It has long been known that skeletal muscle fibers differ in diameter and in their natural color in vivo. The color differences are not apparent in hematoxylin and eosin (H&E)–stained sections. However, histochemical reactions based on oxidative enzyme activity, specifically the succinic dehydrogenase and nicotinamide adenine dinucleotide– tetrazolium (NADH-TR) reactions, confirm the observations seen in fresh tissue and reveal several types of skeletal muscle fibers (Fig. 11.3). The most obvious nomenclature muscle fiber muscle fascicle myofibril epimysium perimysium endomysium aa b FIGURE 11.2 • General organization of skeletal muscle. a. This freeze fracture scanning electron micrograph of an intramuscular connective tissue was obtained from the bovine semitendinous muscle. The specimen was routinely fixed for SEM and subsequently treated according to the cell maceration method with sodium hydroxide to remove muscle cells. Note a delicate honeycomb structure of the endomysium surrounding individual muscle cells. 480. (Reprinted with permission from Nishimura T, Hattori A, Takahashi K. Structural changes in intramuscular connective tissue during the fattening of Japanese Black Cattle: effect of marbling on beef tenderization. J Anim Sci 1999; 77:93–104.) b. This schematic diagram shows the general organization of skeletal muscle and its relation to the surrounding connective tissue. Note the organization of the endomysium that surrounds individual muscle cells (fibers), the perimysium that surrounds a muscle bundle, and the epimysium that surrounds the entire muscle.
FIGURE 11.3 • Cross section of skeletal muscle fibers. This cross section of muscle fibers stained with the NADH-TR reaction demonstrates two fiber types. The deeply stained, smaller muscle fibers exhibit strong oxidative enzyme activity and correspond to the type I slow oxidative fibers. The lighter-staining, larger fibers correspond to the type IIb fast glycolytic fibers. 280. Inset. Portions of the two fiber types at higher magnification. The reaction also reveals the mitochondria that contain the oxidative enzymes. The contractile components, the myofibrils, are unstained. 550. (Original slide specimen courtesy of Dr. Scott W. Ballinger.) to describe these differences is division into red, white, and intermediate fibers.
Skeletal muscle fibers are characterized by speed of contraction, enzymatic velocity, and metabolic activity.
The current classification of skeletal muscle fibers is based on contractile speed, enzymatic velocity of the fiber’s myosin ATPase reaction, and metabolic profile. The contractile speed determines how fast the fiber can contract and relax. Velocity of the myosin ATPase reaction determines the rate at which this enzyme is capable of breaking down ATP molecules during the contraction cycle. The metabolic profile indicates the capacity for ATP production by oxidative phosphorylation or glycolysis. Fibers characterized by oxidative metabolism contain large amounts of myoglobin and an increased number of mitochondria, with their constituent cytochrome electron transport complexes. Myoglobin is an oxygen-binding protein that closely resembles hemoglobin found in erythrocytes and occurs in varying amounts in muscle fibers. It provides a ready source of oxygen for muscle metabolism.
The three types of skeletal muscle fibers are type I (slow oxidative), type IIa (fast oxidative glycolytic), and type IIb (fast glycolytic) fibers.
Three types of fiber are typically found in any given skeletal muscle; the proportion of each type varies according to the functional role of the muscle.