Skeletal muscle fiber


Skeletal muscle fiber cells are noted for their unusual elongated characteristics. Despite their unusual characteristics, they still are identical to the other cells’ organelles. They are comprised of mitochondria, intracellular membranes, glycogen granules just as other muscle cells. Their basic structural differences are found in their striation, as well as their multi nucleus design.


A 12 inch muscle fiber is not uncommon in the human body, which another specific unique characteristic of skeletal muscles. The diameter of a single muscle fiber can range anywhere from 10 micrometers to 100 micrometers. Sarcolemma, a unique cell membrane, completely encases each individual muscle fiber. The cytoplasm of the muscle fiber is determined as the sarcoplasm. Through the sarcoplasm there is an intricate network of membranous channels known as the sarcoplasmic reticulum. Perpendicular to the sarcoplasmic reticulum is the system of transverse tubules. This is often referred to as T tubules. The T tubules run through to the outside of the muscle fiber, leaving through the sarcolemma. Myofibrils are found throughout the muscle fiber. They appear to be additional threads running through the striations of the fibers. Myofibrils run continuously from one end of the muscle fiber to the other, and measure about 1 micrometer in diameter. The myofibrils are so numerous and are crowded so closely together that the other elements of the fibers creation, such as the mitochondria or the intercellular membranes, are forced into the minute spaces in between the myofibrils in the sarcoplasma.

The individual myofibrils are made up of myofilaments, which are small protein fibers. The protein fibers are divided into two categories, thin and thick. The thin fibers are comprised of actin and measure about 6 nm while the thick fibers are comprised of myosin and measure about 16 nm in diameter. The myofilaments are responsible for the dark on light striated visual effect of muscle tissue. The dark striations are labeled A bands while the lighter striations are labeled I bands. When viewed under a highly intense microscope, additional dark striations can be seen within the light striations. These additional dark striations have been labeled Z bands. This pattern indicates the necessary thick and thin variations that is necessary for the subunit of the muscle to experience contraction. The creation of striations in between the Z bands creates the necessary conflict pull of skeletal muscle fibers that allows for and even encourages contraction. Every subunit that is able to create the contraction, more readily recognizable as the striations from one Z band to the next, is known as sarcomeres. A side view of sarcomeres can be viewed when a cross section of the myofibril is taken.


Skeletal Muscle Fibers
Image: Skeletal Muscle Fibers

When viewing the I bands within the myofibril, it can be clearly seen that I bands are considered to run from one edge of the stack of filaments to the next. They take on a lighter shade due to the thin fibers of their creation. Despite the appearance of a distinctive border, the I bands continue past the edge of the stack, interweaving with the thicker myofilaments less obtrusively. The A bands have a darker edge than their center as a result of the overlapping thick and thin filaments at each edge of their stack. The middle section where the hue is obvious lighter is called the H zone, named for the German word for bright, helle. The H zone is comprised of only the thicker filaments. A myofibril cross section is the only way to see the zones in their entirety and as they relate to each other. In any given transverse cross section of the filaments, the darker filaments can be traced all the way through the slide, exposing and highlighting the patterns and relationships discussed here.


Muscle contraction is achieved through the contraction, or shortening, of individual fibers acting as a whole. The muscle then “shortens,” creating a contraction action. The muscle fibers shorten only when they shorten the myofibrils. This is achieved when the distance from one Z line to the next is decreased. The A bands do not shorten when the sarcomers do, but rather they are become more dense, narrowing the space between them. The I bands decrease in length, which successfully represents the increased or decreased distances between the A bands’ successive myomeres. The actin, which creates the I bands, remain the same length whether the muscle is in contraction or relaxed. Thick and thin myofilaments remain the same length regardless of muscle contraction or lack thereof. The sarcomeres are shortened by the action of sliding thick and thin filaments over each other rather than by shortening.

During a muscle contraction, the thin filaments that are found on the edges of each A band are driven deeper into the tissue, creating an exemplified increased in the overlap with the thick filaments. This results in a shortening of the H bands in the center during muscle contractions.
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