sliding filament model

• How many filaments are myofibrils made up of?

  2
• where are myofibrils found?

  skeletal muscle
• what are light bands?

  Where actin and myosin don't overlap- only actin is present (the thinner filament)
• what are dark bands?

  where actin and myosin do overlap - myosin is present as well as actin (the thicker filament)
• what is a light band also referred to as?

  an I band
• what are dark bands also known as?

  an A band
• what biological molecule/macromolecule are muscle fibres made from?

  Proteins
• describe the structure of actin

  • thinner filament
  • two strands twisted around each other
  • contains a binding site for myosin
• describe the structure of myosin

  • thicker filament
  • rod shaped fibres with bulbous heads that project to one side
• what is the z line?

  found at the centre of each light band
• what is a sacromere?

  distance between 2 z lines
• what is a H zone?

  found at the centre of each dark band. only myosin filaments present here
• What does tropomyosin do?

  • Actin binding protein
  • Regulates muscle contraction by blocking myosin binding site
  • So myosin heads can’t bind to actin filaments
• what is the term for the complex that forms when myosin heads bind to actin-myosin binding sites?

  cross bridges
• what is a neuromuscular junction?

  The point where a motor neurone and skeletal muscle fibre meet
• Why are there many NMJs along the length of a muscle?

  • To ensure all muscle fibres contract simultaneously
  • If only one existed, muscle contraction wouldn’t be as powerful and much slower
• What is the name given to the muscle fibres supplied by a single motor neurone?

  A motor unit
• what determines the number of motor units stimulated?

  the size of the force needed

  • strong force needed- large number of motor units stimulated
  • small force needed- small number of motor units stimulated
• describe the process by which an action potential is transmitted across the NMJ

  1. action potential arrives at the NMJ
  2. calcium ion channels open
  3. calcium ions diffuse from the synapse into synaptic knob
  4. this causes synaptic vesicles to fuse with the presynaptic membrane
  5. acetylcholine released into synaptic cleft by exocytosis
  6. Diffuses across synapse and binds to receptors on the postsynaptic membrane (sacrolema), this opens sodium ion channels and results in depolarisation
• what happens to acetylcholine after it binds to the postsynaptic membrane?

  • broken down into choline and ethanoic acid by enzyme acytelcholinesterase
  • this prevents the muscle being overstimulated
  • Choline and ethanoic acid diffuse back into the neurone, where they are recombined into acetylcholine, using the energy provided by mitochondria
• events taking place after the depolarisation of the sacrolemma?

  1. Depolarisation spreads through T tubules, they transmit to sacroplasmic reticulum
  2. AP stimulates Ca ion channels to open, Ca ions diffuse out of SR and flood the sacroplasm
  3. Ca ions bind to troponin, changes shape, moves, pulls on tropomyosin
  4. tropomyosin moves away from the myosin actin binding site on actin filament, exposes it, myosin heads bind forming cross bridge
  5. Myosin head flexes, pulling actin filament along, ATP is released
  6. ATP binds to the head, causing it to detach from filament
  7. ATPase hydrolyses ATP, energy changes it to og position
• at what two moments do we see the myosin head changing shape or flexing?

  • flexes/changes shape when moves along actin filament
  • changes shape when ATPase hydrolyses ATP into ADP and Pi and it moves back to its original position
• what do T tubules do?

  • transmit AP deep into muscle fibre to come into contact with SR, which then releases Ca ions
  • Ca ions then diffuse down their conc grad and bind to troponin
• where does the energy from muscle contraction come from? what is it used for?

  • hydrolysis of ATP into ADP and Pi
  • for movement of myosin heads, and to enable SR to actively reabsorb Ca ions from sacroplasm
• in what direction do myosin filaments pull actin filaments along?

  • towards the centre of the sacromere
  • as z lines come closer together
• what structures in the sliding filament model reduce in size during muscle contraction?

  • I band
  • H zone
  • sacromere shortens as well
  • (A band doesn’t change in length)

  A) H zone

  B) z-line

  C) myosin

  D) actin

  
• describe the structure of actin

  • Actin myosin binding sites
  • Blocked by tropomyosin which is held in place by troponin when the muscle is relaxed
  • Actin filaments arranged in a spiral
  • (Red dots in diagram=troponin)

  
• describe the structure of myosin

  • myosin filaments have globular heads which are hinged- allows them to move back and forth
  • on the head is the bindning site for each of actin and ATP
  • ATPase component which when activated by calcium ions hydrolyse ATP to ADP + Pi
  • At rest, ADP molecules are bound to the myosin heads
  • Tails of several myosin molecules are aligned to form the myosin filament

  
• what does an NMJ look like?

  neurone found making a synapse of a muscle fibre (motor neurone)

  A) sacroplasm

  B) sacroplasmic reticulum

  C) neurone mitochondria

  D) synaptic vesicles