lab11 - 12 Muscular Tissue Objectives In this chapter we...

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69 12 Muscular Tissue Objectives In this chapter we will study the distinctions between muscular deconditioning and disuse atrophy; the neuromuscular disorders botulism, tetanus, and myasthenia gravis; the hereditary disease muscular dystrophy; the musculoskeletal disorder fibromyalgia; and the uses and risks of muscle relaxants. Diagnosing Functional Disorders of the Muscular System Chapter 11 described several muscle disorders that are primarily structural. This chapter focuses on the signs, symptoms, and treatment of muscle disorders that are more physiological in nature. It is important to note that the diagnostic techniques described in chapter 11 are equally useful in diagnosing the conditions discussed in this chapter. Muscle Deconditioning and Disuse Atrophy Lack of physical activity can cause a muscle to atrophy. Muscle deconditioning may become apparent within days to weeks of decreased physical activity. For example, we have all known individuals who were physically fit during their high school days and then became less active over time. The cessation of regular activity causes the muscles to decrease in size, but contrary to what some people say, the muscles themselves do not become “fat.” Rather, a continued high level of caloric intake, coupled with decreased activity, leads to increases in fat synthesis and storage by adipose tissue, which shows up most noticeably as subcutaneous fat in various body regions. Disuse atrophy is a pathological condition in which muscle size is reduced as a result of prolonged inactivity such as bed rest, casting (immobilization of a limb in a cast), or damage to the nerves supplying a muscle (denervation atrophy). Individuals confined to bed have been known to lose up to 3% of their muscle strength per day, and long-term casting can result in the loss of up to 50% of muscle mass. As a result of disuse, the size of the individual muscle fibers and the oxidative capacity of the mitochondria decline. Muscle fibers can enlarge again if use is restored (for example, when a cast is removed), but regrowth may be compromised if the muscle is not used for more than a year. Treatments to prevent and/or minimize disuse atrophy focus on moving the immobilized limb, even if in a very limited way. Some of the more common physical therapy treatments are isometric muscle contractions and passive lengthening exercises. In addition, direct electrical stimulation of the immobilized muscles is sometimes used. In this procedure, small surface electrodes are placed on the skin, and a minimal electrical stimulus is applied to the muscle, causing slight contractions that help maintain the muscle. Neuromuscular Disorders Some muscular dysfunctions result from disorders of the motor neurons in the nervous system or disorders of the neuromuscular junction, the point where a nerve fiber contacts a skeletal muscle fiber. Motor neurons stimulate skeletal muscle fibers to contract by releasing acetylcholine (ACh). Muscle paralysis can
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lab11 - 12 Muscular Tissue Objectives In this chapter we...

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