The myocardium is composed of millions of elongated, striated, multinucleated cardiac muscle cells. These cells are approximately 15 mm X 15 mm X 150 mm long and can be depolarized and repolarized like skeletal muscle. Figure 2.4 below shows a typical group of myocardial muscle cells.
Individual cardiac muscle cells are interconnected by dense structures known as intercalated disks. The cells form a latticework of muscular tissue known as a syncytium.
A multinucleate mass of cardiac muscle cells form a functional syncytium (pronounced sin-sish'e-um). The heart has two separate muscle syncytia. The first is the mass that makes up the two atria and the second is the muscle mass that makes up the two ventricles. Fibrous rings that surround the valves between atria and ventricles separate the two syncytia. When one muscle mass is stimulated, the action potential spreads over the entire syncytium. As a result, under normal
circumstance, both atria contract simultaneously and both ventricles contract simultaneously.
Contraction in myocardium takes 10 to 15 times as long as it takes in average skeletal muscle. Myocardium contracts more slowly because sodium/calcium channels in myocardium are much slower than sodium channel in skeletal muscle during repolarization. In addition, immediately after the onset of the action potential, the permeability of cardiac muscle membrane for potassium ions decreases about five-fold. This effect does not happen in skeletal muscle. This decrease in permeability prevents a quick return of the action potential to its resting level.
The cellular membranes of myocardial cells are polarized in the resting state like any other cells in the body. The resting, transmural electrical potential difference is approximately —90 mV in ventricular cells. The inside of the cell is negative with respect to the outside. This transmembrane potential exists because the cell membrane is selectively permeable to charged particles. Figure 2.5 shows the transmembrane resting potential in a cardiac cell.
The principal electrolyte ions inside of myocardial cells, which are responsible for the transmembrane potential, are sodium, potassium, and chloride. Negative anions associated with proteins and other large molecules are also very important for the membrane potential. They attract the positive potassium ion that can go inside the cell.
The permeability of the membrane to sodium ions is very low at rest, and the ions cannot easily pass through the membrane. On the other hand, permeability of the membrane with respect to both potassium and chloride ions is much higher and those ions can pass relatively easily
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