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IDOWU MARY 13/MHS01/054 HISTOLOGY OF THE MUSCLE TISSUE The muscle tissue is one of the four types of tissues. Structure of the muscle tissue The muscle tissue which is also known as the muscle mass is made up of a large number of individual muscle cells or myocytes. The muscle cells are commonly called muscle fibers because these cells are long and slender in appearance. Skeletal muscle fibers are multinucleated and are arranged parallel to one another with some connective tissue in between. Muscle mass is separated from the neighboring tissues by a thick fibrous tissue layer known as fascia. Beneath the fascia, muscle is covered by a connective tissue sheath called epimysium. In the muscle, the muscle fibers are arranged in various groups called bundles or fasciculi. Connective tissue sheath that covers each fasciculus is called perimysium. Each muscle fiber is covered by a connective tissue layer called the endomysium. Embryologically, the muscle tissue emanates from mesoderm and the mesenchyme. Function of muscle tissue 1. Contraction for locomotion and skeletal movement: Muscle tissue is the tissue that is responsible for motion and movement and is a characteristic of multicellular organisms. In living multicellular organisms the basis for motion mediated by muscle cells is the conversion of chemical energy (ATP) into mechanical energy by the contractile apparatus of muscle cells. The contractile apparatuses include the proteins, actin and myosin [filaments]. It is the interaction of these two proteins that initiates the contraction of muscle cells, which results in movement, eventually. 2. Contraction for propulsion 3. Contraction for pressure regulation. The muscle tissue can be divided into three based on their structure and function namely: 1. Skeletal muscle 2. Smooth muscle 3. Cardiac muscle Also, based on the morphology, muscle tissue can be divided into two namely: 1. Striated muscle 2. Non-striated or smooth muscle Based on function, we have: 1. Voluntary 2. Involuntary A. Skeletal muscle The skeletal muscle tissue is a type of muscle that is protected by the layer of dense connective tissue called Epimysium. It sends septa, called Perimysium into the muscle tissue dividing the tissue into fascicles (bundles). Within the fascicles, we have muscle fibres which are surrounded by a layer of connective tissue, endomysium, rich in reticular fibres and blood vessels. Embryologically, the skeletal muscle originates from the mesoderm. The muscle-forming cells called Myoblasts undergo frequent divisions and coalesce with the formation of a multinucleated, syncytial muscle fibres. The nuclei of the myotube are still located centrally in the muscle fiber. In the course of the synthesis of the myofilaments and myofibrils, the nuclei are gradually displaced to the periphery of the cell Features of the skeletal muscle includes: 1. Skeletal muscle consists of muscle fibers, which are long, cylindrical multinucleated cells that show cross-striations, hence also called striated Muscle. 2. Skeletal muscle cells nuclei are long, oval and usually found at the periphery of the cell under the cell membrane This characteristic nuclear location is helpful in discriminating skeletal muscle from cardiac and smooth muscle, both of which have centrally located nuclei. 3. Their contraction is quick, forceful, and usually under voluntary control. 4. Skeletal muscles have a well-defined neuromuscular junction. 5. The skeletal muscle is innervated by somatic nerves. Skeletal muscle type. Skeletal muscle has three types based on their physiological, biochemical, and histochemical characteristics namely: a. Type I or slow, red oxidative fibers contain many mitochondria and abundant myoglobin, a protein with iron groups that bind O2 and produce a dark red color. They are adapted for slow, continuous contractions over prolonged periods, as required for example in the postural muscles of the back. b. Type IIa or fast, intermediate oxidative-glycolytic fibers have many mitochondria and much myoglobin, but also have considerable glycogen. They utilize both oxidative metabolism and anaerobic glycolysis and are intermediate between the other fiber types both in color and in energy metabolism. They are adapted for rapid contractions and short bursts of activity, such as those required for athletics. c. Type IIb or fast, white glycolytic fibers have fewer mitochondria and less myoglobin, but abundant glycogen, making them very pale in color. They depend largely on glycolysis for energy and are adapted for rapid contractions, but tire quickly. They are typically small, such as the muscles that move the eyes and digits. B. Smooth muscle: They are also known as spindle-shaped cells with centrally placed nuclei. Each of these cells is enclosed by a thin basal lamina and a fine network of reticular fibers. The connective tissues serve to combine the forces generated by each smooth muscle fiber into a concerted action, e.g., peristalsis in the intestine. Note: The largest smooth muscle cells occur in the uterus during pregnancy (12x600 µm). The smallest are found around small arterioles (1x10 µm). Smooth muscle contraction process is slow and not subject to voluntary control, hence called the visceral or involuntary muscle. Embryologically, Smooth muscle cells arise from undifferentiated mesenchymal cells. These cells differentiate first into mitotically active cells, myoblasts, which contain a few myofilaments. Myoblasts give rise to the cells which will differentiate into mature smooth muscle cells. Features of smooth muscle includes: a. b. c. d. Non striated fibers Smooth muscle fibers are elongated, tapering, fusiform cells Lack of well-defined neuromuscular junctions. In addition to contractile activity, smooth muscle cells also synthesize collagen, elastin, and proteoglycans, extracellular matrix (ECM) components normally synthesized by fibroblasts. C. Cardiac muscle: Cardiac muscle, also known as the myocardium, consists of muscle cells, the cardiomyocytes, with one centrally placed oval nucleus. Cardiac muscle is also called involuntary striated muscle. Although equal in ultrastructure to skeletal muscle, the cross-striations in cardiac muscle are less distinct, in part because rows of mitochondria (accounting for about 40% of its cytoplasm) and many lipid and glycogen droplets are found between the myofibrils. Cardiac muscle does not contain cells equivalent to the satellite cells of skeletal muscle. Therefore cardiac muscle cannot regenerate. Features of cardiac muscles: Cardiac muscle has cross-striations and is composed of elongated, branched individual cells that lie parallel to each other. Contraction of cardiac muscle is involuntary, vigorous, and rhythmic. At sites of end-to-end contact are the intercalated disks (which connect the individual cells and permit the conduction of electrical impulses). These structures are found only in cardiac muscle. The cardiac muscle cells are not as long as the skeletal muscle cells. They may be mononucleated or binucleated. In either cases, the nuclei are located centrally in the cell. Clinical correlation: In Skeletal Muscle, although the nuclei are incapable of undergoing mitosis, the tissue can undergo limited regeneration. The source of regenerating cells is the sparse population of mesenchymal satellite cells that lies within the external lamina of each mature muscle fiber. The regenerative capacity of skeletal muscle is limited, however, after major muscle trauma or degeneration. Cardiac Muscle lacks satellite cells and has virtually no regenerative capacity beyond early childhood. Defects or damage (e.g, infarcts) in heart muscle are generally replaced by fibroblast proliferation and growth of connective tissue, forming myocardial scars. Smooth Muscle, composed of simpler, mononucleated cells, is capable of a more active regenerative response. After injury, viable smooth muscle cells undergo mitosis and replace the damaged tissue. Contractile pericytes from the walls of small blood vessels participate in the repair of vascular smooth muscle