Skeletal Muscles

• how does a neuromuscular junction cause a muscle contraction? (detained)

  -action potential arrives at the axon of the motor neurone and causes depolarisation of the neurone

  -Ca2+ vg channels open

  -Ca2+ diffuses in and causes vesiccles containing ACh to fuse with the presynaptic membrane

  -ACh diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma

  - this stimulates ion channels in the sarcolemma to open and allow Na+ to diffuse in

  -sarcolemma depolarises and generates an action potential that passes down the T-tubules towards the centre of the muscle fibre

  -this action potential causes Ca2+ vg channels in the membranes of the sarcoplasmic reticulum to open

  -Ca2+ diffuses out of the sarcoplasmic reticulum and into the sarcoplasm surrounding myofibrils

  -Ca2+ binds to troponin and stimulates them to change shape

  -this causes troponin and tropomyosin proteins to change position on the thin filaments

  -myosin-binding sites are exposed

  -process of muscle contraction can now begin
• antagonistic muscle pairs

  when one muscle contracts the other relaxes
• sarcolemma

  cell surface membrane of a muscle fibre
• sarcoplasma


  cytoplasm of a muscle fibre
• sarcoplasmic reticulum

  endoplasmic reticulum of a muscle fibre
• T-tubules


  extensions of the cell membrane that fold in and penetrate the centre of the muscle fibre

  
• myofibrils


  bundles of actin and myosin filaments which slide past each other during muscle contractions
• why does the sarcoplasm contain lots of mitochondria?

  to produce the ATP requires for muscle contraction
• what protein filaments are myofibrils made up of?

  thick filaments- myosin

  thin filaments- actin
• H band


  thick myosin filaments only
• I band

  thin actin filaments only
• A band


  contains areas with just myosin filaments and areas where myosin and actin filament overlap
• M line


  attachment for myosin filaments
• Z line

  attachment for actin filaments
• sarcomere


  section of myofibril between two Z lines
• structure of a myofibril

  
• structure of thick filaments

  -made up of myosin molecules

  -fibrous part of the myosin molecule anchors the molecule into the thick filament

  -in the thick filament many myosin molecules lie next to each other with their globular heads all pointing away from the M line
• structure of the thin filament

  -made up of actin

  -many actin molecules link together to form a chain

  -2 actin chains twist together to form one thin filament

  -tropomyosin is twisted around the two actin chains

  -troponin is attached to the actin chains at regular intervals
• what protein is twisted around the thin filament (2 actin chains)

  tropomyosin
• what protein is attached to the thin filament (actin chains)

  troponin
• sliding filament theory (detailed answer)


  -action potential arrives at the neuromuscular junction

  -Ca2+ released from the sarcoplasmic reticulum

  -Ca2+ binds to troponin and stimulates them to change shape

  -troponin and tropomyosin change position on the actin filaments

  -myosin binding sited are exposed

  -globular heads of myosin molecules bind to the exposed sites and form a cross-bridge

  -formation of the cross-bridge causes the myosin heads to bend and release ADP + Pi pulling the actin filaments towards the centre of the sarcomere

  -muscle contracts slightly

  -ATP binds to the myosin head causing it to change shape and release from the actin filaments

  -ATP hydrolase hydrolyses ATP into ADP + Pi, this causes the myosin heads to move back to their original positions

  -myosin heads can bind to new binding sites on the actin filament, ones closer to the centre of the sarcomenre

  -myosin heads move again, pulling the actin filaments closer to the centre of the sarcomere

  -muscle slightly contracts

  -ATP binds to the myosin heads again and causes them to detach

  - this continues until the muscle is fully contracted or the ATP runs out

  
• why is ATP needed in muscle contraction?

  ATP provides energy to the myosin heads to allow them to change shape and return to their original position

  It is also required for the active transport of Ca2+ back into the sarcoplasmic reticulum
• Function of phosphocreatine


  is a rapid source of ATP and allows muscles to contract until the mitochondria produce ATP
• How is phosphocreatine a source of ATP?

  donates an organic phosphate to ADP

  
  

  ADP + phosphocreatine--> ATP + creatine
• ATP production and prolonged activity


  Muscle contraction rate must match ATP production from aerobic and anaerobic respiration
• Types of muscle fibres

  fast twitch and slow twitch
• structure of fast twitch fibres

  -have a small amount of myoglobin which stores oxygen and increases the rate of O2 absorption from the capillaries

  -have fewer capillaries so they have a slow supply of oxygen and glucose for aerobic respiration

  -have a large amount of Ca2+ to stimulate contraction
• function of fast twitch fibres

  they contract rapidly

  and are suited to short bursts of high-intensity activity as they fatigue quickly due to the lactate produced from anaerobic respiration
• slow twitch muscle fibres structure

  -have a high amount of myoglobin, haemoglobin and mitochondria which increases the rate of oxygen supply, absorption and aerobic respiration

  -a dense network of capillaries so have a short diffusion distance and a good supply of oxygen and glucose for aerobic respiration
• slow twitch muscle fibres function

  -contract more slowly and are suited for sustained activities

  -fatigue less quickly due to less lactate production so are ideal for endurance

  -rely on ATP for aerobic respiration