Sliding filament model

Key terms

Sliding filament model: movement of actin and myosin filaments in relation to each other to cause contraction

Sliding filament model

• Contraction

  • Myosin filaments pull actin filaments inwards

  • Towards centre of sarcomere

• Light band becomes narrower

• Z lines move closer

  • Shortens sarcomere

• H-zone becomes narrower

• Dark band remains same width

  • Myosin filaments overlap actin filaments

  • Greater amount

• Simultaneous contraction of lots of sarcomeres

  • Myofibrils and muscle fibres contract

  • Enough force to pull on a bone

  • Causes movement

• Sarcomeres return to original length

  • Muscle relaxes

Structure of myosin

• Globular heads which are hinged

  • Allows to move back and forwards

  • Binding site for each actin and ATP

• Tails of myosin molecules aligned together

  • Form myosin filament

Structure of actin

• Actin filaments have binding sites for myosin heads

  • Actin-myosin binding sites
    

• Muscle in resting state

• Actin-myosin sires blocked by tropomyosin

  • Heads can’t bind to actin

  • Filament can’t slide past each other
    

• Muscle stimulated to contract

• Myosin heads form bonds with actin filaments

  • Actin-myosin cross bridges

• Myosin heads flex in unison

  • Pull actin filaments along myosin filament

• Myosin detaches from actin

  • Head returns to original angle

  • Uses ATP

• Myosin reattaches further along actin filament

• Process occurs again

  • Repeated up to 100 times per second

Neuromuscular junction

• Action potential arrives at neuromuscular junction

  • Muscle contraction triggered

  • Point where motor neurone and skeletal muscle fibre meet

• Many neuromuscular junctions along muscle length

  • Ensure muscle fibres contract simultaneously

  • Contraction is powerful and fast

  • One one would mean this isn’t possible

• Muscle fibres supplied by single motor neurone

  • Motor unit

  • Fibres act as single unit

• Strong force needed

  • Large number of motor units stimulated

• Small force needed

  • Small number stimulated

• Action potential reaches neuromuscular junction

  • Stimulates calcium ion channels

    • Open

• Calcium ions diffuse from synapse

  • Into synaptic knob

  • Synaptic vesicles fuse with presynaptic membrane

• Acetylcholine released into synaptic cleft

  • Exocytosis

  • Diffuses across synapse

  • Binds to receptors

    • On postsynaptic membrane

• Opens sodium ion channels

  • Results in depolarisation
    

• Acetylcholine broken down by acetylcholinesterase

  • Into choline and ethanoic acid

• Prevents muscle being overstimulated

• Choline and ethanoic acid diffuse back into neurone

  • Recombined into acetylcholine

  • Uses energy provided by mitochondria

Sarcoplasm

• Depolarisation of sarcolemma travels deep into muscle fibre

  • Spreads through T-tubules

• Contact with sarcoplasm reticulum

• Sarcoplasm reticulum contains stored calcium ions

  • Actively absorbs from sarcoplasm

• Action potential reaches sarcoplasmic reticulum

  • Stimulates calcium ion channels to open

• Calcium ions diffuse down concentration gradient flooding with calcium ions

• Calcium ions bind to troponin

  • Causes it to change shape

• Pulls on tropomyosin

  • Moves it away from actin-myosin binding sites

• Binding sites exposed

  • Myosin head binds to actin filaments

  • Form actin-myosin cross-bridge

• Attached to actin filament

  • Myosin head flexes

  • Pull actin filament along

• Molecule of ADP bound to myosin head released

• ATP molecule binds to myosin head

  • Head detaches from actin filament

• Calcium ions present in sarcoplasm activate ATPase activity of myosin

• Hydrolyses ATP to ADP and phosphate

  • Releases energy

  • Myosin head returns to original position

• Myosin head attaches to another actin-myosin binding site

  • Further along actin filament

  • Cycle repeats

• Cycle continues as long as muscle remains stimulated

• During stimulation

  • Many actin-myosin bridges form and break rapidly

  • Pulls actin filament along

  • Shortens sarcomere

  • Muscle contracts

Energy supply during muscle contraction

• Requires large quantities of energy

• Provided by hydrolysis of ATP into ADP and phosphate

• Energy required

  • Movement of myosin heads

  • Enable sarcoplasmic reticulum to reabsorb calcium ions from sarcoplasm

Aerobic respiration

• ATP used by muscle cells

  • Regenerated from ADP during oxidative phosphorylation

• Chemical reaction takes place inside mitochondria

  • Only occurs in presence of oxygen

• Used for long periods of low-intensity exercise

Interaction of myosin and actin during muscle contraction

• Tropomyosin molecule prevents myosin head from attaching to binding site on actin molecule

• Calcium ions released from endoplasmic reticulum

  • Tropomyosin molecule pulls away from binding sites

• Myosin head

  • Attaches to binding site on actin filament

• Head of myosin changes angle

  • Moving actin filament along

  • ADP molecule released

• ATP molecule fixes to myosin head

  • Detach from actin filament

• Hydrolysis of ATP to ADP by myosin

  • Provides energy for myosin head

  • Resume its normal position

• Head of myosin reattaches to binding site further along actin filament

  • Cycle repeated

Anaerobic respiration

• Very active muscle

  • Oxygen used up more quickly than blood supply can replace it

• ATP generated anaerobically

• Made by glycolysis

  • Pyruvate produced converted into lactic acid

  • Builds up quickly

    • Results in muscle fatigue

• Used for short periods of high-intensity exercise

Creatine phosphate

• Stored in muscle

• Acts as reserve supply of phosphate

  • Available immediately

  • Combine with ADP

  • Reforms ATP

• Generates ATP rapidly

  • Store of phosphate used up quickly

• Muscle relaxes

  • Creatine phosphate store replenished using phosphate from ATP

• Used for short bursts of vigorous exercise