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1

Structure andFunction ofExercising Muscle

chapter 

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Skeletal

Voluntary muscle; controlled consciouslyOver 600 throughout the body

Cardiac

Controls itself with assistance from the

nervous and endocrine systems

Only in the heart

Smooth

Involuntary muscle; controlled unconsciously In the walls of blood vessels and internal

organs

Types of Muscles

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The Basic Structure of Muscle

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Structure of a Single Muscle Fiber 

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Key oints

The Muscle Fiber •  An individual muscle cell is called a muscle fiber.

•  A muscle fiber is enclosed by a plasma

membrane called the sarcolemma.• he cytoplasm of a muscle fiber is called a

sarcoplasm.

• !ithin the sarcoplasm" the #tubules allow

transport of substances throughout the musclefiber and the sarcoplasmic reticulum stores

calcium.

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!n Electron Micrograph of Myofibrils

© Custom Medical Stock Photo

$yofibrils

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!lpha Motor "eurons

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Key oints

The Myofibril• $yofibrils are made up of sarcomeres" the smallest

functional units of a muscle.

•  A sarcomere is composed of filaments of twoproteins" myosin and actin" which are responsible

for muscle contraction.

• $yosin is a thic% filament with a globular head at

one end.•  An actin filament&composed of actin" tropomyosin"

and troponin&is attached to a '#dis%.

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E#ents $eading toMuscle Fiber Contraction

(. A motor neuron releases acetylcholine )ACh*.

+. ACh binds to receptors on the sarcolemma.

,. he action potential triggers release of Ca+

.-. he Ca+ binds to troponin on the actin filament" and

the troponin pulls tropomyosin off the active sites"allowing myosin heads to attach to the actinfilament.

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E%E"TS $E!&'"( T) M*SC$E !CT')"

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!++!"(EME"T )F F'$!ME"TS'" ! S!+C)ME+E

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C)"T+!CT'"( M*SC$E F'BE+

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!CT'" F'$!ME"T

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! Sarcomere in its +elaxed andContracted State

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The Sliding Filament Theory

• !hen myosin cross#bridges are activated" they bind

strongly with actin" resulting in a change in the cross#

bridge.

• he change in the cross#bridge causes the myosinhead to tilt toward the arm of the cross#bridge and drag

the actin and myosin filaments in opposite directions.

• he tilt of the myosin head is %nown as a power stroke.

• he pulling of the actin filament past the myosin resultsin muscle shortening and generation of muscle force.

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Key oints

Muscle Fiber Contraction• $uscle action is initiated by a nerve impulse.• he nerve releases ACh" which allows sodium to

enter and depolarie the cell. If the cell is

sufficiently depolaried" an action potential occurs"which releases stored Ca+ ions.

• Ca+ ions bind with troponin" which lifts thetropomyosin molecules off the active sites on theactin filament. hese open sites allow the myosin

heads to bind to them.

(continued)

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Key oints (continued)

Muscle Fiber Contraction• Once myosin binds with actin" the myosin head tilts

and pulls the actin filament so they slide across each

other.• $uscle contraction ends when calcium is pumped

out of the sarcoplasm to the sarcoplasmic reticulum

for storage.

•

/nergy for muscle contraction is provided when themyosin head binds to A. Aase on the myosin

head splits the A into a usable energy source.

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1ollow needle is inserted into muscle to ta%e a sample.

2ample is mounted" froen" thinly sliced" and e3aminedunder a microscope.

 Allows study of muscle fibers and the effects of acutee3ercise and e3ercise training on fiber composition.

Muscle Biopsy

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! hotomicrograph Sho,ing Type '- Type''a- and Type ''b Muscle Fibers

Photo provided courtesy of the authors.

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Type ' .Slo,/T,itch0 Muscle Fibers

• 1igh aerobic )o3idative* capacity and fatigueresistance

• 4ow anaerobic )glycolytic* capacity and motor unit

strength• 2low contractile speed )((0 ms* and myosin

 Aase

• (0 to (50 fibers per motor neuron

• 4ow sarcoplasmic reticulum development

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Type ''a .Fast/T,itch Type a0Muscle Fibers

• $oderate aerobic )o3idative* capacity and fatigueresistance

• 1igh anaerobic )glycolytic* capacity and motor unit

strength• ast contractile speed )70 ms* and myosin Aase

• ,00 to 500 fibers per motor neuron

• 1igh sarcoplasmic reticulum development

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Type ''b .Fast/T,itch Type b0Muscle Fibers

• 4ow aerobic )o3idative* capacity and fatigueresistance

• 1igh anaerobic )glycolytic* capacity and motor unit

strength• ast contractile speed )70 ms* and myosin Aase

• ,00 to 500 fibers per motor neuron

• 1igh sarcoplasmic reticulum development

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&id ou Kno, 2 2 2 3

he difference in force development between type I

and type II motor units is due to the number of muscle

fibers per motor unit" not the force generated by each

fiber.

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4hat &etermines Fiber Type3

• 8enetics determine which motor neurons innervateindividual muscle fibers.

• $uscle fibers become specialied according to the

type of neuron that stimulates them.• /ndurance training and muscular inactivity may

result in small changes in the percentage of type Iand type II fibers.

•  Aging may alter the distribution of type I and type II

fibers.

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Key oints

Type ' and Type '' Muscle Fibers• 2%eletal muscles contain both type I and type II fibers.•  Aase in type II fibers acts faster" providing energy

for muscle action more 9uic%ly than Aase in type I

fibers.• ype II fibers have a more developed sarcoplasmic

reticulum" enhancing calcium delivery.

(continued)

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Key oints (continued)

Type ' and Type '' Muscle Fibers• $otor units supplying type II fibers are larger

than those supplying type I fibers; thus" type

II motor units can recruit more fibers.• ype I fibers have high aerobic endurance

and are suited to low#intensity enduranceactivities.

• ype II fibers are better for anaerobic ore3plosive activities.

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The !ll/or/"one/+esponse

• or a motor unit to be recruited into activity" themotor nerve impulse must meet or e3ceed thethreshold.

•

!hen this occurs" all muscle fibers in the motor unitact ma3imally.

• If the threshold is not met" no fibers in that unit act.

• $ore force is produced by activating more motorunits.

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Functional Classifications ofMuscles

!gonists are prime movers; they:re responsible for the

movement.

!ntagonists oppose the agonists to prevent

overstretching of them.

Synergists assist the agonists and sometimes fine#tune

the direction of movement.

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TES )F M*SC$E !CT')"

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Factors 'nfluencing Force (eneration

• umber of motor units activated

• ype of motor units activated ) or 2*

• $uscle sie

• Initial muscle length

•

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+!M$'KE +EC+*'TME"T )F F'BE+S

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Key oints

*se of Muscles• $uscles involved in movement can be classified as

agonists" antagonists" and synergists.

•

hree types of muscle action are concentric" static"and eccentric.

• orce production is increased by recruiting moremotor units.

(continued)

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Key oints (continued)

*se of Muscles•  All =oints have an optimal angle at which the muscles

crossing the =oint produce ma3imal force.

•

he angle of ma3imal force depends on the relativeposition of the muscle:s insertion on the bone and theload placed on the muscle.

• 2peed of action affects the amount of force produced.

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+elationship Bet,een Muscle $engtheningand Shortening %elocity and Force

roduction