On page 309 of The Evidence section of The Genius in All of Us, David Shenk states that "in musculature, we are not all created equal. Although on average, humans beings have about a fifty-fifty mix of slow and fast-twitch muscle fibers, some are born with differing proportion", completely going against the GxE model. However, Shenk refutes the claim, stating that "muscles are tremendously adaptive to external stimulus" (309) as well. How can muscles change due to their environmental factors? Cite from the text and additional external sources if necessary. In addition, what abiotic or biotic factors contribute to this adaptation? What events in the past allowed human beings to evolve into having "on average... a fifty-fifty mix of slow and fast-twitch muscle fibers"? How do the structure of the different muscle fibers relate to the task that they must carry out?
~Jimmy Chang
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ReplyDeleteEach muscle fiber is a one long cell with many nuclei. Each fiber contains myofibrils, which are long thin strands of muscle filaments divided into units called sarcomeres. Each sarcomere is composed of a certain arrangement of thin filaments made of two strands of actin and thick filaments made of strands of staggered myosin molecules. Thin filaments at the end of each sarcomere and extend toward the middle but don't reach the thin filaments from the opposite side, and thick filaments are attached at the center of the sarcomere and extend toward, but do not reach, the ends of the sarcomere. When the muscle fiber is relaxed, the filaments partially overlap. In muscle contraction, the filaments slide past each other and increase the amount they overlap, making the sarcomere shorter. This occurs when ATP molecules bind to the myosin heads of the thick filament and activate the myosin molecules into a high energy form. Being in a high energy form allows the myosin molecules to bind to actin and form a cross bridge that pulls the thin filament toward the center of the sarcomere. Different muscle fibers have difference sources of ATP, depending on their function. Fast-twitch muscle fibers contract very quickly for a short amount of time, and they can be oxidative or glycolytic, meaning that their ATP can come from aerobic respiration or glycolysis. Slow-twitch fibers do not contract as quickly as fast-twitch fibers, but they are able to remain contracted for a longer time. Therefore, slow-twitch fibers are oxidative fibers because the quickness of glycolysis compared to aerobic respiration is not necessary, and performing anaerobic respiration to produce enough ATP to sustain a contraction over a long amount of time is inefficient (Campbell 1108-1111).
ReplyDeleteSome research has shown that if a person regularly does activities that require high muscular endurance, fast-twitch glycolytic fibers can adapt and turn into fast-twitch oxidative fibers so that they can sustain longer contractions more efficiently, which makes them better suited to endurance training (Campbell 1111). In response to regular exercise that requires either more fast or slow fibers than a person currently has, “slow-twitch muscle fibers can become transformed into fast-twitch fibers, and vice versa” (Shenk 311). More recent research also shows that exercise affects the expression of genes that make proteins that regulate mitochondria, the main source of ATP for oxidative muscle fibers. By analyzing cells from the thighs of adults after they completed a light or heavy cycling workout, the scientists found that the DNA of the muscle cells was less methylated after the heavy workout than after the light workout, allowing for more transcription of DNA and translation of RNA into proteins to act on mitochondria. This experiment is an example of Christine's point about increasing the amounts of mitochondria, fats, and carbohydrates so that more energy is able to be produced to provide active muscles with enough ATP. Caffeine is an abiotic factor that influences muscle contraction. Caffeine brings more calcium to muscle fibers, and because calcium binds to troponin on thin filaments to open up binding sites for active myosin, caffeine can cause increased muscle contractions, as well as less methylation around the gene that codes for proteins involved in regulation of mitochondria.
(http://www.sciencenews.org/view/generic/id/339000/title/Exercise_brings_on_DNA_changes).
