Cross section of white and red 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 red muscle fibers. The lighter-staining, larger fibers correspond to the white fibers. x280. 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. X550. (Original slide specimen courtesy of Dr. Scott W. Ballinger.)
• Red fibers are small fibers with large amounts of myoglobin and cytochrome complexes and many mitochondria. They make up slow-twitch motor units (a twitch is a single, brief contraction of the muscle). Red fibers have great resistance to fatigue but generate relatively less muscle tension than white fibers. Myosin adenosine triphosphatase (ATPase) activity, essential for contraction, is greatest in red muscle fibers. The large numbers of mitochondria in red fibers are characterized by high levels of oxidative enzymes, as demonstrated by strong succinic dehydrogenase and NADH-TR histochemical staining reactions (see Fig. 10.2). Red fibers are typically found in the limb muscles of mammals and in the breast muscle of migrating birds. More importantly, they are the principal fibers of the long muscles of the back in humans, where they are particularly adapted to the long, slow contractions needed to maintain erect posture.
• White fibers are large fibers with less myoglobin and fewer cytochromes and mitochondria. They make up fast-twitch motor units, fatigue rapidly, and generate a large peak muscle tension. Thus, white fibers are adapted for rapid contraction and precise, fine movements. They constitute most fibers of the extraocular muscles and the muscles that control the movements of the digits. White fibers have a greater number of neuromuscular junctions than do red fibers, thus allowing more precise neuronal control of movements in these muscles.
• Intermediate fibers are of intermediate size. The amount of myoglobin and the number of mitochondria they contain are also intermediate between those of red and white fibers.
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