Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Muscle Tissue
Morphological Organization of Striated Muscle

Muscle tissue accounts for 40–42% of body mass. The primary dynamic function of Muscles is to provide mobility through contraction followed by relaxation. Muscle contraction performs work associated with The conversion of chemical energy into mechanical energy.

Three types of muscle tissue are distinguished: skeletal, cardiac, and Cytology/cytology/32.html">Smooth muscle tissue.

Striated Muscle consists of numerous elongated fibers*, or muscle Cells. Motor nerves enter the muscle fiber at various points and transmit an electrical impulse that triggers contraction. A muscle fiber is generally regarded as a Giant multinucleated Cell covered by an elastic membrane, the sarcolemma (Fig. 20.1). The diameter of a functionally mature striated muscle fiber typically ranges from 10 to 100 µm, and its length often corresponds to the length of the muscle.

Within each muscle fiber, embedded in the semi-liquid sarcoplasm along the length of the fiber—frequently arranged in bundles—are numerous thread-like structures known as myofibrils (usually less than 1 µm in thickness), which exhibit transverse striation just like the fiber as a whole. The transverse striation of the fiber, resulting from the optical heterogeneity of protein substances localized at the same level across all myofibrils, is readily revealed when Skeletal Muscle fibers are examined under a polarizing or phase-contrast microscope.

* White and red muscle fibers are also distinguished. White muscle fibers feature a higher myofibril content and, accordingly, a capacity for more rapid contractions. In red fibers, the myofibril content is relatively lower, while the sarcoplasm content is greater. Red fibers owe their name to their high Myoglobin content. Red muscle fibers are characterized by a more pronounced tonic contraction pattern. In humans, white and red fibers are usually found together within the same muscle.

Class="center">

Fig. 20.1. Structure of a skeletal muscle fiber (after Hasselbach).

I - A-band; II - I-band; III - H-zone; 1 - Z-line; 2 - T-system; 3 - sarcoplasmic reticulum; 4 - opening of the T-system; 5 - Glycogen; 6 - mitochondrion; 7 - sarcolemma.

The sarcoplasm of muscle fibers also contains A number of other structures: Mitochondria, microsomes, Ribosomes, tubules and cisternae of the sarcoplasmic reticulum, various vacuoles, glycogen granules, and lipid inclusions that serve as reserve Energy Sources, etc. (see Fig. 20.1).

A repeating unit of the striated myofibril is the sarcomere—a segment of the myofibril bounded by narrow Z-lines. Each myofibril consists of several hundred sarcomeres. The average length of a sarcomere is 2.5–3.0 µm. In the middle of the sarcomere lies a zone 1.5–1.6 µm long, which appears dark under a phase-contrast microscope. In polarized light, it exhibits strong birefringence. This zone is conventionally referred to as the A-band (anisotropic band). At the center of the A-band lies the M-line, which can be observed only with an Electron microscope. The middle portion of the A-band is occupied by the H-zone, which displays weaker birefringence. Finally, there are isotropic bands, or I-bands, with very weak birefringence. Under a phase-contrast microscope, they appear lighter than the A-bands. The length of the I-bands is about 1 µm. Each of them is divided into two equal halves by the Z-membrane, or Z-line.

Fig. 20.2. Structure of a skeletal muscle sarcomere.

a - schematic diagram of sarcomere structure; b - arrangement of thick and thin filaments (cross-section).

According to current concepts, the A-bands contain thick filaments, composed primarily of the protein Myosin, and thin filaments, which typically consist of the second component of the Actomyosin system, the protein Actin. Thin (actin) filaments originate within each sarcomere at the Z-line, extend across the I-band, penetrate into the A-band, and terminate at the H-zone (Fig. 20.2).

Examination of thin muscle sections under an electron microscope revealed that the protein filaments are arranged in a strictly ordered manner. Thick filaments, 12–16 nm in diameter and approximately 1.5 µm in length, are packed in a hexagonal lattice with a diameter of 40–50 nm and run throughout the entire A-band. Interspersed among these thick filaments are thin filaments 8 nm in diameter, extending from the Z-line for a distance of about 1 µm. Studies of contracting muscle have shown that the I-bands nearly disappear, while the region of overlap between thick and thin filaments increases (in a contracting skeletal muscle, the sarcomere shortens to 1.7–1.8 µm).

According to the sliding filament model proposed by A.F. Huxley and R. Niedergerke, as well as H.E. Huxley and J. Hanson, during myofibril contraction one set of filaments penetrates into the other—that is, the filaments effectively slide past one another, which is the underlying cause of muscle contraction.