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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
Muscle Types

structure and function
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
muscle tendon junction
Muscle tendon junction

muscles and contraction
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.
body movement from muscle lever systems
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
  • Amplifies muscle strength in limited anatomical space.
  • Limits length of contraction

W Herzog, Muscle Mechanics

structural hierarchy 2
Structural Hierarchy 2

structural hierarchy11
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)
  • Skeletal muscle consists of thousands of muscle fibers, the cellular units of muscle.
  • Fibers are densely packed elongated multi-nucleated cells

muscle fiber
Muscle Fiber

Each muscle fiber is made up of thousands of myofibrils

myofibril sarcomere
Myofibril / Sarcomere
  • Myofibrils contain filaments of actin and myosin.
  • Filaments form an ordered array and make up
  • sarcomeres, the functional units of muscle.


sarcomere filament interactions
Sarcomere filament interactions

myofilament structure
Myofilament Structure

molecular basis of muscle contraction
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:

actin myosin contraction
Actin myosin contraction

actin myosin contraction20
Actin myosin contraction

sarcoplasmic reticulum
Sarcoplasmic reticulum

YC Fung, Biomech, 1993

cross bridge cycle
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
cells and formation of myofibers
Cells and formation of myofibers

J Huard et al., JBJS, 2002


J Huard et al., JBJS, 2002

  • 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


J Huard et al., JBJS, 2002

contraction cycle
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

muscle contraction
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
length tension relationship sarcomeres
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

anatomy of leg muscles
Anatomy of leg muscles

Grey’s Anatomy

muscle types32
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.
atp production strategies
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
fast twitch fibers
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.
muscle diseases and pathologies
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