Chapter 9 Muscular System Three Types of Muscle Tissues Skeletal Muscle • usually attached to bones • under conscious control • striated

Cardiac Muscle • wall of heart • not under conscious control • striated •Intercalated discs Smooth Muscle • walls of most viscera, blood vessels, skin • not under conscious control • not striated 1

Functions of Skeletal Muscle •Produce skeletal movement •Maintain posture and body position •Support soft tissues •Guard entrances and exits •Maintain body temperature •Store nutrient reserves 2

Structure of a Skeletal Muscle 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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Connective Tissue Coverings • 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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Skeletal Muscle Fibers •Myofibril-proteins fibers • actin filaments-thin filaments • myosin filamentsthick filaments • sarcomere-functional unit of skeletal muscle 6

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Sarcomere • 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 7

Myofilaments Thick Filaments • composed of myosin • cross-bridges

Thin Filaments • composed of actin • associated with troponin and tropomyosin

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Neuromuscular Junction • also known as myoneural junction • site where an axon and muscle fiber meet • motor neuron-neuron that controls a muscle fiber • motor end plate-area of muscle fiber that forms synapse (junction) with motor neuron

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Neuromuscular Junction •Synapse-area where the neuron communicates with the muscle fiber • synaptic cleft-space between neuron and sarcolemma • synaptic vesicles-store and release neurotransmitters in neuron • neurotransmitters-chemical messengers that carry the signal across the synapse

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Motor Unit • single motor neuron • all muscle fibers controlled by motor neuron

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Stimulus for Contraction • acetylcholine (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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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

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Sliding Filament Model of Muscle Contraction • When sarcromeres

shorten, thick and thin filaments slide past one another • H zones and I bands narrow • Z lines move closer together 14

Cross-bridge Cycling • myosin cross-bridge attaches to actin binding site • myosin cross-bridge pulls thin filament •ADP and phosphate released from myosin • new ATP binds to myosin • linkage between actin and myosin cross-bridge break •ATP splits •myosin cross-bridge goes back to original position

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Relaxation • acetylcholinesterase – rapidly decomposes Ach remaining in the synapse • muscle impulse stops • stimulus to sarcolemma and muscle fiber membrane ceases • calcium moves back into sarcoplasmic reticulum • myosin and actin binding prevented • 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

Major Events of Muscle Contraction and Relaxation

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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 is needed to completely breakdown glucose •myoglobin

• Anaerobic Phase • 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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Oxygen Debt Oxygen debt – amount of oxygen needed by liver cells to use the accumulated lactic acid to produce glucose plus the amount muscle cells require to resynthesize ATP and creatine phosphate and restore their original conditions • oxygen not available • glycolysis continues • pyruvic acid converted to lactic acid • liver converts lactic acid to glucose •Anaerobic threshold or lactic acid threshold 20

Muscle Fatigue • 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 • cramp – sustained, painful, involuntary muscle contraction 21

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Heat Production • by-product

of cellular respiration

• muscle cells are major source of body heat •Helps maintain body temperature •blood transports heat throughout body 22

Muscular Responses 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

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Length-Tension Relationship

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Summation • process by which individual twitches combine • produces sustained contractions • can lead to tetanic contractions

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Recruitment of Motor Units • 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 26

Sustained Contractions •smaller motor units (smaller diameter axons) - recruited first • larger motor units (larger diameter axons) - recruited later • produce smooth movements • muscle tone – continuous state of partial contraction 27

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Types of Contractions • isotonic – muscle contracts and changes length • eccentric – lengthening contraction

• concentric – shortening contraction • isometric – muscle contracts but does not change length

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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

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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Smooth Muscle Fibers 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

Multiunit Smooth Muscle • less organized • function as separate units • fibers function separately • irises of eye • walls of blood vessels

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Smooth Muscle Contraction • 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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Cardiac Muscle • 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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Characteristics of Muscle Tissue

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Skeletal Muscle Actions • 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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Body Movement 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

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Levers and Movement

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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 38

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 39

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Clinical Application • tenanus - sustained powerful contractions of skeletal muscles throughout the body •Caused by Clostridium tetani •Lockjaw •Tetanus shot-vaccine against the toxin

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