Muscle Structure

• Muscle cells fuse together to form a myofibril and share nuclei and cytoplasm (sarcoplasm), many mitochondria and endoplasmic reticulum (sarcoplasmic reticulum).
• Myofibrils bundle together with a shared membrane called sarcomella that has inward-folds called T-tubules so ions can enter the myofibrils. This forms a muscle fibre.
• Muscle fibres bundle together to form a fascicle.
• A muscle is made of:

  • Bundle of fascicles
  • Surrounded by connective tissue (perimysium)
  • With a shared outer wall (epimysium)
• Myofibrils have a striped appearance due to the presence pf regular repeating units called sarcomeres. Each sarcomere is a contracting unit.
• Myofibrils contain 2 proteins:

  • Actin which is made of thin filaments.
  • Myosin which is made of thick filaments.
• The arrangement of myofibril proteins give the sarcomeres their characteristic banding pattern.
• Muscle Structure:

  • I Band: only actin
  • Z Line: centre of I band
  • Z-to-Z: sarcomere
  • A Band: where any myosin is present
  • H Zone: only myosin (no overlap)