MUSCLE

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Slow TwitchMuscles that are used for extended periods of activity, such as standing or walking, they need a consistent energy source. The protein myoglobin stores oxygen in muscle cells, which use oxygen to extract the energy needed for constant activity.

Fast TwitchMuscles that are used for situations where quick bursts of activity are needed are made up of fibers called fast-twitch. These muscles get energy from glycogen.

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Most skeletal muscles contain some mixture of Type I and Type II fibers, but a single motor unit always contains one type or the other, never both

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Fish

float in water and don't need constant muscle energy to support their skeletons

most fish meat is white, with some red meat around the fins and tail, which are used for swimming

the red color of some fish, such as salmon and trout, is due to astaxanthin, a naturally occurring

pigment in the crustaceans they eat

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Cattle

spend a lot of time standing, and so their muscles are constantly being used

therefore, beef has a fairly high concentration of myoglobin and is dark red

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Pigs

spend quite a bit of time standing and roaming around

the pink color of pork is due to myoglobin, but because the animals used for pork are young and small, their muscles are less developed and do less work

so pigs have a lower concentration of myoglobin in their muscles than do cows

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Chickens

spend a lot of time roaming around or standing

their thigh and leg muscles are used constantly, and so the meat from these parts is dark

since they rarely fly, and then only for very short distances, the meat that comes from the breast and wings is white

in contrast, wild birds such as ducks fly a lot; the meat from their breasts and wings is dark

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Humans

have both types of fibers as well, however, unlike animals and fish, humans' fast- and slow-twitch fibers can't be delineated quite so neatly

both types are interspersed throughout the body; the average human has about 50% slow-twitch and 50% fast-twitch fibers

Professional athletes can have a higher percentage of one or the other type

Olympic sprinters may have as much as 80% fast-twitch fibers and long-distance runners may have as much as 80% slow-twitch; weight-lifters need fast-twitch fibers for quick bursts of strength while

long-distance swimmers need the constant movement provided by slow-twitch fibers

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Type I

loaded with mitochondria and depend on cellular respiration for ATP production

resistant to fatigue rich in myoglobin and hence red in color

activated by small-diameter, thus slow-conducting, motor neurons also known as "slow-twitch" fibers

dominant in muscles that depend on tonus, e.g., those responsible for posture

ATPase staining

type II fibers (dark); type I fibers (light).

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Type II

few mitochondria rich in glycogen and

depend on glycolysis for ATP production fatigue easily because of the buildup of lactic acid during glycolysis

low in myoglobin hence whitish in color activated by large-diameter, thus fast-conducting, motor neurons

also known as "fast-twitch" fibers

dominant in muscles used for rapid movement

reacted for enzyme NADH-TR;

type I fibers (dark); type II fibers (light)

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Muscular Dystrophies

Together myosin, actin, tropomyosin, and troponin make up over three-quarters of the protein in muscle fibers. Some two dozen other proteins make up the rest. These serve such functions as attaching and organizing the filaments in the sarcomere and connecting the sarcomeres to the plasma membrane and the extracellular matrix. Mutations in the genes encoding these proteins may produce defective proteins and resulting defects in the muscles. Among the most common of the muscular dystrophies are those caused by mutations in the gene for dystrophin.

The gene for dystrophin is huge, containing 79 exons spread out over 2.3 million base pairs of DNA.

The gene for dystrophin is on the X chromosome, so these two diseases strike males in a typical X-linked pattern of inheritance

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Duchenne muscular dystrophy (DMD)

frame shift mutation no dystrophin is synthesized and DMD, a very severe form of the disease, results

Becker muscular dystrophy (BMD)

If the deletion simply removes certain exons, a shortened protein results that produces BMD, a milder form of the disease

Dystrophin staining

Duchenne Muscular DystrophyNormal Muscle

Western blot of dystrophin from dystrophinopathies

Lane 1: Becker dystrophyLane 2: Becker dystrophyLane 3: NormalLane 4: Duchenne dystrophy

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