8.5 Types of Muscle Fibers

Skeletal muscle fibers can be classified based on two criteria: 1) how fast do fibers contract relative to others; and 2) how do fibers regenerate adenosine triphosphate (ATP). Using these criteria, there are three main types of skeletal muscle fibers recognized (Table 8.5.1). Slow oxidative (SO) fibers (also called slow twitch or Type I fibers) contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative (FO) fibers (also called fast twitch or Type IIa fibers) have relatively fast contractions and primarily use aerobic respiration to generate ATP. Lastly, fast glycolytic (FG) fibers (also called fast twitch or Type IIb/IIx fibers) have relatively fast contractions and primarily use anaerobic glycolysis. Most skeletal muscles in a human body contain all three types, although in varying proportions.

The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibers hydrolyze ATP approximately twice as rapidly as slow fibers, resulting in much quicker cross-bridge cycling (which pulls the thin filaments toward the center of the sarcomeres at a faster rate). The primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. If a fiber primarily produces ATP through aerobic pathways, then it is classified as oxidative. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate.

Oxidative fibers have structural elements that maximize their ability to generate ATP through aerobic metabolism. These fibers contain many more mitochondria than the glycolytic fibers, as aerobic metabolism, which uses oxygen (O₂) in the metabolic pathway, occurs in the mitochondria. SO fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. SO fibers are extensively supplied with blood capillaries to supply O₂ from the red blood cells in the bloodstream. The SO fibers also possess abundant amounts of myoglobin, an O₂-carrying molecule similar to O₂-carrying hemoglobin in the red blood cells. The myoglobin stores some of the needed O₂ within the fibers themselves (and gives SO fibers their red color). All of these features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.

The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. They do not produce high tension, and thus they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.

FO fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between FG fibers and SO fibers. They produce ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibers are useful for this type of movement because they produce more tension than SO fibers but are more fatigue-resistant than FG fibers.

FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly and to produce high levels of tension. Because of their reliance on anaerobic metabolism, they do not possess substantial numbers of mitochondria, have a limited capillary supply, and do not possess significant amounts of myoglobin. This results in a white coloration for muscles containing large numbers of these fibers. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods. Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle.

These different fiber types can be easily identified in poultry. Imagine a turkey. The legs and thighs of the turkey are dark meat, due to their SO fibers and robust supply of blood vessels and myoglobin. Turkeys spend most of their days walking around looking for food, so their legs must be able to work all day without fatiguing. Alternately, turkey breast is white meat, due to its FG fibers and relatively insubstantial supply of myoglobin and lesser blood supply. Turkeys do not fly long distances; they only need to get into trees to roost. Their breast tissue produces strong, rapid contractions, but only for very brief flights.

Glossary Flashcards

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