Muscle contraction (BIOL5)

Question 1

Features of skeletal muscle

Answer 1

• Made up of bundles of muscle fibres:
  
  • sacrolemma - cell membrane of muscle fibre cells
    
    • bits of sacrolemma fold inwards across muscle fibes, stick into sacroplasm - muscle cell cytoplasm
    • folds called T tubles help spread e- impulses across sacroplasm so they reach all parts of muscle fibre
• Sacroplasmic reticulim - network of int membranes runs through sacroplasm
  
  • stores/releases Ca+ needed for muscle contraction
• Muscle fibres have lots of mitochondria to provide ATP needed to contraction
  
  • multinucleate - have many nuclei for protein synthesis (enzymes)
  • have myofibrils - made up of proteins h specialised for contraction

Question 2

Features of myofibrils

Answer 2

• Contain thick/thin myofilaments:
  
  • thick made up of protein myosin
  • thin made up of protein actin
• Under e- microscope, you see alternating dark/light bands:
  
  • dark - have thick myosin filaments; A-bands
  • light - have thin actin filaments; I-bands
• Made up of short units calls sacromeres:
  
  • ends marked w/ Z-line
  • mid of sacromeres is M-line - mid of myosin filaments
  • around M-line is H-zone - only contain myosin

Question 3

Features of the sliding filament theory

Answer 3

• Myosin/actin filaments slide over e/o to make sacromeres contract
  
  • myofilaments don’t contract!
• Simultaneous contraction of sacromeres means myofibrils and muscle fibres contract
  
  • A-bands stay same length
  • I-band gets shorter
  • H-zones get shorter
  • Sacromeres get shorter!
• Sacromeres return to original length as muscle relaxes

Question 4

Features of myosin and actin filaments

Answer 4

• Myosin filaments have hinged globular heads - move back/forth
• Myosin heads have binding sites for actin head & ATP
• Actin filaments have binding sites for myosin heads - actin-myosin binding sites
• Proteins, tropmyosin and troponin found b/w actin filaments:
  
  • attached together, help myofilaments move past e/o
• In resting, actin binding site blocked by tropmyosin, held in place by tropism
  
  • filaments can’t slide past e/o as myosin heads can’t bind to site on actin filament

Question 5

Muscle Contraction

Answer 5

1. AP stimulates muscle cell, sacrolemma depolarised
   
   • ​​​Depolarisation spreads down T-tubles to sacroplasmic reticulum, causes release of Ca+ into sacroplasm
2. Ca+ bind to troponin, cause change in shape. Attached tropomyosin pulled out of actin myosin binding site on actin filament - exposes binding site
3. Myosin head binds to actin filament, forms actin-myosin cross bridge
4. Ca+ activate ATPase, b/d ATP to provide energy for contraction
5. Energy from ATP moves myosin head, pulls action filament along
6. ATP gives energy to break bridge, so myosin head detaches from atin filament
7. Myosin heads reattches to diff binding site on actin filament.
   
   • New actin-myosin filament cross bridge formed and cycle repeated
8. Many bridges form/break v rapidly, pulling actin filaments along
9. Shortens sacromere, causing muscle to contract
10. Cycle continues as long as Ca+ present and bound to troponin

Question 6

When excitation stops…

Answer 6

• Ca+ leave troponin binding site, moved by active transport into sacroplasmic reticulum (need ATP)
  
  • troponin back to original shape, pulling attached tropomyosin w/ them
    
    • tropomyosin molecules block actin-myosin binding site again
    • muscles aren’t contracted as no myosin heads are attached to actin - no bridges!
    • actin filaments slide back to relaxed position - lengthens sacromere

Question 7

Aerobic respiration provides ATP for contraction…

Answer 7

• Most ATP generated via oxidative phosphorylation
• Needs O2

Question 8

Anaerobic respiration provides ATP for contraction…

Answer 8

• ATP made rapidly by glycolysis
• End product is pyruvate, converted into lactate by lactate fermentation

Question 9

ATP-Phosphocreatine (PCr) system in provides ATP for muscle contraction…

Answer 9

• ATP made by phosphorylating ADP - adding P taken from PCr
• PCr stored inside cells
• ATP-PCr generates ATP v quickly
• PCr runs out after few secs
• Is anaerobic (no O2) and alactic (no lactate made!)

Question 10

Features of slow twitch muscle fibres

Answer 10

• Fibres contract slowly
• Used for posture
• Used for long w/o tire
• Energy released slowly by aerobic resp
• Reddish colour - rich in myoglobin

Question 11

Features of fast twitch muscle fibres

Answer 11

• Contract v. quickly
• Used for fast movement, h proportion
• Good for bursts of speed/power
• Energy released quickly by anaerobic resp
• Whitish colour - don’t have much myoglobin