MUSCULAR SYSTEM OUTLINE 10.1 Properties of Muscle Tissue 289 10.2 Characteristics of Skeletal Muscle Tissue 289 10 10.2a Functions of Skeletal Muscle Tissue 289 10.2b Gross Anatomy of Skeletal Muscle 290 10.2c Microscopic Anatomy of Skeletal Muscle 293 10.3 Contraction of Skeletal Muscle Fibers 298 Muscle 10.3a The Sliding Filament Theory 298 10.3b Neuromuscular Junctions 298 10.3c Physiology of Muscle Contraction 301 10.3d Muscle Contraction: A Summary 303 Tissue and 10.3e Motor Units 303 10.4 Types of Skeletal Muscle Fibers 305 10.4a Distribution of Slow, Intermediate, and Fast Fibers 307 10.5 Skeletal Muscle Fiber Organization 307 Organization 10.5a Circular Muscles 307 10.5b Parallel Muscles 307 10.5c Convergent Muscles 307 10.5d Pennate Muscles 307 10.6 Exercise and Skeletal Muscle 309 10.6a Muscle Atrophy 309 10.6b Muscle Hypertrophy 309 10.7 Levers and Joint Biomechanics 309 10.7a Classes of Levers 309 10.7b Actions of Skeletal Muscles 310 10.8 The Naming of Skeletal Muscles 311 10.9 Characteristics of Cardiac and Smooth Muscle 312 10.9a Cardiac Muscle 312 10.9b Smooth Muscle 313 10.10 Aging and the Muscular System 313 10.11 Development of the Muscular System 317 MODULE 6: MUSCULAR SYSTEM mck78097_ch10_288-321.indd 288 2/14/11 3:42 PM Chapter Ten Muscle Tissue and Organization 289 hen most of us hear the word “muscle,” we think of the W muscles that move the skeleton. These muscles are con- 10.2 Characteristics of Skeletal sidered organs because they are composed not only of muscle Muscle Tissue tissue, but also of epithelial, connective, and nervous tissue. Over 700 skeletal muscles have been named and together they form the Learning Objectives: muscular system. 1. Identify the many roles (functions) of skeletal muscle in However, skeletal muscles are not the only places where the body. muscle tissue is found. In fact, muscle tissue is distributed almost 2. Describe the levels of organization in a skeletal muscle. everywhere in the body and is responsible for the movement of 3. Explain how muscles are attached to other body structures. materials within and throughout the body. Without this vital 4. Detail the components of muscle fibers. tissue, the food we eat would not be propelled along the gastro- intestinal tract, the waste products we produce would not be rhyth- Each skeletal muscle is an organ composed of the four tissue mically expelled, and our blood would not be pumped to body types: epithelial, connective, muscle, and nervous. A skeletal mus- tissues. cle is striated (marked by stripes or bands) and usually attached To understand how muscle tissue performs these func- to one or more bones. Individual skeletal muscles vary widely in tions, this chapter examines its gross and microscopic anatomy, shape, ranging from elongated and flat to thick and triangular, or with emphasis on skeletal muscle tissue. The chapter also briefly even circular. A single muscle may be composed of thousands of reviews the characteristics of the other two types of muscle tissue fibers, and each fiber can be as long as the muscle itself. A small (cardiac and smooth), which were introduced in chapter 4. skeletal muscle fiber in a muscle in the toe may have a length of about 100 micrometers (μm) and a diameter of about 10 μm. In contrast, fibers in the muscle on the anterior side of the arm may extend up to 35 centimeters (cm) and have a diameter of about 10.1 Properties of Muscle Tissue 100 μm (i.e., the same thickness as a fine strand of hair). Learning Objective: 1. List and explain the four unique properties of muscle 10.2a Functions of Skeletal Muscle Tissue tissue. Skeletal muscles perform the following functions: Because of their potentially extraordinary length, skeletal ■ Body movement. Bones of the skeleton move when muscles muscle cells are often referred to as muscle fibers. All muscle contract and pull on the tendons which attach the muscles tissue is composed of muscle cells and exhibits certain common to the bones. The integrated functioning of muscles, bones, properties: and joints can produce both highly coordinated movements such as running and relatively simple, localized movements ■ Excitability. Excitability is equated with responsiveness. such as underlining a word in a book. In other words, muscle cells are very responsive to input ■ Maintenance of posture. Contraction of specific skeletal from stimuli. When a muscle cell is stimulated by the muscles stabilizes joints and helps maintain the body’s nervous system, by stretching, or by other stimuli in the posture or position. Postural muscles contract continuously environment, it responds by initiating electrical changes when a person is awake to prevent collapse. (Note that that sweep across its entire plasma membrane and spark when a person falls