Skeletal Muscle Physiology

Susan V. Brooks Herzog

Department of Physiology

University of Michigan

Muscle

Muscle fibers

Muscle fiber

MyofibrilSarcomere

Modified from McMahon, Muscles, Reflexes and Locomotion Princeton University Press, 1984.

A little less than half of the body’smass is composed of skeletal muscle, with most muscles linkedto bones by tendons through which the forces and movements developed during contractionsare transmitted to the skeleton.

Structural hierarchy of skeletal muscle

Sarcomere: functional unit of striated muscle

Electronmicrograph

Myosin filaments

Actin filaments Actin filaments

Sarcomere

A band I bandI band

Z line

1 m

Modified from Vander, Sherman, Luciano Human Physiology, McGraw-Hill.

thin filamentlattice

overlapregion

center ofsarcomere

thick filamentlattice

Cross-sectional views of:

Myosin is a hexamer:2 myosin heavy chains4 myosin light chains

C terminus

2 nm

Coiled coil of two helices

Myosin is a molecular motor

Myosin S1 fragmentcrystal structure

Ruegg et al., (2002) News Physiol Sci 17:213-218.

NH2-terminal catalytic (motor) domain

neck region/lever arm

Nucleotide binding site

Myosin head: retains all of the motor functions of myosin,i.e. the ability to produce movement and force.

Modified from Vander, Sherman, Luciano Human Physiology, McGraw-Hill.

Working stroke produced by opening and closing of the nucleotide binding site, resultingin rotation of the regulatory domain (neck) about a fulcrum (converter domain).

Sub-nanometer rearrangements atactive site are geared up to give 5-10 nm displacement at the end of the lever arm.

Hypothetical model of theswinging lever arm

Ruegg et al., (2002) News Physiol Sci 17:213-218.

Po

wer S

troke

The lever movement drives displacement of the actin filament relative to the myosin head (~5 nm), and by deforming internal elastic structures, produces force (~5 pN).

Thick and thin filaments interdigitate and “slide” relative to each other.

How striated muscle works: The Sliding Filament Model

From Vander, Sherman, Luciano Human Physiology, McGraw-Hill.

Chemomechanical coupling – conversion of chemical energy(ATP about 7 kcal/mole) into force/movement.

• ATP is unstable thermodynamically

• Two most energetically favorable steps: 1. ATP binding to myosin 2. Phosphate release from myosin

• Rate of cycling determined by M·ATPase activity and external load

Adapted from Goldman & Brenner (1987) Ann Rev Physiol 49:629-636.

Shortening velocity dependent on ATPase activity

Different myosin heavy chains (MHCs) have different ATPase activities.

There are at least 7 separate skeletal muscle MHC genes…arranged in series on chromosome 17.

Two cardiac MHC genes located in tandem on chromosome 14.

The slow cardiac MHC is the predominant gene expressed in slow fibersof mammals.

Goldspink (1999) J Anat 194:323-334.

Peak power obtained at intermediate loads and intermediatevelocities.

Power output: the most physiologically relevant marker of performance

Power = work / time= force x distance / time= force x velocity

Figure from Berne and Levy, PhysiologyMosby—Year Book, Inc., 1993.

• shortening

• isometric

• lengthening

(Isotonic: shortening against fixed load, speed dependent on M·ATPase activity and load)

Three potential actions during muscle contraction:

Most likely to causemuscle injury

Biceps muscle shortensduring contraction

Biceps muscle lengthensduring contraction

Modified from Vander, Sherman, Luciano Human Physiology, McGraw-Hill.

Motor Units: motor neuron and the muscle fibers it innervates

Spinalcord • The smallest amount of

muscle that can be activated voluntarily.

• Gradation of force in skeletalmuscle is coordinated largely by the nervous system.

• Recruitment of motor unitsis the most important means of controlling muscle tension.

To increase force:1. Recruit more M.U.s2. Increase freq. (force –frequency)

• Since all fibers in the motor unit contract simultaneously, pressures for gene expression(e.g. frequency of stimulation, load) are identical in all fibersof a motor unit.

From Matthews GG Cellular Physiology of Nerve and Muscle Blackwell Scientific Publications.

Modified from Vander, Sherman, Luciano Human Physiology, McGraw-Hill.

