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

Z-Disc M-line 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

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)fascicle B)fiber C)myofibril D)sarcomere

According to sliding filament theory, during a contraction the distance between the M and Z lines A)increases B)decreases C)stays the same D)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,  l Series Parallel

f,  l Series F=?  L=? A)F = f ;  L =  l B)F = 3f ;  L = 3  l C)F = 3f ;  L =  l D)F = f ;  L = 3  l E)don’t understand

f,  l f,  L  L=n  l F,  l F=nf f,  l Series Parallel A)F = f ;  L =  l B)F = 3f ;  L = 3  l C)F = 3f ;  L =  l D)F = f ;  L = 3  l E)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/cm 3 ) = g/cm 3 l=fiber length (cm) V= Muscle volume = M/ 

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) F max = 5 units; V max = 2 units/s ii) F max = 2 units; V max =5 units/s iii) F max =5cos10 o units; V max =2cos10 o 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 deg B)F = 472 N, angle = 25.4 deg C)F = 428 N, angle = 4.17 deg D)F = 428 N, angle = E)I don’t understand

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

Time Stimulation (Action potentials) SingleLow frequencyHigh frequency Twitch Fused Tetanus Unfused Tetanus Force 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 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 – P o is maximum tetanic force – Length of muscle at Po is muscle’s optimal length

Rest length (%) Relative force Force-Length Relationship

Rest length (%) Relative force Force-Length Relationship

Rest length (%) Relative force Force-Length Relationship

Rest length (%) Relative force Force-Length Relationship

Rest length (%) Relative force Force-Length Relationship

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% P o 0% V max 95% P o 1% V max 90% P2.2% V max 75% P o 6.3% V max 50% P o 16.6% V max 25% P o 37.5% V max 10% P o 64.3% V max 5% P o 79.1% V max 0% P o 100% V max

Shortening Contractions Force decreases with velocity

Knee Shank Thigh Knee extensor muscles in shortening contraction during knee extension

Knee Shank Thigh Isometric Contractions

Isometric

Knee Shank Thigh Active and Lengthening)

Lengthening Contractions Higher force (160%!) Velocity-independent Don’t know why Important – Common – Selective for soreness and injury – Muscle strengthening greatest

How will the force-angle curves change for different muscle actions? Force 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 P0P0 Shortening Velocity 0 0 Prestretch No prestretch Frog knee flexor (semitendinosis) From Cavagna & Citterio, Prestretch effect lasts for a limited time

Data from Gregor et al , (fig Enoka) Velocity (mm/s)

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

PrestretchShorten Extensor stretch-shorten cycle in countermovement jump

Prestretch occurs in a variety of activities Jumping with countermovement Running Other examples?