Chapter 49

After studying this chapter, you should be able to:

• Understand the general biochemical features of skeletal, cardiac, and smooth muscle contraction.
• Know the biologic effects of nitric oxide (NO).
• Indicate the different metabolic fuels required for a sprint and for the marathon.
• Know the general structures and functions of the major components of the cytoskeleton, namely microfilaments, microtubules, and intermediate filaments.
• Understand the bases of malignant hyperthermia Duchenne and Becker muscular dystrophies, inherited cardiomyopathies, the Hutchinson–Gilford syndrome (progeria), and several skin diseases due to abnormal keratins.

Proteins play an important role in movement at both the organ (eg, skeletal muscle, heart, and gut) and cellular levels. In this chapter, the roles of specific proteins and certain other key molecules (eg, Ca2+) in muscular contraction are described. A brief coverage of cytoskeletal proteins is also presented.

Knowledge of the molecular bases of a number of conditions that affect muscle has advanced greatly in recent years. Understanding of the molecular basis of Duchenne-type muscular dystrophy was greatly enhanced when it was found that it was due to mutations in the gene encoding dystrophin (see case history no. 7 in Chapter 57). Significant progress has also been made in understanding the molecular basis of malignant hyperthermia, a serious complication for some patients undergoing certain types of anesthesia. Heart failure is a very common medical condition, with a variety of causes; its rational therapy requires understanding of the biochemistry of heart muscle. One group of conditions that cause heart failure are the cardiomyopathies, some of which are genetically determined. NO has been found to be a major regulator of smooth muscle tone. Many widely used vasodilators—such as nitroglycerin, used in the treatment of angina pectoris—act by increasing the formation of NO. Muscle, partly because of its mass, plays major roles in the overall metabolism of the body.

Muscle is the major biochemical transducer (machine) that converts potential (chemical) energy into kinetic (mechanical) energy. Muscle, the largest single tissue in the human body, makes up somewhat less than 25% of body mass at birth, more than 40% in the young adult, and somewhat less than 30% in the aged adult. We shall discuss aspects of the three types of muscles found in vertebrates: skeletal, cardiac, and smooth. Both skeletal and cardiac muscles appear striated upon microscopic observation; smooth muscle is nonstriated. Although skeletal muscle is under voluntary nervous control, the control of both cardiac and smooth muscle is involuntary.

### Sarcoplasm of Muscle Cells Contains ATP, Phosphocreatine, & Glycolytic Enzymes

Striated muscle is composed of multinucleated muscle fiber cells surrounded by an electrically excitable plasma membrane, the sarcolemma. An individual muscle fiber cell, which may extend the entire length of the muscle, contains a bundle of many myofibrils arranged in parallel, embedded in intracellular fluid termed sarcoplasm. Within this fluid is contained glycogen, the high-energy compounds ATP and phosphocreatine, and ...

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