Chapter 6 The Muscular System 167 Copyright Goodheart-Willcox Co., Inc. T he human body has several different types of muscles, each with different structural characteristics. Each type of muscle is adapted to perform different physiological functions. Muscle Categories The three major categories of muscles are skeletal, smooth, and cardiac muscle. This section examines the important structural and functional differences among the three types of muscles. Skeletal Muscle The skeletal muscles attach to bones and are largely responsible for voluntary body movements. Skeletal muscle is also known as striated muscle due to the prominent cross-stripes, or striations, that can be seen when examining this tissue under a microscope (Figure 6.1A). A third name, voluntary muscle, is appropriate because this type of muscle is stimulated by consciously directed nerve activity. An individual skeletal muscle cell is referred to as a muscle fiber because of its thread-like shape. Muscle fibers include many nuclei and vary considerably in length and diameter. Some fibers run the entire length of a muscle others are much shorter. The number of muscle fibers present at birth is genetically determined, so it varies from person to person. Skeletal muscle fibers grow in length and diameter from birth to adulthood. Adults can increase their fiber diameter, and their strength, through resis- tance training with a few repetitions of heavy loads on a regular basis over a period of time. As Figure 6.2 shows, skeletal muscle is highly organized. The cell membrane of the muscle fiber is called the sarcolemma. The sarcolemma of each muscle fiber is covered by the endomysium, a fine, protective sheath of connective tissue. Groups of muscle fibers are bundled together by a strong fibrous membrane called a perimysium into a unit known as a fascicle. All of the fascicles in a muscle are enclosed by the epimysium, which is a thick, tough connective tissue. The epimysium connects at both ends of the muscle with either a cordlike tendon composed of extremely strong connective tissue or a flat, sheetlike aponeurosis. Tendons and aponeuroses directly connect each muscle to a bone, cartilage, or other connective tissue. Recall from Chapter 5 that tendons differ from ligaments in that ligaments connect bone to bone. Smooth Muscle In contrast to skeletal muscle fibers, smooth muscle cells are small, spindle-shaped, and nonstriated. These muscles are involuntary (not under conscious control), and they have a single nucleus (Figure 6.1B). Also known as visceral muscle, this type of muscle is found in the walls of many internal organs, such as the stomach, intestines, urinary bladder, arteries, and respiratory passages. Smooth muscle cells are arranged in layers, with one layer running lengthwise and the other surrounding the organ in which the muscles are contained. The coordinated, alternate contracting and relaxing of these layers changes the size and shape of the organ and can aid in moving the contents of the organ. Moving food through the digestive system, emptying the bladder, and changing the diameter of the blood vessels are examples of the important functions of smooth muscles. SECTION 6.1 Types and Functions of Muscle Tissue Objectives • Describe the structural characteristics of the three categories of muscle. • Explain the functions of each type of muscle tissue. Key Terms agonist antagonist aponeurosis concentric contraction contractility eccentric contraction elasticity endomysium epimysium extensibility fascicle irritability isometric contraction muscle fiber perimysium sarcolemma