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. MyofibrilsD. 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)
Z-DiscM-line
<--I-Band---><--------------------A-Band--------------->
<-H-Zone->
<--I-Band--->
Sarcomere:
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
According to sliding filament theory, during a contraction the distance
between the M and Z lines
A) increasesB) decreasesC) stays the sameD) 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
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 myofilamentstendon and aponeuroses
PE: Parallel Elastic Elementconnective 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, l
f, l f, l f, l
Series
F=?L=?
A) F = f ; L = lB) F = 3f ; L = 3lC) F = 3f ; L = lD) F = f ; L = 3lE) don’t understand
f, l
f, LL=nl
F,lF=nf
f, l
f, l
f, l f, l
f, l
f, l
Series
Parallel
A) F = f ; L = lB) F = 3f ; L = 3lC) F = 3f ; L = lD) F = f ; L = 3lE) don’t understand
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)• =muscle density (g/cm3) = 1.056 g/cm3
• l=fiber length (cm)• V= Muscle volume = M/
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
Significance of Architecture
• Clever design– Same functional component can yield so many
different motors
• Muscles designed for a purpose– Perhaps this simplifies the control
ProblemImagine 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
Enoka Fig 1.6
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) F = 472 N, angle = 64.5 degB) F = 472 N, angle = 25.4 degC) F = 428 N, angle = 4.17 degD) F = 428 N, angle = 85.82E) I don’t understand
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
TimeStimulation(Action potentials)
Single Low frequency High frequency
Twitch
FusedTetanusUnfused
TetanusFor
ce
Neural Stimulation
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 greaterB)5 Hz and greaterC)10 Hz and greaterD)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
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
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-VelocityRelative Force Velocity100% 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
Lengthening Contractions
• Higher force (160%!)• Velocity-independent• Don’t know why• Important
– Common– Selective for soreness and
injury– Muscle strengthening greatest
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
Activelengthening(prestretch)
Activeshortening
Force
P0
Shortening Velocity0
0
Prestretch
Noprestretch
Frog knee flexor(semitendinosis)From Cavagna &
Citterio, 1974.
Prestretch effectlasts for a limitedtime
SSC
• Muscle can produce more power if actively stretched before it is allowed to shorten
• Can also lower metabolic cost
Immediately after being stretched
Resting length
Crossbridges (and/or titin?) act like springs: after being stretched, higher F per xbridge
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