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

Skeletal Muscle Fiber Structure

■ 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.)

Striated Muscle Polarized Light

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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • esko
    Do tendons surrounding the epimysium?
    5 years ago
  • BOBBY
    What type of skeletal muscle fiber has glycogen granules?
    5 years ago
  • Rita
    Why skeletal muscle fibers have multiple nuclei?
    5 years ago
  • SAMUEL
    What do the i in i band stand for in the muscle fiber?
    5 years ago
  • andrea sanger
    How are skeletal muscles and muscle fibers structure?
    5 years ago
  • nicholas white
    Where is the deep fascia on muscle fiber?
    5 years ago
  • Aleardo Onio
    Why do skeletal muscles have multiple nuclei?
    5 years ago
  • Blanco Hogpen
    Do muscle fibers of skeletal muscles contain several nuclei?
    5 years ago
  • nicola mazzanti
    Why do skeletal muscles have multiple nuclei and mitochondria?
    4 years ago
  • NIKLAS
    Which muscle type has syncytial structure?
    4 years ago
  • elisabeth
    Why does skeletal muscle have nuclei and mitochondria?
    3 years ago
  • kibra
    What is syncytial skeletal muscle?
    2 years ago
  • Vittoria
    Why does skeletal muscle have multiple nucleus?
    2 years ago
  • Maxima Rumble
    Why do skeletal muscle cells have multiple nuclei?
    7 months ago
  • Nunzio
    Which of the structures is surrounded by the connective tissue sheath known as the perimysium?
    4 months ago

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