Functions and Types of Muscles
All muscles, regardless of the particular type, can contract-that is, shorten. When muscles contract, some part of the body or the entire body moves. Humans have three types of muscles: smooth, cardiac, and skeletal (Fig. 7.1). The contractile cells of these tissues are elongated and therefore are called muscle fibers.
Figure 7.1 Types of muscles. The three types ofmuscles in the body have the appearance and characteristics shown here.
Smooth muscle is located in the walls of hollow internal organs, and its involuntary contraction moves materials through an organ. Smooth muscle fibers are spindle-shaped cells, each with a single nucleus (uninucleated). The cells are usually arranged in parallel lines, forming sheets. Smooth muscle does not have the striations (bands of light and dark) seen in cardiac and skeletal muscle. Although smooth muscle is slower to contract than skeletal muscle, it can sustain prolonged contractions and does not fatigue easily.
Cardiac muscle forms the heart wall. Its fibers are uninucleated, striated, tubular, and branched, which allows the fibers to interlock at intercalated disks. Intercalated disks permit contractions to spread quickly throughout the heart. Cardiac fibers relax completely between contractions, which prevents fatigue. Contraction of cardiac muscle fibers is rhythmical; it occurs without outside nervous stimulation or control. Thus, cardiac muscle contraction is involuntary.
Skeletal muscle fibers are tubular, multinucleated, and striated. They make up the skeletal muscles attached to the skeleton. Skeletal muscle fibers can run the length of a muscle and therefore can be quite long. Skeletal muscle is voluntary because its contraction is always stimulated and controlled by the nervous system.
Connective Tissue Coverings
Muscles are organs, and as such they contain other types of tissues, such as nervous tissue, blood vessels, and connective tissue. Connective tissue is essential to the organization of the fibers within a muscle (Fig. 7.2). First, each fiber is surrounded by a thin layer of areolar connective tissue called the endomysium. Blood capillaries and nerve fibers reach each muscle fiber by way of the endomysium. Second, the muscle fibers are grouped into bundles called fascicles. The fascicles have a sheath of connective tissue called the perimysium. Finally, the muscle itself is covered by a connective tissue layer called the epimysium. The epimysium becomes a part of the fascia, a layer of fibrous tissue that separates muscles from each other (deep fascia) and from the skin (superficial fascia). Collagen fibers of the epimysium continue as a strong, fibrous tendon that attaches the muscle to a bone. The epimysium merges with the periosteum of the bone.
Functions of Skeletal Muscles
This page concerns the skeletal muscles, and therefore it is fitting to consider their functions independent of the other types of muscles:
Skeletal muscles support the body. Skeletal muscle contraction opposes the force of gravity and allows us to remain upright. Some skeletal muscles are serving this purpose even when you think you are relaxed.
Skeletal muscles make bones and other body parts move. Muscle contraction accounts not only for the movement of limbs but also for eye movements, facial expressions, and breathing.
Skeletal muscles help maintain a constant body temperature.
Skeletal muscle contraction causes ATP to break down, releasing heat that is distributed about the body.
Skeletal muscle contraction assists movement in cardiovascular and lymphatic vessels. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels.
Skeletal muscles help protect internal organs and stabilize joints.
Muscles pad the bones that protect organs, and they have tendons that help hold bones together at joints.
Figure 7.2 Connective tissue of a skeletal muscle. a. Trace the connective tissue of a muscle from the endomysium to the perimysium to the epimysium, which becomes a part of the deep fascia and from which the tendon extends to attach a muscle to the periosteum of a bone. b. Cross section of the arm showing the arrangement of the muscles, which are separated from the skin by fascia. The superficial fascia contains adipose tissue. c. Photomicrograph of muscle fascicles from the tongue where the fascicles run in different directions. (c.s. = cross section; l.s. = longitudinal section).