• Produce skeletal movement • Maintain posture and body position • Support soft tissues • Guard entrances and exits • Maintain body temperature • Store nutrient reserves
Skeletal Muscle • usually attached to bones • under conscious control • striated
Cardiac Muscle
Smooth Muscle
• wall of heart • not under conscious control • striated • Intercalated discs
• walls of most viscera, blood vessels, skin • not under conscious control • not striated 1
Structure of a Skeletal Muscle
Connective Tissue Coverings
Skeletal Muscle • organ of the muscular system - skeletal muscle tissue - nervous tissue - blood - connective tissues • fascia-tissue that surrounds the entire muscle • tendons-dense connective tissue cord that connect muscles to bones • aponeuroses-sheet-like tendon that connects muscles to bones
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Skeletal Muscle Fibers
• epimysium-membrane that surrounds the whole muscle • perimysium-membrane that surrounds fascicles • fascicles-groups of muscle fibers(cells) • endomysium-membrane that that surrounds individual muscle cells • muscle fibers-muscle cells • myofibrils-protein fibers inside muscle cells • thick and thin filaments
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Skeletal Muscle Fibers
• sarcolemma-cell membrane of a
muscle fiber • sacroplasm-cytoplasm of a muscle fiber • sarcoplasmic reticulum-stores calcium ions • transverse tubule-carries muscle impulse from sarcolemma to searcplasmic reticulum • triad • pair of cisternae of sarcoplasmic reticulum • transverse tubule
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• Myofibril-proteins fibers • actin filaments-thin filaments • myosin filamentsthick filaments • sarcomere-functional unit of skeletal muscle 5
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Sarcomere
Myofilaments
• I bands-thin only • A bands-thick and thin • H band-thick only • Z lines-edge of sarcomer • M line-mid-line of sarcomere • zone of overlapwhere thick and thin filaments overlap
Thick Filaments • composed of myosin • cross-bridges
Thin Filaments • composed of actin • associated with troponin and tropomyosin
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Neuromuscular Junction
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Neuromuscular Junction
• also known as myoneural junction
• Synapse-area where the neuron communicates with the muscle fiber
• site where an axon and muscle fiber meet
• synaptic cleft-space between neuron and sarcolemma
• motor neuron-neuron that controls a muscle fiber
• synaptic vesicles-store and release neurotransmitters in neuron
• motor end plate-area of muscle fiber that forms synapse (junction) with motor neuron
• neurotransmitters-chemical messengers that carry the signal across the synapse 9
Motor Unit
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Stimulus for Contraction
• single motor neuron • all muscle fibers controlled by motor neuron
• acetylcholine
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(ACh) • nerve impulse causes release of ACh from synaptic vesicles • ACh binds to ACh receptors on motor end plate • generates a muscle impulse • muscle impulse eventually reaches sarcoplasmic reticulum and the cisternae
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Sliding Filament Model of Muscle Contraction
Excitation Contraction Coupling • muscle impulses cause sarcoplasmic reticulum to release calcium ions into cytosol • calcium binds to troponin to change its shape • position of tropomyosin is altered • binding sites on actin are exposed • actin and myosin molecules bind
• When sarcromeres
shorten, thick and thin filaments slide past one another • H zones and I bands narrow • Z lines move closer together 13
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Cross-bridge Cycling
Relaxation
• myosin cross-bridge attaches to actin binding site • myosin cross-bridge pulls thin filament
• acetylcholinesterase – rapidly decomposes Ach remaining in the synapse • muscle impulse stops
• ADP and phosphate released from myosin
• stimulus to sarcolemma and muscle fiber membrane ceases
• new ATP binds to myosin
• calcium moves back into sarcoplasmic reticulum
• linkage between actin and myosin cross-bridge break
• myosin and actin binding prevented
• ATP splits • myosin cross-bridge goes back to original position
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Major Events of Muscle Contraction and Relaxation
• muscle fiber relaxes • Rigor mortis-muscle contract and stay contracted as calcium leaks 16 out and there is no ATP to break the cross bridges
Energy Sources for Contraction 1) Creatine phosphate
2) Cellular respiration
• creatine phosphate – stores energy that quickly converts ADP to ATP
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Oxygen Supply and Cellular Respiration
Oxygen Debt Oxygen debt – amount of oxygen needed by liver cells to use the
• Oxygen is needed to completely breakdown glucose • myoglobin
accumulated lactic acid to produce glucose plus the amount muscle cells require to resynthesize ATP and creatine phosphate and restore their original conditions
• Anaerobic Phase
• oxygen not available • glycolysis continues • pyruvic acid converted to lactic acid • liver converts lactic acid to glucose • Anaerobic threshold or lactic acid threshold
• glycolysis • occurs in cytoplasm • produces little ATP
• Aerobic Phase • citric acid cycle • electron transport chain • occurs in the mitochondria • produces most ATP • myoglobin stores extra oxygen
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Muscle Fatigue
