Skeletal Muscle Fiber-Definition and Structure

Each muscle consists of muscle bundles. Each muscle bundle is further composed of skeletal muscle fiber or muscle cells. Each muscle fiber is a long cylindrical cell the diameter of the muscle fiber cell is 10-100 micrometers. It has the following structure;

Nuclei

Muscle fiber cells contain multiply oval nuclei. These nuclei are arranged just beneath the sarcolemma. 

Sarcolemma

The outer membrane of the muscle cell is called sarcolemma. But it contains a large amount of stored glycogen and myoglobin. Myoglobin is a unique oxygen-binding protein. It has a red pigment that stores oxygen.

Myofibrils

Each muscle fiber contains a large number of myofibrils, 1-2 micrometer in diameter. These fibers run in a parallel direction to the muscle fiber they extend the entire length of the cell. Sarcolemma encloses bundles of myofibrils. 

Sarcomere

The small contractile unit of the myofibrils is called a sarcomere. A sarcomere is the region of myofibrils between two successive Z-lines. The myofibril contains myofilament. Each sarcomere has a series of dark and light bands. These bands can be seen along the length of each myofibril. There are the following parts of a sarcomere.

A-band

The dark bands are anisotropic so they are called A bands. An anisotropic compound can polarize visible light.

I-band

The light when called I band. They are the isotropic or nonpolarizing band. These give the cell are striped appearance. 

H-zone

Each  A-band has a lighter strip in its mid section. This lighter strip is called the H zone. H stands for Hele, it means bright. 

M-line

The H zone is bisected by Dark Line called the M line. 

Z-line

The I band has a midline called Z- the line. Z stands for zwish means between. 

Structure of Myofilament

Myofilaments are composed of thick and thin filaments;

Thick filament (Myosin)

The central thick filament extends to the entire length of the band. A thick filament is about 16 nm in diameter. It is composed of Myosin protein. Each Myosin molecule has a tail. The tail ends in two globular heads.

Myosin tail consists of 2 long polypeptide chains coiled together. The heads link the thick and thin filament together during a contraction so they are sometimes called cross bridges. 

Thin Filament

The filament is 7 to 8 nanometers. it is composed of 3 proteins.

Actin

Thin filaments are composed of the chiefly acting molecule. The thin filament extends across the I-band and partially into the A band. The actin molecule is arranged in two chains. These chains twist around to surround each other like twisted double strands of pearls. 

Tropomyosin

Two strands of another protein tropomyosin also twist around the acting. 

Troponin

The other major protein in the thin filament is troponin. It is three polypeptide complexes. One binds to actin, another binds to tropomyosin, and the third binds calcium ions. Myosin filament is surrounded by 6 actin filaments on each end. 

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