Cardiac Conduction - MedComic.com

Introduction:

The cardiac conduction system is responsible for the organized transmission of electrical impulses in the heart. This system consists of a network of cells that transmits electrical potentials from the atria to the ventricles.

The Conduction Circuit:

The sinoatrial node, which is located in the right atrium, is responsible for the generation of electrical impulses. These impulses are then transmitted through atrial conduction tissue to the atrioventricular node. The atrioventricular node causes a slight delay in transmission, and then allows the signals to travel to the cells of the interventricular septum. The conduction fibers in the interventricular septum are known as the bundle of His. These fibers divide into the left and right bundle branches, which transmit electrical impulses to the left and right ventricles, respectively.

Properties of the Conduction System:

The ability of the conduction system to stimulate the heart to contract in an organized fashion stems from its intrinsic properties. These properties include: excitability, conductivity, and automaticity.

-Excitability: Refers to the ability of a cell to respond to an electrical stimulus.

-Conductivity: Refers to the ability of each cell of the conduction system to conduct individual electrical impulses from one cell to another.

-Automaticity: Refers to an individual cell’s ability to “self-excite” without any impulse from an outside source.

The conductive tissue that has the fastest automaticity acts as the pacemaker of the heart. In a normal heart, the sinoatrial node has the highest automaticity and, thus, acts as the primary pacemaker of the heart. However, if one pacemaker in the heart fails to act, the conduction tissue with the next fastest rate will gain control of the pacing function. Each component of the conductive system has its own intrinsic rate of self-excitation, as follows:

-The sinoatrial node has an intrinsic rate of 60-100 beats per minute.

-The atrioventricular node has an intrinsic rate of 40-60 beats per minute.

-The ventricular conduction tracts have an intrinsic rate of 15-40 beats per minute.

The spontaneous generation of cardiac electrical signals, as well as their propagation, is due to alterations in electrolyte concentrations. The major electrolytes that are involved in this process include: Sodium, Calcium, Potassium, Magnesium, and Chloride.

Sodium:

Sodium plays a vital role in automaticity, which is a property characterized by the heart’s ability to generate spontaneous, repetitive contraction stimuli.

Calcium:

Calcium is also essential for automaticity and, in addition, plays a vital role in cardiac contractions.

Potassium:

Potassium’s role in the heart is to reset the repetitive firing system so that it can quickly become active after each electrical stimulus it generates.

Magnesium and Chloride:

The roles of Magnesium and Chloride are unclear, but low levels of these electrolytes impair the replacement and functioning of Sodium, Calcium, and Potassium, thus altering overall heart activity.

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