A myofiber is a multi-nucleated single muscle cell. Grouped together into bundles known as fascicles, and sheathed in connective tissue, myofibers are the basic cellular unit of skeletal muscle. Also known as muscle fibers, myofibers are large, highly specialized cells that are mostly packed with contractile elements. These cells can be broadly classified as either fast twitch or slow twitch, based on the speed at which contraction occurs, and further categorized based on the metabolic processes used to power cellular activities.
While most animal cells typically contain a single nucleus per cell, myofibers contain many. Muscle tissue is mostly complete at birth, and although cells might continue to increase in size, they do not usually multiply by mitosis the way most cells other do. As they grow larger, it becomes increasingly difficult for a single nucleus to govern the entire cell. This is known as the myonuclear domain theory. When a muscle fiber grows, the myonuclear domain theory dictates that additional nuclei are needed to keep up with the increase in cell size.
Surrounding each myofiber are undifferentiated cells known as satellite cells. Similar to stem cells, these cells are able to take on a number of forms. When muscle cells are stimulated to grow, the process triggers immune and hormonal responses that stimulate nearby satellite cells to increase in number and begin differentiation. They are then incorporated into the muscle fiber as needed, and eventually become part of the muscle cell itself.
The speed of muscular contraction within a single myofiber is determined largely by the activity of a particular enzyme within the cell. ATPase governs the rate at which the energy intermediary adenosine triphosphate (ATP) is broken down to release phosphate ions, which in turn power cellular contraction. Higher ATPase activity leads to faster muscular contraction. Fast twitch muscle cells are associated with a higher level of ATPase activity, while slow-twitch muscle cells experience a lower level of it.
Muscle cells can be further divided based on predisposition for particular metabolic processes. Most cells power activity by some combination of glycolysis and oxidative phosphorylation. Glycolysis is the process by which cells break down carbohydrates to form ATP. This normally occurs within the cytoplasm of the cell with limited oxygen present, and can create lactic acid as a byproduct.
Oxidative phosphorylation, by contrast, occurs in the mitochondria of the myofiber, and consumes a great deal of available oxygen. Oxidative phosphorylation is a more efficient process than glycolysis, yielding significantly more ATP per unit of nutrients than glycolysis, and doing so without producing the muscle-fatiguing lactic acid. As a result, fibers using this method are more resistant to fatigue than glycolytic fibers.
Normally, both metabolic processes occur in all muscle cells, but most myofiber types are better equipped for one process than the other. Oxidative fibers require significantly more oxygen than glycolytic fibers, and are therefore rich in the oxygen-binding protein myoglobin. Oxygenated myoglobin tends to give muscle fibers a characteristic red hue, and as a result oxidative fibers are often referred to as red fibers. Glycolytic fibers, by contrast, do not have the same concentration of myoglobin, and are often termed white fibers.
In general, slow twitch muscle fibers primarily employ the more efficient oxidative phosphorylation, and are termed Type I fibers. They are associated with muscles that perform low-energy activities over a long period of time, such as the muscles of the neck or the stabilizer muscles of the body's core. Among athletes, this type of muscle fiber is predominant in the muscles of highly-specialized endurance athletes, such as marathon runners.
Fast twitch muscle fibers can employ either glycolysis or oxidative phosphorylation. Like the slow twitch fibers, oxidative fast twitch fibers, known as type IIa fibers, are packed with mitochondria and myoglobin. Glycolytic fast twitch fibers, known as type IIx, possess an abundance of available glycogen, are adapted to brief bursts of intense power, and are common in the muscle tissue of power athletes, like sprinters and power lifters.