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•  Each myofibril is made up of contractile sarcomeres

Myofibrils consist of repeating contractile units called sarcomeres, which are made of two protein myofilaments

  • The thick filament (myosin) contains small protruding heads which bind to regions of the thin filament (actin)
  • Movement of these two filaments relative to one another causes the lengthening and shortening of the sarcomere

Each individual sarcomere is flanked by dense protein discs called Z lines, which hold the myofilaments in place

  • The actin filaments radiate out from the Z discs and help to anchor the central myosin filaments in place

The recurring sarcomeres produce a striated (striped) pattern along the length of the skeletal muscle fibres

  • The centre of the sarcomere appears darker due to the overlap of both actin and myosin filaments (A band)
  • The peripheries of the sarcomere appear lighter as only actin is present in this region (I band)
  • The dark A band may also contain a slightly lighter central region where only the myosin is present (H zone)

Structure of a Sarcomere (and associated banding pattern)


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•  Drawing labelled diagrams of the structure of a sarcomere

When drawing a diagram of a sarcomere it is important to remember the following conventions:

  • The myosin filaments are the thick filaments and should be represented as being thicker than the actin filaments
  • The myosin filaments should include protruding heads (myosin heads form cross-bridge attachments with actin)
  • The striated banding pattern should be identified (A band = dark region ; I band = light region)

Labelled Diagram of a Sarcomere

sarcomere drawing