| Muscle consists of two types of striated muscle cells; fast-glycolytic and slow-oxidative fibers
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| Striated muscle cells are generally classified by their physiological contractile properties (fast versus slow) that are determined by the level of ATPase activity and the primary metabolic source of ATP synthesis (anaerobic vs. aerobic). The muscle type is closely related to muscle function. In skeletal muscle this comparison is easily seen with muscles whose contraction is necessary to continuously maintaining posture versus muscle contraction for infrequent-burst activities. The two striated muscle types are readily distinguished in skeletal muscle by coloring. Fast-glycolytic muscle is white in appearance because of less blood flow, lower mitochondrial density, and decreased myoglobin content than slow-twitch oxidative muscle, which is red. Fast-glycolytic fibers also have lower fat content and increased glycogen stores. The fast-glycolytic fibers, rely on glycogen and anaerobic glycolysis for short bursts of contraction when additional muscle force is required such as in the 'fight or flight' stress response. These muscle fibers are not capable of sustaining contraction for long periods, when compared to slow-oxidative fibers. In contrast, slow-oxidative fibers are well perfused with blood, rich in mitochondria (cytochromes) and myoglobin. This muscle type has the ability to sustain low-intensity contractions for long periods. Slow muscle uses fatty acid oxidation for ATP synthesis, which requires mitochondria. Cardiac muscle, which is continuously contracting, has many contractile and metabolic characteristics that are similar to slow-oxidative skeletal muscle. Cardiac muscle is well perfused with blood, rich in mitochondria, and relies largely on oxidative metabolism of circulating fatty acids.
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