Structure of Skeletal Muscle Fibers

Structure of Skeletal Muscle Fibers

A single cell that can rapidly contract in response to stimulation and relaxes when the stimulation ceases is known as a skeletal muscle fiber (FIGURE 9-3). These fibers are thin, elongated cylinders with rounded ends. They are very large, with a diameter ranging from 10 to 100 μm. This is up to 10 times larger than an average body cell. The length may be up to 30 cm, and skeletal muscle fibers consist of hundreds of fused embryonic cells. The cell membrane (sarcolemma) lies above the cytoplasm (also known as sarcoplasm), with many contraction­. The sarcoplasm also contains large amounts of glycosomes and myoglobin. Myoglobin is similar to hemoglobin, which is the pigment that transports oxygen in the blood. The collective struc-tures that comprise myofibrils make up approximately 80% of the cellular volume of muscle fibers.

Myofibrils

The sarcoplasm is made up of many thread-like ­myofibrils arranged parallel to each other. One mus-cle fiber contains between several hundred and several thousand rod-like myofibrils.

Filaments

Myofilaments have thick filaments composed of myosin and thin filaments composed of actin. These filaments are organized so they appear as ­striations— areas of alternating colored bands of skeletal muscle fiber. The repeating patterns of striation units that appear along each muscle fiber are referred to as sarcomeres­, which are the functional units of skeletal muscle. Muscles are basically considered to be collec-tions of sarcomeres.

Each skeletal muscle fiber contains hundreds to thousands of myofibrils. Skeletal muscle fibers are the longest types of muscle cells. They contain smaller, rod-shaped myofilaments. The light bands (I bands) are made up of thin filaments of actinsmall, oval-shaped mitochondria and nuclei. Skeletal muscle fibers have multiple nuclei. The advantage of these fibers is that they can produce large amounts of the enzymes and structural proteins needed for attached to round sheets known as Z lines (or Z discs). The Z lines or discs are made up of mostly alpha-actinin, a protein that anchors the thin filaments. The dark bands (A bands) are made up of thick filaments of myosin that overlap thin filaments of actin. The central thick filaments contain myosin and extend the entire length of the A bands. The thin filaments contain actin and extend across the I bands, part of the way into the A bands. When a muscle fiber is intact, the A bands and I bands are almost perfect in their alignment. The ability of a muscle to be stretched is known as extensibility.

There is a central region (H zone) of thick fila-ments, with a thickened area (the M line) that con-sists of proteins holding them in place. The H zone appears less dense because the thin filaments do not extend into it. The M line is slightly darker than the H zone because its fine protein strands hold the nearby thick filaments together. The zone of overlap is a dark area where thin filaments lie between thick filaments. Three thick filaments surround every thin filament, and then six thin filaments surround every thick filament. Myofilaments connect to the sar-colemma and are held in an aligned pattern at the Z discs and M lines. Sarcomeres extend from one Z line to another Z line. Other proteins form the structure of myofibrils. Elastic filaments are com-posed of very large proteins known as titins. Each titin extends from the Z disc to the thick filament, forms the core of the thick filament, and attaches to the M line. Titin binds thick filaments in place, keeping the A bands organized and helps muscle cells to return to normal shape after being stretched. This process is called elasticity. Dystrophin is an important structural protein that links the thin fila-ments to the proteins of the sarcolemma. Filaments and sarcomeres are also bound by proteins such as myomesin, nebulin, and C proteins.

Myosin molecules are made up of two protein strands with globe-shaped cross-bridges that project outward. Groups of many myosin molecules make up a myosin (thick) filament. Actin molecules are globe-shaped with a binding site that attaches to myosin cross-bridges. Groups of many actin molecules twist in double strands (helixes) to form an actin (thin) fila-ment, which includes the proteins known as troponin and tropomyosin.

Polypeptide strands of the rod-shaped tropo-myosin protein, at rest, prevent actin–myosin inter-action. Tropomyosin spirals around the actin core, providing stiffening and stabilization. One subunit of the round three-polypeptide complex troponin protein molecule binds to tropomyosin, forming the troponin–tropomyosin complex. Another subunit binds to G-actin to hold the complex in position. A third subunit has a receptor binding a calcium ion. When the muscle is at rest, intracellular calcium is very low and the binding site is empty. Contractions cannot occur unless the position of the troponin– tropomyosin complex changes to expose the active sites on filamentous actin (F-actin). The position change occurs when calcium ions bind to recep-tors on the troponin molecules. When sarcomeres shorten within a skeletal muscle fiber, a skeletal muscle contracts. This occurs because of the cross-bridges pulling on the thin filaments of F- actin. Each strand of F-actin is made up of two rows of 300–400 globular molecules of G-actin. Strands of tropomyo-sin cover the G-actin active sites and prevent actin– myosin interaction. A molecule of tropomyosin is a double-stranded protein covering seven active sites, which is bound to one molecule of troponin halfway down its length. A troponin molecule is made up of three globular subunits.

The sliding filament modelor theory is so named because of the way sarcomeres shorten, with thick and thin filaments sliding past each other toward the center of the sarcomere, from both ends ( FIGURE 9 -4). If cross-bridges generate enough tension on thin fil-aments, shortening occurs. When the cross-bridges become inactive, contraction stops, tension decreases, and the muscle fiber relaxes. Thin and thick filaments overlap only at the ends of A bands. Therefore, the sliding filament model states that when contraction occurs, thin filaments slide past thick filaments. Actin and myosin filaments overlap more, and myosin on thick filaments connects with myosin-binding sites on actin in the thin filaments. Cross-bridges are portions of myosin molecules. Sliding begins, and cross-bridge attachments form and then break several times during each contraction. They generate tension and move the thin filaments toward the center of the sarcomere. Because this occurs throughout sarcomeres in the cell at the same time, the muscle cell shortens. As thin filaments slide centrally, attached Z discs are pulled toward the M line.

Myosin filaments contain the enzyme ATPase in their globe-shaped portions. This enzyme catalyzes the breakdown of ATP to both adenosine diphosphate (ADP) and phosphate, releasing energy. The myosin cross-bridges act as ATPases during the contraction cycle of muscle. They assume a “cocked” position, binding to actin to pull on the thin filament. After the pulling occurs, the cross-bridge is released from actin before the ATP splits. The cycle repeats as long as there is enough ATP for energy and muscular stim-ulation occurs.

To understand further, the following steps occur as a muscle cell shortens:

■■ I bands shorten.

■■ Distances between successive Z discs shorten.

■■ H zones (H bands) disappear.

■■ Contiguous A bands move closer together with out changing their length.

1. Describe connective tissue associated with skeletal muscle tissue.

2. Describe the structural components of a sarcomere.

3. Explain the reason why skeletal muscle fibers appear striated.