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NAME: SAFIA BELLO MIKAILU MATRIC NO: 14/MHS01/035 DEPARTMENT: MEDICINE AND HEALTH SCIENCES THE HISTOLOGY OF MUSCLE AS A TISSUE The muscle or rather muscle tissue is composed of cells that are highly specialized to shorten in length by contractility. This muscle tissue is made of cells that are called myocytes. These myocytes are specialized for contraction with apparatus of actin and myosin proteins. Elongated in one direction are the muscle fibers. These muscles function in: contraction for locomotion and skeletal movement, for propulsion and for pressure regulation. Morphologically, muscle tissues are of two (2) types: (i) striated (ii) non-striated. Functionally, there are also two (2) types of muscle tissue; (i) voluntary (ii) involuntary. Histologically, the muscle tissue is of three (3) types: (i) skeletal muscle (ii) cardiac muscle (iii) smooth muscle. However, the skeletal muscle is a striated and voluntary muscle, present mainly in limbs and in relation to body wall. It is formed by fusion of multiple myoblasts during embryonic life and mostly originates from somatic mesoderm. The basic unit of the skeletal muscle is long, cylindrical fiber. Microscopically, the skeletal muscle fibers are arranged parallel to each other having alternate dark and light bands (cross striations). The skeletal muscle is multinucleated, placed peripherally beneath the sarcolemma. It has the myofibrils as the contractile elements and the sarcomere as its fundamental contractile unit. The myofilaments contain thick myosin and actin filaments. Each thick filament of this skeletal muscle is surrounded by six (6) thin filaments in hexagonal fashion. It has accessory proteins necessary for proper contraction: titin, alpha, actinin, nebulin, tropomodulin, dystrophin. The motor fibbers and the sensory fibbers are the nerve supply to the skeletal muscle. Sensitive stretch receptors called neuromuscular spindles are present within nearly all skeletal muscles. These spindles consist of a connective tissue capsule, in which are found modified muscle fibers called intrafusal fibers and numerous nerve endings, surrounded by a fluid-filled space. The neuromuscular spindles monitor the changes (distension) in the muscle lengths and activate complex reflexes to regulate muscle activity. Skeletal muscle is surrounded by a dense, irregular connective tissue layer called epimysium.From epimysium, a less dense irregular connective tissue layer, called perimysium, extends inward and divides the interior of the muscle into smaller bundles called fascicles; each fascicle is thus surrounded by perimysium. A thin layer of reticular connective tissue fibers, called endomysium, invests individual skeletal muscle fibers. An illustration of the skeletal muscle is shown below. More so, the cardiac muscle is striated and involuntary, present exclusively in the heart. It originates in the splanchnopleuric mesoderm. The cardiac muscle is supplied by the autonomic nervous system. Microscopically, the cardiac muscle consists of long and thick branching muscle fibers which may appear as Y shaped. It also consists of centrally placed single oval nucleus, faint transverse striations and also A and I bands along with Z discs present. The terminal ends of adjacent cardiac muscle fibers show characteristic and densestaining, end-to-end junctional complexes called intercalated disks. These disks are special attachment sites that cross the cardiac cells at irregular intervals in step like fashion. Located in the intercalated disks are the gap junctions that enable ionic communication and continuity between adjacent cardiac muscle fibbers. An illustration of the cardiac muscle is shown below. Furthermore, the smooth muscles have a wide distribution and are found in numerous hollow organs; gastro-intestinal walls, blood vessels, urinary bladder, ureter, uterine tube, arrector pili muscle of hair follicles, etc. Smooth muscle fibers also contain contractile actin and myosin filaments; however, they are not arranged in the regular, crossstriated patterns that are visible in both the skeletal and cardiac muscle fibers. As a result, these muscle fibers appear smooth or non-striated. Smooth muscle fibers are also involuntary muscles and are, therefore, under autonomic nervous system and hormonal control. Microscopically, smooth muscle consists of elongated spindle shaped fibers, centrally placed single elongated nucleus. Its adjacent cells are in contact with each other through gap junctions. As shown below; REFERENCES: www.clinicme.com www.buzzle.com