Muscle Tissue

 Muscle tissue is specialized for contraction. There are three types of muscle tissue: skeletal, cardiac, and smooth. Skeletal muscle is often called voluntary muscle because its contraction is consciously controlled. Both skeletal and cardiac muscles are striated; they have striations, or stripes, that extend across the width of the muscle cell. These striations are produced by a characteristic arrangement of contractile proteins, and for this reason skeletal and cardiac muscle have similar mechanisms of contraction. Smooth muscle lacks these striations and has a different mechanism of contraction.

Three skeletal muscle fibers showing the characteristic
light and dark cross striations. Because of this feature, skeletal muscle is also called striated muscle.

Skeletal Muscle

Skeletal muscles are generally attached to bones at both ends by means of tendons; hence, contraction produces movements of the skeleton. There are exceptions to this pattern, however. The tongue, superior portion of the esophagus, anal sphincter, and diaphragm are also composed of skeletal muscle, but they do not cause movements of the skeleton.

Beginning at about the fourth week of embryonic development, separate cells called myoblasts fuse together to form skeletal muscle fibers, or myofibers (from the Greek myos, meaning “muscle”). Although myofibers are often referred to as skeletal muscle cells, each is actually a syncytium, or multinucleate mass formed from the union of separate cells. Despite their unique origin and structure, each myofiber contains mitochondria and other organelles common to all cells.

The muscle fibers within a skeletal muscle are arranged in bundles, and within these bundles the fibers extend in parallel from one end to the other of the bundle. The parallel arrangement of muscle fibers allows each fiber to be controlled individually: one can thus contract fewer or more muscle fibers and, in this way, vary the strength of contraction of the whole muscle. The ability to vary, or “grade,” the strength of skeletal muscle contraction is obviously needed for precise control of skeletal movements.

Human cardiac muscle. Notice the striated appearance and dark-staining intercalated discs

Cardiac Muscle

Although cardiac muscle is striated, it differs markedly from skeletal muscle in appearance. Cardiac muscle is found only in the heart, where the myocardial cells are short, branched, and intimately interconnected to form a continuous fabric. Special areas of contact between adjacent cells stain darkly to show intercalated discs, which are characteristic of heart muscle.

The intercalated discs couple myocardial cells together mechanically and electrically. Unlike skeletal muscles, therefore, the heart cannot produce a graded contraction by varying the number of cells stimulated to contract. Because of the way it is constructed, the stimulation of one myocardial cell results in the stimulation of all other cells in the mass and a “wholehearted” contraction.

A photomicrograph of smooth muscle cells. Notice that these cells contain single, centrally located nuclei and lack striations.

Smooth Muscle 

As implied by the name, smooth muscle cells  do not have the striations characteristic of skeletal and cardiac muscle. Smooth muscle is found in the digestive tract, blood vessels, bronchioles (small air passages in the lungs), and in the ducts of the urinary and reproductive systems. Circular arrangements of smooth muscle in these organs produce constriction of the lumen (cavity) when the muscle cells contract. The digestive tract also contains longitudinally arranged layers of smooth muscle. The series of wavelike contractions of circular and longitudinal layers of muscle known as peristalsis pushes food from one end of the digestive tract to the other.