Ultrastructure of Striated Muscle (Cambridge International A-Level Biology)

Spec point — 15.1.11 · describe the ultrastructure of striated muscle with reference to sarcomere structure using electron micrographs and diagrams

Ultrastructure of striated muscle

• Striated muscle makes up the muscles in the body that are attached to the skeleton

  • 'Striated' means it is striped/streaky in appearance

Skeletal striated muscle as seen under the microscope, after staining Image by Nephron, CC BY-SA 3.0, via Wikimedia Commons

• 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 from the equivalent parts of a normal cell:

  • Cell surface membrane = sarcolemma
  • Cytoplasm = sarcoplasm

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

Myofibrils

• 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 band and line

The structure of a myofibril