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
- Contractile protein
- Regulatory protein
- Structural protein
- 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.
- 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.
- 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.