Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 69

Muscle tissue obtains energy as adenosine triphosphate (ATP) and phosphocreatine kinase from both the aerobic metabolism of fatty acids and glucose and the anaerobic glycolysis. Skeletal muscle cells can be divided into three subtypes based on their metabolic and histochemical characteristics as well as their myosin heavy‐chain subcomponents. Type I fibers, also known as slow‐twitch fibers or “red” fibers, are small in size, contain many mitochondria and large amounts of myoglobin and cytochromes, and have high type I myosin heavy‐chain content (Figure 3.5). Their glycolytic enzyme content is low. Myoglobin is an iron protein that binds O₂ and is the feature that makes these fibers appear dark red in color. Type I fibers obtain their energy primarily from aerobic oxidative phosphorylation of fatty acids. As a consequence, they are adapted for slow contractions over prolonged periods. Postural muscles of the back contain many type I fibers.

In contrast, type IIb, or fast‐twitch fibers or “white” fibers, have an opposite metabolic profile. They are also large in size. They have abundant glycogen for the generation of APT via anaerobic metabolism, but fewer mitochondria and less myoglobin, giving them a pale whitish color. As a consequence, they depend on anaerobic glycolysis for energy and are adapted mainly for rapid contractions, although they undergo rapid physiological fatigue. These fibers are recruited during short‐duration high‐intensity activity, such as short sprints and maximum weight lifting.

Physiologically intermediate between slow and fast fibers are type IIa or intermediate fibers. They are also intermediate in size. They have many mitochondria and high myoglobin content and also contain a high amount of glycogen. Since they utilize both oxidative metabolism and anaerobic glycolysis, they are adapted for both rapid contracts and short bursts of energy. They are also intermediate in color and energy metabolism. There is high percentage of type IIa fibers in muscles used during sustained power activities, such as sprinting 400 m.

Experiments using myosin heavy‐chain isoform immunostaining has also revealed an additional type of fiber, type IIx, that does not stain with antibodies against type I or II antibodies (and are thus unstained, as shown in Figure 3.5) (Pierobon‐Bormioli, Sartore, Libera, Vitadello, & Schiaffino, 1981; Schiaffino, 2010). Interestingly, if nerves to slow and fast type fibers are exchanged experimentally, the fibers change their morphological and physiological features to conform to the innervating nerve.