Muscles

• Myofibrils

  Repeated functional units called sarcomeres
• Sarcomere

  • Ends marked by Z lines
• Relaxed sarcomere

  1. Z-line - thin actin filaments anchored here

  2. Distance between 2 Z-lines = 1 sarcomere

  3. M-line - thick myosin

  4. A band - A length of myosin with some overlapping actin, forms the dark region

  5. I band - contains actin only (light region)

  6. H zone - contains myosin only
• Muscle contraction - sliding filament theory

  1. Thin actin filaments slide inwards over the thicker myosin filaments

  2. Shortening of each sarcomere along the whole length of myofibril

  3. Muscle fibre becomes shorter and the muscle contracts
• Sliding filament theory


  Explanation for how muscle contraction occurs
• What happens when a muscle contracts

  • Sarcomeres become shorter (Z-lines move closer together)
  • Light (Isotropic) bands become shorter (I-Band narrows)
  • Dark (Anisotropic) bands stay the same length
  • H zone becomes narrower
• Roles in myofibril contraction

  • Sarcoplasmic reticulum
  • Tropomyosin
  • Ca2+
  • Actin and myosin
  • ATP
• Sarcoplasmic reticulum


  Contains high concentration of Ca2+ in relaxed muscle
• Muscle contraction


  1. Nerve stimulation

  2. Calcium channels in sarcoplasmic reticulum open

  3. Ca2+ floods into sarcoplasm
• Tropomyosin

  Blocks myosin binding sites on actin filaments when muscle is relaxed
• Muscle contraction

  1. Ca2+ diffuses into myofibrils sarcoplasm

  2. Tropomyosin moves out of binding sites on actin

  3. Ca2+ activates ATPase in myosin head

  4. Myosin heads bind to actin filaments forming cross bridges

  5. Myosin heads change position/angle, pulling actin filaments

  6. Myosin heads detach and reattach further along actin filament

  7. ATP binding allows myosin head to detach from actin
• ATP hydrolysis

  • Provides energy for myosin head to change angle and pull actin
  • Provides energy for myosin head to detach from actin and return to original position
  • Provides energy for reabsorption of Ca2+ into sarcoplasmic reticulum
• Ways of generating ATP for muscle contraction

  • Aerobic respiration - oxidative phosphorylation in mitochondria
  • Anaerobic respiration - ATP made by glycolysis
  • ATP-phosphocreatine (PCr) system
• Muscle contraction requires considerable energy supplied by hydrolysis of ATP to ADP and Pi
• Fast twitch muscle fibres


  • Contract very quickly and produce powerful contractions but only for a short period
  • Good for short bursts of speed and power
  • High proportions in muscles which need to do short bursts of intense activity
  • Energy released quickly through anaerobic respiration using glycogen
  • Adapted with thicker and more numerous myosin filaments, high concentration of glycogen and enzymes for anaerobic respiration, store of PCr, few mitochondria or blood vessels, little myoglobin
• Slow twitch muscle fibres

  • Contract more slowly than fast-twitch fibres and provide less powerful contractions but over a longer period
  • Can work for longer without getting tired
  • High proportions in back and calves muscles used for posture
  • Energy released slowly through aerobic respiration
  • Adapted with large store of myoglobin, rich supply of blood vessels, many mitochondria, mitochondria near edge of muscle fibres