Purkinje's fibers in the heart

Our heart is a muscle that has a completely unique mechanism of contraction. Inside it is a complex system of specific cells (pacemakers), which has a multi-level system for monitoring work. It includes, among other things, Purkinje fibers. They are located in the myocardium of the ventricles and are responsible for their synchronous contraction.

General anatomy of the conducting system

The conduction system of the heart is conventionally divided into four parts by anatomists. The first part includes the sinus-atrial (sinoatrial) node. It is a combination of three bundles of cells that generate pulses at a frequency of eighty to one hundred and twenty times per minute. Such a rate of heart contractions allows maintaining sufficient blood circulation in the body, oxygen saturation and metabolic rate.

If for some reason the first pacemaker can not perform its functions, the atrioventricular (atrioventricular) node enters the case. 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 of sixty to eighty beats and is considered to be the driver of the second-order rhythm.

The next level of the conducting system is the bundle of His and the fibers of Purkinje. They are located in the interventricular septum and braid the apex of the heart. This makes it possible to rapidly spread electrical impulses along the ventricular myocardium. The speed of generation varies from forty to sixty times per minute.

Blood supply

Parts of the conductive system, which are located in the atria, receive nutrients from isolated sources, separately from the rest of the myocardium. The sinoatrial node nourishes one or two small arteries that pass 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 the branch of the right coronary artery. It, in turn, gives many small branches that form a dense arterial-venous network on this site of the atrial tissue.

The bundle of Giesa and Purkinje fibers are also fed 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 of the artery. Here, too, a dense network of capillaries is formed, which densely interlace the cardiomyocytes.

Cells of the first type

The differences in the cells that enter the conductive 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 that have a large nucleus and many long processes intertwined. Several neighboring cells are considered as a cluster, united by a common basal membrane.

Beams of myofibrils are arranged to generate contractions in the internal environment of P cells. These elements occupy not less than a quarter of the entire cytoplasmic space. Other organelles are randomly located inside the cell and less than in ordinary cardiomyocytes. And the cytoskeleton tubes, on the other hand, are densely packed and support the shape of the rhythm drivers.

Of these cells there is a sino-atrial node, but the rest of the elements, including the Purkinje fibers (whose histology will be described below), have a different structure.

Cells of the second type

They are also called transient or latent rhythm drivers. Wrong shapes, shorter than conventional cardiomyocytes, but have a greater thickness, contain two cores, and the cell wall has deep recesses. Organelles in these cells are larger than in the cytoplasm of P cells.

The contraction threads are extended along the long axis of the cell. They are thicker and have many sarcomeres. This allows them to be drivers of second-order rhythm. These cells are located in the atrioventricular node, and the fasciculus and Purkinje fibers on the micro preparations are represented by cells of the third type.

Cells of the third type

Histologists have identified several types of cells in the terminal sections of the conduction system of the heart. According to the classification considered here, cells of the third type will have a similar structure with those that make Purkinje fibers in the heart. They are more voluminous, compared with other drivers of rhythm, long and wide. The thickness of the myofibrils is not the same on all sections of the fiber, but the sum of all the contractile elements is greater than in the usual cardiomyocyte.

Now we can compare cells of the third type with those that make Purkinje fibers. Histology (the preparation obtained from tissues on the apex of the heart) of these elements is significantly different. The nucleus has an almost rectangular shape, and the contractile fibers are developed rather weakly, they have many branches and are connected with each other. In addition, they are not oriented clearly along the length of the cage and are located at large intervals. A meager amount of organelles, which are located around myofibrils.

Differences in the frequency of the generated pulses and their speed require a phylogenetically developed mechanism for synchronizing the contraction process in all parts of the heart.

Histological differences of the conducting system from cardiomyocytes

Cells of the second and third type 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. The 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 there is an opposite picture. Due to this feature, the reduction in oxygen delivery is more easily tolerated by pacemakers, including Purkinje fibers. The preparation of the conductive system after treatment with reactive substances shows a high activity with cholinesterase and lysosomal enzymes.