The Structure of Skeletal Muscle (AQA A Level Biology)

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

• 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

The structure of a myofibril