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Name: Isamotu Ayoola Temitope Department: Medicine and Surgery Matric no: 14/MHS01/073 Course: Histology of basic tissues Course code: ANA 203 HISTOLOGY OF MUSCLE TISSUE Muscle tissue has a unique histological appearance which enables it to carry out its function. In this particle, we will examine the histology and how it relates to contractility. Muscle tissue is composed of elongated cells specialized for contraction and movement. Muscle tissue is composed of cells differentiated for optimal use of the universal cell property termed contractility. Microfilaments and associated proteins together generate the forces necessary for cellular contraction, which drives movement The body contains three types of muscle tissue: (a) skeletal muscle, (b) smooth muscle, and (c) cardiac muscle. (Same magnification) within certain organs and the body as a whole. Nearly all muscle cells are of mesodermal origin and they differentiate mainly by a gradual process of cell lengthening with simultaneous synthesis of myofibrillar proteins. Three types of muscle tissue can be distinguished on the basis of morphologic and functional characteristics and the structure of each type is adapted to its physiologic role Types of Muscle There are three types of muscle: Skeletal – Unlike most tissues, skeletal muscle does not consist of individual cells. Rather it is formed from huge, multinucleate muscle fibers, which develop by fusion of many individual embryonic cells called myoblasts. Muscle fibers can grow by continued nuclear division within the fibers. Although skeletal muscle fibers are thus not proper, individual cells, the term "muscle cell" is commonly used to refer to one multinucleate fiber. Each individual skeletal muscle fiber extends over much of the length of the muscle in which it resides (up to many centimeters), with a uniform diameter that is typically around 50 µm. In contrast, cardiac muscle (which like skeletal muscle is also striated) and smooth muscle both consist of single, discrete cellular units, each with its own nucleus. Cardiac – A form of striated muscle that is found only in the heart. Identifying features are single nuclei and the presence of intercalated discs between the cells. it also has cross-striations and is composed of elongated, branched individual cells that lie parallel to each other. At sites of end-to-end contact are the intercalated disks, structures found only in cardiac muscle. Contraction of cardiac muscle is involuntary, vigorous, and rhythmic. Cardiac muscle has several distinct characteristics. Cardiac muscle is striated, like skeletal muscle. (For brief description of striations, see skeletal muscle.) Cardiac muscle consists of distinct, individual cells, unlike skeletal muscle (which consists of immensely long, multinucleate fibers). Cardiac muscle cells are attached end-to-end by specialized junctions called intercalated discs. Smooth – Smooth muscle consists of individual cells (leiomyocytes), each cell with its own nucleus. The function of smooth muscle also differs substantially from that of striated muscle. Neurotransmitter activitation of smooth muscle is fairly diffuse. There are no discrete, well-defined neuromuscular junctions. The motor endings of autonomic axons, where neurotransmitter is released, are not closely associated with individual smooth muscle fibers. Electrical activitation of smooth muscle is passed from cell to cell by gap junctions. Smooth muscle of the gut can generate intrinsic rhythmic contraction, independent from direct neural control. Input from the autonomic nervous system increases or decreases the level of this spontaneous activity. (In certain other locations, such as the iris of the eye, neural control is more direct and precise.) Consult your textbook for more information, including molecular details of contraction and control mechanisms. Each smooth muscle cell (or "muscle fiber") is just a few microns in diameter but may be two hundred microns long. The nucleus is also elongated, often cigar-shaped. In histologic sections, the full length of smooth muscle nuclei is only apparent when the plane of section is aligned with the long axis of the cells. Muscle Fibers As observed with the light microscope, longitudinally sectioned skeletal muscle fibers show cross-striations of alternating light and dark bands (Figure 10–7). The darker bands are called A bands (anisotropic or birefringent in polarized light); the lighter bands are called I bands (isotropic, do not alter polarized light). In the TEM each I band is seen to be bisected by a dark transverse line, the Z line (Ger. Zwischenscheibe, between the discs). The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z line to Z line (Figure 10–8) and is about 2.5 m long in resting muscle. Myosin is a much larger complex (molecular mass ~500 kDa). Myosin can be dissociated into two identical heavy chains and two pairs of light chains. Myosin heavy chains are thin, rod-like molecules (150 nm long and 2–3 nm thick) made up of two heavy chains twisted together as myosin tails. Small globular projections at one end of each heavy chain form the heads, which have ATP binding sites as well as the enzymatic capacity to hydrolyze ATP (ATPase activity) and the ability to bind actin. The four light chains are associated with the head. Several hundred myosin molecules are arranged within each thick filament with their rodlike portions overlapping and their globular heads directed toward either end.