Physiological profiles of motor units:all fibers in a motor unit are of the same fiber type

Slow motor units contain slow fibers: • Myosin with long cycle time and therefore uses ATP at a slow rate.• Many mitochondria, so large capacity to replenish ATP.• Economical maintenance of force during isometric contractions and efficient performance of repetitive slow isotonic contractions.

Fast motor units contain fast fibers: • Myosin with rapid cycling rates. • For higher power or when isometric force produced by slow motor units is insufficient. • Type 2A fibers are fast and adapted for producing sustained power. • Type 2X fibers are faster, but non-oxidative and fatigue rapidly. • 2X/2D not 2B.

Modified from Burke and Tsairis, Ann NY Acad Sci 228:145-159, 1974.

Frequency of recruitment

Load

inactivity

controls

strengthtrained

endurancetrained

Continuum of Physical Activity

Muscle is plastic!

Muscle “adapts” to meet the habitual level of demand placed on it, i.e. level of physical activity.

Level of physical activity determined by the frequency of recruit-ment and the load.

Increase muscle use – endurance training – strength training(cannot be optimallytrained for both strengthand endurance)

Decrease muscle use – prolonged bed rest – limb casting – denervation – space flight.

Adapted from Faulkner, Green and WhiteIn: Physical Activity, Fitness, and Health, Ed. Bouchard, Shephard and Stephens

Human Kinetics Publishers, 1994

Endurance training

Little hypertrophy but major biochemical adaptations within muscle fibers.

Increased numbers of mitochondria; concentration and activities of oxidative enzymes (e.g. succinate dehydrogenase, see below).

Control 12-weekstreadmill running

Succinate dehy-drogenase (SDH)activity: Low activity lightHigh activity dark

Images courtesy of John Faulkner and Timothy White

A calcineurin dependent transcriptional pathway appears to control skeletal muscle fiber type.

• Calcineurin is a Ca2+-regulated serine/threonine phosphatase.

• Caclineurin dephosphorylates nuclear factor of activated T cells (NFAT) transcription factors.

• Dephosphorylated NFATs translocate to the nucleus where combinatorially with other factors they activate transcription.

• A second target of calcineurin is the transcriptional co-activator, peroxisome-proliferator-activated receptor- co-activator-1 (PGC-1).

• Activation of calcineurin in skeletal myocytes selectively up-regulates slow-fiber-specific gene promoters and the effect enhanced with PGC-1 expression

• PGC-1 activates mitochondrial biogenesis. Lin et al. (2002) Nature 418:797-801

A calcineurin dependent transcriptional pathway appears to control skeletal muscle fiber type.

• Cyclosporin is widely used clinically to prevent rejection of transplanted tissues; patients develop skeletal muscle myopathy and loss of oxidative capacity.

• Cyclosporin (and FK-506) are specific inhibitors of calcineurin and thereby block T cell activation.

• Cyclosporin administration to intact animals promotes slow-to-fast fiber transformation.

Increased use: strength training

Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns.

Long term gains almost solely the result of hypertrophy i.e. increased size.

Rommel et al. (2001) Nature Cell Biology 3, 1009.

The PI(3)K/Akt(PKB)/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy/atrophy.

• Application of IGF-I to C2C12 myotube cultures induced both increased width and phosphor-ylation of downstream targets of Akt (p70S6 kinase, p70S6K; PHAS-1/4E-BP1; GSK3) but did NOT activate the calcineurin pathway.

• Treatment with rapamycin almost completely prevented increase in width of C2C12 myotubes.

• Treatment with cyclosporin or FK506 does not prevent myotube growth in vitro or compensatory hypertrophy in vivo

• Recovery of muscle weight after following reloading is blocked by rapamycin but not cyclosporin.

Control Prolonged

bed rest

Disuse causes atrophy -- USE IT OR LOSE IT!

Individual fiber atrophy (loss of myofibrils) with no loss in fibers.

Effect more pronounced in Type II fibers.

“Completely reversible” (in young healthy individuals).

ATPase activity:Type I fibers light Type II fibers dark

Images courtesy of John Faulkner

Performance Declines with Aging --despite maintenance of physical activity Performance Declines with Aging --despite maintenance of physical activity

Age (years)

10 20 30 40 50 60

Pe

rfo

rman

ce (

% o

f pe

ak)

0

20

40

60

80

100

Shotput/DiscusMarathonBasketball (rebounds/game)

D.H. Moore (1975) Nature 253:264-265. NBA Register, 1992-1993 Edition

Number of motor units declines during aging- extensor digitorum brevis muscle of human beings

Campbell et al., (1973) J Neurol Neurosurg Psych 36:74-182.

