Skeletal muscle

A skeletal muscle cell is a multinucleated syncytium

In skeletal muscle, each muscle cell, more commonly called a muscle fiber, is actually a multinucleated syncytium. A muscle fiber is formed during development by the fusion of small, individual muscle cells called myoblasts. When viewed in cross section, the mature multinucleated muscle fiber reveals a polygonal shape with a diameter of 10 to 100 ¡xm. Their length varies from almost a meter, as in the sartorius muscle of the lower limb, to as little as a few millimeters, as in the stapedius muscle of the middle ear. (Note: A muscle fiber should not be confused with a connective tissue fiber; muscle fibers are cellular elements, whereas connective tissue fibers are extracellular products of connective tissue cells.)

The nuclei of a skeletal muscle fiber are located in the cytoplasm immediately beneath the plasma membrane, also called the sarcolemma. In the past, the term sarcolemma was used to describe a thick "membrane" that was thought to be the cytoplasmic boundary of the muscle cell. It is now known that the thick sarcolemma actually represents the plasma membrane of the cell, its external lamina, and the surrounding reticular lamina.

A skeletal muscle consists of striated muscle fibers held together by connective tissue

The connective tissue that surrounds both individual muscle fibers and bundles of muscle fibers is essential for force transduction. At the end of the muscle, the connective tissue continues as a tendon or some other arrangement of collagen fibers that attaches the muscle, usually, to bone. A rich supply of blood vessels and nerves travels in the connective tissue.

The connective tissue associated with muscle is named according to its relationship with the muscle fibers:

• Endomysium is the delicate layer of reticular fibers that immediately surrounds individual muscle fibers. Only small-diameter capillaries and the finest neuronal branches are present within the endomysium, running parallel to the muscle fibers.

• Perimysium is a thicker connective tissue layer that surrounds a group of fibers to form a bundle or fascicle. Fascicles are functional units of muscle fibers that tend to work together to perform a specific function. Larger blood vessels and nerves travel in the perimysium.

• Epimysium is the sheath of dense connective tissue that surrounds a collection of fascicles that constitutes the muscle (see Fig. 10.1a). The major vascular and nerve supply of the muscle penetrates the epimysium.

There are three types of skeletal muscle fibers: red, white, and intermediate

Skeletal muscle fibers differ in diameter and in their natural color in vivo. The color differences are not apparent in hematoxylin and eosin (H&E)-stained sections. However, special cytologic and histochemical reactions based on oxidative enzyme activity, specifically the succinic dehydrogenase and nicotinamide adenine dinucleotide-tetrazolium (NADH-TR) reactions, confirm the observations of fresh tissue and reveal several types of skeletal muscle fibers (Fig. 10.2). The most obvious are red fibers, white fibers, and intermediate fibers. The histochemical staining and enzyme activity of these three types of muscle fibers reflect their functional differences. Typically, all three fiber types are present in any given muscle. The proportion of each type varies according to the functional role of the muscle.

Fiber type is mainly attributable to myoglobin content and mitochondrial number

Myoglobin is an oxygen-binding protein that closely resembles hemoglobin found in erythrocytes and occurs in varying amounts in muscle fibers. It provides a ready source of oxygen for muscle metabolic reactions. Classification of skeletal muscle fibers into red, white, and intermediate fibers reflects the myoglobin content and the number of mitochondria with their constituent cytochrome electron transport complexes. These complexes are essential for oxidative phosphorylation to produce adenosine triphosphate (ATP), the energy source for muscle.

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