ANSWERS: 2
  • You could compare it to a good biology book, and have a look at which muscle tissue looks similar. Remarkably, a lot of the book's photo's are great representations of the slides. Sorry, but I don't know any off the top of my head. Need to go to the library or the uni library if you are studying there. Or look on the web.
  • The 3 types of muscle tissue are cardiac, smooth, and skeletal. Cardiac muscle cells are located in the walls of the heart, appear spindle-shaped, and are under involuntary control. Smooth muscle fibers are located in walls of hollow visceral organs, except the heart, appear striated, and are also under involuntary control. Skeletal muscle fibers occur in muscles which are attached to the skeleton. They appear striated in appearance and are under voluntary control. Muscle tissue is made of "excitable" cells that are capable of contraction. Of all of the different tissue types, muscle tissue is the most abundant in most animals. Three types of muscle tissue can be identified histologically: skeletal muscle, cardiac muscle and smooth muscle. The fibres of skeletal muscle and cardiac muscle exhibit cross striations at the light microscope level and they are both referred to as striated muscle. Skeletal muscle Skeletal muscle constitutes the muscle that is attached to the skeleton and controls motor movements and posture. There are a few instances where this type of muscle is restricted to soft tissues: the tongue, pharynx, diaphragm and upper part of the esophagus. (Some people use the term visceral striated muscle in the foregoing examples, but since it is identical in structure to the muscle that moves the skeleton, we won't bother with the extra term.) Skeletal muscle fibres (cells) are actually a multinucleated syncytium formed by the fusion of individual small muscle cells or myoblasts, during development. They are filled with longitudinally arrayed subunits called myofibrils. The myofibrils are made up of the myofilaments myosin (thick filaments) and actin (thin filaments). The striations reflect the arrangement of actin and myosin filaments and support structures. The individual contractile units are called sarcomeres. A myofibril consists of many sarcomeres arranged end to end. The entire muscle exhibits cross-striations because sarcomeres in adjacent myofibrils and muscle fibers are in register. The most obvious feature in longitudinal sections of skeletal muscle is the alternating pattern of dark and light bands, called respectively the A (anisotropic) and I (isotropic) band. The I band is bisected by a dense zone called the Z line, to which the thin filaments of the I band are attached. The nuclei are located peripherally, immediately under the plasma membrane (sarcolemma). The thickness of individual muscle fibres varies (depending for example on location in the body and exercise) but each fibre is of uniform thickness throughout its length. Skeletal muscle fibres do not branch. Connective tissue elements surround muscle fibres. Individual muscle fibres are surrounded by a delicate layer of reticular fibres called the endomysium. Groups of fibres are bundled into fascicles by a thicker CT layer called the perimysium. The collection of fascicles that constitutes one muscle is surrounded by a sheath of dense CT called the epimysium, which continues into the tendon. Blood vessels and nerves are found in the CT associated with muscle. The endomysium contains only capillaries and the finest neuronal branches. At the light microscope level, a number of features distinguish cardiac from skeletal muscle. Cardiac muscle cells have only one or two nuclei, which are centrally located. The myofibrils separate to pass around the nucleus, leaving a perinuclear clear area (not always evident in standard preparations). This clear area is occupied by organelles, especially mitochondria (which are of course not visible in LM). As in skeletal muscle, individual muscle fibres are surrounded by delicate connective tissue. Numerous capillaries are found in the connective tissue around cardiac muscle fibres. Cardiac muscle cells are joined to one another in a linear array. The boundary between two cells abutting one another is called an intercalated disc. Intercalated discs consist of several types of cells junctions whose purpose is to facilitate the passage of an electrical impulse from cell to cell and to keep the cells bound together during constant contractile activity. Unlike skeletal muscle fibres, cardiac muscle fibres branch and anastomose with one another. Although made up of individual fibres, heart muscle acts as a functional syncytium during contraction for the efficient pumping of blood. Specialized fibres, called Purkinje fibres, arise from the atrioventricular node and travel along the interventricular septum toward the apex of the heart, sending branches into the ventricular tissue. Purkinje fibres are of larger diameter than ordinary cardiac fibres, with fewer myofibrils and an extensive, well-defined clear area around the nucleus. They conduct impulses at a rate about four times faster than that of ordinary cardiac fibres and serve to coordinate the contraction of the atria and ventricles.

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