AQA A Level Biology

Revision Notes

6.3.2 The Structure of Skeletal Muscle

Ultrastructure of Striated Muscle

  • Striated muscle makes up the muscles in the body that are attached to the skeleton
  • Striated muscle is made up of muscle fibres
  • A muscle fibre is a highly specialised cell-like unit:
    • Each muscle fibre contains an organised arrangement of contractile proteins in the cytoplasm
    • Each muscle fibre is surrounded by a cell surface membrane
    • Each muscle fibre contains many nuclei – this is why muscle fibres are not usually referred to as cells
  • The different parts of a muscle fibre have different names to the equivalent parts of a normal cell:
    • Cell surface membrane = sarcolemma
    • Cytoplasm = sarcoplasm
    • Endoplasmic reticulum = sarcoplasmic reticulum (SR)
  • The sarcolemma has many deep tube-like projections that fold in from its outer surface:
    • These are known as transverse system tubules or T-tubules
    • These run close to the SR
  • The sarcoplasm contains mitochondria and myofibrils
    • The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction
    • Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction
  • The membranes of the SR contain protein pumps that transport calcium ions into the lumen of the SR

The ultrastructure of striated muscle and of a section of muscle fibre


  • Myofibrils are located in the sarcoplasm
  • Each myofibril is made up of two types of protein filament:
    • Thick filaments made of myosin
    • Thin filaments made of actin
  • These two types of filament are arranged in a particular order, creating different types of bands and line

Myofibrils parts & descriptions table

Myofibrils parts & descriptions table, downloadable AS & A Level Biology revision notes

Structure of a myofibril (1), downloadable AS & A Level Biology revision notesStructure of a myofibril (2), downloadable AS & A Level Biology revision notes

The structure of a myofibril

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Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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