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Skeletal Muscle - 1 Muscle Types Gross Anatomy Structure and Function Skeletal muscle represents the largest tissue mass in the body (40-45% body weight) Composite structure Muscle cells Nerves Blood vessels

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Muscle types l.jpg
Muscle Types

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Gross Anatomy

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Structure and Function

  • Skeletal muscle represents the largest tissue mass in the body (40-45% body weight)

  • Composite structure

    • Muscle cells

    • Nerves

    • Blood vessels

    • Extra cellular connective tissue

      • Aponeurosis

      • Tendon (with interdigitating junctions)

  • Basic unit

    • Muscle fiber (myofiber)

    • Cytoplasm of myofiber is sarcoplasm

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Muscle tendon junction

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Muscles and contraction

  • Contract when stimulated by muscle-nerve pulses at motor unit of peripheral nervous system

  • Produce body movement, bones serve a levers, joints serve as fulcrum

  • Muscles stabilize joints

    • Pull only and do not push, arranged in opposition

    • Agonist and antagonist muscles balance force

  • Move eyes

  • Produce facial expression

  • Chewing

  • Etc.

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Body movement frommuscle lever systems

  • Third class lever has muscle force between fulcrum (joint) and load (limb and limb loading)

  • 3rd class is common lever system in body with 1st class as antagonist

  • Amplifies limb motion for relatively small muscle contraction

  • Requires high muscle loads relative to limb loading

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  • Amplifies muscle strength in limited anatomical space.

  • Limits length of contraction

W Herzog, Muscle Mechanics

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Structural Hierarchy 2

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Structural Hierarchy

  • Fascicles

    • Bundles of muscle fibers

    • Confined in sheath (perimysium)

  • Fibers (10-60 m)

    • Up to 30 cm long

  • Myofibril (~1m)

    • Made up of contactile myofilaments

    • Functional units show striations (sarcomeres)

  • Sarcomeres (2.5 m length)

    • Actin (5 nm dia) and myosin (12 nm dia)

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  • Skeletal muscle consists of thousands of muscle fibers, the cellular units of muscle.

  • Fibers are densely packed elongated multi-nucleated cells

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Muscle Fiber

Each muscle fiber is made up of thousands of myofibrils

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Myofibril / Sarcomere

  • Myofibrils contain filaments of actin and myosin.

  • Filaments form an ordered array and make up

  • sarcomeres, the functional units of muscle.

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Sarcomere filament interactions

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Myofilament Structure

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Molecular basis of muscle contraction

  • Sliding filament mechanism

    • Thin filaments (actin) slide toward center of sarcomere (A band) pulling their respective Z lines together (shortening the sarcomere)

    • Filaments do not change length (effectively)

    • Filaments are pulled forward in ratcheting action of thick filament (myosin) cross-bridges

  • Cross-bridges

    • Myosin has globular head that makes up cross-bridge

    • Actin has binding sites for globular myosin cross-bridge

    • Tropomyosin obstructs binding sites

    • Troponin holds tropomyosin in place

Animation of this process at:

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Actin myosin contraction

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Actin myosin contraction

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Sarcoplasmic reticulum

YC Fung, Biomech, 1993

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Cross-bridge cycle

  • To form a cross-bridge:

    • Ca2+ is released from long tubules of sarcoplasmic reticulum

    • Ca2+ binds to troponin

    • Allows topomyosin thread to reconfigure

    • Exposes binding site

    • # of sites determined by concentration of Ca2+

  • Cross-bridges bend to pull actin filament inward

  • When maximum range of bending reached, bridge connection is broken

    • Globular head returns to oblique angle

    • Connection to new binding site can be established

  • Numerous cycles are required for complete shortening

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Cells and formation of myofibers

J Huard et al., JBJS, 2002

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J Huard et al., JBJS, 2002

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  • Motor unit (MU) consists of all fibers innervated by one single motor nerve fiber

    • Small precise muscles 2-3 muscle fibers/MU

    • Large muscles, up to 1000 muscle fibers/MU

Feedback via muscle spindles to sense tension in the sensory peripheral nervous system

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J Huard et al., JBJS, 2002

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Contraction cycle

Action potential (AP) at neuromuscular junction Muscles can not push, they may only only CONTRACT (pull)A muscle contraction is called a muscle TWITCH

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Muscle contraction

  • To increase strength of contraction

    • Recruit more motor units

    • Increase stimulation frequency (wave summation)

  • Efficiency of muscle contraction

    • 20-25% of metabolic energy becomes mechanical work

    • 75-80% becomes heat

  • Isotonic contractions – same force

  • Isometric contractions – “same” length

  • Eccentric contractions – lengthening

  • Concentric contractions – shortening

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Length-tension relationship (sarcomeres)

  • Optimum overlap

  • Few available binding sites

  • No available binding sites

  • Fewer binding sites due to overlap

  • Not continuous F-L curve

  • Isometric forces at max stimulation

  • at various lengths

W Herzog, Muscle Mechanics

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Anatomy of leg muscles

Grey’s Anatomy

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Muscle types

  • Two main types of fibers

  • Differ in the mechanism they use to produce ATP

  • Amount of each type varies from muscle to muscle and from person to person

    • Red ("slow-twitch") fibers have more mitochondria, store oxygen in myoglobin, rely on aerobic metabolism, have a greater capillary to volume ratio and are associated with endurance; these produce ATP more slowly. Marathon runners tend to have more red fibers, generally through a combination of genetics and training.

    • White ("fast-twitch") fibers have fewer mitochondria, are capable of more powerful (but shorter) contractions, metabolize ATP more quickly, have a lower capillary to volume ratio, and are more likely to accumulate lactic acid. Weightlifters and sprinters tend to have more white fibers.

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ATP Production Strategies

  • Aerobic – ATP produced by breakdown of precursors in the presence of O2

    • High efficiency pathway but cannot proceed without O2

  • Anaerobic – Anaerobic respiration (glycolysis) produces ATP w/o O2

    • Less efficient than Aerobic respiration

    • Produces the undesirable Lactic Acid, which produces muscle ache after strenuous exercise

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Fast twitch fibers

  • Fast fibers come in three varieties, types IIa, IIx and IIb.

  • Type IIa is very common fiber in humans

  • Type IIx fibers (used to be called, confusingly, type IIB) are our fastest fibers.

  • Type IIb fibers predominate in the fast muscle of small mammals that have to accelerate their limbs very fast against little load.

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Muscle diseases and pathologies

  • Blunt injury

  • Tears

  • Muscle pulls

    • Usually damage at muscle-tendon junction or muscle-aponeurosis junction

  • Myasthenia gravis

    • Autoimmune disease which involves neuromuscular junction characterized by impaired neural impulse transmission.

  • Duchenne’s muscular dystrophy

    • Most common MD is deficiency of dystrophin, an integral plasma membrane protein that links various structural proteins to membrane. Associated with degeneration of skeletal muscle

  • Myotonic dystrophy

    • Genetic muscle disease associated with extreme muscle wasting

  • Myositis

    • Inflammatory muscle diseases (infectious and immune)

  • Poliomyelitis

    • Infectious disease causing muscle weakness

  • Amyotrophic lateral sclerosis

    • Neurological disease that attacks neurons for controlling voluntary muscles

  • Cerebral palsy

    • Neurological disorders that appear in infancy and permanently affect muscle coordination and body movement