Anatomic Organization and Conduction Pathways
Excitation in the heart propagates in a strict sequential order: from the atria to the ventricles. The conduction system consists of nodal generators and emerging fiber bundles.
- Sinoatrial (SA) node (Node of Keith and Flack). Located in the upper wall of the right atrium. This is the primary pacemaker.
- Internodal tracts. Three bundles extending from the SA node through the atrial walls. They interconnect the atria and transmit the signal to the next node.
- Atrioventricular (AV) node (Node of Aschoff-Tawara). Located in the floor 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 right and left bundle branches. It conveys excitation to the ventricular myocardium.
- Purkinje fibers. Terminal ramifications of the bundle branches that directly transmit the impulse to the working ventricular myocytes.
Physiology of Automatism
The cells of the conduction system are functionally similar to nervous tissue. They exhibit automatism—the ability to spontaneously generate electrical impulses (described in biophysics as auto-oscillations).
Automatism is driven by a specialized ionic mechanism:
- During diastole, immediately following repolarization, membrane sodium channels are closed.
- A gradual membrane "leak" begins: $Na^+$ ions slowly enter the cell.
- This causes a progressive decline in the transmembrane potential.
- Once the threshold potential is reached, $Na^+$ channels open fully, triggering a complete action potential (depolarization).
The SA node serves as the primary pacemaker (60–70 impulses per minute). The AV node also has pacemaking activity, but at a lower rate—about 40 impulses per minute. The baseline rate is modulated by neural and humoral factors.
Histology: Atypical Cardiomyocytes
All structures of the conduction system are built from atypical cardiomyocytes. Due to a critically low content of myofibrils, mitochondria, T-tubules, and L-type channels, they possess virtually no contractile ability, but feature exceptionally high excitability.
There are three types of these cells, differing in their degree of automatism and conduction capacity:
| Cell Type | Location | Structure and Functions |
|---|---|---|
| P-cells (Pacemaker cells) | Center of SA node, present in AV node | Small, polygonal. Minimal myofibrils, no T-tubules. True auto-oscillatory system (60–70 imp/min). |
| Transitional cells | Center of AV node, periphery of SA node | Cylindrical. Intermediate features: short T-tubules and more myofibrils (capable of contraction). Intrinsic rate ~40 imp/min. |
| Purkinje fibers | Bundle branches, Purkinje network | Large, oval. No T-tubules or cross-striations. Specialized for rapid impulse conduction. |
Morphology of Purkinje Fibers
These structures lie between the endocardium and myocardium and penetrate deep into the heart muscle. They do not form standard cylindrical fibers with intercalated discs; instead, cells connect via gap junctions (nexuses) and are surrounded by thin connective tissue sheaths.
Under light microscopy (standard H&E staining), Purkinje fibers stand out due to their large size and pale cytoplasm. This is because their myofibrils are sparse and scattered, while their cytoplasm is packed with glycogen granules. These glycogen reserves render the cells highly resistant to hypoxia.
In tissue sections, Purkinje fibers visually resemble adipocytes (fat cells). The key distinction under H&E staining is that Purkinje cells are slightly darker than fat cells. For definitive differential diagnosis, the azan stain is used: Purkinje fibers stain pinkish-blue, whereas adipocytes remain completely unstained (white).