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Heat Production • by-product
• inability to contract after persistent, prolonged use • commonly caused from • decreased blood flow • ion imbalances across the sarcolemma • accumulation of lactic acid (most common cause) • Psychological loss of desire to continue the exercise
of cellular respiration
• muscle cells are major source of body heat • Helps maintain body temperature • blood transports heat throughout body
Threshold Stimulus • minimal strength required to cause contraction Recording a Muscle Contraction • myogram • twitch • latent period • period of contraction • period of relaxation • refractory period • all-or-none response 23
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Summation
Recruitment of Motor Units
• process by which individual twitches combine • produces sustained contractions • can lead to tetanic contractions
• recruitment - increase in the number of motor units activated • whole muscle composed of many motor units • more precise movements are produced with fewer muscle fibers within a motor unit • as intensity of stimulation increases, recruitment of motor units continues until all motor units are activated 25
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Types of Contractions
Sustained Contractions
• isotonic – muscle contracts and changes length
• smaller motor units (smaller diameter axons) - recruited first
• eccentric – lengthening contraction
• concentric – shortening contraction • isometric – muscle contracts but does not change length
• larger motor units (larger diameter axons) - recruited later • produce smooth movements • muscle tone – continuous state of partial contraction 27
Smooth Muscle Fibers
Fast and Slow Twitch Muscle Fibers Slow-twitch fibers (type I) • always oxidative • resistant to fatigue • red fibers • most myoglobin • good blood supply • Many mitochondria • aerobic Fast-twitch glycolytic fibers (type IIb) • white fibers (less myoglobin) • poorer blood supply • susceptible to fatigue • Few mitochondria • anaerobic
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Fast-twitch fatigueresistant fibers (type IIa) • intermediate fibers • oxidative • intermediate amount of myoglobin • pink to red in color • resistant to fatigue
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Compared to skeletal muscle fibers • shorter • single, centrally located nucleus • elongated with tapering ends • myofilaments randomly organized • lack striations • lack transverse tubules • sarcoplasmic reticula not well developed
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Types of Smooth Muscle Visceral Smooth Muscle • single-unit smooth muscle • sheets of muscle fibers • fibers held together by gap junctions • exhibit rhythmicity • exhibit peristalsis • walls of most hollow organs
Smooth Muscle Contraction
Multiunit Smooth Muscle • less organized • function as separate units • fibers function separately • irises of eye • walls of blood vessels
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Cardiac Muscle
• Resembles skeletal muscle contraction • interaction between actin and myosin • both use calcium and ATP • both are triggered by membrane impulses
• Different from skeletal muscle contraction • smooth muscle lacks troponin • smooth muscle uses calmodulin • two neurotransmitters affect smooth muscle • acetlycholine and norepinephrine • hormones affect smooth muscle • stretching can trigger smooth muscle contraction • smooth muscle slower to contract and relax • smooth muscle more resistant to fatigue • smooth muscle can change length without changing tautness
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Characteristics of Muscle Tissue
• located
only in the heart • striated muscle fibers joined together by intercalated discs • Single nucleus per cells • Cisternae are less developed and store less calcium transverse tubules are larger • fibers branch • network of fibers contracts as a unit • self-exciting and rhythmic • longer refractory period than skeletal muscle
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Body Movement
Skeletal Muscle Actions
Four Basic Components of Lever 1. rigid bar – bones 2. fulcrum – point on which bar moves; joint 3. object - moved against resistance; weight 4. force – supplies energy for movement; muscles
• origin – immovable end • insertion – movable end • prime mover (agonist) – primarily responsible for movement • synergists – assist prime mover • antagonist – resist prime mover’s action and cause movement in the opposite direction
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Levers and Movement
Life-Span Changes • Changes in muscular system first begin to appear in one’s 40’s • myoglobin, ATP, and creatine phosphate decline • by age 80, half of muscle mass has atrophied • adipose cells and connective tissues replace muscle tissue • exercise helps to maintain muscle mass and function 37
Clinical Application
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Clinical Application
Myasthenia Gravis • autoimmune disorder • receptors for ACh on muscle cells are attacked • weak and easily fatigued muscles result • difficulty swallowing and chewing • ventilator needed if respiratory muscles are affected • treatments include • drugs that boost ACh • removing thymus gland • immunosuppressant drugs • antibodies
• tenanus - sustained powerful contractions of skeletal muscles throughout the body • Caused by Clostridium tetani • Lockjaw • Tetanus shot-vaccine against the toxin