AGE-ASSOCIATED ATROPHY DUE TO BOTH…

Individual fiber atrophy (which may be at least

partially preventable and reversible through exercise).

Loss of fibers

(which as yet appears irreversible).

Motorneuronloss

Centralnervoussystem

Motor unit remodeling with agingMotor unit remodeling with aging

Muscle

• Fewer motor units• More fibers/motor unit

AG

ING

Mean Motor Unit Forces:• FF motor units get smaller in old age and decrease in number• S motor units get bigger with no change in number• Decreased rate of force generation and POWER!!

FF FI FR S

Ma

xim

um

Iso

me

tric

Fo

rce

(mN

)

0

25

50

75

100

125

150

175

200

225

AdultOld

Motor Unit Classification Kadhiresan et al., (1996) J Physiol 493:543-552.

• Muscles in old animals are more susceptible to contraction- induced injury than those in young or adult animals.

Muscle injury may play a role in the development of atrophy with aging.

• Muscles in old animals show delayed and impaired recovery following contraction-induced injury.

• Following severe injury, muscles in old animals display prolonged, possibly irreversible, structural and functional deficits.

Inju

red

fib

ers

(%

tota

l)

0

5

10

15

20control *passiveisometriclengthening

* different from zero (p<0.05)

Only lengthening contractions result in damaged fibers

Other Measures of Contraction-Induced Injury

enzyme release from degenerating muscle fibers

in human beings, subjective reports of muscle soreness

in the absence of fatigue, a decrease in the development of force

immediate mechanical disruption observed by EM.

Koh & Brooks (2001) Am J Physiol 281:R155-R161.

“Ghost” fiber 3 days after initial injury

Faulkner, Brooks and Zerba (1995) J Gerontol 50:B124-B129.

Repair through activation of satellite cells

Myology (Sanes, McGraw-Hill, 1994)

Perry and Rudnicki (2000) Frontiers in Bioscience 5:D750-67.

4 days after damage

2 weeks after damage

4 weeks after damagewith irradiation

A single prior exposure to a protocol of lengtheningcontractions reduced the force deficit and damaged fibers

Force Deficit(% control)

Injured Fibers(% total)

0

10

20

30

40

50

60%non-trained

*

2 weeks post

*

* p < 0.05

Koh & Brooks (2001) Am J Physiol 281:R155-R161.

Degeneration-regeneration not necessary to provide musclesprotection from contraction-induced injury

• Despite the increase in susceptibility to injury with aging, and the decreased ability to recover, muscles in old animals can be conditioned for protection from injury.

• Maintenance of conditioned fibers, particularly in muscles of elderly people, may prevent inadvertent damage during contractions.

Koh & Brooks (2001) Am J Physiol 281:R155-R161. Force deficit Injured fibers

For

ce d

efic

it (%

con

trol

)

0

10

20

30

40

50

60 Injured fibers (% total)

0

10

20

30

40

50

60non-trained trained passive trained isometric

* *

*

* different from non- -trained (p<0.05)

Microstructure

Modified from Squire, Muscle: Design, Diversity, and Disease Benjamin/Cummings, 1986Originally from Lazarides (1980) Nature 283:249-256.

• Proteins localized in the nucleus, cytosol, cytoskeleton, sarcolemma, and ECM.

Cohn and Campbell (2000) Muscle Nerve 23:1459-1471.

• Since the discovery of dystrophin, numerous genetic disease loci have been linked to protein products and to cellular phenotypes, generating models for studying the pathogenesis of the dystrophies.

Muscular Dystrophy:A frequently fatal disease of muscle deterioration• Muscular dystrophies have in the past been classified based on subjective and sometimes subtle differences in clinical presentation, such as age of onset, involvement of particular muscles, rate of progression of pathology, mode of inheritance.

(Some components of the dystrophin glycoproteincomplex are relativelyrecent discoveries, so onecannot assume that all players are yet known.)

DGC

dystrophindystroglycan ( and )sarcoglycans (, , , )syntrophins (, 1)dystrobrevins (, )sarcospanlaminin-2 (merosin)

Cohn and Campbell (2000) Muscle Nerve 23:1459-1471.

Dystrophin function: transmission of force to extracellular matrix