Chapter 9 Muscular System. Functions of Skeletal Muscle. Structure of a Skeletal Muscle. Connective Tissue Coverings. Skeletal Muscle Fibers

Chapter 9 Muscular System Functions of Skeletal Muscle Three Types of Muscle Tissues • Produce skeletal movement • Maintain posture and body positi...
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Chapter 9 Muscular System

Functions of Skeletal Muscle

Three Types of Muscle Tissues

• 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

•  cramp – sustained, painful, involuntary muscle contraction 21

Muscular Responses

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Length-Tension Relationship

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

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