Myofibril

Each muscle fiber contains several hundred to several thousand myofibrils. These are the contractile elements of skeletal muscle. Myofibrils appear as long strands of still smaller subunits – the sarcomeres.

Striations and the Sarcomere

Under a light microscope, skeletal muscle fibers have a distinctive striped appearance. Because of these markings, skeletal muscle is also called striated muscle. This striation also is seen in cardiac muscle, so it too can be considered striated muscle.

As you can see in the picture, you will see the color difference between A-bands, and I-bands. Each dark A-band has a lighter region in its center, the H-zone, which is visible only when myofibril is relaxed. There is a dark line in the middle of H-zone called the M-line. I-bands are interrupted by a dark stripe referred to as the Z-disk, also known as the Z-line.

A sarcomere is the basic functional unit of a myofibril and the basic contractile unit of muscle. Each myofibril is composed of numerous sarcomeres joined end to end at the Z-disks. Each sarcomere includes what is found between each pair of Z-disks, in this sequence:

- An I-band(light zone)

- An A-band(dark zone)

- An H-zone(in the middle of the A-band)

- An M-line in the middle of the H-zone

- The rest of the A-band

- A second I-band

Looking at individual myofibrils through an electron microscope, one can differentiate two types of small protein filaments that are responsible for muscle contraction. The thinner filaments are composed primarily of actin, and the thicker filaments are primarily myosin. The striations seen in muscle fibers result from alignment of these filaments. The light I-band indicates the region of the sarcomere where there are only thick filaments. The dark A-band represents the regions that contain both thick and thin filaments. The H-zone is the central portion of the A-band and contains only thick filaments. The absence of thin filaments causes the H-zone to appear lighter than the adjacent A-band. In the center of the H-zone is the M-line, which is composed of proteins that serve as the attachment site for the thick filaments and assist in stabilizing the structure of the sarcomere. Along with two additional proteins, titin and nebulin, they provide points of attachment and stability for the thin filaments.

Thick filaments

About two-thirds of all skeletal muscle protein is myosin, the principal protein of the thick filament. Each myosin filament typically is formed by about 200 myosin molecules.

Each myosin molecule is composed of two protein strands twisted together. One end of each strand is folded into a globular head, called the myosin head. Each thick filament contains many such heads, which protrude from the thick filament to form cross-bridges that interact during muscle contraction with specialized active sites on the thin filaments. There is an array of fine filaments, composed of titin, that stabilizes the myosin filaments along their longitudinal axis. Titin filaments extend from the Z-disk to the M-line.

Thin filaments

Each thin filament, although often reffered to simply as an actin filament, is actually composed of three different protein molecules – actin, tropomyosin, and troponin. Each thin filament has one end inserted into a Z-disk with the opposite and extending toward the center of the sarcomere, lying in the space between the thick filaments. Nebulin, an anchoring protein for actin, coextends with actin and appears to play a regulatory role in mediating actin and myosin interactions(see in picture). Each thin filament contains active sites to which myosin heads can bind.

Actin forms the backbone of the filament. Individual actin molecules are globular proteins(G-actin) and join together to form strands of actin molecules. Two strands then twist into a helical pattern, much like two strands of pearls twisted together.

Tropomyosin is a tube-shaped protein that twists around the actin strands. Troponin is a more complex protein that is attached at regular intervals to both the actin strands and the tropomyosin. Tropomyosin and troponin work together in an intricate manner along with calcium ions to maintain relaxation or initiate contraction of the myofibril.

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney