![]() Placed end to end, these sarcomeres form long bands called myofibrils. The basic units of muscle are the contractile proteins actin and myosin arranged in sarcomeres. Know what exactly is a muscle cell and what's the difference between a muscle fiber and a muscle fibril. Note also the the cytoplasm of the muscle cells has a stippled, punctate appearance which is due to the bundling of thick and thin filaments into myofibrils as mentioned above. In transverse sections of skeletal muscle cells (slide 58), observe the cylindrical shape of the cells (fibers) and the peripherally-located nuclei. This is because different myofibrils may not be aligned. ![]() Note that sometimes the cross striations are not aligned all the way across the cell. A less regular longitudinal striping can sometimes be seen within the muscle fibers and is due to the bundling together of the thick and thin filaments into myofibrils, which are arranged parallel to the long axis of the cell.Some good areas of skeletal muscle showing cross striations can be found in slide 058thin View Image. However, they can be seen clearly on some areas of almost all slides, and it is just necessary to do some looking around for a favorable area on your slide. ![]() It is admittedly difficult to see the Z line and especially the H zone with the light microscope. The dark Z line which bisects the I band. The lighter H zone which bisects the A band.A sarcomere consists of the structures between two Z lines. Cross striations can be seen, and are due to the structure of the sarcomere.Note that each cell contains large numbers of nuclei. Peripheral position of the elongate nuclei just inside of the sarcolemma (plasma membrane).This is why it is important to look at other features of the cell to determine what exactly it is. Cells with peripheral nuclei that are cut at an angle can appear to have centrally located nuclei. You should be able to recognize each type of muscle in both planes of section.Īgain the answer to the question is in the plane of section. In contrast, cardiac and smooth muscles are called involuntary muscles because their contractions are either spontaneously generated or are under the control of the autonomic nervous system.Įach of these three types of muscle has a characteristic appearance in both cross and longitudinal sections. Skeletal muscle is also called voluntary muscle, because its contraction is under conscious neural control. Such an arrangement is not seen in smooth muscle cells. This regular arrangement imparts a cross-striated (or striped) appearance. Skeletal and cardiac muscle cells are called striated muscle because of the very regular arrangement of their intracellular contractile units, sarcomeres, at the light microscope (LM) and electron microscope (EM) levels. There are three major types of muscle, and their structure reflects their function. Be familiar with the regenerative potential of each muscle type.Understand where stem cells are located in skeletal muscle and be able to identify their location at both the light and EM levels. ![]()
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