3. Structure of myofiber

Structure of myofiber:
 These are the indivisual cells of muscle which are covered by endomysium and they are multinucleated.
 The plasma membrane is called sarcolemma.
 The cytoplasm is called sarcoplasm
 Each muscle fiber contains 1000s of cylindrical structures known as myofibrils which are contractile elements of cell. In between these myofibrils mitochondrias are seen .
 Each myofibril is surrounded by sarcoplasmic reticulum . These act as the store house of Ca2+ which is the essential element for muscle contraction.


 These Ca2+ are transported into the cytosol via openings called T-tubules (transverse tubule ).

Elements of myofibrils:
Each myofibrils are composed of 3 types of proteins. They are

  1. Contractile protein
  2. Regulatory protein
  3. Structural protein
  4. Contractile protein:
     They consist of actin (thin filament) and myosin (thick filament)
     Thick filament having darker A- band (anisotropic) and thin filament having lighter I-band (isotropic) .
     There is overlapping of thick and thin filaments which give striation.
  5. Regulatory protein:
     They are consists of troponin and tropomyosin which are present in actin filament.
     The main function of these two are to block the active site of actin at rest position.
  6. Structural protein:
     They include Z-line, M-line and H- band.
     Z- line is the non contractile protein which run across the muscle fibers to which myofibrils are attached. This bisects the I-band.
     The distance between these two Z-lines are called sarcomere which is the smallest contractile unit of muscle fiber.
     M- lines are present at the centre of A-band of the sarcomere.
     H- band is the band in which no overlapping is seen means it is the total length of the myosin filament.
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