As seen under a polarization microscope, the A-Band in skeletal muscle fibres is so named because it is anisotropic in its refractive index which is a characteristic of orderly crystalline structure. On the other hand, the I-Band is so named because of its isotropic nature, as far as its refractive index is considered, which is characteristic of Amorphous substances or substances which lack long-range order. Both are filamentous (as opposed to globular) in the sarcomere (Actin is present as F-Actin).
But then why this difference in the characteristics of refractive index, which is indicative of order (i.e crystalline or amorphous?) in their molecular structure?
Moreover, is the H-Band anisotropic?
If not, then the anisotropy of the remaining part of A-Band must be a consequence of the relative arrangement of Actin and Myosin which are independently isotropic, and hence cannot give a possible isotropic arrangement.
If yes, then that indicates that the myosin filaments are anisotropic in themselves and actin filaments are isotropic. But, in the region of their overlapping (A-Band minus H-Band), the overlap is between an anisotropic (myosin) and an isotropic (actin) component which shouldn't be anisotropic because the randomness of actin filaments (i.e. its isotropicity) should make the entire configuration isotropic?
I am sorry if I am wrong in my understanding of isotropic and anisotropic characteristics, or if my question sounds too "physics-related", but this has been bothering me for quite some while now.
Here is a directly related question I asked on physics.se. It partly solves the problem of possible arrangement of isotropic elements to generate anisotropy.