Conduction System Of Heart

"Conduction System Of Heart"
The mighty, nonstop heart keeps up its rhythm because of a carefully choreographed
dance of electrical impulses called the conduction system that has the power to produce
a spontaneous rhythm and conduct an electrical impulse. Four structures play
key roles in this dance — the sinoatrial node, atrioventricular node, atrioventricular
bundle, and Purkinje fibers.Each is formed of highly tuned modified cardiac muscle.
Rather than both contracting and conducting impulses as other cardiac muscle does,
these structures specialize in conduction alone, setting the pace for the rest of the heart.
Following is a bit more information about each one:

1 Sinoatrial node: This node really is the pacemaker of the heart. Located at the
junction of the superior vena cava and the right atrium, this small knot, or mass,
of specialized heart muscle initiates an electrical impulse that moves over the
musculature of both atria, causing atrial walls to contract simultaneously and
emptying blood into both ventricles. It’s also called the S-A node, sinoauricular
node, and sinus node.

2 Atrioventricular node: The impulse that starts in the S-A node moves to this mass
of modified cardiac tissue that’s located in the septal wall of the right atrium. Also
called the A-V node, it directs the impulse to the A-V bundles in the septum.

3 Atrioventricular bundle: From the A-V node, the impulse moves into the atrioventricular
bundle, also known as the A-V bundle or bundle of His (pronounced
“hiss”). The bundle breaks into two branches that extend down the sides of the
interventricular septum under the endocardium to the heart’s apex.

4 Purkinje fibers: At the apex, the bundles break up into terminal conducting fibers,
or Purkinje fibers, and merge with the muscular inner walls of the ventricles. The
pulse then stimulates ventricular contraction that begins at the apex and moves
toward the base of the heart, forcing blood toward the aorta and pulmonary artery.

One of the best ways to detect cardiac tissue under a microscope is to look for undulating
double membranes called intercalated discs separating adjacent cardiac muscle
fibers. Gap junctions in the discs permit ions to pass between the cells, spreading the
action potential of the electrical impulse and synchronizing cardiac muscle contractions.
Potential problems include fibrillation, a breakdown in rhythm or propagation of
the impulses that causes individual fibers to act independently, and heart block, an
interruption that causes the atria and ventricles to take on their own rates of contraction.
Usually the atria contract faster than the ventricles.

A healthy heart makes a “lub-dub” sound as it beats. The first sound (the “lub”) is
heard most clearly near the apex of the heart and comes at the beginning of ventricular
systole (the closing of the atrioventricular valves and opening of the semilunar
valves). It’s lower in pitch and longer in duration than the second sound (the “dub”),
heard most clearly over the second rib, which results from the semilunar valves closing
during ventricular diastole. Defects in the valves can cause turbulence or regurgitation
of blood that can be heard through a stethoscope. Called murmurs, these sounds
indicate imperfect closure of one or more valves.


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