Sarcomere- Definition, Structure, Diagram, and Functions

A sarcomere is a complex multicomponent biological system and functional unit of striated muscle which plays a vital role in transforming the chemical energy released upon the ATP hydrolysis into mechanical work. Skeletal muscles are made up of the basic unit called a sarcomere and all voluntary movement is initiated by this skeletal muscle. The main purpose of a sarcomere is to coordinate muscle contraction by the shortening and extension of the sarcomere.

  • The movement of the animals is notably smooth and complex and the contractile properties are its defining characteristics.
  • Movement of the muscle requires a change in muscle length and this ability of change is found in the sarcomere.
  • When observed in a microscope skeletal muscle consists of stripes called striation.
  • These stripes are seen due to the presence of alternating patterns of light and dark bands corresponding to the different filaments.
  • The stripes are formed by interlocking the fibers that consist of each sarcomere.
  • The basic component that forms muscle tissue is tubular fibers also called myofibrils.
  • Myofibrils are essential polymers that are fibrous and long, which consist of two different types of proteins that stack on each other including myosin and actin.
  • Myosin is thick with a globular head, whereas actin is a thin filament and the action between the actin and myosin myofilament leads to muscle contraction.

Structure of Sarcomere

When the muscle fiber is viewed under the microscope, then the fibers appear to be of varying length stacked patterns. The actin and myosin myofilament arrange parallel to each other. The sliding filament theory is followed whenever a muscle in our body contracts, which means myosin and actin filament slide against each other, and the muscle contracts. This interaction between the myofilaments leads to the yield of contractile force. However, for the contraction of muscle, there is a need for a certain kind of unit. The unit that is required for lengthening and shortening the flexible muscle is the sarcomere.

Structure of Sarcomere
Structure of Sarcomere, created with biorender.

The sliding theory was first proposed by a group of scientists who were able to visualize actin and myosin filaments by the use of high-resolution microscope and filament strains. They visualized the lengthening of the sarcomere in its relaxed state and shortening in its contracted state leading to the discovery of sarcomere zones.

Sarcomere consists of different filaments along with the light and dark bands.

The structure of the sarcomere is described with the dark and the light band.

  • A bands (or anisotropic bands): It is also called the dark band and contains the whole thick filament (myosin).
  • I bands (or isotropic bands): it is called the light band that contains only the thin filament (actin). The thin filament lies between the two thick filaments.
  • Z disc: it is the area where two actin filaments connect and transverse the I bands. Similarly, sarcomere can also be described as the structure between the two z discs.
  • M line: M line contains the protein called myomesin and it marks the center of the sarcomere.
  • H zone: it is the area between the M line and Z disc and this zone contains only the myosin.

We can remember filament as thin and thick in the following way

  • I being a thin letter so it contains only thin filament and becomes easier to remember.
  • H is a thick letter so it contains only the thick filament.

Function of Sarcomere

  • One of the functions of the sarcomere is muscle contraction.
  • The sarcomere also helps in muscle flex by shortening and lengthening the sarcomere brought by the action of myosin and actin myofilament.


  1. Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997. Section II, The Organization, Structure, and Function of Muscle. Available from:

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