The heart's electrical system, also called the cardiac conduction system, includes three main parts: the sinoatrial (SA) node, the atrioventricular (AV) node, and the His-Purkinje system. The SA node generates electrical signals that travel through the heart and cause the chambers to contract and pump blood through the body in a coordinated, rhythmic manner called sinus rhythm. Electrical signals pass from the atria to the ventricles through the AV node, which causes a delay allowing the atria to contract and empty before the ventricles. The signals then travel through the His-Purkinje system to cause synchronized ventricular contraction and pumping of blood out of the heart.

1. Electrical Activity
of the Heart
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2. 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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3. Parts of the Electrical System
Your 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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4. 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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5. Electrical Signals and Blood Flow
The 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.5

6. Four Steps of Cardiac Conduction
Step 1: Pacemaker Impulse Generation
The sinoatrial (SA) node contracts generating nerve impulses
that travel throughout the heart wall.
This causes both atria to contract.
Step 2: AV Node Impulse Conduction
The 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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7. Step 3: AV Bundle Impulse Conduction
The 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 Conduction
At 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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9. Spread of Excitation Through the
Heart 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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10. 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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11. 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 top
In this sense, ventricular contraction is reminiscent of
how one should squeeze a tube of toothpaste—from the
bottom up.
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