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Electrical Activity of the Heart

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The heart's electrical system creates the signals that tell your heart when to beat .

And your heartbeat is what pumps blood throughout your body .

The heart's electrical system is also called the cardiac conduction system.

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Parts of the Electrical SystemYour heart's electrical system includes three important part:

1 .S-A node (sinoatrial node) — known as the heart's natural pacemaker, the S-A node has special cells that create the electricity that makes

your heart beat .2 .A-V node (atrioventricular node) — the A-V node

is the bridge between the atria and ventricles .Electrical signals pass from the atria down to the ventricles through the A-V node.

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3 .His-Purkinje system — the His-Purkinje system carries the electrical signals throughout the ventricles to make them contract. The parts of the His-Purkinje system include :o His Bundle (the start of the system) o Right bundle branch o Left bundle branch o Purkinje fibers (the end of the system)

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Electrical Signals and Blood FlowThe S-A node normally produces 60-100 electrical signals per minute — this is your heart rate, or pulse. With each pulse, signals from the S-A node follow a natural electrical pathway

through your heart walls .The movement of the electrical signals causes your heart's chambers to contract and relax.In a healthy heart, the chambers contract and relax in

a coordinated way, or in rhythm .When your heart beats in rhythm at a normal rate ,it is called sinus rhythm.

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Four Steps of Cardiac ConductionStep 1: Pacemaker Impulse GenerationThe sinoatrial (SA) node contracts generating nerve impulses that travel throughout the heart wall .This causes both atria to contract .

Step 2: AV Node Impulse ConductionThe atrioventricular (AV) node lies on

the right side of the partition that divides the atria, near the bottom of the right atrium .

When the impulses from the SA node reach the AV node they are delayed for about a tenth of a second. This delay allows the atria to contract and empty their contents first .

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Step 3: AV Bundle Impulse ConductionThe impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.Step 4: Purkinje Fibers Impulse ConductionAt the base of the heart the atrioventricular bundles start to divide further into Purkinje fibers. When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract .

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Spread of Excitation Through theHeart Muscle

As impulses propagate through the heart muscle, they travel in an orderly pattern as a kind of wave

this wave of excitation spreads, contraction of the muscle follows.The wave of excitation starts at the SA node and then spreads outward through the atria .

The wave then “funnels” through the atrioventricular bundle by way of the AV node, which acts as a kind of bottleneck due to the relative slowness of impulse conduction in this region.

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This delay is essential for efficient cardiac function:

It allows the wave of excitation to spread completely through the atria before it reaches the ventricles, ensuring that atrial contraction is complete before ventricular contraction starts.Given that the function of atrial contraction is to drive blood into the ventricles, ventricular contraction would work against the pumping action of the atria if no such delay occurred.Once impulses reach the bundle branches and the Purkinje fibers, they are carried relatively quickly to the lower portion of the ventricles.

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From there, the wave of excitation fans out through the entire ventricular muscle. Thus ventricular contraction begins at the apex and spreads upward .

This pattern makes sense when you consider that blood exits the ventricles from the topIn this sense, ventricular contraction is reminiscent ofhow one should squeeze a tube of toothpaste—from the bottom up.

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Made by

Ahmed A. Abdulwahab

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