muscle structure

• Skeletal muscle is made up of blocks of muscle fibres
• Each muscle fibre measures 100um in diameter
• Each muscle fibre is multinucleated
• Each muscle fibre is surrounded by a cell surface membrane called the sarcolemma
• The cytoplasm of a muscle fibre is called the sarcoplasm
• The sarcoplasm contains numerous mitochondria which provide ATP for muscle contraction
• The sarcoplasm also contains a specialized endoplasmic reticulum called sarcoplasmic reticulum, which is a store of calcium ions
• Within each muscle fibre are numerous threads called myofibrils
• Myofibrils have a striped appearance
• Each myofibril is made up of a chain of contractile units called sarcomeres
• Sarcomeres also have a striped or banded appearance
• The striped or banded appearance of sarcomeres is due to the presence of two contractile protein myofilaments which overlap
• The thicker protein myofilament is called myosin
• The thinner protein myofilament is called actin
• structure of myofibrils

  A) sarcomere

  B) Z line

  C) thin filaments

  D) thick filaments

  E) H zone

  F) I band

  G) a band

  
• Sarcomere


  One Z line to another. a contractual unit
• A band

  Consists of the H band and the myosin and actin overlapping
• I band 


  Only actin present had a central Z line
• H zone 


  Composed of just myosin with an M band at the centre
• Z line 


  The line between two adjacent sarcomeres where the actin filaments are anchored. is the centre of the I band
• myosin is made up of two heavy chains and four light chains
• myosin consists of an elongated tail region attached to a globular head via a flexible neck structure
• myosins functions to move material within a cell
• myosin causes muscle contractions
• myosin converts chemical energy in the form of ATP to mechanical energy thus generating force and movement
• actin is made up of two protein strands that twist to form a simple helix which is surrounded by a protein called tropomyosin which helps the actin filaments move past each other
• actin has a important role in muscle contraction and cell movements
• actin maintains and controls cell shape
• troponin has three subunits- TnC, TnI, TnT which are regulatory proteins
• troponin is attached to the protein tropomyosin
• during excitation calcium ions bind to TnC in troponin
• troponin interacts with tropomyosin to unblock active sites between the myosin filaments and actin allowing cross-bridge cycling
• tropomyosin consists off two chains of a-helical coiled protein
• tropomyosin associates end-to-end and binds to both sides of the actin filament
• tropomyosin prevents the muscle from contracting at the wrong time
• tropomyosin prevents actin and myosin filaments from making contact with each other
• at rest troponin molecules hold tropomyosin fibres so that they cover the myosin-binding sites on actin
• troponin has Ca2+ binding sites
• if myosin binding sites are blocked then the muscle can't contract
• when Ca2+ binds to troponin the shape changes causing movement of troponin, releasing tropomyosin and exposing myosin-binding sites on actin