Ultrastructure of Striated Muscle (Cambridge (CIE) A Level Biology): Revision Note

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

Striated Muscle Appearance (Stained)

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

  • 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 Diagram

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

Myofibrils Parts & Descriptions Table

Myofibril Structure Diagram

The structure of a myofibril