Structure of a Skeletal Muscle

A skeletal muscle is an organ of the muscular system. It is composed primarily of skeletal muscle tissue, nervous tissue, blood, and connective tissues.

Connective Tissue Coverings

An individual skeletal muscle is separated from adjacent muscles and held in position by layers of dense connective tissue called fascia (fash'e-ah). This connective tissue surrounds each muscle and may project beyond the end of its muscle fibers to form a cordlike tendon. Fibers in a tendon intertwine with those in the periosteum of a bone, attaching the muscle to the bone. In other cases, the connective tissues associated with a muscle form broad, fibrous sheets called aponeuroses (ap"o-nu-ro'sez), which may attach to the coverings of adjacent muscles (figs. 9.1 and 9.2).

A tendon, or the connective tissue sheath of a tendon (tenosynovium), may become painfully inflamed and swollen following an injury or the repeated stress of athletic activity. These conditions are called tendinitis and tenosynovitis, respectively. The tendons most commonly affected are those associated with the joint capsules of the shoulder, elbow, hip, and knee, and those involved with moving the wrist, hand, thigh, and foot.

The layer of connective tissue that closely surrounds a skeletal muscle is called the epimysium. Another layer of connective tissue, called the perimysium, extends inward from the epimysium and separates the muscle tissue into small sections. These sections contain bundles of skeletal muscle fibers called fascicles (fasciculi). Each muscle fiber within a fascicle (fasciculus) lies within a layer of connective tissue in the form of a thin covering called endomysium (figs. 9.2 and 9.3). Layers of

Skeletal Muscle Structure

Aponeuroses

Skeletal muscles

Tendons

Tendons attach muscles to bones, whereas aponeuroses attach muscles to other muscles.

Aponeuroses

Skeletal muscles

Tendons

Tendons attach muscles to bones, whereas aponeuroses attach muscles to other muscles.

Aponeuroses

Axon of motor neuron

Figure

Axon of motor neuron

Figure

A skeletal muscle is composed of a variety of tissues, including layers of connective tissue. Fascia covers the surface of the muscle, epimysium lies beneath the fascia, and perimysium extends into the structure of the muscle where it separates muscle cells into fascicles. Endomysium separates individual muscle fibers.

Electron Micrograph Muscular Tissue

Scanning electron micrograph of a fascicle (fasciculus) surrounded by its connective tissue sheath, the perimysium. Muscle fibers within the fascicle are surrounded by endomysium (320x).

Figure

Scanning electron micrograph of a fascicle (fasciculus) surrounded by its connective tissue sheath, the perimysium. Muscle fibers within the fascicle are surrounded by endomysium (320x).

connective tissue, therefore, enclose and separate all parts of a skeletal muscle. This arrangement allows the parts to move somewhat independently. Also, many blood vessels and nerves pass through these layers.

The fascia associated with each individual organ of the muscular system is part of a complex network of fasciae that extends throughout the body. The portion of the network that surrounds and penetrates the muscles is called deep fascia. It is continuous with the subcutaneous fascia that lies just beneath the skin, forming the subcutaneous layer described in chapter 6 (p. 175). The network is also continuous with the subserous fascia that forms the connective tissue layer of the serous membranes covering organs in various body cavities and lining those cavities (see chapter 6, p. 169).

A compartment is the space that contains a particular group of muscles, blood vessels, and nerves, all tightly enclosed by fascia. The limbs have many such compartments. If an injury causes fluid, such as blood from an internal hemorrhage, to accumulate within a compartment, the pressure inside will rise. The increased pressure, in turn, may interfere with blood flow into the region, reducing the supply of oxygen and nutrients to the affected tissues. This condition, called compartment syndrome, often produces severe, unrelenting pain. Persistently elevated compartmental pressure may irreversibly damage the enclosed muscles and nerves. Treatment for compartment syndrome may require immediate intervention by a surgical incision through the fascia (fasciotomy) to relieve the pressure and restore circulation.

Skeletal Muscle Fibers

Recall from chapter 5 (p. 160) that a skeletal muscle fiber is a single muscle cell (see fig. 5.28). Each fiber forms from many undifferentiated cells that fuse during development. Each resulting multinucleated muscle fiber is a thin, elongated cylinder with rounded ends that attach to the connective tissues associated with a muscle. Just beneath the muscle cell membrane (sarcolemma), the cytoplasm (sarcoplasm) of the fiber contains many small, oval nuclei and mitochondria. The sarcoplasm also has abundant, parallel, threadlike structures called myofibrils (mi"o-fi'-brilz) (fig. 9.4a)

The myofibrils play a fundamental role in the muscle contraction mechanism. They contain two kinds of protein filaments: Thick filaments composed of the protein myosin (mi'o-sin), and thin filaments composed primarily of the protein actin (ak'tin). The organization of these filaments produces the alternating light and dark striations characteristic of skeletal muscle (and cardiac muscle) fibers. The striations form a repeating pattern of units called sarcomeres (sar'ko-merz) along each muscle fiber. The myofibrils may be thought of as sarcomeres joined end to end. (fig. 9.4a).

