PROCESS

• ACTION OF MYOSIN HEADS?

  • bulbous heads of myosin filaments form cross-bridges with actin filaments
  • by attaching to binding sites on actin filaments
  • flex in unison
  • pull actin filaments along myosin filaments
  • become detached
  • use ATP to return to original angle
  • re-attach further along actin filament
• STAGES OF MUSCLE CONTRACTION?

  1. Muscle stimulation
  2. Muscle contraction
  3. Muscle relaxation
• PROCESS OF MUSCLE STIMULATION?

  1. Action potential reaches many neuromuscular junctions simultaneously
  2. Ca2+ protein channels open
  3. Ca2+ diffuse into synaptic knob
  4. Causes synaptic vesicles to fuse with presynaptic membrane
  5. Release ACh into cleft
  6. ACh diffuses across cleft
  7. ACh binds with receptors on muscle sarcolemma
  8. Sarcolemma is depolarised
• PROCESS OF MUSCLE CONTRACTION?

  1. AP enters fibre via T-tubules (in contact with SR)
  2. SR contains Ca2+ AT in from sarcoplasm
  3. = lower Ca2+ conc. in sarcoplasm
  4. AP opens VGCa2+PC on SR
  5. Ca2+ diffuse into sarcoplasm down CG
  6. = tropomyosin molecules blocking binding sites on AF to pull away
  7. ADP molecules attached to MH help bind to AF = cross-bridges
  8. Once attached, change angle
  9. Pull AF along
  10. Release ADP
  11. ATP attaches to each head = detach from AF
  12. Ca2+ activate ATPase to hydrolyse ATP into ADP
  13. = Energy for for MH to return to original position
  14. MH with attached ADP reattaches further along AF
• WHAT ARE T-TUBULES?

  extensions of sarcolemma that branch throughout the sarcoplasm
• WHAT CAUSES THE PROCESS OF MUSCLE CONTRACTION TO REPEAT?

  cycle repeats as long as Ca2+ concentration in myofibril remains high
• HOW DOES THE PROCESS OF MUSCLE CONTRACTION ACTUALLY CAUSE THE MUSCLE TO CONTRACT?

  • myosin molecules joined tail to tail in two oppositely facing sets
  • so movement of one set of myosin heads is in the opposite direction to the other set
  • therefore actin filaments also move in opposite directions
  • pulling them towards each other (shortening distance between Z-lines)
• PROCESS OF MUSCLE RELAXATION?

  1. Nervous stimulation ceases
  2. Ca2+ actively transported back into sarcoplasmic reticulum (using energy from ATP hydrolysis)
  3. Reabsorption of Ca2+ = tropomyosin blocks actin filament again
  4. Myosin heads unable to bind to actin filaments
  5. Contraction ceases, muscle relaxes
  6. Force from antagonistic muscles can pull actin filaments out from between myosin (to a certain point)