Our heart is a muscle that has a completely unique contraction mechanism. Inside it is a complex system of specific cells (pacemakers), which has a multi-level system for monitoring work. It also includes Purkinje fibers. They are located in the myocardium of the ventricles and are responsible for their synchronous contraction.
General anatomy of the conduction system
The conducting system of the heart is conditionally divided by anatomists into four parts. The sinus-atrial (sinoatrial) node belongs to the first part. It is a combination of three bundles of cells that generate impulses at a frequency of eighty to one hundred and twenty times per minute. This heart rate allows you to maintain sufficient blood circulation in the body, its saturation with oxygen and metabolic rate.
If for some reason the first pacemaker cannot perform its functions, the atrioventricular (atrioventricular) node comes into play. It is located on the border of the chambers of the heart in the median septum. itaccumulation of cells sets the frequency of contractions in the range from sixty to eighty beats and is considered a second-order pacemaker.
The next level of the conduction system is the bundle of His and Purkinje fibers. They are located in the interventricular septum and braid the apex of the heart. This makes it possible to quickly propagate electrical impulses through the ventricular myocardium. The generation rate varies from forty to sixty times per minute.
Blood supply
Parts of the conduction system that are located in the atria receive nutrients from separate sources, separate from the rest of the myocardium. The sinoatrial node is fed by one or two small arteries that run through the thickness of the walls of the heart. The peculiarity lies in the presence of a disproportionately large artery that passes through the middle of the node. This is a branch of the right coronary artery. It, in turn, gives many small branches that form a dense arterial-venous network in this area of the atrial tissue.
The bundle of His and Purkinje fibers also receive nutrition from the branches of the right coronary artery (interventricular artery) or directly from it itself. In some cases, blood can enter these structures from the circumflex artery. Here, too, a dense network of capillaries is formed, which tightly braid the cardiomyocytes.
Cells of the first type
The differences in the cells that make up the conducting system are due to the fact that they perform different functions. There are three main types of cells.
Leading pacemakers are P-cells or cells of the first type. Morphologically, these are small muscle cells with a large nucleus and many long processes intertwined with each other. Several adjacent cells are considered as a cluster united by a common basement membrane.
To generate contractions, bundles of myofibrils are located in the internal environment of P-cells. These elements occupy at least a quarter of the entire space of the cytoplasm. Other organelles are randomly located inside the cell and are fewer than in ordinary cardiomyocytes. And the tubules of the cytoskeleton, on the contrary, are located tightly and maintain the shape of the pacemakers.
The sinoatrial node consists of these cells, but the rest of the elements, including the Purkinje fibers (the histology of which will be described below), have a different structure.
Cells of the second type
They are also called transient or latent pacemakers. Irregularly shaped, shorter than normal cardiomyocytes but thicker, contain two nuclei, and have deep grooves in the cell wall. There are more organelles in these cells than in the cytoplasm of P-cells.
Contractile filaments are extended along the long axis of the cell. They are thicker and have many sarcomeres. This allows them to be second order pacemakers. These cells are located in the atrioventricular node, and the His bundle and Purkinje fibers on micropreparations are represented by cells of the third type.
Cells of the third type
Histologists have identified several types of cells in the terminal parts of the conduction system of the heart. According to the classification considered here, the cells of the third type will have a similar structure with those that make up the Purkinje fibers in the heart. They are more voluminous compared to other pacemakers, long and wide. The thickness of myofibrils is not the same in all parts of the fiber, but the sum of all contractile elements is greater than in a normal cardiomyocyte.
Now you can compare the cells of the third type with those that make up the Purkinje fibers. The histology (a preparation obtained from tissues at the apex of the heart) of these elements differs significantly. The nucleus has an almost rectangular shape, and the contractile fibers are rather poorly developed, have many branches and are connected to each other. In addition, they are not clearly oriented along the length of the cell and are located at large intervals. A meager amount of organelles that are located around the myofibrils.
Differences in the frequency of generated impulses and the speed of their conduction require a phylogenetically developed mechanism for synchronizing the process of contraction in all parts of the heart.
Histological differences between the conduction system and cardiomyocytes
Cells of the second and third type have more glycogen and its metabolites than ordinary cardiomyocytes. This feature is designed to provide a sufficient degree of plastic function and cover the nutritional needs of cells. Enzymes responsible for glycolysis and glycogen synthesis are much more activein the cells of the conducting system. In the working cells of the heart, the opposite picture is observed. Due to this feature, a decrease in oxygen delivery is more easily tolerated by pacemakers, including Purkinje fibers. The preparation of the conducting system after treatment with chemically active substances shows high activity with cholineserase and lysosomal enzymes.
