CARDIAC AUTOMATISM: ANATOMY, HOW IT OCCURS - ANATOMY AND PHYSIOLOGY - 2024

The cardiac automaticity is the ability of the myocardium cells themselves beating. This property is unique to the heart, since no other muscle in the body can disobey the orders dictated by the central nervous system. Some authors consider chronotropism and cardiac automatism as physiological synonyms.

Only higher organisms possess this characteristic. Mammals and some reptiles are among living things with cardiac automatism. This spontaneous activity is generated in a group of specialized cells that produce periodic electrical oscillations.

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Although the exact mechanism through which this pacemaker effect is initiated is not yet known, it is known that ion channels and intracellular calcium concentration play a fundamental role in its functioning. These electrolytic factors are vital in the dynamics of the cell membrane, which triggers action potentials.

For this process to be carried out without alterations, the indemnity of the anatomical and physiological elements is vital. The complex network of nodes and fibers that produce and conduct stimulus through the entire heart must be healthy to function properly.

Anatomy

Cardiac automatism has a highly intricate and specialized group of tissues with precise functions. The three most important anatomical elements in this task are: the sinus node, the atrioventricular node and the Purkinje fiber network, the key characteristics of which are described below:

Sinus node

The sinus node or sinoatrial node is the heart's natural pacemaker. Its anatomical location was described more than a century ago by Keith and Flack, locating it in the lateral and superior region of the right atrium. This area is called the Venous Sinus and is related to the entrance door of the superior vena cava.

The sinoatrial node has been described by several authors as a banana-shaped, arc-shaped, or fusiform structure. Others simply do not give it a precise shape and explain that it is a group of cells dispersed in a more or less delimited area. The most daring even describe the head, body and tail, like the pancreas.

Histologically, it is composed of four different types of cells: pacemakers, transitional cells, working cells or cardiomyocytes, and Purkinje cells.

All these cells that make up the sinus or sinoatrial node have intrinsic automatism, but in a normal state, only the pacemakers are imposed at the time of generating the electrical impulse.

Atrioventricular node

Also known as the atrioventricular node (AV node) or Aschoff-Tawara node, it is located in the interatrial septum, near the opening of the coronary sinus. It is a very small structure, with a maximum of 5 mm on one of its axes, and it is located in the center or slightly oriented towards the upper vertex of Koch's triangle.

Its formation is highly heterogeneous and complex. Trying to simplify this fact, the researchers have tried to summarize the cells that compose it in two groups: compact cells and transitional cells. The latter are intermediate in size between the working and the pacemaker of the sinus node.

Purkinje fibers

Also known as Purkinje tissue, it owes its name to the Czech anatomist Jan Evangelista Purkinje, who discovered it in 1839. It is found throughout the ventricular muscle below the endocardial wall. This tissue is actually a collection of specialized heart muscle cells.

The subendocardial Purkinje plot has an elliptical distribution in both ventricles. Throughout its course, branches are generated that penetrate the ventricular walls.

These branches can meet each other, causing anastomoses or connections that help to better distribute the electrical impulse.

How is it produced?

Cardiac automatism depends on the action potential that is generated in the muscle cells of the heart. This action potential depends on the entire electrical conduction system of the heart that was described in the previous section, and on the cellular ion balance. In the case of electrical potentials, there are variable functional voltages and charges.

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The cardiac action potential has 5 phases:

Phase 0:

It is known as the rapid depolarization phase and depends on the opening of fast sodium channels. Sodium, a positive ion or cation, enters the cell and abruptly changes the membrane potential, going from a negative charge (-96 mV) to a positive charge (+52 mV).

Phase 1:

In this phase, the fast sodium channels are closed. It occurs when the membrane voltage changes and is accompanied by a small repolarization due to movements of chlorine and potassium, but conserving the positive charge.

Phase 2:

Known as plateau or "plateau". In this stage, a positive membrane potential is conserved without important changes, thanks to the balance in the movement of calcium. However, there is slow ion exchange, especially potassium.

Phase 3:

During this phase rapid repolarization occurs. When the fast potassium channels open, it leaves the interior of the cell, and being a positive ion the membrane potential changes towards a negative charge violently. At the end of this stage a membrane potential between -80 mV and -85 mV is reached.

Phase 4:

Resting potential. In this stage the cell remains calm until it is activated by a new electrical impulse and a new cycle begins.

All these stages are fulfilled automatically, without external stimuli. Hence the name of Cardiac Automation. Not all cells of the heart behave in the same way, but the phases are usually common among them. For example, the action potential of the sinus node lacks a resting phase and must be regulated by the AV node.

This mechanism is affected by all the variables that modify cardiac chronotropism. Certain events that can be considered normal (exercise, stress, sleep) and other pathological or pharmacological events usually alter the automatism of the heart and sometimes lead to severe diseases and arrhythmias.

References

1. Mangoni, Matteo and Nargeot, Joël (2008). Genesis and Regulation of the Heart Automaticity. Physiological Reviews, 88 (3): 919-982.
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6. Sánchez Quintana, Damián and Yen Ho, Siew (2003). Anatomy of the cardiac nodes and the specific atrioventricular conduction system. Revista Española de Cardiología, 56 (11): 1085-1092.
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8. Wikipedia (2018). Cardiac Action Potential. Recovered from: en.wikipedia.org