Homeostasis in the Muscular System
Skeletal muscles contribute to maintaining temperature homeostasis in the body by generating heat. Muscle contraction requires energy and produces heat as a byproduct of metabolism. All types of muscle produce heat, but because of the large amount of skeletal muscle present in the body, skeletal muscle contributes most greatly to heat production. This is very noticeable during exercise, when sustained muscle movement causes body temperature to rise. In cases of extreme cold, shivering produces random skeletal muscle contractions to generate heat as part of the negative feedback mechanism of maintaining body temperature.
Our body can use skeletal muscle contractions to maintain body temperature when we are cold, but excessive contractions can lead to the body overheating to the point that the body’s metabolic reactions are interrupted. This can occur in a condition called malignant hyperthermia, which develops in genetically susceptible individuals who are administered a specific combination of anesthetic agents during surgery. In these individuals, a drastic increase in skeletal muscle calcium leads to sustained contractions and heat generation. Because the individuals are anesthetized, they have little ability to cool themselves. If proper interventions are not administered, they will die due to a greatly increased body temperature. Because this condition is genetic, patients are asked prior to surgery if there is a family history of such problems occurring.
Muscle Homeostasis and Growth
Physical training alters the appearance of skeletal muscles and can produce changes in muscle performance. Conversely, a lack of use can result in decreased performance and muscle appearance. As you learned earlier, mature muscle cells grow from hypertrophy, not cell division. The loss of structural proteins and muscle mass occurs during atrophy. Cellular components of muscles can also undergo changes in response to changes in muscle use.
Although atrophy due to disuse can often be reversed with exercise, muscle atrophy that comes with age is irreversible. This is why even highly trained athletes succumb to declining performance with age, although extensive training may slow the decline. This is especially noticeable in sports that require an explosion of strength and power over a very short period of time. Examples of these kinds of sports include sprinting, competitive weight lifting, gymnastics and diving. The effects of age are less noticeable in endurance sports such as marathon running or long-distance cycling. Age-related muscle atrophy is called sarcopenia. As muscles age, muscle fibers die, and they are replaced by connective tissue and adipose tissue. Because those tissues cannot contract as muscle can, muscles lose the ability to produce powerful contractions. The decline in muscle mass causes a loss of strength, including strength required for posture and mobility. This may be caused by a reduction in the proportion of FG fibers that hydrolyse ATP quickly to produce short, powerful contractions. Muscles in older people sometimes possess greater numbers of SO fibers, which are responsible for longer contractions and do not produce powerful movements. There may also be a reduction in motor units, resulting in fewer fibers being stimulated and less muscle tension being produced.
Some athletes attempt to boost their performance by using various agents that may enhance muscle performance. Anabolic steroids are one of the more widely known agents used to boost muscle mass and increase power output. Anabolic steroids are a form of testosterone, a male sex hormone that stimulates muscle formation, leading to increased muscle mass. They have been used by athletes in many sports, but sprinting is one sport in which the effects of steroids are readily apparent. Because a 100-meter dash can last less than 10 seconds, incredible amounts of power need to be created by the muscles. Increasing the muscle mass increases the numbers of actin and myosin cross-bridges, increasing the power that can be produced by a muscle, which provides a competitive advantage in a sport measured in hundredths of seconds. Similarly, creatine has become a substance used by some athletes to increase power output. Because creatine phosphate provides quick bursts of ATP to muscles in the initial stages of contraction, increasing the amount available to cells is thought to produce more ATP and therefore increase explosive power output. However, both creatine and steroids are banned in sports and they can be extremely harmful to other systems of the body as well as to long-term muscle health.
Muscle and Blood Flow
Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions. The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen. Myoglobin is found in the sarcoplasm and stores oxygen. Endurance training can also trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen and remove metabolic waste. To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase, maintaining a shorter distance for the diffusion of nutrients and gases. All of these cellular changes result in the ability to sustain low levels of muscle contraction for greater periods of time without fatiguing.
Endurance athletes also engage in drug use, but instead of trying to add muscle mass or produce power, they focus on substances that increase muscle endurance and reduce fatigue. This includes trying to boost the availability of oxygen to muscles to increase aerobic respiration by using substances such as erythropoietin, or EPO. EPO is a hormone that triggers the production of red blood cells, which carry oxygen in the blood. This oxygen can then be used by muscles for aerobic respiration. Human growth hormone is another commonly used agent, and although it can facilitate building muscle mass, its main role is to promote the healing of muscle and other tissues after strenuous exercise. Increased growth hormone allows for faster recovery after muscle damage, reducing the rest required after exercise, and allowing for more sustained high-level performance. As with creatine and steroids, these substances are harmful to the body and can negatively impact the homeostasis of other systems.
As discussed previously, there are a number of proteins that are important in regulating and carrying out the contractile process. Thus there are genetic disorders that lead to altered protein formation and dysfunction of the muscular system. Such dysfunction tends to affect many body systems, of which the respiratory system may be most important. Without the ability to appropriately contract the skeletal muscles of respiration, people cannot live very long. The skeletal muscular system is also very dependent upon a constant supply of ATP. Like many other systems, this requires the intracellular pathways to convert glucose into energy.
Thiamine (vitamin B1) is a necessary cofactor in the production of ATP from glucose. Deficiencies in thiamine and other vitamins can lead to extreme muscle weakness as well as neurological problems. Thiamine deficiency leads to a disease called beriberi, which was common in people who ate white rice as their major food source. Beriberi can affect skeletal and cardiac muscles as well as the nervous system. Eating a balanced diet and the increase of vitamin-enriched foods have eliminated beriberi from developed countries, but many disorders of vitamin deficiency exist in developing areas.
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