ECG Level 1 Tutorial: Basic Electric Stuff
Tutorial: Basic Electric Stuff
This module will teach the basics of electrical physiology in the heart.
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Tutorial: Basic Electric Stuff Action potentials
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Action potentials
During a heartbeat, the electrical properties of the cells in the heart change. The small microscopic electrical changes of each cell add together to generate an electrical signal that can be measured on the skin.

Understanding the electrical changes of each cell requires understanding action potentials. An action potential is a description of the electrical change in one individual cell as the voltage of the cell changes. The voltage changes because ions (which carry an electrical charge) move into and out of the cell. When the ions move, they carry their charge into or out of the cell and this changes the voltage of the cell.

If a positive ion, such as sodium moves into the cell, the cell becomes more positively charged. This process is called depolarization. Cells of the heart (and nervous system too) are all negatively charged at baseline, so in a sense they are starting from a "polarized" state. Adding a positive charge de-polarizes this baseline so that is why it is called depolarization.

Making the inside of the cell more negative, either by moving a positive charge out, or moving a negative charge (like chloride) in, results in repolarization. Both de- and re- polarization are important concepts to understand for ECG's.

Below is an image of an atrial or ventricular myocyte depolarization and repolarization. I won't go into details of ions here because I want you to focus on the fact the cell starts off with a negative voltage, switches to a positive voltage (depolarized) and then returns to the negative charge (repolarizes).

The action potential

When a single myocyte depolarizes, it transmits this voltage change to an adjacent myocyte. This second myocyte receives the electrical signal and also depolarizes and repolarizes. Through this mechanism, the electrical charge is very quickly transmitted throughout the entire heart. In a normal heart, the electrical charge moves through the heart in a very predictable manner, sometimes moving quickly and sometimes moving slowly. We will discuss those details in upcoming lessons.