VPB 121: Veterinary Physiology - II (2+1)

Electron microscopic structures of muscle fibres

ELECTRON MICROSCOPIC STRUCTURE OF MUSCLE FIBRES

• Myofibril
  • Muscle fibres are made up of several hundred to several thousand of parallely arranged myofibrils having the special function of contraction. Each myofibril posses the contractile proteins myosin (thick) and actin (thin) myofilaments results in regular repetition of dark and light bands in the skeletal and cardiac muscles. Each muscle fibre contains about 1500 molecules of myosin and 3000 molecules of actin.
• Myosin is a large protein molecule composed of 6 polypeptide chains 2 of which are 100% α - helically exist. Well distinct Head with binding site for Actin and is equipped with ATPase system.They have Hinges to alter the position of the head. The twisted end show the tail. 
•  Properties of myosin molecular are
  • It has enzymatic ability to split ATP and release energy. Enzyme is myosin ATPase. Actin and Mg²⁺ are required for activating these enzymes.
  • Myosin binds with actin and the actin myosin complex is specifically dissociated by ATP.
  • Myosin spontaneously aggregates to form dimers which can aggregate again to form native thick filaments.
    • The active sites for the myosin ATPase are activity and the sites on myosin that bind to actin are both located in the heads of the myosin molecule and each of the myosin molecule has two such sites.
• Actin: much smaller molecule than myosin and contains only one polypeptide chain per molecule. Actins are found in aggregations which form a double stranded helical filament. It has been calculated that one thick filament contains around 500 myosin molecules and each thin filament contains 340-380 actin monomers.
• Tropomyosin: molecules are rod shaped they lie in the 2 grooves of the double stranded actin filament and aggregate end to end to produce two strands of tropomyosin running enter length of this filament.

• Troponin: More ellipsoidal or globular molecule, binds to a particular site on the tropomyosin molecule. Troponin has an important role as switch for muscle contraction. It has got 3 sub unit polypeptide chains called,
  • Troponin – T (This chain has site for attachment with tropomyosin)
  • Troponin – I (Inhibits the actin activated myosin ATP ase)
  • Troponin – C (binding sites (4 numbers) for Ca²⁺).
    • Troponin – I attach not only with troponin C and T and but also with actin in the absence of Ca²⁺. Troponin C attaches itself to Troponin T and I but not with actin.
    • The electron microscopic view of light bands indicates actin filaments represented as “I” bands because they are isotropic to polarised light.
• The dark bands of myosin is represented by “A” Band, which are anisotropic to polarised light. “H” zone represents partial overlapping the myosin filaments with actin filament. Small projections of myosin filaments called cross-bridges protrude out of the myosin filaments along the entire length except the very centre of the myosin filament.
• The centre of I bands contain a dark line called the “z line” or “z disc” where the ends of the actin filaments are attached. The z-discs are filamentous proteins, located in a vertical position in the myofibril. That portion of the myofibril between two successive z-discs is called as “sarcomere”. The sarcomere is the physiological contractile unit of muscle.
• `H' Band of the myofibril is a dark line seen at the centre of “A” band is rarely seen in normally functioning muscle. It is well marked only during relaxation beyond physiological limit.