ELECTRICAL CONDUCTION SYSTEM OF THE HEART: ORGANIZATION AND FUNCTIONING - ANATOMY AND PHYSIOLOGY - 2025

The electrical conduction system of the heart, or rather excitation-conduction, is a set of myocardial structures whose function is to generate and transmit from its site of origin to the myocardium (cardiac muscle tissue) the electrical excitation that triggers each cardiac contraction (systole).

Its components, which are spatially ordered, which are activated sequentially and conduct at different speeds, are essential for the genesis (initiation) of cardiac excitation and for the coordination and rhythmicity of the mechanical activity of the different myocardial areas during cardiac cycles.

Schematization of the electrical conduction system of the human heart (Source: Madhero88 (original files); Angelito7 (this SVG version); via Wikimedia Commons)

These components, named in the order of their sequential activation during a cardiac cycle, are: the sinoatrial node, three internodal fascicles, the atrioventricular (AV) node, the bundle of His with its right and left branches, and Purkinje fibers..

Major failures in the electrical conduction system of the heart can lead to the development of cardiac pathologies in humans, some more dangerous than others.

Anatomical organization of the heart

Diagram of the human heart showing its parts (Source: Diagram_of_the_human_heart_ (cropped) _pt.svg: Rhcastilhosderivative work: Ortisa via Wikimedia Commons)

To understand the importance of the functions of the excitation-conduction system, it is necessary to bear in mind some aspects of the heart, whose contractile function is the responsibility of the myocardial working mass organized into two components: one atrial and the other ventricular.

The muscular tissue (myocardium) of the atria is separated from that of the ventricles by fibrous tissue on which the atrioventricular valves are located. This fibrous tissue is non-excitable and does not allow the passage of electrical activity in any sense between atria and ventricles.

The electrical excitation that gives rise to the contraction originates and diffuses in the atria and then passes to the ventricles, so that in cardiac systole (contraction) the atria contract first and then the ventricles. This is so thanks to the functional arrangement of the excitation-conduction system.

Sinoatrial node (sinus, SA) and cardiac automatism

Skeletal muscle fibers need nervous action to trigger an electrical excitation in their membranes to contract. The heart, for its part, contracts automatically, generating by itself and spontaneously the electrical excitations that allow it to contract.

Normally cells have an electrical polarity that implies that their interior is negative with respect to the exterior. In some cells this polarity can disappear momentarily, and even reverse. This depolarization is an excitation called the action potential (AP).

Schematic of an action potential (Source: en: Memenen via Wikimedia Commons)

The sinus node is a small anatomical structure of elliptical shape and about 15 mm in length, 5 mm in height and about 3 mm in thickness, which is located in the posterior part of the right atrium, near the mouth of the vena cava in this chamber.

It is made up of a few hundred modified myocardial cells that have lost their contractile apparatus and have developed a specialization that allows them to spontaneously experience, during diastole, a progressive depolarization that ends up unleashing an action potential in them.

This spontaneously generated excitation spreads and reaches the atrial myocardium and ventricular myocardium, also exciting them and forcing them to contract, and is repeated as many times a minute as the value of the heart rate.

The cells of the SA node communicate directly with and excite neighboring atrial myocardial cells; this excitation spreads to the rest of the atria to produce atrial systole. The conduction velocity is here 0.3 m / s and atrial depolarization is completed in 0.07-0.09 s.

The following image shows a wave from a normal electrocardiogram:

Internadal fascicles

The sinus node leaves three fascicles called internodal because they communicate with this node with another called atrioventricular (AV) node. This is the path that excitation takes to reach the ventricles. The velocity is 1 m / s and the excitation takes 0.03 s to reach the AV node.

Atrioventricular (AV) node

The atrioventricular node is a nucleus of cells located in the posterior wall of the right atrium, in the lower portion of the interatrial septum, behind the tricuspid valve. This is the mandatory pathway of excitation that goes to the ventricles and cannot use the non-excitable fibrous tissue that gets in the way.

In the AV node, a cranial or superior segment is recognized whose conduction speed is 0.04 m / s, and a more caudal segment with a speed of 0.1 m / s. This reduction in conduction velocity causes the passage of excitation to the ventricles to be delayed.

The conduction time through the AV node is 0.1 s. This relatively long time represents a delay that allows the atria to complete their depolarization and contract before the ventricles, completing the filling of these chambers before they contract.

Bundle of His or atrioventricular bundle and its right and left branches

The most caudal fibers of the AV node cross the fibrous barrier that separates the atria from the ventricles and travel a short course down the right side of the interventricular septum. Once the descent begins, this set of fibers is called the bundle of His or atrioventricular bundle.

After descending 5 to 15 mm, the bundle divides into two branches. A right follows its path towards the tip (apex) of the heart; the other, left, pierces the septum and descends the left side of it. At the apex, the branches curve up the inner lateral walls of the ventricles until they reach the Purkinje fibers.

The initial fibers, those that cross the barrier, still have a low conduction speed, but are quickly replaced by thicker and longer fibers with high conduction speeds (up to 1.5 m / s).

Purkinje fibers

They are a network of fibers diffusely distributed throughout the endocardium that lines the ventricles and that transmits the excitation that leads the branches of the bundle of His to the fibers of the contractile myocardium. They represent the last stage of the specialized excitation conduction system.

They have different characteristics from those of the fibers that make up the AV node. They are longer and thicker fibers even than the contractile fibers of the ventricle and show the highest conduction velocity among the components of the system: 1.5 to 4 m / s.

Due to this high conduction velocity and the diffuse distribution of the Purkinje fibers, excitation reaches the contractile myocardium of both ventricles simultaneously. It could be said that a Purkinje fiber initiates the excitation of a block of contractile fibers.

Ventricular contractile myocardium

Once excitation reaches the contractile fibers of a block through a Purkinje fiber, conduction continues within the succession of contractile fibers organized from endocardium to epicardium (the inner and outer layers of the heart wall, respectively). The excitement appears to radially pass through the thickness of the muscle.

The conduction velocity within the contractile myocardium is reduced to about 0.5-1 m / s. Since the excitation reaches all sectors of both ventricles simultaneously and the path to be traveled between the endocardium and the epicardium is more or less the same, total excitation is reached in about 0.06 s.

Synthesis of speeds and driving times in the system

The conduction velocity in the atrial myocardium is 0.3 m / s and the atria complete depolarization in a period between 0.07 and 0.09 s. In the internodal fascicles, the velocity is 1 m / s and the excitation takes about 0.03 s to reach the AV node from when it begins in the sinus node.

At the AV node, the velocity varies between 0.04 and 0.1 m / s. The excitation takes 0.1 s to pass through the node. The speed in the bundle of His and its branches is 1 m / s and rises to 4 m / s in the Purkinje fibers. The conduction time for the His-branches-Purkinje path is 0.03 s.

The conduction velocity in the contractile fibers of the ventricles is 0.5-1 m / s and the total excitation, once it starts, is completed in 0.06 s. Adding the appropriate times shows that the excitation of the ventricles is reached 0.22 s after the initial activation of the SA node.

The consequences of the combination of speeds and times in which the passage of the excitation is completed by the different components of the system are two: 1. the excitation of the atria occurs first than that of the ventricles and 2. these are activated synchronously producing an efficient contraction to expel blood.

References

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