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MUSCLES Muscles Fibres Types There are three types of muscles in the human body - skeletal, smooth and cardiac, which differ in their structure, innervation, and function. This lab deals almost exclusively with the skeletal muscle system, which also has three different types categorized by their maximal shortening velocity and the major pathway used to form ATP. There are fast and slow fibers, and glycolytic or oxidative fibers. Skeletal muscle has myosin isozymes which differ in their maximal rates of ATP splitting, which affects the rate of cross-bridge cycling, and hence the rate of contraction. Fast fibers have isozymes with high ATP-ase activity, while slow fiber isozymes have low activity. Oxidative fibers are supplied with numerous small blood vessels, lots of myoglobin, many mitochondria, and thus a high capacity for oxidative phosphorylation (the regeneration of ATP from various energy stores). The myoglobin affects the diffusion rate for oxygen, and also gives the muscle its characteristic red appearance. Because of all of this cellular capability for ATP production, oxidative fibers fatigue far slower than glycolytic fibers. Glycolytic fibers have low mitochondria levels, but large stores of glycogen and glycolytic enzymes. Because they are typically not as rich in myoglobin, they are white in appearance. Glycolytic fibers are also somewhat larger in diameter, and can produce more force because of this greater cross-sectional area. Below are listed the various muscle types with some of their properties. A similar and more complete table can be found in Vander. Fiber Type ATP-ase Metabolism Fatigue Color Slow-oxidative Low Oxidative Slow Red Fast-oxidative Hi Oxidative Intermediate Pink Fast-glycolytic Hi Glycolytic Fast White Muscle Tension The action-potential frequency critically affects the force produced because The amount of tension (or force) developed by a muscle is dependent upon the number of fibers and the tension developed by each fiber. The number of fibers used depends on the motor unit organization of the muscle and the number of active motor units. The tension in the individual fibers is determined by fiber length, fiber diameter, fatigue, and the stimulation frequency. The fiber length changes as the muscle is stretched out, or relaxed. Because there must be some overlap between the thick and thin filaments, the muscle can not be pulled too far apart. Likewise, the muscle cannot achieve maximum force generation if the thin filament themselves overlap, because the maximum number of cross-bridge sites is not presented. The optimum length is that which simply positions the thick and thin filaments for greatest non-interfering overlap. Fiber diameter is influenced by fiber type (fast or slow), with glycolytic fibers have larger cross-sectional areas and greater force production. Fatigue is also influenced by fiber type, (see above), and duration of activity. multiple stimuli, if timed close enough together (in other words, with a relative high frequency) will add up in summation. With one single stimulus, the muscle produces one twitch. If two stimuli are given, the muscle twitches twice. If the second stimuli is timed so that it just overlaps the first, the two will add to produce greater tension. With multiple stimuli of high frequency (little time between each) the muscle can achieve tetanus, or a sustained contraction. Tetanus tension can be 3-5 times that of a single twitch. The frequency required for maximal tetanus varies with fiber type because of the different contraction times.