Skeletal Muscle (DP IB Biology: HL)

• Muscles in the body that are attached to the skeleton and aid movement are called skeletal muscles
• Other muscle types include:
  • Cardiac muscle which is found in the heart
  • Smooth muscle is found in the blood vessels and organs
• Skeletal muscle is striated as it has a stripy appearance when viewed under a microscope
• Striated muscle cells are bundled up into fibres which are surrounded by a single plasma membrane called the sarcolemma
• The fibres are highly specialised cell-like units
  • Each muscle fibre contains:
    • An organised arrangement of contractile proteins in the cytoplasm
    • Many nuclei – this is why muscle fibres are not usually referred to as cells
    • Specialised endoplasmic reticulum called the sarcoplasmic reticulum (SR) which stores calcium and conveys signals to all parts of the fibre at once using protein pumps in the membranes
    • Specialised cytoplasm called 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 sarcolemma (muscle fibre membrane) 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 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 lines

Myofibrils Parts & Descriptions Table