skeletal muscle components

Each of your skeletal muscles is a separate organ composed of hundreds to thousands of cells, which are called muscle fibers (myocytes) because of their elongated shapes. Thus, muscle cell and muscle fiber are two terms for the same structure. Skeletal muscle also contains connective tissues surrounding muscle fibers, and blood vessels and nerves (Figure 10.1). To understand how contraction of skeletal muscle can generate tension, you must first understand its gross and microscopic anatomy.

Connective Tissue Components

Connective tissue surrounds and protects muscular tissue. The subcutaneous tissue or hypodermis, which separates muscle from skin (see Figure 11.21), is composed of areolar connective tissue and adipose tissue. It provides a pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles. The adipose tissue of the subcutaneous tissue stores most of the body’s triglycerides, serves as an insulating layer that reduces heat loss, and protects muscles from physical trauma. Fascia (FASH-ē-a = bandage) is a dense sheet or broad band of irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other organs of the body. As you will see, fascia holds muscles with similar functions together (see Figure 11.21). Fascia allows free movement of muscles; carries nerves, blood vessels, and lymphatic vessels; and fills spaces between muscles.

Three layers of connective tissue extend from the fascia to protect and strengthen skeletal muscle (Figure 10.1):

Epimysium (ep-i-MĪZ-ē-um; epi- = upon) is the outer layer, encircling the entire muscle. It consists of dense irregular connective tissue.

Perimysium (per-i-MĪZ-ē-um; peri- = around) is also a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called muscle fascicles (FAS-i-kuls = little bundles), also called muscle fasciculi. Many muscle fascicles are large enough to be seen with the naked eye. They give a cut of meat its characteristic “grain”; if you tear a piece of meat, it rips apart along the muscle fascicles.

Clinical Connection

Fibromyalgia

Fibromyalgia (fī-brō-mī-AL-jē-a; -algia = painful condition) is a chronic, painful, nonarticular rheumatic disorder that affects the fibrous connective tissue components of muscles, tendons, and ligaments. A striking sign is pain that results from gentle pressure at specific “tender points.” Even without pressure, there is pain, tenderness, and stiffness of muscles, tendons, and surrounding soft tissues. Besides muscle pain, those with fibromyalgia report severe fatigue, poor sleep, headaches, depression, irritable bowel syndrome, and inability to carry out their daily activities. There is no specific identifiable cause. Treatment consists of stress reduction, regular exercise, application of heat, gentle massage, physical therapy, medication for pain, and a low-dose antidepressant to help improve sleep.

A S E M micrograph at 720 times magnification shows partly unraveled skeletal muscle fiber with densely packed myofibril. The myrofibrils composing the muscle fibers have a string-like texture. A three-part diagram shows the components of skeletal muscle tissue. The first part is an orientation diagram of a transverse plane through the humerus. In the second part, skeletal muscle and tendons are attached to the bone. The belly of the skeletal muscle extends from the tendon, and is covered by periosteum. The outer epimysium of the muscle is identified, as is the perimysium covering the individual muscle fascicles. A myofibril is projecting from a muscle fiber. The third part shows the details of an individual muscle fascicle, including a blood capillary and somatic neuron. The sarcoplasm, sarcolemma, and myofibril, and filament extend from a striated muscle fiber wrapped in endomysium.

FIGURE 10.1 Organization of skeletal muscle and its connective tissue coverings.

A skeletal muscle consists of individual muscle fibers bundled into muscle fascicles and surrounded by three connective tissue layers that are extensions of the fascia.

Functions of Muscular Tissues

Producing motions.

Stabilizing body positions.

Storing and moving substances within the body.

Generating heat (thermogenesis).

Q Which connective tissue coat surrounds groups of muscle fibers, separating them into muscle fascicles?

Endomysium (en′-dō-MĪZ-ē-um; endo- = within) penetrates the interior of each muscle fascicle and separates individual muscle fibers from one another. The endomysium is mostly reticular fibers.

The epimysium, perimysium, and endomysium are all continuous with the connective tissue that attaches skeletal muscle to other structures, such as bone or another muscle. For example, all three connective tissue layers may extend beyond the muscle fibers to form a ropelike tendon that attaches a muscle to the periosteum of a bone. An example is the calcaneal (Achilles) tendon of the gastrocnemius (calf) muscle, which attaches the muscle to the calcaneus (heel bone) (shown in Figure 11.22c). When the connective tissue elements extend as a broad, flat sheet, it is called an aponeurosis (ap-ō-noo-RŌ-sis; apo- = from; -neur- = a sinew). An example is the epicranial aponeurosis on top of the skull between the frontal and occipital bellies of the occipitofrontalis muscle (shown in Figure 11.4a, c).

