Types of muscle fibers

False negative muscle contraction tests are very rare. To date, a negative muscle contraction test rules out MH. A false negative test can be explained by the presence of two types of muscle fibers in a MH susceptible patient the response being dependent on the proportion of the two types of muscle fibers. The K-type designation is used to describe a patient who has a positive joint halothane-caffeine contracture, but a negative separate halothane or caffeine contracture. Whether K-type individuals are MH-susceptible or not is a controversial issue. 

Table 49-11. Types of muscle fibers and major fuel sources used by a sprinter and by a marathon runner.

Two major types of muscle fibers are found in humans white (anaerobic) and red (aerobic). The former are particularly used in sprints and the latter in prolonged aerobic exercise. During a sprint, muscle uses creatine phosphate and glycolysis as energy sources in the marathon, oxidation of fatty acids is of major importance during the later phases. Nonmuscle cells perform various types of mechanical work carried out by the structures constituting the cytoskeleton. These strucmres include actin filaments (microfilaments), micrombules (composed primarily of a- mbulin and p-mbulin), and intermediate filaments. The latter include keratins, vimentin-like proteins, neurofilaments, and lamins. 

Distinguish among the three types of muscle fibers slow-twitch oxidative, fast-twitch oxidative, and fast-twitch glycolytic... 

Two major differences between the types of muscle fibers are ... 

As such, three types of muscle fibers (see Table 11.1) exist ... 

The same distinctions hold in humans. Sprinters and marathon runners have different proportions of muscle fibers, and therefore different metabolisms. Sprinters have relatively more fast white fibers, and can run very rapidly, but not for long distances. Marathon runners, on the other hand, have more slow red fibers and can carry out aerobic metabolism for very long periods of time, although they can t go as fast. Well-trained, world-class runners may have as much as 90 percent of their leg muscle of one type or the other, depending on their sport. Some sports, such as basketball and soccer, involve both aerobic endurance and anaerobic sprinting these athletes tend to have both types of muscle fiber. Untrained individuals have about 50 percent of each type. The relative contributions of training and heredity to each type of metabolism remain unknown, although both play some part. 

Type of muscle fiber Type I Red muscle Aerobic exercise Type II White muscle Anaerobic exercise... 

Fiber type. Skeletal muscle has several distinct fiber types. Type I is used primarily for aerobic activity, whereas type II is specialized for short, intense bursts of activity. How could you distinguish between these types of muscle fiber if you viewed them with an electron microscope ... 

Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiber is the most common. Intrafusal fibers, which make up the central region of the muscle, are weaker than extrafusal fibers. Skeletal muscles fibers are additionally characterized as fast or slow based on their activity patterns. Fast, also called white, muscle fibers contract rapidly, have poor blood supply, operate anaerobically, and fatigue rapidly. Slow, also called red, muscle fibers contract more slowly, have better blood supplies, operate aerobically, and do not fatigue as easily. Slow muscle fibers are used in movements that are sustained, such as maintaining posture. 

When one examines muscle tissue that has been surgically removed, one finds two predominant types of muscle fibers. Fast twitch muscle fibers are large, relatively plump, pale cells. 

Relative paudty of one type of muscle fiber in a patient s biopsy can be caused, hypothetically, by (a) preferential impairment of the corresponding type of LMNs or Schwann cells (b) if both LMN types are equally abnormal, preferentially more successful sprouting and reinnervation ability of the opposite type of LMNs (c) if there are large groups of both types of muscle fibers in a chronic reinnervation sitnation, biopsy sampling could produce a non-representative impression of paudty or (d) preferential myopathic loss of that type of muscle fiber. (We use the term fiber-type paucity and not fiber-type predominance because it is more likely that the muscle fibers that are too few reflect the abnormal status.)... 

Importantly, the precise electrical stimulus protocol chosen also impacts the training effect on the muscle. In particular, the frequency of stimulus delivered likely determines the types of muscle fibers activated. Slow-twitch fibers normally have a firing frequency of -10 Hz, whereas fast-twitch fibers have a firing frequency of 30 Hz (97). A TCEMS stimulus frequency up to 10 Hz would, therefore, preferentially activate slow-twitch fibers and may selectively improve resistance to fatigue (101), whereas frequency greater than 30 Hz may activate both types of fibers or may selectively recruit fast-twitch fibers and... 

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