Brent Cornell

    
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)

    
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