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Updated: Mar 5 2022

Muscle Biology & Physiology

Images Epimysium_moved.jpg[1]_moved.jpg
  • Overview
    • Key topics of this chapter include
      • Gross anatomy
      • Muscle contraction
      • Muscles type
      • Muscle metabolism
      • Types of contraction
      • Muscle training 
      • Nutritional training
      • Muscle injury
  • Gross Anatomy
    • Myotendinous junction
      • weak link in muscle and often site of tears (especially with eccentric contraction)
      • involution of muscles cells maximized surface area for attachment
    • Noncontractile elements
      • Epimysium surrounds muscle bundles
      • Perimysium surrounds muscle fascicles
      • Endomysium surrounds individual fibers
  • Muscle Contraction
    • Contractile elements
      • derived from myoblasts
      • the muscles fiber (an elongated cell) is the basic unit of contraction
      • a myofibril is a collection of sarcomeres
    • Sarcomere composition
      • filaments
        • thick myosin filaments
        • thin actin filaments
      • bands
        • H band is myosin only
        • I band is actin only
        • A band is both actin and myosin
        • Z line flanks each sarcomere and acts as site of attachment for actin filament
        • during muscle contraction
          • A band stays the same length
          • I band reduces in length
          • H zone reduces in length
    • Action stimulation
      • nerve cell body delivers electrical signal to motor endplate (junction between muscle and nerve)
        • nerve action potentials are started with passage of sodium ions through voltage gated channels
      • Ach is released and diffuses across synaptic cleft to bind to Ach receptor
        • myasthenia gravis patient has shortage of Ach receptors
        • botox blocks release of Ach from end plate
      • Ach binding triggers depolarization of sarcoplasmic reticulum and release of calcium into muscles cytoplasm
      • excitation-contraction coupling
        • in low calcium environment
          • tropomyosin blocks myosin-binding sites on actin
        • in high calcium environment
          • calcium binds to troponin (on thin filaments) leading to a configuration change of tropomyosin (on thin filaments)
          • exposing myosin-binding sites on actin filament
          • actin forms cross-bridges to myosin, and the ATP breakdown, the two fibers contract past one another
    • Types of muscle contraction
      • isometric
        • muscle contracts with constant length (e.g. pushing against an immovable object)
      • isokinetic
        • muscle contracts with constant speed (requires specific equipment like cybex machines)
      • plyometric
        • rapid lengthening followed by contraction of muscle groups (e.g. jumping up and down on boxes)
      • isotonic - muscle contract with constant tension
        • concentric
          • muscle shortens during contraction (e.g. biceps curl)
        • eccentric
          • muscle lengthens during contraction (e.g. "negative" of a biceps curl)
    • Force generation
      • force generated is most dependent on muscle cross-sectional area
      • muscle fiber size increases with strength conditioning
    • Contraction speed
      • duration and speed of contraction is most dependent on fiber type
  • Muscle Types
      • Type I vs. Type II muscles
      • Type I muscle
      • (slow twitch - ST)
      • "slow red ox muscles"
      • Type II muscle
      • (fast twitch - FT)
      • Metabolism
      • Aerobic/oxidative
      • Anerobic/glycolytic
      • Energy source
      • Aerobic system (oxidative phosphorolation via Krebs cycle)
      • ATP-CP system
      • Exercise duration
      • Endurance (distance running)
      • Low strength of contraction
      • Low speed of contraction
      • First to atrophy with deconditoning
      • High strength of contraction
      • High speed of contraction (large force generation per cross sectional area)
      • Fatigue rapidly
      • Sprinting is example
      • Note
      • High yield ATP
      • Requires O2 and thus more vascular
      • Increase mitochondria in cells
      • High yield ATP (increased ATPase)
      • Low intramuscular triglycerine stores
  • Metabolic Systems
    • Three systems are used to generate energy for muscles
      • ATP-CP anaerobic system
        • (adenosine triphosphate-creatinine phosphate system, "phosphagen system")
          • basis for creatine phosphate supplementation (main side effect: muscle cramping)
        • used for intense metabolic exercise lasting less than 20 seconds (e.g., 100 meter sprint)
        • converts carbohydrates stored within muscle into energy
        • anaerobic (does not use oxygen and does not produce lactate)
        • formulas
          • ATP –» ADP + P + energy
          • ADP –» AMP + P + energy
      • lactic anaerobic system (lactic acid metabolims)
        • intense muscle activity lasting 20 to 120 seconds (e.g., 400 meter sprint)
        • involves hydrolysis of one glucose molecule
        • formula
          • glucose –» lactic acid + energy
      • aerobic system
        • used in longer duration and lower intensity exercises
        • Krebs cycle generates ATP from glucose and fatty acids through oxidative phosphorylation
  • Muscle Injury
    • Muscles soreness
      • caused by edema and inflammation in the connective tissue
        • neutrophils are the most abundant cells early on after acute injury
          • generates free radicals that possibly increase muscle damage
      • worse with unaccustomed eccentric exercise
      • peaks at 24-48 hours
      • elevated CK levels seen in serum
    • Muscles strain
      • occur at myotendinous junction (off during eccentric contraction which produces highest forces in skeletal muscle)
      • pathoanatomy in inflammation followed by fibrosis
    • Muscle atrophy
      • caused by disuse or nerve injury
      • leads to fatty infiltration and increased fatigability
      • muscles crossing a single joint atrophy faster
      • loss of cross-sectional area leads to decreased force generation
      • use of Angiotensin-II receptor blockade increases muscle regeneration after contusion which decreases fibrosis
        • proposed mechanism is IGF-1 blockade decreasing apoptosis cascade
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