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39 Cards in this Set
- Front
- Back
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Skeletal Muscle
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Each muscle is served by one artery, onenerve, and one or more veins
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Endomysium
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fine areolar connective tissue surrounding each muscle fiber (cell)
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Perimysium
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fibrous connective tissuesurrounding fascicles (groups of muscle fibers)
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Epimysium
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dense regular connective tissuesurrounding entire muscle
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Skeletal Muscle: Attachments
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Muscles attach: Indirectly— connective tissue wrappingsextend beyond the muscle as a ropeliketendon to bone (or sheetlike aponeurosis) Directly— epimysium of muscle is fused tothe periosteum of bone or perichondriumof cartilage
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Microscopic Anatomy of a Skeletal MuscleFiber (Cell)
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Cylindrical cell 10 to 100 m in diameter, up to30 cm long
PACKED with Myofibrils (very important toremember) Multiple peripheral nuclei Many mitochondria |
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Myofibrils
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Densely packed, rodlike elements
• ~80% of cell volume • Exhibit striations: perfectly alignedrepeating series of dark A bands and lightI bands |
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Sarcomere...think Zzz
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• Smallest contractile unit (functional unit)of a muscle fiber
• The region of a myofibril between twosuccessive Z discs • Composed of thick and thin myofilamentsmade of contractile proteins Each sarcomere extends fromone Z disc to the next. |
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Features of a Sarcomere
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Z disc: coin-shaped sheet of proteins that:
1) anchors the thin filaments 2) connects myofibrils to one another Thick filaments: run the entire length of an A band Thin filaments: run the length of the I band and partway into the A band H zone: lighter midregion where filaments do not overlap M line: line of protein myomesin that holds adjacent thick filaments together |
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Ultrastructure of Thick Filament
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• Composed of the protein myosin
• Myosin tails contain: • 2 interwoven, heavy polypeptide chains • Myosin heads contain: Can bind to thin filaments Binding sites for ATP can be thought of as ATPase enzymes |
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Ultrastructure of Thin Filament
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-Twisted double strand of fibrous protein composed of G actin proteins = F actin
-F actin consists of G (globular) actin subunits -Each G actin bears active sites for myosinhead attachment during contraction -Tropomyosin and troponin: regulatoryproteins bound to actin |
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Sarcoplasmic Reticulum (SR)
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• Network of smooth endoplasmic reticulumsurrounding each myofibril
• Functions in the regulation of intracellularCa2+ levels • SR wraps around sarcomere at terminalcisternae |
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T Tubules
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-Continuous with the sarcolemma (i.e. part of the cell membrane)
- PATHWAY to the extracellular environment! -Penetrate the cell’s interior at each A band–I band junction onevery sarcomere - Associate with the paired terminal cisternae to form triads thatencircle each sarcomere |
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Triad Relationships
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• T tubules conduct impulses deep intomuscle fiber
• T tubule and SR cisternae membranes areLOADED with integral membrane proteins: T tubule proteins: voltage sensors SR proteins: channels that regulate Ca2+ release from the SR cisternae into the CYTOPLASM |
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Contractions
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• The generation of force
• Does not necessarily cause shortening of the muscle fiber • Shortening of the muscle fiber occurswhen tension generated by cross bridgeson the thin filaments exceeds forcesopposing shortening |
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Sliding Filament Model of Contraction
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• During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line
• As H zones shorten and disappear, sarcomeres shorten, musclecells shorten, and the whole muscle shortens |
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Requirements for Skeletal MuscleContraction
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1. Activation: neural stimulation at aneuromuscular junction
2. Excitation-contractioncoupling: Generation and propagation of an action potential along the sarcolemma Final trigger: a brief rise in intracellularCa2+ levels |
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Events at the Neuromuscular Junction
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-Skeletal muscles are stimulated bysomatic motor neurons
-Axons of motor neurons travel from thecentral nervous system via nerves toskeletal muscles -Each axon forms several branches as itenters a muscle -Each axon branch forms a neuromuscularjunction with a single muscle cell |
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Neuromuscular Junction
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-Axon terminal and muscle fiber areseparated by a gel-filled space called thesynaptic cleft
- Synaptic vesicles of axon terminal containthe neurotransmitter acetylcholine (ACh) - Junctional folds of the sarcolemma containACh receptors • Nerve impulse arrives at axon terminal • ACh is released and binds with receptors on the sarcolemma • Electrical events lead to the generation ofan action potential |
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Destruction of Acetylcholine
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• ACh effects are quickly terminated by theenzyme acetylcholinesterase
• Prevents continued muscle fibercontraction in the absence of additionalstimulation |
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Events in Generation of a MUSCLE FIBERAction Potential
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1. Local muscle fiber depolarization = endplate potential: ACh binding opens chemically (ligand)gated ion channels
- Simultaneous diffusion of Na+ (inward)and K+ (outward) More Na+ diffuses, so the interior of thesarcolemma becomes less negative • Local depolarization – end plate potential 2. Generation and propagation of an actionpotential: - End plate potential spreads to adjacentmembrane areas - Voltage-gated Na+ channels openNa+ influx decreases the membrane voltage toward a critical threshold -If threshold is reached, an action potential is generated |
