i. Sarcomere is the functional unit of myofibril. It has specific arrangement of actin and myosin filaments. The components of sarcomere are organized into variety of bands and zones. Actin and myosin are referred as contractile proteins. Actin is called as thin filament whereas myosin in called as thick filament.
The structure of sarcomere:
ii. ‘A’ band – dark bands present at the centre of sarcomere and contain myosin as well as actin. ‘H’ zone or Hensen’s zone – light area present at the centre of ‘A’ band ‘M’ line – present at the centre of ‘H’ zone ‘I’ band – light bands present on the either side of ‘A’ band containing only actin Z’ line – adjacent ‘I’ bands are separated by ‘Z’ line.
iii. Sliding filament theory:
It was put forth by H.E Huxley and A.F Huxley. It is also known as ‘Walk along theory’ or Ratchet theory.
- According to the sliding filament theory, the interaction between actin and myosin filaments is the basic cause of muscle contraction. The actin filaments are interdigitated with myosin filaments.
- The head of the myosin is joined to the actin backbone by a cross bridge forming a hinge joint. From this joint, myosin head cannot tilt forward or backward. This movement is an active process as it utilizes ATP.
- Myosin head contains ATPase activity. It can derive energy by the breakdown of ATP molecule. This energy can be used for the movement of myosin head.
- During contraction, the myosin head gets attached to the active site of actin filaments and pull them inwardly so that the actin filaments slide over the myosin filaments. This results in the contraction of muscle fibre.
