Chapter 9

Types of Muscle TissueDerived from mesodermSkeletal

Skeletal, striated, voluntary, multinucleatedFast, fatigued contractionsAdaptable (paperclip vs textbook)

SmoothVisceral, nonstriated, involuntary, uninucleatedSlow, sustained contractions

CardiacCardiac, striated, involuntary, uninucleatedContraction rate stabilized by pacemaker cells

Neural control can alterIntercalated discs

Functional CharacteristicsExcitability

Receive and respond to a stimulus (pH or NT) Electrical impulse along the sarcolemma

ContractibilityShorten and thicken w/ appropriate stimulation

ExtensibilityStretch or extend without damage

ElasticityReturn to normal shape after a stretch

Muscle FunctionProduce movement

Sk: locomotion, manipulation, and responseSm: squeeze substances throughCar: keep blood moving

Maintain posture and positionAdjustments to stay erect or seated despite gravity

ProtectionEncloses viscera and forms valves (control)

Generate heatContractions keep body temp at 98.6

Stabilize jointsPull on bones for movement, but strengthen joints

Gross Anatomy of Skeletal MuscleDiscrete organs of all 4 tissue typesNerves and blood

1 nerve, 1 artery, & 1+ vein per muscle Enter centrally; 1 nerve ending per muscle fiber (cell) Constant O2 and nutrients b/c contractions are high E

demandConnective tissue

Support and reinforce3 layers (internal to external)

Endomysium: cover muscle fiber Perimysium: cover fasicle Epimysium: cover muscle

AttachmentsDirect: epimysium fused to periosteumIndirect: epimysium beyond muscle = tendon

SarcolemmaSacroplasm

Glycogen, myoglobin, and mitochondriaMyofibrils

Actin (thin) and myosin (thick) proteins arranged into repeating sarcomeres

Sarcoplasmic reticulum (SR) Smooth ER surrounding myofibrils

TriadsTerminal cisternae

SR is enlarged and joins with T tubules; occur in pairsTransverse (T) tubules

Deep indentions of sarcolemma into sarcoplasm; conduct Ca2+ into cell

Microscopic Anatomy

Sarcomere OrganizationSmallest functional unit of skeletal muscle fiberA bands dark b/c contain thick and thin filaments

H zone is lighter middle because it lacks thin filaments

M line created by a protein that link thick filamentsI bands light b/c contain thin filaments only

Z line connect thin filaments together in a zig zag pattern Marks end of sarcomere

Zone of overlap6 thin surround 1 thick; 3 thick surround 1 thin

Thick filamentsBundles of myosin proteinsComposed of a rod-like tail and globular head

Head forms cross bridges; attach to site on actin; contain ATPases

Thin filamentsTwisted strands of F actin, composed of G actin

G actin contains ‘active sites’ where myosin can attachTropomyosin forms stiffening chains that cover ‘active sites’Troponin holds the tropomyosin in place

Changes shape to expose active sites

G-actin = pearl, F-actin = strand, tropomysin = strands together

Myofilament Structure

Sliding Filament TheoryDuring contraction,

sarcomeres (not filaments) shorten

Z lines closer, shortening sarcomere

H band and I band narrow

Zone of overlap increases

A band doesn’t change

Sliding MechanismCross bridges detached

Tropomyosin blocks ‘active sites’

Active site exposed cross bridge attachCa2+ binds troponin shape

changeMyosin head pivots toward M

line thin filaments to centerCross bridges detach and

mysoin reactivated ATP binds ATPase resets

Neuromuscular Junction (NMJ)Innervation of muscle fiber by an

axon terminal1 NMJ per muscle fiberMotor unit: motor neuron and

all muscle fibers innervated Fewer fibers = more precise Number determines strength of

muscleSeparated by a synaptic cleftAxon terminal houses synaptic

vesicles filled with acetylcholine (Ach) Impulse opens Ca2+ channels to

releaseMotor end plate is depression

in the sarcolemma for the axonContains Ach receptors

Propogates an action potential (AP)

http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure7m.jpg

Introduction to Action Potentials Resting membrane is polarized (charge separation)NT binds = opens gated ion channels (Na + and K +)Depolarize cell (less ‘–’ or more ‘+’) locally

Spreads throughout plasma membrane in wavesInitiates AP

Adjacent Na + open more depolarization to threshold

Na + close, K + open = repolarizationRefractory period because no stimuli can initiateResets electrical condition to resting state

Na + /K + pump restore ionic conditionAll or none response, b/c unstoppable once started

Excitation – Contraction CouplingStimuli releases ACh, depolarizes

end plateAP propagated down T tubulesTermianl cisternae of SR release

Ca 2+

Electrical signal raise Ca2+ levels by opening Ca2+ channels

Ca2+ binds troponin, removing tropomyosin block

Contraction occurs (see earlier)Ca2+ levels decrease, tropomyosin

replaced = relaxationATP dependent Ca2+ pump into SR

Repeat with stimulation

Skeletal Muscle ContractionsMuscle Tone

Alternating active motor units while muscle at rest

No active movements producedStabilize joints and maintain postureEnsure response ready

IsotonicTension constant as muscle length

changesIsometric

Tension increases to peak, but muscle length unchanged Moving a load greater than developable

tensionConcentric : force w/shorteningEccentric : force w/lengthening

(gastrocnemius & hills)

Muscle TwitchResponse to a single stimulationQuick contract, relax cycle in 3 phasesLatent period

Excitation – coupling is occurringMuscle tension increases, but no

contractionContraction period

Cross bridges activePeak tension, muscle shortens

Relaxation periodReentry Ca2+ into SRMuscle tension to zero, resting

Varies between muscle typesStrength depends on # of motor units,

recruitment

Contraction varies depending on circumstanceWave summation (time)2 stimuli in rapid succession = larger contraction 2nd

timeRefractory period unalteredTetanus (speed)Sustained contraction w/ or w/o partial relaxationUnfusedFused

Treppe Increase tension with repeated contractionsWarming up stronger later to same stimulus

Graded Muscle Responses

Muscle Metabolism

Muscle DisordersMyasthenia gravis: autoimmune disease, loss of Ach

receptorsRigor mortis: ATP depletion prevents cross bridge

detachmentAtrophy: degeneration of muscle from disuseDuchenne muscular dystrophy: sex-linked disease

that destroys muscleHernia: organ protrudes through abdominal wallMyalgia: muscle painFibromyositis: inflammation of a muscle and CT

coveringsStrain: excessive stretching and tearing of muscle or

tendon