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Chapter 9

ch. 9 muscle notes.doc

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ENGL 1010
Laura Sterrett

• Muscle fibers- elongated muscles, only in skeletal and smooth muscle cells • Myo and mys- muscle • Sarco- flesh, refers to muscle • Skeletal muscle tissue- skeletal muscle fibers that attach to and cover bony skeleton, longest muscle cells, controlled voluntarily, have stripes/striations • Voluntary muscle- only type of muscle subject to conscious control, can also be activated by reflexes • Cardiac muscle tissue- in the heart, striated, involuntary movement • Smooth muscle tissue- found in walls of hollow visceral organs, forces fluids and other substances through body channels • Excitability-also known as responsiveness or irritability, property of muscle tissue, ability to receive and respond to a stimulus, usually a chemical, generates electrical impulse along sarcolemma, causing cell to contract • Sarcolemma- plasma membrane of muscle cell • Contractility- ability to shorten forcibly when adequately stimulated • Extensibility- ability to be stretched or extended • Elasticity- ability of muscle fiber to recoil and resume resting length after stretch • Properties of muscle tissue- excitability, contractility, extensibility, elasticity • Functions of muscle tissue- produces movement, maintains posture, stabilizes joints, and generates heat, protect more fragile internal organs, forms valves • Skeletal muscle accounts for at least 40% of body mass • Skeletal muscle- discrete organ made of several kinds of tissues, includes skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue, cannot contract without neural stimulation • Every muscle is usually served by one nerve, one artery, and one or more veins • Endomysium- fine sheath of connective tissue made of areolar and reticular fibers (within the muscle) that surrounds each individual muscle fiber • Fascicles- bundles of endomysium-wrapped muscle fibers • Perimysium- layer of fibrous connective tissue that surrounds each fascicle • Epimysium- dense irregular connective tissue surrounding entire muscle • Muscle contraction is pulling on sheaths which pull the bone to be moved • Origin- bone that is less movable that the muscle is connected to • Insertion- bone that is more movable that the muscle is connected to • Direct attachment- fleshy, epimysium of muscle fused to periosteum of bone or perichondrium of cartilage • Indirect attachments- tissue wrappings extend as a tendon or aponeurosis • Tendon/aponeurosis- anchors muscle to connective tissue covering of bone or cartilage or to fascia of other muscles, tough, small, more common • Sarcoplasm- similar to cytoplasm, but in a muscle fiber, contains large numbers of glycosomes which are stored glycogen, and myoglobin • Myoglobin- red pigment that stores oxygen • Myofibrils- dense, hundreds in a single muscle fiber, 80% of muscle cell volume, contain contractile elements of skeletal muscle cells • A and I bands- dark A bands aligned with light I bands to create muscle striations • H zone- bisected vertically by M line, appears less dense because thin filaments don’t extend into this region • M line- bisects H zone vertically • Z disc- midline interruption of I band, darker • Sarcomere- region of a myofibril between two successive Z discs, smallest contractile unit • Myofilaments- two types are thick and thin filaments • Thick filaments- extend entire length of A band, about 300 myosin molecules per filament, heads have ATPase enzymes to split ATP and generate energy for contraction • Thin filaments- extend across I band partway into A band, anchored by Z disc, six thin filaments surround each thick filament, composed of actin • Myosin- compose most thick filaments, rod like tail and two interwoven globular heads • Cross bridges- heads of myosin link thick and thin filaments together • Actin- compose thin filaments, chief place where myosin heads attach for contraction • Tropomyosin- in thin filament, stiffen actin core by spiraling around it in pairs, block myosin binding sites on actin so myosin heads can’t bind • Troponin- three-polypeptide complex, one of which (TnI) is inhibitory subunit that binds to actin, TnT binds to tropomyosin to position it on actin, TnC binds calcium ions • Tropomyosin and troponin help control myosin- actin interactions • Elastic filament- composed of titin, extends from Z disc to thick filament, attaches to M line, holds thick filaments in place, assists muscle cell to spring back into shape • Titin- giant protein that makes of elastic filament • Dystrophin- links thin filaments to integral proteins or sarcolemma • Sarcoplasmic reticulum (SR)- smooth endoplasmic reticulum, consists of tubules mostly running longitudinally along myofibrils, and terminal cisternae, regulates intracellular levels of ionic calcium, stores and releases Ca on demand when stimulated to contract • Terminal cisternae- occur in pairs, perpendicular cross channels at A band – I band junctions, part of sarcoplasmic reticulum • T tubule- sarcolemma penetrates cell interior at each A band – I band junction, encircle each sarcomere • Triads- successive groupings of three membranous structures including terminal cisterna, T tubule, and terminal cisterna • Contraction- tension generated by cross bridges on thin filaments exceeds forces opposing shortening • Sliding filament theory of contraction- thin filaments slide past thick ones so actin and myosin filaments overlap to a higher degree • Action potential- along sarcolemma, causes short-lived rise in intracellular calcium levels which is the final trigger for contraction • Neuromuscular junction- axon of each motor neuron divides as it enters muscles, each axon ending has short branches to form neuromuscular junction with single muscle fiber • Synaptic cleft0 rich in glycoproteins and collagen fibers • Synaptic vesicles- small membranous sacs containing neurotransmitter acetylcholine • Motor end plate- trough like part of muscle fiber’s sarcolemma that helps form neuromuscular junction, highly folded • Ach receptors- located in millions of junctional folds that have a high surface area at motor end plate • Acetylcholinesterase- beaks down Ach into acetic acid and choline, prevents continued muscle fiber contraction in absence of additional neural stimulation • Resting sarcolemma is polarized with inside being negative • Depolarization- more sodium diffuses into muscle cell than potassium diffuses out, interior of sarcolemma becomes less negative • 1. Membrane areas by depolarized motor end plate are depolarized, opens voltage-gated sodium channels to initiate action potential 2. Action potential moves along sarcolemma, local depolarization wave spreads, sodium ions continue to diffuse in 3. Repolarization, sodium channels close and voltage-gated potassium channels open • Refractory period- cell cannot be stimulated again until repolarization is complete • Repolarization restores electrical conditions of resting state • Sodium potassium pump restores ionic conditions or resting state • Contraction lasts about 100 ms or more and outlasts electrical event that triggers it because active transport of calcium ions back into the SR takes longer than the release • Excitation-contraction coupling- transmission of an action potential along sarcolemma leads to sliding myofilaments, events occur between action potential and beginning of mechanical activity 1. 1. Action potential goes down sarcolemma and T tubules 2. AP in T tubules of triads causes activates voltage-sensitive receptors, trigger calcium release from terminal cisternae of SR into cytosol 3. Calcium ions bind to troponin and troponin changes shape, removes blocking action of tropomyosin to expose actin active sites 4. Myosin heads attach and detach to actin pulling actin filaments towards sarcomere center, powered by ATP 5. Calcium removed by active transport after action potential ends 6. Tropomyosin reblocks actin sites, contraction ends • Muscle fiber contraction: 1. 1. Myosin binds to actin sites to form cross bridge 2. Phosphate is released to myosin goes from high energy state to low energy state, pulling thin fila
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