Nerve and Blood Supply

Skeletal muscles are well supplied with nerves and blood vessels. Generally, an artery and one or two veins accompany each nerve that penetrates a skeletal muscle. The neurons that stimulate skeletal muscle to contract are somatic motor neurons. Each somatic motor neuron has a threadlike axon that extends from the brain or spinal cord to a group of skeletal muscle fibers (see Figure 10.9d). The axon of a somatic motor neuron typically branches many times, each branch extending to a different skeletal muscle fiber.

Microscopic blood vessels called blood capillaries are plentiful in muscular tissue; each muscle fiber is in close contact with one or more blood capillaries (see Figure 10.9d). The blood capillaries bring in oxygen and nutrients and remove heat and the waste products of muscle metabolism. Especially during contraction, a muscle fiber synthesizes and uses considerable ATP (adenosine triphosphate). These reactions, which you will learn more about later on, require oxygen, glucose, fatty acids, and other substances that are delivered to the muscle fiber in the blood.

Microscopic Anatomy of a Skeletal Muscle Fiber

The most important components of a skeletal muscle are the muscle fibers themselves. The diameter of a mature skeletal muscle fiber ranges from 10 to 100 µm.* The typical length of a mature skeletal muscle fiber is about 10 cm (4 in.), although some are as long as 30 cm (12 in.). Because each skeletal muscle fiber arises during embryonic development from the fusion of a hundred or more small mesodermal cells called myoblasts (MĪ-ō-blasts) (Figure 10.2a), each mature skeletal muscle fiber has a hundred or more nuclei. Once fusion has occurred, the muscle fiber loses its ability to undergo cell division. Thus, the number of skeletal muscle fibers is set before you are born, and most of these fibers last a lifetime.

Sarcolemma, T Tubules, and Sarcoplasm

The multiple nuclei of a skeletal muscle fiber are located just beneath the sarcolemma (sar′-kō-LEM-ma; sarc- = flesh; -lemma = sheath), the plasma membrane of a muscle fiber (Figure 10.2b, c). Thousands of tiny tube-shaped invaginations of the sarcolemma, called T tubules (transverse tubules), tunnel in from the surface toward the center of each muscle fiber. Because T tubules are open to the outside of the fiber, they are filled with interstitial fluid. Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber. This arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant.

Within the sarcolemma is the sarcoplasm (SAR-kō-plazm), the cytoplasm of a muscle fiber. Sarcoplasm includes a substantial amount of glycogen, which is a large molecule composed of many glucose molecules (see Figure 2.16). Glycogen can be used for synthesis of ATP. In addition, the sarcoplasm contains a red-colored protein called myoglobin (mī-ō-GLŌB-in). This protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. Myoglobin releases oxygen when it is needed by the mitochondria for ATP production. The mitochondria lie in rows throughout the muscle fiber, strategically close to the contractile muscle proteins that use ATP during contraction so that ATP can be produced quickly as needed (Figure 10.2c).

Clinical Connection

Muscular Hypertrophy, Fibrosis, and Muscular Atrophy

The muscle growth that occurs after birth occurs by enlargement of existing muscle fibers, called muscular hypertrophy (hī-PER-trō-fē; hyper- = above or excessive; -trophy = nourishment). Muscular hypertrophy is due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles. It results from very forceful, repetitive muscular activity, such as strength training. Because hypertrophied muscles contain more myofibrils, they are capable of more forceful contractions. During childhood, growth hormone and other hormones stimulate an increase in the size of skeletal muscle fibers. The hormone testosterone promotes further enlargement of muscle fibers.

A few myoblasts do persist in mature skeletal muscle as satellite cells (see Figure 10.2a, b). Satellite cells retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers. However, when the number of new skeletal muscle fibers that can be formed by satellite cells is not enough to compensate for significant skeletal muscle damage or degeneration, the muscular tissue undergoes fibrosis, the replacement of muscle fibers by fibrous scar tissue.

Muscular atrophy (AT-rō-fē; a- = without, -trophy = nourishment) is a decrease in size of individual muscle fibers as a result of progressive loss of myofibrils. Atrophy that occurs because muscles are not used is termed disuse atrophy. Bedridden individuals and people with casts experience disuse atrophy because the flow of nerve impulses to inactive skeletal muscle is greatly reduced, but the condition is reversible. If instead its nerve supply is disrupted or cut, the muscle undergoes denervation atrophy. Over a period of 6 months to 2 years, the muscle shrinks to about one-fourth its original size, and its fibers are irreversibly replaced by fibrous connective tissue.