Anatomical organization and conduction pathways
Excitation in the heart propagates in a strictly sequential manner: from the atria to the ventricles. The conduction system consists of pacemaker nodes and bundles extending from them.
- Sinus node (Sinoatrial node / Keith-Flack node). Located in the upper wall of the right atrium. This is the primary pacemaker.
- Internodal tracts. Three bundles extending from the sinus node within the atrial walls. They connect the atria to each other and transmit the signal to the next node. Collectively, these tracts are referred to as the Keith-Flack bundle.
- Atrioventricular node (Aschoff-Tawara node). Located in the lower wall of the right atrium, near the interatrial septum.
- Bundle of His (Atrioventricular bundle). Originates from the AV node, enters the interventricular septum, and divides into two branches (right and left). Its function is to connect with the ventricular myocardium.
- Purkinje fibers. These are the terminal ramifications of the bundle of His branches that directly transmit the impulse to working myocardial cells.
Physiology of automaticity
The cells of the conduction system are functionally similar to nervous tissue. They exhibit automaticity — the ability to spontaneously generate electrical impulses (in biophysics, this phenomenon is described as self-sustained oscillation).
The basis of automaticity is a special ion mechanism:
- During the resting phase, immediately after repolarization is complete, sodium channels on the cell membrane are closed.
- A gradual "leakage" of the membrane begins: $Na^+$ ions slowly enter the cell.
- This leads to a gradual decrease in the transmembrane potential.
- As soon as the potential reaches a critical threshold, $Na^+$ channels open fully, and full excitation (depolarization) occurs.
The leading pacemaker is always the sinus node (60–70 impulses per minute). The AV node also has pacemaker activity, but its rate is lower — about 40 impulses. The basal rate can be further modulated by neural and humoral factors.
Histology: atypical cardiomyocytes
All structures of the system are built from atypical cardiomyocytes. Due to the critically low content of myofibrils, mitochondria, T-tubules, and L-type channels, they are practically devoid of contractile ability, but are distinguished by extremely high excitability.
Three types of such cells are distinguished, which differ in the degree of automaticity and signal conduction capacity:
| Cell type | Location | Structure and functions |
|---|---|---|
| P-cells (pacemaker cells) | Basis of the sinus node, present in the AV node | Small, polygonal. Minimal myofibrils, no T-tubules. True self-oscillatory system (60–70 impulses/min). |
| Transitional cells | Basis of the AV node, periphery of the sinus node | Cylindrical. Intermediate position: short T-tubules and many myofibrils are present (capable of contraction). Inherent automaticity — 40 impulses/min. |
| Purkinje cells | Branches of the bundle of His, Purkinje fibers | Large, oval. No T-tubules and no cross-striations. Conduct the impulse. |
Morphology of Purkinje fibers
These structures are located both between the endocardium and myocardium and penetrate deep into the thickness of the heart muscle. They do not form typical cylindrical bundles with intercalated discs; instead, cells are connected via gap junctions (nexuses) and surrounded by thin layers of connective tissue.
Under light microscopy (standard hematoxylin and eosin staining), Purkinje fibers stand out by their large size and light cytoplasm. This is because their myofibrils are sparsely and randomly scattered, and a huge amount of glycogen granules accumulates inside. Glycogen reserves make these cells extremely resistant to hypoxia.
In preparations, Purkinje fibers visually resemble fat cells (adipocytes). The main difference with hematoxylin-eosin staining is that Purkinje cells are still somewhat darker than fat. For reliable differential diagnosis, the azan method is used: with this staining, Purkinje fibers become pinkish-blue, while adipocytes remain absolutely white (unstained).
Briefly: answers to topical questions
What are the names of the three internodal tracts connecting the sinus and atrioventricular nodes?
The combination of the three internodal tracts extending from the sinus node in the atrial walls is designated as the Keith-Flack bundle.
Their function: to connect the atria to each other and link the nodes of the heart's conduction system.
Which ion currents and channels provide the slow diastolic depolarization phase in pacemaker cells?
The slow diastolic depolarization phase is provided by a decrease in potassium permeability and inward sodium and calcium currents.
During diastole, the following mechanisms interact:
- during diastole, membrane permeability to $K^+$ gradually decreases, which leads to a decrease in the potassium equilibrium potential and depolarization of the overall membrane potential;
- during the interspike period, there is a constant slow inward current of $Na^+$ and, to a lesser extent, $Ca^{2+}$;
- the inward $Na^+$ current passes through specific $I_f$ channels and initiates $Ca^{2+}$ entry;
- at the end of slow diastolic depolarization, an additional increase in inward $Na^+$ and $Ca^{2+}$ currents occurs, which accelerates spontaneous depolarization.
Class and exam questions
Why doesn't the AV node normally set the rhythm if it possesses the property of automaticity?
In a healthy heart, the intrinsic slow activity of the AV node (about 40 impulses per minute) is constantly interrupted and suppressed by more frequent impulses arriving from the sinus node. The transitional cells of the AV node simply do not have time to generate their action potential.
How to distinguish Purkinje fibers from adipocytes on a microslide?
Both structures look large and light, but with hematoxylin and eosin staining, Purkinje fibers are slightly darker. An absolutely precise method is azan staining: Purkinje cells stain pinkish-blue, while fat cells do not stain at all (remain white).
Are there T-tubules in the conduction system of the heart?
In a typical form, they are absent. T-tubules are completely absent in P-cells (sinus node) and in Purkinje fibers. Short T-tubules can only be found in transitional-type cells.
What else to study on this topic
- Humoral and neural regulation of pacemaker activity (sympathetic and parasympathetic influences).
- Ultrastructure of nexuses and features of intercellular signal transmission in conduction pathways.
- Ion currents in the repolarization and plateau phase: deep analysis of pacemaker action potentials.
- Histochemical methods for detecting glycogen in heart tissues (PAS reaction).
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