 There are 3 main types of muscles.
1. Striated muscle fibres or skeletal muscles.
2. Unstriped or smooth muscle fibres or visceral muscles.
3. Cardiac muscles
Each actin filament is made of two ‘F’  or filamentous actins. These are helically wound to each other. The ‘F’ actin is a polymer of monomeric
‘G’ (Globular) actins. Two filaments of another protein, tropomyosin are also present close to the ‘F’ actins throughout its length. Troponin a complex protein is found at regular intervals on the tropomyosin.Â
How are the thick and thin filaments arranged ?
Thin or Actin filaments :
The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin.  Actin filaments are thinner and are called thin filaments. The myosin filaments are called thick filament. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line. The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.
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Hypertrophy : Increase in the size of muscles is called hypertrophy. Increase in the size of muscle may occur due to exercise. Due to repeated and forceful contraction of muscles, mass of muscle increases and so does the number of mitochondria, filaments in sarcomeres and amount of sarcoplasm.
Atrophy : Decrease in the size of muscle is called atrophy. Atrophy is due to immobilization of muscles. Mass of muscles decreases due to decrease in number of mitochondria, filaments in sarcomeres and amount of sarcoplasm. An injury to a nerve which supplies a muscle can also cause atrophy
The striated appearance is due to the distribution pattern of two important proteins – Actin and
Myosin. The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin proteins.  The proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. Actin filaments are thinner while the myosin filaments, are thicker. An elastic fibre called ‘Z’ line which bisects the I band. The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line. The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.Â
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