The striation pattern of skeletal muscle has two main parts. The first, the I bands (the light bands), are composed of thin actin filaments held by direct attachments to structures called Z lines, which appear in the center of the I bands. The second part of the striation pattern consists of the A bands (the dark bands), which are composed of thick myosin filaments overlapping thin actin filaments (fig 9.4b).

Note that the A band consists not only of a region where thick and thin filaments overlap, but also a slightly

— Skeletal muscle fiber

■ Sarcoplasmic reticulum

Z line

Myosin (thick) Actin (thin) filaments filaments

— Skeletal muscle fiber

■ Sarcoplasmic reticulum

Z line

Myosin (thick) Actin (thin) filaments filaments

Myofibril Structure Zone

- Myofibril iii»i

- Myofibril

H zone rh

Z line

M line

H zone rh

Z line

M line

Figure 9.4

(a) A skeletal muscle fiber contains numerous myofibrils, each consisting of (b) repeating units called sarcomeres. The characteristic striations of a sarcomere are due to the arrangement of actin and myosin filaments.

lighter central region (H zone) consisting only of thick filaments. The A band includes a thickening known as the M line, which consists of proteins that help hold the thick filaments in place (fig. 9.4b). The myosin filaments are also held in place by the Z lines but are attached to them by a large protein called titin (connectin) (fig. 9.5). A sarcomere extends from one Z line to the next (figs. 9.4 and 9.5).

Thick filaments are composed of many molecules of myosin. Each myosin molecule consists of two twisted protein strands with globular parts called cross-bridges (heads) that project outward along their lengths. Thin filaments consist of double strands of actin twisted into a helix. Actin molecules are globular, and each has a binding site to which the cross-bridges of a myosin molecule can attach (fig. 9.6).

Two other types of protein, tropomyosin and troponin, associate with actin filaments. Tropomyosin molecules are rod-shaped and occupy the longitudinal grooves of the actin helix. Each tropomyosin is held in place by a troponin molecule, forming a tropomyosin-troponin complex (fig. 9.6).

Within the sarcoplasm of a muscle fiber is a network of membranous channels that surrounds each myofibril and runs parallel to it. These membranes form the sarcoplasmic reticulum, which corresponds to the endoplasmic reticulum of other cells (see figs. 9.2 and 9.4). A set of membranous channels, the transverse tubules (T-tubules), extends into the sarcoplasm as invaginations continuous

Sarcomere

Sarcomere

Sarcom Tubule

Thin (actin) filament

Figure 9.5

Titin

Thick

(myosin)

filament

Thin (actin) filament

Figure 9.5

A sarcomere (16,000x).

Titin

Thick

(myosin)

filament

Myosin Filament

-Actin filament

- Myosin filament

Troponin

Tropomyosin Myosin molecule

Actin molecule

-Actin filament

- Myosin filament

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Essentials of Human Physiology

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  • sheryl
    Does a muscle attach with tendon or aponeurosis?
    4 years ago
  • belba
    How are actin and myosin arranged within in myofibril?
    4 years ago
  • joan
    How connective tissue attach muscles to bone?
    4 years ago
  • ville
    How many blood vessels are in the skeletal muscles?
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  • Nina
    How are tissues arranged is muscle from fascia down to the myofilaments?
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    What types of blood vessels skeletal muscle?
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  • fabiola
    Which of the following tissue layers in muscle is continuous with the fascia?
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  • haben
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  • mira
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  • Asmara Mebrahtu
    What layers of skeletal muscle is attached to fascia?
    3 years ago
  • klaudia
    What type of burn extends into the muscle, blood vessels and nerves?
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  • Virginia Clarkson
    Is a blood vessel a skeletal muscle a cardiac muscle or smooth muscle?
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  • Tewelde
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  • Castore
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    Why do muscles have blood vessles & nerves?
    2 years ago
  • matti
    Why do blood vessels surrond the muscle fibers?
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    What is a molecule inside of the skeletal system?
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  • sara
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  • Flaviana Endrizzi
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  • Mikaela
    What group of blood vessels surround the muscle fiber?
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  • Libera Maria Pinto
    Which structure encloses each lung including its blood vessels and nerves?
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  • jessica
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  • Fulgenzio
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  • KENZIE
    Do blood vessels in the human have pain fibers?
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  • niklas zweig
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  • jennifer
    Which heart muscles contain nerves and blood vessels?
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  • clark
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    1 year ago
  • alexander
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  • Valentin
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  • ashlyn
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  • Thomas
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  • quarantino
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  • Paavo
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  • reima
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    12 months ago
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  • Mandy Ehrlichmann
    WHAT connective tissue that lacks nerves and blood vessels is:?
    10 months ago
  • Helen
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    7 months ago
  • Satu
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  • RIAN
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    Is there blood vessels that run through muscles of animals like: cornish hen?
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