Muscles and Motor Locomotion

  • Muscle Movement

    • Muscles do work by contracting
    • Skeletal muscles come in antagonistic pairs
      • Flexor vs. extensor
    • Contracting = shortening
      • Move skeletal parts
    • Tendons
      • Connect bone to muscle
    • Ligaments
      • Connect bone to bone
  • Structure of Striated Skeletal Muscle

    • Muscle fiber
    • Muscle cell
      • Divided into sections = sarcomeres
    • Sarcomere
    • Functional unit of muscle contraction
    • Alternating bands of thin (actin) and thick (myosin) protein filaments
  • Actin

    • Complex of fibers
    • Brain of actin molecules and tropomyosin fibers
      • Tropomyosin fibers secured with troponin molecules
  • Myosin

    • Single protein
    • Myosin molecule
      • Long protein with globular head
  • Thick and thin filaments

    • Myosin tails aligned together and heads pointed away from center of sarcomere

  • Interaction of Thick and Thin Filaments

    • Cross bridges
    • Connections formed between myosin heads and actin
    • Cause muscle to shorten
  • Muscle Cell Organelles

    • Sarcoplasm
    • Muscle cell cytoplasm
    • Contains many mitochondria
    • Sarcoplasmic reticulum
    • Organelle similar to ER
      • Network of tubes
    • Stores Ca2+
    • Ca2+ released from sarcoplasmic reticulum through channels
    • Ca2+ restored to sarcoplasmic reticulum by Ca2+ pumps
      • Pump Ca2+ from cytosol
      • Pumps use ATP
  • Muscle at Rest

    • Interacting proteins
    • At rest, troponin molecules hold tropomyosin fibers, so that they cover the myosin-binding sites on actin
      • Troponin has Ca2+ binding sites
  • Motor Neurons

    • Motor neuron triggers muscle contraction
    • Release acetylcholine (Ach) neurotransmitter
  • Nerve Trigger of Muscle Action

    • Nerve signal travels down T-tubule
    • Stimulates sarcoplasmic reticulum of muscle cell to releases stored Ca2+
    • Flooding muscle fibers with Ca2+
  • Ca2+ Triggers Muscle Action

    • At rest, tropomyosin blocks myosin-binding sites on actin
    • Secured by troponin
    • Ca2+ binds to troponin
    • Shape change accuses movement of troponin
    • Releasing tropomyosin
    • Exposes myosin-binding sites on actin
  • How Ca2+ Controls Muscle

    • Sliding filament mode
    • Exposed actin binds to myosin
    • Fibers slide past each other
      • Ratchet system
    • Shorten muscle cell
      • Muscle contraction
    • Muscle doesn’t relax until Ca2+ is pumped back into sarcoplasmic reticulum
      • Requires ATP
  • How It All Works

    • Action potential causes Ca2+ release from sarcoplasmic reticulum
    • Ca2+ binds to troponin
    • Troponin moves tropomyosin, uncovering myosin binding site on actin
    • Myosin bonds actin
    • Uses ATP to ratchet each time
    • Releases and bonds to next actin
    • Myosin pulls actin chain along
    • Sarcomere shortens
    • Z discs move closer together
    • Whole fiber shortens
    • Contraction
    • Ca2+ pumps restore Ca2+ to sarcoplasmic reticulum
  • Fast Twitch and Slow Twitch Muscles

    • Slow twitch muscle fibers
    • Contract slowly, but keep going for a long time
      • More mitochondria for aerobic respiration
      • Less sarcoplasmic reticulum
      • Ca2+ remain in cytosol longer
    • Long distance runner
    • “Dark” meat = more blood vessels
    • Fast twitch muscle fibers
    • Contract quickly, but get tired rapidly
      • Store more glycogen for anaerobic respiration
    • Sprinter
    • “White” meat
  • Muscle Limits

    • Muscle fatigue
    • Lack of sugar
      • Lack of ATP to restore Ca2+ gradient
    • Low O2
      • Lactic acid drops pH which interferes with protein function
    • Synaptic fatigue
      • Loss of acetylcholine
    • Muscle cramps
    • Build up of lactic acid
    • ATP depletion
    • Ion imbalance
      • Massaging or stretching increases circulation
  • Diseases of Muscle Tissue

    • ALS
    • Amyotrophic lateral sclerosis
    • Lou Gehrig’s disease
    • Motor neurons degenerate
    • Myasthenia gravis
    • Autoimmune
    • Antibodies to acetylcholine receptors
  • Botox

    • Bacteria Clostridium botulinum toxin
    • Blocks release of acetylcholine
    • Botulism can be fatal
  • Rigor Mortis

    • No life, no breathing
    • No breathing, no O2
    • No O2, no aerobic respiration
    • No aerobic respiration, no ATP
    • No ATP, no Ca2+ pumps
    • Ca2+ stays in muscle cytoplasm
    • Muscle fibers continually contract
    • Eventually, tissues breakdown and relax
    • Measure of time of death

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