Although on average humans have a 50-50 mix of fast and slow-twitch muscle fibers, different muscles of the body are made up of different combinations of the two types. Eye muscles, for example, are made up of all fast-twitch fibers because eye movements need to be quick, but lower leg muscles and back muscles that maintain posture are composed mostly of slow-twitch fibers because they need endurance rather than speed to function ideally (cont. next post)
Adele Padgett adele.padgett@gmail.com
(http://www.bbc.co.uk/science/humanbody/body/factfiles/fastandslowtwitch/soleus.shtml). Also, the average percentages of fast and slow muscle fiber do not tell that much about the evolutionary history of muscle fibers because, Shenk quotes from Jesper Andersen's article “Muscle, Genes, and Athletic Performance,” “humans show great variation in this regard; we have encountered people with a slow fiber percentage as low as 19 percent and as high as 95 percent in the quadriceps muscle,” a muscle that usually contains about the same amount of fast and slow-twitch muscle fibers. Humans may have on average about 50% fast-twitch fibers and 50% slow-twitch fibers because the wide variation across the human population balances out to about equal total amounts of fast and slow fibers across the whole population. Andersen's article includes a bar graph that visually represents the relative amounts of fast and slow muscle fibers in people with spinal injuries, world class sprinters, average couch potatoes, average active people, middle distance runners, world class marathon runners, and extreme endurance athletes. The percentage of fast-twitch muscle fibers is very high in the person with the spine injury and decreases steadily across the other groups to a very low percentage in the extreme athlete. The slow twitch muscle fiber percentages are the exact opposite, balancing out against the fast fiber percentages. There are evolutionary advantages for both types of muscle fibers, and some muscle fibers of both types are needed to live healthily, so there was no natural selection to favor one type of muscle fiber over another type (http://www.learningmethods.com/downloads/pdf/muscle,.genes.and.athletic.performance—scientific.american.sept.2000.pdf).
ReplyDeleteI disagree with Christine that changes in muscle fibers reflect the theme interdependence in nature because changes in human muscle fibers aren't vital to being an important part of an ecosystem, which is what the theme is about. Instead, I would relate changes in muscle fibers to the theme of structure and function. The function of muscle fibers is to contract, and this is clearly seen in the sliding filament model of their structure. Slow muscle fibers (which are usually oxidative and contain mitochondria and appear red because of myoglobin and because they are close to capillaries to get oxygen) are structurally different from glycolytic fast-twitch fibers (which don't need mitochondria, myoglobin, or oxygen) because they get ATP from glycolysis.
Adele Padgett adele.padgett@gmail.com
Well first of all, the fact that humans are born with different proportions of slow and fast-twitch muscle fibers doesn’t completely go against the G X E model. The G X E model displays how both genetics and the environment play important roles in an individual’s development. It’s not saying only environmental factors decide outcomes and acknowledges genetics play just as important role in future athletic ability, intelligence, etc. At the core of this is interdependence in nature at play. It is “the dependence of every form of life on other living things and on the natural resources in its environment, such as air, soil, and water” (http://kids.britannica.com/comptons/article-273213/ecology). This is based on the ecology level but can also be applied to the genetics level. The phenotype of an individual is dependent on both inherited genes and the biotic and abiotic factors in the environment. This concept is displayed by the muscular structure of humans. Genetics may determine initial muscular structure but environmental factors have the potential of changing that structure. Yet even as environmental factors change the muscular structure, they often interact with genes in order to do so. Therefore, genes and environment go hand in hand with development.
ReplyDeleteThe ways muscles can change due to environmental factors are discussed in The Genius in All of Us. The key to all of the changes is that muscles need to be pushed beyond the ordinary level of exertion in order for muscles fibers to change. One way that muscles change is through neural response, “an increase in the neural drive stimulating muscle contraction” (310). This is caused by an increase in the level of exertion, which causes a greater demand of efficiency from the muscles. For example, in order for people to start lifting weights suddenly, a neural response must occur for the muscles in the arm to contract and lift the weights. For muscles in the arm that are unused to exerting the amount of energy needed to lift weights, this neural response helps both the brain and the arm muscles adapt to lifting weights.
Another way for muscles to change is through genetic responses, displaying the G X E model. External environmental factors, such as an increase in certain types of exercise, initiate genetic responses in the cells of muscle fibers. For example, “in response to extended (aerobic) exercise – e.g. jogging – there is a genetic response in the nucleus of each cell fiber…increasing the number of mitochondria and provoking an increase in surrounding capillaries and the accumulation of fats and carbohydrates” (310). Fats and carbohydrates are sources of energy in which the mitochondria can harness for ATP, and an increase in capillaries increases the amount of blood and oxygen that reaches the muscles. Therefore, this response helps adapt to aerobic, endurance activities as energy can be supplied for over a longer period of time. A similar genetic response to overload/resistance exercise makes muscle fibers become strong and larger. This increases the strength of short bursts of energy used in exercises such as weight-lifting.
