Each individual muscle fibre has the following specialised features designed to facilitate muscle contraction:

• They are multinucleate (fibres form from the fusion of individual muscle cells and hence have many nuclei)
• They have a large number of mitochondria (muscle contraction requires ATP hydrolysis)
• They have a specialised endoplasmic reticulum (it is called the sarcoplasmic reticulum and stores calcium ions)
• They contain tubular myofibrils made up of two different myofilaments – thin filament (actin) and thick filament (myosin)
• The continuous membrane surrounding the muscle fibre is called the sarcolemma and contains invaginations called T tubules

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)

Can you draw a Sarcomere?