Initiation of Cardiac Contraction: The Role of the SA Node
The contraction of the human
heart is initiated by a wave of depolarization that begins at the sino-atrial
(SA) node, a specialized group of cardiac muscles embedded in the upper wall of
the right atrium. This SA node is often referred to as the "heart's
pacemaker" as it originates each heartbeat. The wave of depolarization
passes over the right and left atria, causing their contraction, and then reaches
the atrio-ventricular (AV) node.
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| Sinoatrial Node |
Conduction of the Depolarization Wave: AV Node, Bundle of His, and
Purkinje Fibers
From the AV node, the
depolarization wave passes through the AV bundle, which has a bundle branch to
each ventricle. The AV bundle is also known as the bundle of His and is located
in the ventricular septum. It further divides into Purkinje fibers when reaching
the apex of the heart. These specialized cardiac muscle fibers conduct the
depolarization wave rapidly over the surface of the ventricles, leading to
their contraction.
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| Bundle of His |
Cardiac Contraction: A Myogenic Process
Unlike most muscles that
contract in response to impulses from nerves, the heart can continue to beat
rhythmically even after its nerve supply has been severed. This is because
cardiac muscle is myogenic, meaning its rhythmical contraction arises from
within the muscle itself. The periodic and spontaneous electrical excitation of
the cells of the SA node generates the cardiac impulse that triggers the
contraction of the heart.
The Parts of the Heart Involved in Producing Heartbeat
There are several
specialized structures in the heart that play a role in producing the
heartbeat:
1.
Sino-Atrial (SA) Node: Located in the upper wall of the right
atrium, the SA node initiates each heartbeat and is often referred to as the
"heart's pacemaker". It has developed from the sinus venosus and has
become a part of the atrium, hence its name.
2.
Atrio-Ventricular (AV) Node: Located near the junction
of the right atrium and right ventricle, the AV node is another specialized
group of cardiac muscle fibers that acts as a relay between the atria and the
ventricles.
3.
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 further divide into Purkinje fibers
when reaching the apex of the heart. These fibers are extended into the walls
of the ventricles and play a crucial role in conducting the depolarization wave
and triggering ventricular contraction.
The Beating Mechanism of the Heart
The contraction of the heart
is initiated by the periodic and spontaneous electrical excitation of the cells
of the SA node, which generates the cardiac impulse. This impulse then spreads
over both the left and right atria, causing their muscle cells to contract.
However, the electrical impulse cannot be directly transferred to the
ventricles due to the presence of valves and nonconductive tissues.
Contraction of the Ventricles and the Slight Delay
The electrical impulse
eventually reaches the AV node, from where it passes to the bundle of His and
then to the right and left branches of the bundles of His. Finally, it reaches
the Purkinje fibers, which are extended into the walls of the ventricles. As
the impulse reaches the Purkinje fibers, the ventricles contract. However,
there is a slight delay of approximately 0.1 seconds before the ventricles
start to contract.
The reason for this delay is
twofold. Firstly, the Atria-Ventricular Node (AV Node), located near the
junction of the right atrium and right ventricle, acts as a delay mechanism.
The electrical impulse from the SA node takes about 0.1 second to reach the AV
node, and then it is delayed for a short period of time before it is allowed to
pass through the bundle of His or AV bundle, which is a specialized group of
muscles in the ventricular septum. This delay allows the atria to complete
their contraction and empty their contents into the corresponding ventricles
before the ventricles start to contract.
Secondly, the cells that
carry the electrical impulse from the atria to the ventricles, including the
bundle of His and Purkinje fibers, have small diameters. This results in the
slow propagation of depolarization, causing a natural delay in the contraction
of the ventricles. This delay is important in coordinating the contraction of
the atria and ventricles, ensuring that the heart functions efficiently as a
pump.
Recording the Electrical Activity of the Heart - Electrocardiogram
(ECG)
The electrical activity of
the heart can be recorded and analyzed using an electrocardiogram (ECG). An ECG
is a written record that provides valuable information about the electrical
functioning of the heart. By placing electrodes on the body surface at specific
locations, usually on the limbs and chest, the electrical currents generated by
the heart during each cardiac cycle can be detected, amplified, and recorded.
Abnormalities in the Heart's Electrical Activity
Malfunction of the heart can
result in abnormal electrical currents, which in turn can produce abnormal ECG
patterns. ECG abnormalities can provide important diagnostic information about
various heart conditions, such as arrhythmias, ischemia, and conduction
disturbances. ECGs are a vital tool in diagnosing and monitoring heart
conditions, allowing healthcare professionals to make informed decisions about
patient care.
The contraction of the human
heart is a complex and coordinated process initiated by the SA node, which acts
as the heart's natural pacemaker. The electrical impulse generated by the SA
node travels through the atria, AV node, bundle of His, and Purkinje fibers,
ultimately leading to the contraction of the ventricles and the pumping of
blood. The delay in ventricular contraction is essential for efficient heart
function, and the electrical activity of the heart can be recorded and analyzed
using an ECG, providing valuable information for diagnosing and managing heart
conditions.
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