shorten
Active
lengthening
(prestretch)
Active
shortening
Force
Prestretch
P0
Frog knee flexor
(semitendinosis)
From Cavagna &
Citterio, 1974.
Prestretch effect
lasts for a limited
time
No
prestretch
0
0
Shortening Velocity
Velocity (mm/s)
Data from Gregor et al. 1988., (fig. 6.36 Enoka)
SSC
• Muscle can produce more power if actively
stretched before it is allowed to shorten
• Can also lower metabolic cost
Resting length
Immediately after being stretched
Crossbridges (and/or titin?) act like springs:
after being stretched, higher F per xbridge
Extensor stretch-shorten cycle
in countermovement jump
Prestretch
Shorten
Prestretch occurs in a variety of
activities
• Jumping with countermovement
• Running
• Other examples?