structure of skeletal muscle

• (largest to smallest)

  muscle --> muscle bundles --> muscle fibres --> myofibril --> myofilament --> actin + myosin
• muscle bundles are surrounded by sheath of connective tissue called perimysium with allow bundles to function as individual units and for adjacent bundles to slide easily over one another when contracting
• muscle bundles are held together by a sheath of connective tissue called epimysium which tapers towards the end and blend to form the tendon
• muscle fibres are long cylindrical cells that lie parallel to each other and contain many nuclei. they vary in length with a diameter between 10 and 100 micrometres
• muscle fibres are surrounded by sarcolemma, a thin, transparent plasma membrane which contains the sarcoplasm (cytoplasm)
• Myofibril are thread-like structures that lie in the sarcoplasm of a muscle cell. they lie parallel to each other, running the length of the fibre
• myofibril are made of sarcomeres, the contractile unit created by striations
• myofibril are surrounded by the sarcoplasmic reticulum (a tubular network) which is the storage site for calcium ions. these ions are released during contraction
• myofilaments are one of the structures that make up myofibril and are made of protein. they are the actual unit involved in contraction
• two types of myofilaments

  • thick myofilament - composed mainly of the protein myosin
  • thin myofilament - composed mainly of the protein actin

  these give a banded effect to the muscles which creates the striated appearance
• when the muscle fibre is supplied with sufficient energy in the form of ATP, and activated by nerve impulses, the thick and thin protein filaments slide past each other to shorten the myofibril