Contractile Apparatus of Striated Muscle

The muscle cell is a fiber A2) approximately 10 to 100 ^m in diameter. Skeletal muscles fibers can be as long as 15 cm. Meat "fibers" visible with the naked eye are actually bundles of muscle fibers that are around 100 to 1000 ^m in diameter (^ A1). Each striated muscle fiber is invested by a cell membrane called the sarcolemma, which surrounds the sarcoplasm (cytoplasm), several cell nuclei, mitochondria (sarcosomes), substances involved in supplying O2 and energy (^ p. 72), and several hundreds of myofibrils.

So-called Z lines or, from a three-dimensional aspect, Z plates (plate-like proteins; ^ B) subdivide each myofibril (^ A3) into ap-prox. 2 ^m long, striated compartments called sarcomeres (^ B). When observed by (two-dimensional) microscopy, one can identify alternating light and dark bands and lines (hence the name "striated muscle") created by the thick myosin II filaments and thin actin filaments (^ B; for myosin I, see p. 30). Roughly 2000 actin filaments are bound medially to the Z plate. Thus, half of the filament projects into two adjacent sarcomeres (^ B). The region of the sarcomere proximal to the Z plate contains only actin filaments, which form a so-called I band (^ B). The region where the actin and myosin filaments overlap is called the A band. The Hzone solely contains myosin filaments (ca. 1000 per sarcomere), which thicken towards the middle of the sarcomere to form the M line (Mplate). The (actin) filaments are anchored to the sarcolemma by the protein dystrophin.

Each myosin filament consists of a bundle of ca. 300 myosin-II molecules (^ B). Each molecule has two globular heads connected by flexible necks (head and neck = subfragment S1; formed after proteolysis) to the filamentous tail of the molecule (two intertwined a-helices = subfragment S2) (^ C). Each of the heads has a motor domain with a nucleotide binding pocket (for ATP or ADP + Pi) and an actin binding site. Two light protein chains are located on each neck of this heavy molecule (220 kDa): one is regulatory (20kDa), the other essential (17 kDa). Conformational changes in the head-neck segment allow the myosin head to "tilt" when interacting with actin (slidingfilaments; ^ p. 62).

Actin is a globular protein molecule (G-actin). Four hundered such molecules join to form F-actin, a beaded polymer chain. Two of the twisted protein filaments combine to form an actin filament (^ B), which is positioned by the equally long protein nebulin.

Tropomyosin molecules joined end-to-end (40 nm each) lie adjacent to the actin filament, and a troponin (TN) molecule is attached every 40 nm or so (^ B). Each troponin molecule consists of three subunits: TN-C, which has two regulatory bindings sites for Ca2+ at the amino end, TN-I, which prevents the filaments from sliding when at rest (^ p. 62), and TN-T, which interacts with TN-C, TN-I, and actin.

The sarcomere also has another system of filaments (^ B) formed by the filamentous protein titin (connectin). Titin is more than 1000 nm in length and has some 30 000 amino acids (Mr > 3000 kDa). It is the longest known polypeptide chain and comprises 10% of the total muscle mass. Titin is anchored at its carb-oxyl end to the M plate and, at the amino end, to the Z plate (^ p. 66 for functional description).

The sarcolemma forms a T system with several transverse tubules (tube-like invaginations) that run perpendicular to the myofibrils (^ p. 63 A). The endoplasmic reticulum (^ p. 10 ff.) of muscle fibers has a characteristic shape and is called the sarcoplasmic reti-culum (SR; ^ p.63 A). It forms closed chambers without connections between the intra-and extracellular spaces. Most of the chambers run lengthwise to the myofibrils, and are therefore called longitudinal tubules (^ p. 63 A). The sarcoplasmic reticulum is more prominently developed in skeletal muscle than in the myocardium and serves as a Ca2+ storage space. Each T system separates the adjacent longitudinal tubules, forming triads (^ p. 63 A, B).

i— A. Ultrastructure of striated muscle fibers i— A. Ultrastructure of striated muscle fibers

Myofibril Myocytes Differentiation

100-1 000^m 1 Bundle of fibers

10-100 I^m 2 Muscle fiber (myocyte)

3 Myofibril

100-1 000^m 1 Bundle of fibers i— B. Sarcomere structure

10-100 I^m 2 Muscle fiber (myocyte)

3 Myofibril

Sarcomere Three Dimensions
(After D.M.Warshaw)

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