Physiology of Muscle Contraction

• Sarcolemma


  Membrane that surrounds the muscle cell
• Sarcoplasm


  Cytoplasm inside the muscle cell
• Muscle structure


  1. Muscle fibre

  2. Sarcolemma

  3. Myofibril

  4. Myofilaments
• Myofibrils


  • Around 1-2 μm in diameter
• Myofibrils


  • Contain tiny myofilament proteins
• Myofibrils


  • Cause the striations on skeletal muscle
• Due to the alignment of myofilaments in the myofibrils the muscle fibre appear striated in a microscope because the light is passing through only where there are gaps between the myofilaments
• A Band (dark band)


  The overlap of the Actin filaments and the Myosin myofilaments
• I Band (light band)

  Overlap of Titin and Actin
• Sarcomere


  Made of one dark band and half a light band where it appears some myofilaments are overlapping and places where its not
• Sarcomere


  The smallest functional unit in muscle cell
• M line


  Proteins that connect to the myosin filaments to keep them in place
• H Zone


  Region that is on either side of the M Line that contains thick myosin filaments ONLY
• Muscle contraction


  1. Z discs move closer to the M Line

  2. Actin and myosin overlap even further

  3. Sarcomere shortens
• Myosin filaments


  • Made up of myosin
  • Have a double head like bulge and a tail
  • Each myosin head contains an Actin binding site which allows them to attach to actin = crossbridge
• Crossbridge


  Generates skeletal contraction via ATP
• Actin (Thin Filament)


  • G Actin (goblet): actin subunit that contain binding site for myosin heads
  • F Actin (fibrous): actin filament
• Two long actin filaments intertwine (double strand of pearls)
• Tropomyosin


  Thin protein rods that wrap around actin filaments to stabilise them (block the binding sites at rest)
• Troponin


  • 3 polypeptide complex = 3 components
  • Tn (I): inhibitory subunit that bins to Actin (ie activates during rest)
  • Tn (T): binds to Tropomyosin to help position itself onto Actin Filaments
  • Tn (C): binds to Calcium ions
• Muscle contraction


  1. Calcium gets released from the Sarcoplasmic Reticulum

  2. Binds to Troponin C

  3. Exposes the actin binding sites for myosin binding heads
• Sarcoplasmic Reticulum


  • An elaborate network of smooth rough endoplasmic reticulum that create interconnecting tubules surrounding each myofibril
  • Stores ionic calcium (releases Ca on demand to stimulate contraction)
  • Communicate at the H zone longitudinally
  • Terminal Cisternae: other tubules perpendicular to where the A band and I band meet which always occurs in pairs
• 1. Tubules


  • Bits connected to the sarcolemma which protrude deep into the muscle cell = elongated tubes
  • Facilitate 'spread of action'
  • Spread through the deepest region, this way T-tubules act as a rapid messaging system to myofibrils to ensure that all sarcomere contract in sync to perform contraction
• Sliding Filament Mechanism


  1. At rest, thin and thick filaments overlap only at the ends of the A band

  2. During contraction thin filaments slide past thick filaments so that the actin and myosin filaments overlap to a greater degree

  3. Titin the elastic protein attaching myosin to the Z discs DOES change length

  4. Sliding begins when myosin heads bind to binding sites on actin filaments

  5. Cross bridge forms between myosin and actin and break reputedly during a singular contraction

  6. Thin filaments slide centrally

  7. Z discs are pulled toward the M line

  8. H Zones disappear

  9. I Bands Shorten (due to Titin)

  10. Sarcomere shortens
• Excitation-contraction Coupling


  1. Action potential travels down t-tubules

  2. Stimulates the Sarcoplasmic Reticulum to release Calcium Ions

  3. Calcium binds to Troponin

  4. Tropomyosin shift

  5. Reveals binding sites on actin

  6. Myosin head bind to Actin = Contraction
• Power stroke


  1. ADP and Phosphate molecule are released

  2. Myosin heads pivot and bend (bending is the myosin's state of low energy)

  3. This fuels the myosin pulling on the actin filament towards the M line
• Cross Bridge Detachment


  If ATP is present, the myosin head attaches to ATP and releases from the actin
• ATP Hydrolysis


  ATP is broken down into ADP and Phosphate molecule so myosin head can return to 'High energy position' ready to attach to actin
• These steps can only occur if ATP is available and Calcium can bind to Troponin
• Calcium is constantly pumped back to the Sarcoplasmic Reticulum which will decrease level in the cytosol hence = muscle relation as tropomyosin return to original position