Skeletal Muscles

• Muscles: effector organs which respond to nervous stimulation by contracting
• Smooth muscle - found in the walls of the gut and blood vessels
• Skeletal Muscles - make up most of our body muscle and attached to bones. These act under voluntary control
• Structure of the Thick Filament

  • Within a myofibril and made up of myosin molecules

  → Fibrous protein molecules with a globular head 

  → Fibrous part of the myosin molecule anchors the molecule into the thick filament 
• Thin Filament

  • Within a myofibril and made up of actin molecules

  → Globular protein molecules 

  → Many actin molecules link together to form a chain 

  → Two actin chains twist together to form one thin filament 

  • A fibrous protein known as tropomyosin is twisted around the two actin chains 
• Slow Twitch Fibres

  • Contract Slowly
  • Good for endurance (e.g. marathon)
  • Also for posture (many slow twitch fibres in your calves)
• Adaptations of Slow Twitch Fibres

  • Many mitochondria
  • Energy released slowly through aerobic respiration
  • Many blood vessels for oxygen diffusion
  • Thinner for shorter diffusion distance
  • Have more myoglobin - red colour - more oxygen
• Myoglobin: : red coloured protein that associates oxygen
• Fast Twitch Fibres

  • Rapid contraction
  • Good for short busts of speed/exercise
  • Tire easily/fatigue
• Adaptations of Fast Twitch Fibres

  • Few mitochondria and blood vessels
  • Energy released quickly through anaerobic respiration
  • Have lots of glycogen
  • Thicker, more myosin
  • Have stores of phosphocreatine
  • Little myoglobin - white colour - less oxygen
• What is a Motor Unit ?

  • A motor unit is formed when all muscles fibres supplied by a single motor neurone act together as a single functional unit
  • This structure provides control over the force that the muscle exerts

  → If a small force is needed only a few units are stimulated 

  → If a large force is needed a larger number of units are stimulated 
• • A myofibril is made up of two types of protein: actin and myosin that form a sarcomere
• Sarcomere Structure

  A) I band

  B) A band

  C) Z line

  D) M line

  E) H zone

  F) Actin

  G) Myosin

  
• A band: total width of the myosin
• I band: only thin actin filaments present
• H zone: only thick myosin filaments present
• M line: middle point of the myosin and attachment for myosin filaments
• Z line: Indicate the parameters of the sarcomere and attachment for actin filaments
• What happens to the sarcomere when a muscle contracts ?

  • A band remains constant as myosin is not moving
  • Z lines pulled closer together
  • I band and H zone become smaller
• Phosphocreatine: can rapidly generate ATP from ADP in anaerobic conditions
• Describe the roles of calcium ions and ATP in the contraction of a myofibril 

  1. Calcium ions diffuse into myofibrils from sarcoplasmic reticulum 
  2. Cause movement of tropomyosin on actin
  3. Movement causes exposure of the binding sites on the actin 
  4. Myosin heads attach to binding sites on actin 
  5. Hydrolysis of ATP (on myosin heads) causes myosin heads to bend 
  6. Pulling actin molecules 
  7. Attachment of a new ATP molecule to each myosin head causes myosin heads to detach   
• Role of ATPase

  • Sarcoplasm contains an enzyme ATPase which is activated by calcium ions to hydrolyse ATP on the myosin head into ADP and releases enough energy for the myosin head to return to its original position
• Describe the role of Tropomyosin in myofibril contraction (2)          

  • Moves out of the way when calcium ions binds 
  • Allowing myosin to bind to actin/crossbridge formation
• Describe the role of Myosin in myofibril contraction (2)

  • Head of myosin binds to actin and moves actin past 
  • Myosin detaches from actin and re-sets 
  • This uses ATP