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 of work control. Purkinje fibers also enter into it. They are located in the myocardium of the ventricles and are responsible for their synchronous contraction.
General anatomy of the conduction system
The conduction system of the heart is conditionally divided by anatomists into four parts. The first part includes the sinus-atrial (sinoatrial) node. It is a combination of three bundles of cells that generate impulses with a frequency of eighty to one hundred twenty times per minute. This speed of heart contractions 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 his functions, the atrioventricular (atrioventricular) node comes into play. It is located on the border of the heart chambers in the median septum. This accumulation 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 conducting system is the bundle of His and Purkinje fibers. They are located in the interventricular septum and braid the top of the heart. This makes it possible to quickly propagate electrical impulses throughout the ventricular myocardium. The generation rate varies from forty to sixty times per minute.
Blood supply
The 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 pass through the thickness of the walls of the heart. The peculiarity is the presence of a disproportionately large artery, which passes through the middle of the node. This is a branch of the right coronary artery. She, in turn, gives many small branches that form a dense arterial-venous network in this area of atrial tissue.
The bundle of His and Purkinje fibers also receive power from the branches of the right coronary artery (interventricular artery) or directly from it. In some cases, blood can enter these structures from the envelope 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 enter the conduction system are due to the fact that they perform different functions. There are three main types of cells.
The leading pacemakers are P-cells or cells of the first type. Morphologically, these are small muscle cells having a large nucleus and many long processes intertwined with each other. Several neighboring 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 total space of the cytoplasm. Other organelles are randomly located inside the cell and there are fewer than in ordinary cardiomyocytes. And the cytoskeletal tubes, on the contrary, are densely arranged and maintain the shape of pacemakers.
The sino-atrial node consists of these cells, but the remaining elements, including 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. Irregular shapes, shorter than regular cardiomyocytes, but have a greater thickness, contain two nuclei, and there are deep notches in the cell wall. There are more organelles in these cells than in the cytoplasm of P-cells.
The contractile threads are elongated 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.
Third type cells
Histologists have identified several types of cells in the terminal sections of the cardiac conduction system. 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 with other pacemakers, long and wide. The thickness of myofibrils is not uniform in all sections of the fiber, but the sum of all contractile elements is greater than in a conventional cardiomyocyte.
Now we can compare cells of the third type with those that make up Purkinje fibers. Histology (a drug obtained from tissues at the apex of the heart) of these elements is significantly different. The nucleus has an almost rectangular shape, and the contractile fibers are poorly developed, have many branches and are interconnected. In addition, they are not clearly oriented along the length of the cell and are located at large gaps. The meager number of organelles that are located around myofibrils.
Differences in the frequency of the generated pulses and the speed of their conduction require a phylogenetically developed mechanism for synchronizing the contraction process in all parts of the heart.
Histological differences between the conduction system and cardiomyocytes
Cells of the second and third types have a greater amount of glycogen and its metabolites than conventional cardiomyocytes. This feature is designed to provide a sufficient degree of plastic function and to cover the needs of cells in nutrients. Enzymes responsible for glycolysis and glycogen synthesis are much more active in the cells of the conducting system. In the working cells of the heart, the opposite picture is observed. Due to this feature, reduced pace of oxygen delivery is easier for pacemakers, including Purkinje fibers. The preparation of the conductive system after treatment with chemically active substances shows high activity with cholinesterase and lysosomal enzymes.