chapter 9. types of muscle tissue derived from mesoderm skeletal skeletal, striated, voluntary,...
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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