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MUSCLE FIBER Action Potential movesalong the MUSCLE FIBER
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-Local depolarization wave continues tospread, changing the permeability of thesarcolemma
-Voltage-regulated Na+ channels open inthe adjacent patch, causing it to depolarizeto threshold |
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Events in Generation of a MUSCLE FIBERAction Potential
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3. Repolarization:
- Na+ channels close and voltage-gated K+ channels open -K+ efflux rapidly restores the restingpolarity - MUSCLE FIBER cannot be stimulatedand is in a refractory period untilrepolarization is complete -Ionic conditions of the resting state arerestored by the Na+-K+ pump |
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Excitation-Contraction (E-C) Coupling
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• Sequence of events by which transmissionof a MUSCLE FIBER action potential alongthe sarcolemma leads to sliding of themyofilaments
• Latent period: Note this is NOT refractory period! • Time when E-C coupling events occur • Time between AP initiation and thebeginning of contraction |
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Events of Excitation-Contraction (E-C)Coupling
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- AP is propagated along sarcomere to Ttubules; remember T tubules represent theOUTSIDE of the cell
- Voltage-sensitive proteins stimulate Ca2+release from sarcoplasmic reticulum (SR) • Ca2+ is necessary for contraction |
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Role of Calcium (Ca2+) in MUCSLE FIBER
Contraction |
• At low intracellular Ca2+ concentration:
• Tropomyosin blocks the active sites on actin • Myosin heads cannot attach to actin • Muscle fiber relaxes |
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Role of Calcium (Ca2+) in Contraction
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• At higher intracellular Ca2+ concentrations:
• Ca2+ binds to troponin • Troponin changes shape and movestropomyosin away from active sites • Events of the cross bridge cycle occur • When action potential stimulation ceases,Ca2+ is pumped back into the SR andcontraction ends |
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Cross Bridge Cycle
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• Continues as long as the Ca2+ signal andadequate ATP are present
• Cross bridge formation—high-energymyosin head attaches to thin filament • Working (power) stroke—myosin headpivots and pulls thin filament toward Mline • Cross bridge detachment— ATP attaches tomyosin head and the cross bridgedetaches • “Cocking” of the myosin head— energy fromhydrolysis of ATP cocks the myosin head(changes the proteins shape) into thehigh-energy state |
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Muscle Mechanics
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1. Same principles apply to contraction of a single fiber and a whole muscle
2. Contraction produces tension, the forceexerted on the load or object to bemoved 3. Isometric and Isotonic Contraction 4. Force and duration of contraction vary inresponse to stimuli of differentfrequencies and intensities |
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Isometric contraction
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no shortening;muscle tension increases but does notexceed the load
- Tension increasesto the muscle’s capacity, but the muscleneither shortens nor lengthens |
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Isotonic contraction
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-muscle shortensbecause muscle tension exceeds the load
- Muscle changes inlength and moves the load |
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Motor unit
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- a motor neuron and all (fourto several hundred) muscle fibers itsupplies
- All neurons outside the central nervoussystem conduct impulses along hairlikecytoplasmic extensions, the axon. Theaxons connecting your spinal cord to yourfoot can be as much as 1 m long (althoughonly a few micrometers in diameter). -Muscle fibers from a motor unit are spreadthroughout the muscle so that a singlemotor unit causes weak contraction ofentire muscle -Motor units in a muscle usually contractasynchronously; helps prevent fatigue |
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Small or Large Motor Units
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• Small motor units in muscles that controlfine movements (fingers, eyes)
• Large motor units in large weight-bearingmuscles (thighs, hips) |
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Muscle Twitch
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-Response of a muscle to a single, briefthreshold stimulus (involves electricalstimulation of ONE MOTER UNIT)
• A single stimulus results in a singlecontractile response—a muscle twitch |
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Three Phases of a Muscle Twitch
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• Three phases of a twitch:
1. Latent period: events of excitation- contraction coupling 2. Period of contraction: cross bridgeformation; tension increases 3. Period of relaxation: Ca2+ reentry into theSR; tension declines to zero *Different strength and duration of twitchesare due to variations in metabolic propertiesand enzymes between muscles |
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Graded Muscle Responses
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-Variations in the degree of musclecontraction
- Responses are graded by: 1. Changing the frequency of stimulation 2. Changing the strength of the stimulus |
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Response to Change in Stimulus Frequency
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• Increase frequency of stimulus (muscle doesnot have time to completely relax betweenstimuli)
• Ca2+ release stimulates further contraction temporal (wave) summation • Further increase in stimulus frequency unfused (incomplete) tetanus • If stimuli are given quickly enough, fused(complete) tetany results |
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Threshold stimulus
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-stimulus strength atwhich the first observable muscle contractionoccurs
- Contraction force is precisely controlled byrecruitment (multiple motor unit summation),which brings more and more muscle fibersinto action |
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Response to Change in Stimulus Strength
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• The recruitment process is not random!Instead it is dictated by the Size Principle
• Size principle: as the intensity of the electricstimulus increases to the whole muscle,motor units with larger and larger musclefibers (yes, larger cells) are recruited • The largest Motor units are controlled bythe largest, least excitable neurons |
