What are myofibrils made up of?

  • Myofibrils contain bundles of thick and thin myofilaments that move past each other to make muscles contract.
  • The thick myofilaments are made of the protein myosin and the thin myofilaments are made of the protein actin.

Myofibrils under a microscope:

Under an electron microscope, you will see a pattern of alternating dark and light bands. The dark bands contain the thick myosin filaments and some overlapping thin actin filaments- these are called A-bands. Light bands contain thin actin filament only - these are called I-bands. 

A myofibril is made up of many short units called sarcomeres. The end of each sarcomere is marked with a Z-line. In the middle of each sarcomere is an M-line. This is the middle of the myosin filament. Around the M-line is the H-zone. The H-zone only contains myosin filaments.

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