| Talk in general about the cardiac muscle tissue. | The middle part of the heart wall (the myocardium) is formed of a muscular tissue associated to an interstitial connective tissue rich in blood and lymphatic vessels and in nerve fibers. The myocardium is composed largely of striated muscular cells (cardiomyocytes). These cells allow, by their continuous rhythmic, spontaneous and involuntary contraction, blood propulsion and the emptying of cardiac cavities
The myocardium is mainly constituted of muscle fibers, anastomosed and separated by thin septa of loose connective tissue rich in capillaries. |
| Talk about the cardiomyocyte. | Cardiac muscle cells (or cardiomyocytes) are long cylindrical cells with ramified extremities. Their sarcoplasm resembles that of skeletal fibers, with some particularities: every cardiac muscle cell has one elongated centrally placed nucleus. There are fusiform sarcoplasm zones around the nucleus, containing numerous mitochondria, a small Golgi apparatus, localized at one pole of the nucleus, some lipid droplets, glycogen granules |
| Talk about the cardiac muscle fibrils | The structure and ultrastructure of myofibrils are identical to those of skeletal striated muscular cells, and the molecular contraction mechanism is basically the same. Cardiac muscle fibers form 2 distinct contractile networks: the atrial network and the ventricular network |
| Talk about the sarco-tubular system of the cardiac muscle cell. | The sarco-tubular system of myocardiac cells differs from that of skeletal striated cells. The sarcoplasmic reticulum is less developed than that of the skeletal striated muscle cells. It is formed of longitudinal tubules (L tubules) and extends between myofilaments and beneath the sarcolemma and does not contain a triad. The T tubules of the cardiac cell are more numerous and larger in diameter than those of the skeletal fiber, they invaginate at the Z-line level of myofibrils, not at the A- I junction level. Every T tubule is generally associated to an L tubule, and together they form a dyad. |
| Talk about the sarcolemma of the cardiomyocytes | The sarcolemma of cardiomyocytes is limited by a basal lamina, and is devoid of motor end plates and neuromuscular junctions. As cardiomyocytes are linked through their extremities, the sarcolemma presents cellular junctions called intercalated discs: they are membrane zones that attach adjacent cells to each other. Intercalated discs are localized at the Z line level, connect myofibrils of adjacent cells and contain gap junctions that allow rapid spread of excitation from one cell to the other. |
| What are the characteristics of the cardiomyocyte? | Cardiomyocytes are characterized by an oxidative metabolism, a slow contraction, and a dependence on the extracellular Ca++ level. |
| Talk about the heterogeneity of cardiac muscle cells (contractile) | Contractile cardiomyocytes: These are the majority of cardiac cells. They are classified into ventricular cardiomyocytes and atrial cardiomyocytes. The latter are smaller than ventricular cells. Their T tubules are less developed and the contractile units are less numerous. |
| Talk about the heterogeneity of cardiac muscle cells (myoendocrine) | Myoendocrine cells: They are localized at the level of the atria. They do not contain much contractile material and perform an endocrine function. Their Golgi apparatus and their rough ER are well developed, and they contain secretory granules containing the atrial natriuretic factor (ANF) that decreases arterial pressure. |
| Talk about the heterogeneity of cardiac muscle cells (Cardionector) | Cardionector cells: They are modified cardiac cells specialized in the initiation and conduction of contraction of the myocardium. They differ according to their anatomic localization:
- Nodal cells (cells of the sino-atrial node and atrio-ventricular node): they are small, contain very few myofibrils, and are devoid of T tubules.
- Cells of the bundle of His: they are larger than nodal cells. Their myofibrils are well organized and parallel.
- Purkinje cells: they are huge and contain very few myofibrils. |
