Answer
There are three types of myofilaments in each myofibril; thick filaments, thin filaments and elastic filaments.
1. Thick filaments
A thick filament is made up of several hundreds of molecules of the protein myosin (mol wt. 210-250 kDa). Each molecule is shaped like a golf club and consists of two intertwined molecular chains with bent and projecting globular heads (double head) and a tail or shank. The filament itself resembles a bundle of golf clubs in a helical arrangement with the heads on one half angled to the left, and the heads of the other half of bundle angled to the right. In between the bundle presents a bare region.
2. Thin filaments
Each thin filament is made of two intertwined F-actin molecules ( fibrous actin). These F-actin molecules look like a string necklace with beads of spheroid G actin beads. Each G-actin has a site for the binding of the head of a myosin molecule.
Tropomyosin: Thin filaments also incorporate several other proteins, for example, tropomyosin, troponin. Tropomyosin is an important part of the structure of the thin filament. When the muscle fiber is relaxed, tropomyosin blocks active sites for calcium binding in G -actin molecules.
Troponin is another molecule found in thin filaments. These are small molecules bound to tropomyosin; they have binding sites for calcium.
Work Step by Step
3. Elastic filaments
These filaments are about 1 nm in diameter. They form the core of each thick filament and anchor it
to a Z disc at one end, and an M-line at the other. Elastic filaments incorpote the protein titin which is one of the largest human proteins ( approx. 3,000 kD). Titin confers passive elasticity on muscle fibers. It prevents overstretching, and enables recoil after stretching ; this property enables the myofibrils to return to their resting shapes after muscle movements.
