The Heart in Action

How the heart Contracts? Contraction of human heart is initiated by a wave of depolarization at begins at the SA node. After passing over the right and left atria and causing their contraction, the wave of depolarization reaches the AV node. From there it passes through the AV bundle which has a bundle branch to each ventricle. From the tip of the ventricles, the depolarization is conducted rapidly over their surface by the branching Purkinje fibers.

How the Heart Beats?

Most muscles contract as a result of impulses reaching them from nerves. This is not, however, true of the heart, which will continue beating rhythmically even after its nerve supply has been severed. The heart will go on beating after it has been cut right out of the body. Cardiac muscle is therefore mynogenic i.e. its rhythmical contraction arises from within the muscle itself.

Parts of the heart involved in producing heart beat

Sino-Atrial node (SA Node): There is a specialized, plexus of cardiac
muscles embedded in the upper wall of the right atrium. It is close to where·
the vena cavae enter the atrium. This plexus of cardiac muscles is called sino
atrial (SA) node. SA Node is called the heart's pace maker as it originates
each heart beat. The SA node has been developed from the sinus venousus

and has become a part of the atrium, so it is called Sino atrial node. 

Atrio-Ventricular node (AV .Node): There is another specialized group of' cardiac muscle fibers called AV node. It is present near the junction of right atrium and right ventricle.

Bundle of His and Purkinje Fibers: The AV node is connected to a strand of
specialized muscles in the ventricular septum known as the bundle of His or
AV bundle. The bundle branches divide to the right and left, on reaching the
apex of the heart each branch further divides into Purkinje fibers.

Beating Mechanism: The contraction of the heart is initiated by the periodic
and spontaneous electrical excitation of the cells of the SA Node i.e cardiac

impulse. The resulting wave of electrical excitation passes over both the left

and right atria' and causes their muscle cells to contract.

Contraction of Ventricles: The electric impulse cannot be transferred to
the ventricles, directly due to the presence of valves and nonconductive
tissues, The wave eventually reaches the AV node. From the AV node it goes

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to the bundle of His and then to the right and left branches of bundles of His.

From these two branches, the electrical impulse passes to the Purkinje
fibers. These Purkinje fibers are extended into walls of the ventricles. So as
the impulse reaches the Purkinje fibers, the ventricles contract.

Reason for the slight delay between the a rial and ventricular
contraction

The wave of electrical impulse does not immediately spread to the
ventricles from the SA node. Almost O.1 second passes before the
ventricles start to contract. The reason for the delay is that the atria of the
heart are separated from the ventricles by connective tissues which cannot
propagate a wave of electrical excitation.

Secondly the cells that carry the wave of impulse from atria to the
ventricle have small diameter. Thus they propagate the depolarization slowly,
causing the delay of contraction of ventricles. This delay permits the atria to
finish the emptying the contents into the corresponding ventricles before the
ventricles start to contract.

ECG: As each wave of contraction spreads through the heart, electrical
current spreads into the tissues surrounding the heart and onto the body
surface. By placing the electrodes on the body surface on the opposite sides
of the heart, the electrical activity can be amplified and recorded by an
electrocardiograph.

The written record produced is called an electrocardiogram or ECG.

Malfunction of the heart causes abnormal currents, which in turn produce an
abnormal ECG.