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==What does the ECG register?==
;The electrocardiogram: An electrocardiogram (ECG or EKG) is a register of the heart's electrical activity.
Just like skeletal muscles, heart muscles are electrically stimulated to contract. This stimulation is also called ''activation'' or ''excitation''. Cardiac muscles are electrically charged at rest. The inside of , the cell is negatively charged relative to the outside (resting potential). If the cardiac muscle cells are electrically stimulated , they depolarize (the resting potential changes from negative to positive) and contract. The electrical activity of a single cell can be registered as the [[action potential]].As the electrical impulse spreads through the heart, the electrical field changes continually in size and direction. The ECG is a graph of these electrical cardiac signals in the heart.
==The ECG represents the sum of the action potentials of millions of cardiomyocytes==
The individual [[action potential|action potentials]] of the individual cardiomyocytes are averaged. The final result , which is shown on the ECG , is actually the average of billions of microscopic electrical signals.During the depolarization , sodium ions stream into the cell. Subsequently , the calcium ions stream into the cell. These calcium ions cause the actual muscular contraction. Finally the potassium ions stream out of the cell. During the repolarization the ion concentration returns to its precontraction state. On the ECG, an action potential wave coming towards toward the electrode is shown as a positive (upwards) signal. Here the ECG electrode is represented as an eye.
[[Image:Ion_currents_en.png|thumb|left|300px|Ion currents of the cardiomyocytes]]
[[Image:Hart_cells_en.png|thumb|right|300px|The heart consists of approximately 300 trillion cells]]
| <small>This movie shows the contraction of a single (rabbit) heart cell. The glass electrode measures the electrical current in the heart cell (with the[[w:Patch_clamp|patch-clamp method]]). The electrical signal is written in blue and shows the actionpotentialaction potential. ''Courtesy of Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands''.</small>
==The electric discharge of the heart==
[[Image:conduction_system_en.png|thumb|300px|The conduction system of the heart]]
'''The sinoatrial node (SA node) contains the fastest physiological pacemaker cells of the heart; therefore , they determine the [[Rate|heart rate]].'''
'''First the [[heart|atria]] depolarize and contract. After that the [[heart|ventricles]]''' depolarize and contract.
The electrical signal between the atria and the ventricles goes from the sinus node via the atria to the AV-node (atrioventricular transition) to the His bundle and subsequently to the right and left bundle branchbranches, which end in a dense network of Purkinje fibers.The depolarization of the heart results in an electrical force which has a direction and magnitude: ; an electrical vector. This vector changes every millisecond of the depolarization. In the animation vectors for atrial depolarization, ventricular depolarization and ventricular repolarization are shown.
[[Image:Epi_endo_en.png|thumb|300px| The QRS complex is formed by the sum of the electric avtivity of the inner (endocardial) and the outer (epicardial) cardiomyocytes]]
[[Image:Qrs-shapes.png|thumb|300px| Example of the different QRS configurations]]
The [[P_wave_morphology|'''P wave''']] is the result of the atrial depolarization. This depolarization starts in the SA (sinoatrial) node. The signal produced by pacemaker cells in the SA node is conducted to the right and left atria. Normal atrial repolarisation repolarization is not visible on the ECG (but can be visible during [[atrial infarction]] and [[pericarditis]]).
The [[QRS_morphology|'''QRS complex''' ]] is the average of the depolarization waves of the inner (endocardial) and outer (epicardial) cardiomyocytes. As the endocardial cardiomyocytes depolarize slightly earlier than the outer layers, a typical QRS pattern occurs (figure).
One heart beat consists of an atrial depolarization --> atrial contraction --> p-wave, ventricular depolarization --> ventricular contraction --> ORS-complex and the resting phase (including the repolarization during the T-wave) between two heart beats.
Have a look at this excellent [[ animation of the heart cycle]]
The origin of the '''U wave''' is unknown. This wave possibly results from "afterdepolarizations" of the ventricles.
** LA - left arm
** RA - right arm
** N - neutral, on the right leg (= electrical earth , or point zero , to which the electrical current is measured)
** F - foot, on the left leg
It makes no difference whether the electrodes are attached proximal or distal on the extremities. ''However'', it is best to be uniform in this. (eg. do not attach an electrode on the left shoulder and one on the right wrist).

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