Skeletal Muscle

• Functions:

  • movement (by pulling on bones to make the skeleton move)
  • Maintain posture
  • support for soft tissues
  • control of opening and exists (conscious control of urination, defecation and swallowing)
  • Maintenance of body temperature (muscles use energy and some is converted to heat)
  • nutrient reserve
• Gross organisation of skeletal muscle

  A) Blood vessel

  B) perimysium

  C) epimysium

  D) Muscle fiber (cell)

  E) fascicle (wrapped by perimysium)

  F) Endomysium (betweem fibers)

  G) tendon

  H) bone

  
• Skeletal muscle fibre:

  • up to 30cm long
  • several myoblast cells fuse early in development to form a muscle fiber, a muscle fibre contains many myofibrils
  • Multinucleates (nuclei located under sarcolemma - plasma membrane)
  • long (extends length of skeletal muscle)
  • striated appearance because of aligned sarcomeres
• Myofibrils:

  • comprised of many myofilaments (thick and thin filaments) arranged into sarcomeres (about 10,000 arranged end to end)
  • thin filaments = acin
  • thick filaments = myosin
• Sarcoplasmic reticulum:

  • type of endoplasmic reticulum which store Ca2+
• T-tubules

  • continuous with sarcolemma - transfers action potential to the myofibrils
• Sarcomere:

  A) I band

  B) A band

  C) Z line (discs)

  D) Titin

  E) Z line

  F) Zone of overlap

  G) H bands

  H) m Line

  I) Thin filament

  J) Thick filament

  
• M-line

  • middle line
  • attaches thick filaments together
• A-band

  • dark bands
  • contains myosin and actin
• I band

  • light band
  • only contains actin
• H band

  • contains only myosin
• Z line (z discs)

  • joins adjacent sarcomeres together with alpha actinin
• Zone of overlap

  • region where actin and myosin overlap
• Sliding filament model of muscle contraction

  • Thick (myosin) filaments interact with thin (actin) filaments and pull the thin filaments towards the M line
• Thick filaments

  • made of myosin , many myosin molecules wound together
  • titin prevents over-extension of sarcomere
• Thin filament

  made of actin

  regulatory proteins attached (troponin and tropomyosin

  CONTRACTION:

  • myosin must bind to actin
  • when relaxed, the myosin binding binding site on actin is covered by tropomyosin
  • When stimulates to contract, calcium enters the muscle
  • Ca2+ binds to troponin which causes tropomyosin to uncover myosin binding sites on actin
  • myosin can now bind to actin and cause contraction
• Sliding filament mechanism

  1. Action potential arrives

  2. Calcium released from sarcoplasmic reticulum

  3. Calcium ions bind to troponin

  4. Tropomyosin moves and uncovers myosin binding sites on actin

  5. Cross bridge formation - myosin binds to actin

  6. Power stroke - stored energy in myosin head is released, myosin head pivots, ADP and phosphate are released, the head bends, pulling the actin

  7. Cross bridges break because ATP binds myosin head

  8. Myosin reactivation - myosin head splits ATP into ADP+P, myosin head 're-cocked' ready for another contraction cycle

  9. Contraction cycle repeats if calcium is still present
• Skeletal muscle contraction

  A) Muscle fiber

  B) Myofibrils

  C) motor neuron synapse (neuromuscular junction)

  D) sarcoplasmic reticulum - store calcium

  E) sarcolemma

  
• An action potential travelling down a motor neuron eventually stimulates calcium release from the sarcoplasmic reticulum
• T-tubules:

  The action potential spreads along the sarcolemma and down t-tubules deep into the muscle fibre- stimulates Ca2+ release
• Overview

  A) ACh

  B) synaptic terminal of motor neuron

  C) synaptic cleft

  D) T tubule

  E) Plasma membrane

  F) Sarcoplasmic reticulum

  G) Ca2+ pump

  H) Ca2+

  I) Cytosol

  J) Ca2+

  
• How does the contraction cycle end?

  • acetylcholein broken in synaptic cleft
  • Calcium no longer release from sarcoplasmic reticulum
  • Calcium ions are pumped from the cytoplasm back into the sarcoplasmic reticulum
  • Calcium detaches from the troponin complex on actin, so tropomyosin covers myosin binding site, preventing muscle contraction