Going back to the proportions of slow and fast-twitch muscle fibers, it also has been observed that certain types of training can change that proportion, supporting Shenk’s claim that these proportions are not set in stone. Slow-twitch fibers “are more efficient at using oxygen to generate more ATP for continuous, extended muscle contractions over a long time,” explaining why they are ideal for endurance sports (http://sportsmedicine.about.com/od/anatomyandphysiology/a/MuscleFiberType.htm). They also fire more slowly than fast twitch fibers, hence the name “slow-twitch fibers”. Because slow-twitch fibers assist in endurance sports, it makes sense that intense endurance training, like running, can transform fast-twitch fibers into slow-twitch muscles since slow-twitch fibers would be more useful.
Christine Zhao (c_zhao@ymail.com)
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ReplyDeleteOn the other hand, fast-twitch fibers use anaerobic energy for short, quick, and strong muscle contractions over a short amount of time. This is ideal for strength-based, short-burst sports like weight-lifting. Therefore, intense training of this type of sport would transform slow-twitch fibers into fast-twitch fibers. Since fast-twitch fibers would be more useful, this allows the body to be as efficient as possible and not waste storing energy in slow-twitch fibers (and vice versa).
Slow and fast-twitch fibers also provide an example of the relationship between structure and function: “The structural levels from molecules to organisms ensure successful functioning in all living organisms and living systems” (http://apbio12007.blogspot.com/2007/11/theme-5-relationship-of-structure.html). Indeed structure and function often go hand in hand. Slow-twitch fibers “contain large amounts of myoglobin, many mitochondria and many blood capillaries” (http://www.brianmac.co.uk/muscle.htm). “Myoglobin has oxygen attached to it, which provides extra oxygen for the muscle to keep up a high level of activity for a longer period of time” (http://www.nlm.nih.gov/medlineplus/ency/article/003664.htm). Many mitochondria help supply a constant supply of usable energy in the form of ATP and many blood capillaries increase the amount of blood that can get to the muscles. This structure of slow-twitch fibers therefore reflects its function in long, endurance activities. On the other hand fast-twitch fibers found in humans “contain a low content of myoglobin, relatively few mitochondria, relatively few blood capillaries and large amounts glycogen” (http://www.brianmac.co.uk/muscle.htm). This structure that is the opposite of the structure of slow-twitch fibers reflects the opposite function of fast-twitch fibers. They generate ATP by anaerobic metabolic processes (thus less of a need for mitochondria) and are useful instead for short bursts of strength.
The immense ability of muscles fibers to change reveals an adaptation evolved in humans. Evolution is “the theory that groups of organisms change with passage of time, mainly as a result of natural selection, so that descendants differ morphologically and physiologically from their ancestors”(http://necsi.edu/projects/evolution/cover/evolution_cover.html). Abiotic or biotic factors could have acted as external pressures that made muscles fibers having this ability to change a selective advantage, thus leading to adaptation and evolution. One such factor could have been the location of a civilization and the availability of resources in that location. For example, in civilizations surrounded by land, water was a major factor in survival and reproduction. Individuals might have had to run to distant areas in order to obtain water. In individuals whose muscles fibers could react to the endurance running, some fast-twitch fibers would have transformed into slow-twitch fibers. This increased proportion of slow-twitch fibers would have helped the individual run more efficiently to get water, increasing his/her chances of survival and reproduction. On the other hand, those whose muscle fibers didn’t adapt to the endurance running would have had a more difficult time obtaining water. They would have also wasted more energy, thus decreasing chances of survival and reproduction.
Christine Zhao (c_zhao@ymail.com)
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ReplyDeleteIt may also be an evolutionary adaptation for humans to initially have "on average... a fifty-fifty mix of slow and fast-twitch muscle fibers" (309). The increased proportion of slow or fast-twitch muscle fibers is only a selective advantage in certain scenarios because an increase in the proportion of one type of fiber means a decrease in the other type of fiber. Unlike athletes, average people don’t usually require a lot of short-bursts of strength or long, enduring supplies of energy. Instead, they usually require an energy level somewhere between those two extremes. Also, the proportion of muscle fibers most advantageous to an individual is only known after the individual is born. Environmental factors, such as expectations, necessity, and interest, are what decide whether an individual would need more slow-twitch fibers or more fast-twitch fibers. Therefore, it’s more advantageous to start with a fifty-fifty mix and then allow environmental factors to change that proportion as needed.
Christine Zhao (c_zhao@ymail.com)