Structure of Skeletal Muscles

The fibrous connective tissue proteins within the tendons extend around the muscle in an irregular arrangement, forming a sheath known as the epimysium (epi = above; my = muscle). Connective tissue from this outer sheath extends into the body of the muscle, subdividing it into columns, or fascicles (these are the "strings" in stringy meat). Each of these fascicles is thus surrounded by its own connective tissue sheath, which is known as the perimysium (peri = around).

Dissection of a muscle fascicle under a microscope reveals that it, in turn, is composed of many muscle fibers, or myofibers.

Each is surrounded by a plasma membrane, or sarcolemma, enveloped by a thin connective tissue layer called an endomy-sium (fig. 12.1). Since the connective tissue of the tendons, epimysium, perimysium, and endomysium is continuous, muscle fibers do not normally pull out of the tendons when they contract.

PfJ Duchenne's muscular dystrophy is the most severe of the muscular dystrophies, afflicting I out of ^ jt fi 3,500 boys each year. This disease, inherited as an X-linked recessive trait involves progressive muscular wasting and usually results in death by the age of 20. The product of the defective gene is a protein named dystrophin, which is associated with the plasma membrane of skeletal muscle fibers (the sarcolemma). Using this information, scientists have recently developed laboratory tests that can detect this disease in fetal cells obtained by amniocentesis. This research has been aided by the development of a strain of mice that exhibit an equivalent form of the disease. When the "good genes" for dystrophin are inserted into mouse embryos of this strain, the mice do not develop the disease. Insertion of the gene into large numbers of mature muscle cells, however, is more difficult, and so far has met with only limited success.

Despite their unusual elongated shape, muscle fibers have the same organelles that are present in other cells: mitochondria, endoplasmic reticulum, glycogen granules, and others. Unlike most other cells in the body, skeletal muscle fibers are multinucleate— that is, they contain multiple nuclei. This is because, as described in chapter 1, each muscle fiber is a syncytial structure. That is, each muscle fiber is formed from the union of several embryonic myoblast cells. The most distinctive feature of skeletal muscle fibers, however, is their striated appearance when viewed microscopically (fig. 12.2). The striations (stripes) are produced by alternating dark and light bands that appear to span the width of the fiber.

The dark bands are called A bands, and the light bands are called I bands. At high magnification in an electron microscope,

Periosteum covering the bone

Tendon

Fascia

Periosteum covering the bone

Tendon

Fascia

■ Figure 12.1 The structure of a skeletal muscle.

The relationship between muscle fibers and the connective tissues of the tendon, epimysium, perimysium, and endomysium is depicted in the upper figure. Below is a close-up of a single muscle fiber.

Nuclei

Nuclei

■ Figure 12.2 The appearance of skeletal muscle g fibers through the light microscope. The striations are produced by alternating dark A bands and light I bands. (Note the peripheral location of the nuclei.)

■ Figure 12.2 The appearance of skeletal muscle g fibers through the light microscope. The striations are produced by alternating dark A bands and light I bands. (Note the peripheral location of the nuclei.)

thin dark lines can be seen in the middle of the I bands. These are called Z lines (see fig. 12.6). The labels A, I, and Z—derived in the course of early muscle research—are useful for describing the functional architecture of muscle fibers. The letters A and I stand for anisotropic and isotropic, respectively, which indicate the behavior of polarized light as it passes through these regions; the letter Z comes from the German word Zwischenscheibe, which translates to "between disc." These derivations are of historical interest only.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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