asleep in, say, a lecture hall, the head internal events leading to muscle contraction. droops forward and the mouth rolls open.) ■ Contractility. Stimulation of muscle cells generates ■ Temperature regulation. Energy is required for muscle tension within the cell (contraction), which may cause the tissue contraction, and heat is always produced as a waste cell to shorten. This shortening results in either a pull on product of this energy usage. Most of this heat maintains our bones of the skeleton or the movement of specific body normal body temperature. Note that when you exercise, you parts. feel warmer; the heat you feel is produced by your working ■ Elasticity. A contracted muscle cell recoils to its resting muscles. Likewise, you shiver when you are cold because length when the applied tension is removed. Thus, elasticity your muscles are contracting and relaxing to produce heat. is not the muscle’s ability to stretch, but its ability to return ■ Storage and movement of materials. Circular muscle to its original length when tension is released. bands, called sphincters (sfingk ́ter; sphinkter = a band), ■ Extensibility. A muscle cell must be capable of extending contract at the openings, or orifices (or ́i-fis; orificium = in length in response to the contraction of opposing opening), of the gastrointestinal and urinary tracts. These muscle cells. For example, when you flex your elbow sphincters may be voluntarily closed (to store the material joint, you are contracting the biceps brachii on the within an organ) or opened (to facilitate movement of anterior side of your arm and extending the triceps materials). brachii on the posterior side of your arm. The reverse ■ Support. Skeletal muscle is sometimes arranged in flat sheets is true when you extend your elbow joint. During these or layers, such as along the walls of the abdominal cavity movements, muscle cells undergo alternating cycles of and the floor of the pelvic cavity. These sheets of muscle contraction and extension. protect the organs and support their weight within the abdominopelvic cavity. In fact, one of the primary functions WHATW DID YOU LEARN? of your abdominal muscles is to hold your abdominal organs ●1 What are the properties of muscle tissue? in place! mck78097_ch10_288-321.indd 289 2/14/11 3:01 PM 290 Chapter Ten Muscle Tissue and Organization Tendon Deep fascia Artery Skeletal muscle Nerve Vein Epimysium Perimysium Nuclei Perimysium Endomysium Artery Vein Nerve Muscle fiber Fascicle (b) Fascicle Endomysium Muscle fiber Endomysium Nuclei Myofibrils (a) Muscle Sarcoplasm Sarcolemma Satellite cell (c) Muscle fiber Figure 10.1 Structural Organization of Skeletal Muscle. (a) A skeletal muscle is ensheathed within a connective tissue layer called the epimysium. (b) Each fascicle (bundle of muscle fibers) is wrapped within a connective tissue layer called the perimysium. (c) Each muscle fiber is surrounded by a delicate connective tissue layer termed the endomysium. 10.2b Gross Anatomy of Skeletal Muscle composed of myofilaments. Table 10.1 illustrates the levels of In general, a skeletal muscle is composed of numerous skeletal organization in a skeletal muscle. muscle fibers, blood vessels and nerves, and connective tissue sheets that surround the muscle fibers and connect the muscle Connective Tissue Components to bone. While an individual muscle fiber runs the length of a Three concentric layers of connective tissue, composed of collagen muscle, it takes groupings of many muscle fibers to form the and elastic fibers, encircle each individual muscle fiber, groups of width of a muscle. If you look at a cross section (figure 10.1), muscle fibers, and the entire muscle itself. These layers provide each skeletal muscle is composed of fascicles (fas ́i-kl; fascis = protection, sites for distribution of blood vessels and nerves, and a bundle), which are bundles of muscle fibers. Muscle fibers, in means of attachment to the skeleton. The three connective tissue lay- turn, contain cylindrical structures called myofibrils, which are ers are the endomysium, the perimysium, and the epimysium. mck78097_ch10_288-321.indd 290 2/15/11 10:55 AM Chapter Ten Muscle Tissue and Organization 291 Table 10.1 Organizational Levels of Skeletal Muscle Level Appearance Description Connective Tissue Covering Muscle Multiple fascicles housing Epimysium many muscle fi bers, connective tissue coverings, blood vessels, nerve fi bers Muscle Muscle fiber Fascicle Epimysium Fascicle A bundle of muscle fi bers Perimysium separated from other bundles Muscle fiber of fi bers by a dense irregular connective tissue covering Fascicle Perimysium Muscle Fiber Elongated, multinucleated, Endomysium (muscle cell) cylindrical fi ber (cell); Endomysium contains myofi brils, separated from