Muscles

• Structure of a muscle

  = Made up of a bundle of muscle fibres, each muscle fibre (cell- many cells fused together that share a sarcoplasm, nuclei, sarcoplasmic reticulum.)

  • Myofibril which is made up of units of sarcomere, made of actin and myosin filaments
• Slow twitch fibres

  • Fibres contract slowly
  • Provide less powerful contractions but over a longer period.
  • Used for endurance activities as they don't tire easily.
  • Gain their energy from aerobic respiration.
  • Rich in myogloblin, bright red protein which stores oxygen
  • Rich blood vessel supply and mitochondria.
  • Found in large proportions in muscles, help maintain posture.
• Fast twitch fibres

  • Fibres contract quickly
  • Produce powerful contractions but for shorter periods.
  • Used for short bursts of speed and power as they tire easily.
  • Gain energy from anaerobic respiration.
  • Pale coloured as they have low levels of myoglobin and blood vessels.
  • Contain more, ticker, myosin filaments
  • Store creatine phosphate
  • Found in high proportion of muscles eg: biceps, eyes
• Sources of ATP:

  • Aerobic respiration.
  • Anaerobic respiration.
  • Addition of a phosphate group to ADP. Phosphate group comes from creatine phosphate.
• Neuromuscular junction

  1 electrical impulses cause depolarisation of presynaptic membrane, v-gated calcium ion channels open and calcium ions diffuse in.

  2 vesicles fuse to presynaptic membrane and releases neurotransmitter in neuromuscular junction by exocytosis.

  3 neurotransmitters bind to receptor sites on sodium channels on sarcolemma.

  4 sodium ion channels open and sodium ions diffuse in.

  5 depolarisation in muscle causes v-gated calcium ion channels to open on sarcoplasmic reticulum.

  6 calcium ions diffuse out which causes the muscle to contract.
• Mitochondria in muscle to produce energy for calcium pump on sarcoplasmic reticulum so there is higher concentration of calcium ions inside the SR.
• Actin:

  • Troponin
  • Tropomyosin
  • G-actin
• • Calcium ions bind to and move tropomyosin so the binding site on actin is exposed.  
  • ADP and Pi on myosin head soit can bind to actin. 
  • Myosin binds to actin and ADP and Pi is released during power stroke. 
  • ATP binds to myosin head so it releases actin. 
  • Hydrolysis of ATP releases energy to get myosin to recovery position.