MUSCULAR TISSUE
Author: Dr. R. Menaka
....................................................................................................................
There are three types of muscles are distinguishable histologically
1. Smooth
2. Skeletal
3. Cardiac
Smooth muscles: consists of fusiform or spindle cell 15 to 500 microns in length with abundant cytoplasm in whose central thickest position the nucleus lies. The nucleus is oval elongated or rod-shaped. The nuclei of contracted muscle cells usually have a folded outline. The cytoplasm contains the usual orgaanoids and appears more or less homogenous in routine sections. The smooth muscle cells increase in the pregnant uterus where the length may exceed half m.m. (500 microns). The smooth muscle is not richly supplied with blood vessels. Capillaries are seen in between group of cells.
The smooth muscle is found in the wall of the alimentary canal from the stomach to anus, gall bladder, trachea, bronchial tree, ureter, urinary bladder, prostate and cowper's glands, oviduct, uterus, vagina, blood vesse;s and larger lymphatics, hair follicles. In the eye, it is found in the iris and ciliary body.
Skeletal muscles:
The independent units of skeletal muscle are called as muscle fibers. The fibers have many nuclei and are larger than most cells. Fibers are grouped together in to bundles or fasciculi. Larger muscles are composed of many fasciculi. The skeletal muscle fibers are elongated and cylindrical and 1.5 cm in length and 10-15 microns in diameter. The thickness varies with species.
The skeletal muscle fiber shows a characteristic striated appearance due to regular alternation of dark and light stripes or striations across. They are produced by alternating light and dark segments of longitudinally arranged elements, the myofibrillae. The myofibrils are 1.2 microns in diameter. They are visible within the fibers of teased freshed muscle as well as in fixed preparations. Similar segment of adjacent fibrillae are placed side by side in an intact fiber to form characteristic cross striations seen in the fiber as a whole.
In a cross section of a fiber the cut ends of myofibrillae are visible as definite areas with individual size and shape. They are separated from each other by narrow spaces which represent the sarcoplasm. In routine preparations the fibrils appear to be arranged in regular groups' known as cohanheimsfield.
In the cross striations of the fibrillae the dark band is called the A or Q band (anisotropic). The light band is designated I or J band (isoyropic). Each of these band is bisected by a narrow line, tat in the I band stains deeply and is designated "Z" line and the bisecting the A band is pale and is called H zone. The Z line is also called Krause's membrane. The portion of fibril between two successive "Z" line is called a sarcomere. In the relaxed condition, the sarcomere is to 3 microns. Each myofibril is made up of thin, thread like elements myofilaments. Two types of myofilaments have been observed. One is thick and extends from one end of the A band to the other. These are myosin filaments. The thin filaments from either side of the "Z" line extended across the adjacent I band into the "A" band as far as the H zone. These are made up of actin and tropomyosin. Thus, the cross striations seen with the light microscope are related to the distribution of myofilaments which can be seen only under electron microscope.
Cardiac Muscle:
Myocardium or or heart muscle is composed of an interlacing network of muscle fibers (cells). The muscle net is rregularly and incompletely divided into bundles and laminae which wind about the heart in the form of spirals. The fibers within a bundle are mostly parallel but the bundles themselves cut bin longitudinal, transverse and oblique planes.
The muscle substance is composed of fibers or cells which show cross striations like skeletal muscle and are 9 to 20 microns in diameter, connected with each other by anatomosing branches. The myofibrils resemble that of skeletal muscle but the cross striations are closer together and not very distinct. These fibrillae are thicker and more closely packed together at the periphery of the fiber but may be so fine and sparse in the center that there appears to be anaxial core of sarcoplasm. In cross section the arrangement o\f cut ends of fibrillae frequently suggests short parallel bands or the spokes of a wheel nuclei are in the interior near the central axis. They are oval and quite large. There is an accumulation of sarcoplasm around the nucleus which contains mitochondria, fat droplets etc. A sarcolemma or plasmalemma is present similar to that of the skeletal muscle.
Cardiac muscle also shows intensely staining transverse bands at certain levels and these are known as intercalated discs. These are 0.5 to 1 micron thick that is less than a cross striations or sarcomere. They are strongly refractile in fresh muscle, but deeply stained in fixed material. They run in a straight line across fibers but frequently are irregular or broken into step formations. They are the cell membranes of adjacent cells meeting to each other at cell junctions.