 This video will cover the following objective from muscle physiology. Distinguish between fast glycolytic, fast oxidative, and slow oxidative muscle fibers. Explain the characteristics of each. Compare the effects of aerobic versus resistance exercise on muscles. Define fatigue, atrophy, sarcopenia, hypertrophy, and hyperplasia. When an action potential spreads through a muscle fiber, it stimulates a single all-or-nothing response known as a twitch. But when a muscle is stimulated at high frequency, twitches can overlap leading to summation and increase in tension with increasing frequency of stimulation. Tetanus is when twitches are at such a high frequency, the muscle doesn't relax before the next twitch arrives leading to a smooth sustained contraction. However, at high frequency stimulation with sustained stimulation, eventually muscle tension will decline through a process known as fatigue. So fatigue is a decline in muscle tension as a result of previous contractile activity and physiological muscle fatigue results from depletion of ATP inside of muscle fibers. In contrast, psychological fatigue occurs when the nervous system decreases stimulation of the muscle. This could be a result of pain, coming from the muscle, or it could be a result of loss of focus and motivation. Muscle fibers are classified into three major types. Type 1 slow oxidative fibers have a relatively slow minus an ATPase and primarily rely on aerobic cellular respiration to provide ATP. Fast glycolytic fibers, or type 2a fibers, have a fast myosin ATPase and primarily rely on anaerobic cellular respiration to provide ATP. And type 2b muscle fibers are fast oxidative fibers, which have a fast myosin ATPase and primarily rely on aerobic cellular respiration to provide ATP. So slow oxidative fibers have a high concentration of mitochondria, which are responsible for aerobic cellular respiration. They also have a high amount of capillaries, the small blood vessels where the exchange of nutrients and waste occurs. Slow oxidative fibers also have a high concentration of myoglobin, the protein that stores oxygen inside of muscle fibers, and a relatively low concentration of glycogen, the carbohydrate that serves as a storage form for glucose. Slow oxidative fibers are also relatively smaller in diameter compared to the other muscle fiber types. Together, these features lead to a relatively high resistance to fatigue in slow oxidative muscle fibers. In contrast, fast glycolytic fibers have a low amount of mitochondria, a low amount of capillaries, a low amount of myoglobin, and a high storage of glycogen. Fast glycolytic fibers are also relatively large, but fast glycolytic fibers are relatively vulnerable to fatigue. They have a low resistance to fatigue. So the fast glycolytic fibers can respond quickly and forcefully. However, they cannot sustain contraction for a long period of time, they fatigue easily. Then the fast oxidative fibers are intermediate between slow oxidative and fast glycolytic, in that the fast oxidative fibers have intermediate resistance to fatigue. They can produce more rapid contractions, but they cannot sustain those contractions as long as the slow oxidative fibers. Here we see the histology of skeletal muscle tissue that contains all of the major muscle fiber types. We can see the slow oxidative fibers are darker in the image as a result of the high concentration of myoglobin. The fast oxidative fibers have a lighter color but still a reddish color as a result of an intermediate concentration of myoglobin, and fast glycolytic fibers that have a very low concentration of myoglobin appear white. Aerobic cellular respiration fuels slow oxidative fibers to enable endurance exercise. Aerobic exercise such as long distance running stimulates the growth of new blood vessels, a process called angiogenesis, and also stimulates the production of the protein myoglobin. Long distance runners have a relatively large number of slow oxidative fibers and relatively few fast oxidative and fast glycolytic fibers. It is still unclear if this reflects adaptation to exercise training. Aerobic exercise may stimulate the switching of fiber types where some fast oxidative fibers may switch to become slow oxidative fibers. However, it is also possible that individuals that naturally have a higher number of slow oxidative fibers have a preference for endurance exercise. In contrast, resistance exercise involves rapid forceful contractions, and resistance exercise such as weight lifting is the type of exercise that bodybuilders use to increase muscle mass. Bodybuilders have a relatively large number of fast glycolytic fibers. The large fibers that produce rapid forceful contractions, and they have relatively fewer fast oxidative and slow oxidative fibers. Now, as bodybuilders are performing resistance exercise leading to increased muscle mass, the muscles grow through a mechanism known as hypertrophy, where the individual muscle fibers become larger. Each individual muscle fiber, as it hypertrophes, develops more myofibrils within each cell. Hypertrophy is when a tissue grows as a result of the cells increasing in size. So, hypertrophy is the response to resistance exercise that causes our muscles to become larger. The individual myofibers inside of our muscles become larger as they have a larger number of myofibrils inside of each myofiber. So, each cell inside of a skeletal muscle organ becomes larger, but the number of cells does not increase. In contrast, hyperplasia is when a tissue increases in size as a result of cell division. So, smooth muscle grows through pregnancy as a result of hyperplasia, where cell division produces more smooth muscle cells within the tissue. In contrast to hypertrophy and hyperplasia that increase muscle mass, atrophy is a mechanism that leads to decreased muscle mass. So, atrophy is a decline in the mass of a tissue and atrophy can result from loss of use. So, if you don't use your muscles, if you don't exercise, your muscles will atrophy. Sarcopenia is a specific type of atrophy. Sarcopenia is normal age-related muscle atrophy.