Structure of cardiac muscle
The ultrastructure of the contractile apparatus and the mechanism of contraction largely correspond to that seen in skeletal muscle cells. Although equal in ultrastructure to skeletal muscle, the cross-striations in cardiac muscle are less distinct, in part because rows of mitochondria and many lipid and glycogen droplets are found between myofibrils.
In contrast to skeletal muscle cells, cardiac muscle cells often branch at acute angles and are connected to each other by specialisations of the cell membrane in the region of the intercalated discs. Intercalated discs invariably occur at the ends of cardiac muscle cells in a region corresponding to the Z-line of the myofibrils (the last Z-line of the myofibril within the cell is "replaced" by the intercalated disk of the cell membrane). In the longitudinal part of the cell membrane, between the "steps" typically formed by the intercalated disk, we find extensive GAP junctions.
T-tubules are typically wider than in skeletal muscle, but there is only one T-tubule set for each sarcomere, which is located close to the Z-line. The associated sarcoplasmatic reticulum is organised somewhat simpler than in skeletal muscle. It does not form continuous cisternae but instead an irregular tubular network around the sarcomere with only small isolated dilations in association with the T-tubules.
Cardiac muscle does not contain cells equivalent to the satellite cells of skeletal muscle. Therefore cardiac muscle cannot regenerate.
Sections of cardiac muscle - Alizarin Blue, Whipf's polychrome, iron haematoxylin, H&E
| Cardiac Muscle, human - H&E Use a low magnification to find a part of the tissue in which the cardiac muscle cells are cut longitudinally. At high magnification you should see striations and the large nuclei of the cardiac muscle cells. If you follow the course of individual cardiac muscle cells you will note fine, darker lines which seem to cross the fibres. These are the intercalated discs which connect the individual muscle cells mechanically and permit the conduction of electrical impulses between the cells. A light streak of cytoplasm is often visible extending from the poles of the nucleus. This part of the cytoplasm does not contain myofibrils, and it appears very light in transversely cut cardiac muscle cells. Myofibrils are often visible in transversely cut cells. Their visible separation reflects the large numbers of mitochondria located between them. Also, the large number capillaries reflect the need of a good blood supply to the constantly active muscle cells. Draw longitudinally cut cardiac muscle cells which show all the features mentioned. Label the features in your drawing, and include an suitable scale. |
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