Skeletal Muscles

• Antagonistic muscle pairs

  Pairs of muscles which pull in opposite directions - as one muscle contracts (agonist), the other relaxes (antagonist)
• Gross structure of skeletal muscle

  • Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils)
  • Arrangement ensures there is no point of weakness between cells
  • Each bundle is surrounded by endomysium: loose connective tissue with many capillaries
• Microscopic structure of skeletal muscle

  • Myofibrils: site of contraction
  • Sarcoplasm: shared nuclei & cytoplasm with lots of mitochondria & endoplasmic reticulum
  • Sarcolemma: folds inwards towards sarcoplasm to form transverse (T) tubules
• Ultrastructure of a myofibril

  • Z-line: boundary between sarcomeres
  • I-band: only actin - under microscope = light
  • A-band: overlap of actin & (full length of myosin) - under microscope = dark
  • M line: only myosin
• Sliding filament theory

  1. Myosin head with ADP attached forms a cross bridge with actin

  2. Power stroke: myosin head changes shape & loses ADP, pulling actin over myosin

  3. ATP attaches to myosin head, causing it to detach from actin

  4. ATPase hydrolyses ATP→ADP (+Pi) so myosin head can return to its original position

  5. Myosin head re-attaches to actin further along filament
• Sliding filament action

  Causes a myofibril to shorten as: Myosin heads flex in opposite directions = actin filaments are pulled towards each other, Distance between adjacent sarcomere Z lines shortens, Sliding filament action occurs up to 100 times per second in multiple sarcomeres
• Pieces of evidence that support the sliding filament theory

  • H-zone narrows & I-band narrows
  • Z-lines get closer (sarcomere shortens)
  • A-zone remains same width (proves that myosin filaments do not shorten)
• Role of Ca2+ ions in muscle contraction

  1. Action potential moves throughT-tubules in the sarcoplasm; Ca2+ channels in the sarcoplasmic reticulum open

  2. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum

  3. Ca2+ binds to troponin, triggering conformational change in tropomyosin

  4. Exposes binding sites on actin filaments so actinomyosin bridges can form (Myosin heads attach to binding sites on actin)
• During muscle relaxation

  1. Ca2+ is actively transported back into the endoplasmic reticulum

  2. Tropomyosin blocks the actin binding site
• Role of ATP in muscle contraction

  1. Hydrolysis of ATP (on myosin heads) causes myosin heads to bend

  2. Bending pulling actin molecules

  3. Attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites)
• Role of phosphocreatine in muscle contraction

  Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited
• Calculating length of 1 sarcomere

  View thin slice of muscle under optical microscope, Calibrate eyepiece graticule, Measure distance from the middle of 1 light band to the middle of another
• Slow-twitch skeletal muscle fibres

  • Found on sustained contraction sites, Role: long-duration contraction; well-adapted to aerobic respiration to prevent lactate buildup, Glycogen store: many terminal ends can be hydrolysed to release glucose for respiration, Contain myoglobin: higher affinity for oxygen than haemoglobin at lower partial pressures, Many mitochondria: aerobic respiration produces more ATP, Surrounded by many blood vessels: high supply of oxygen & glucose
• Fast-twitch skeletal muscle fibres

  • Found on sites of short-term,rapid, powerful contraction, Role: powerful short-term contraction; well-adapted to anaerobic respiration, Large store of phosphocreatine, More myosin filaments & Thicker myosin filaments, High concentration of enzymes involved in anaerobic respiration, Extensive sarcoplasmic reticulum: rapid uptake & release of Ca2+