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Conducting system of the heart dentistry hb2 dr magdi
1. Conduction system of the heart
Presented 2010 / 2011 by: Dr Magdi El Sersi
Assistant Prof of Medical Physiology
Basic Medical Sciences Department
Ext. 7243
E mail:
melsersi@sharjah.ac.ae
3. IMPULSE GENERATION AND
CONDUCTION SYSTEM
The heart contains special
excitatory and conductive
system each capable of
discharging at regular
intervals and of conducting
impulses
Some of cardiac
myocytes lose the ability
to contract and become
specialized, instead, for
generating action
potentials
4. Non contractile cells
specialised regions
• Sinoatrial (SA) node
– Pacemaker
• Posterior wall of the right
atrium; inferior to entrance of
the superior vena cava
• Consists of cells that produce
impulses in a constant rhythm
5. This is the pacemaker that
initiates each heartbeat and
determines the heart rate.
Signals from the SA node
spread throughout the atria,
as shown by the yellow
arrows .
6. 2. Atrioventricular node
• Atrioventricular
(AV) Node
– Located near the lower
interatrial septum, at the
junction of the atria and
ventricles
7. • AV Bundle (Bundle of His)
A pathway by which signals leave the AV node.
• Right and Left Bundle Branches
– Interventricular septum
• Purkinje Fibers
– Network spreads within the muscle bundles of the
ventricle walls
– Denser and more elaborate in left ventricle
– Causes entire ventricular myocardium to
depolarize; ventricles contract
• Atria are electrically isolated from the
ventricles
– Electrical excitation can pass from the atria to the
ventricles at the AV node ONLY
– Separated by a region of electrically inert
connective tissue
8. Impulses normally starts in the heart’s pacemaker,
the SA node.
From there, it spreads in all directions
through the atria.
This cause the atria to contract.
When impulses reach the
AV node, it relays them by
Way of the bundle of His
Sinoatrial and Purkinje fibers to the
ventricles causing them to
Node contract
Atrioventricular
Node
9. Firing of the SA node excites atrial
myocytes and stimulates the two atria to
contract almost simultaneously.
The conduction velocity of
action potentials in the atrial
muscle is about 0.5 m/sec.
10. Normally, the only pathway
available for action potentials to
enter the ventricles is through a
specialized region of cells
(atrioventricular node, or AV
node).
The AV node is a highly
specialized conducting tissue
that slows the impulse
conduction considerably (to
about 0.05 m/sec) thereby
allowing sufficient time for
complete atrial depolarization and
contraction (systole) prior to
ventricular depolarization and
contraction.
11. The impulses then enter at the Bundle of
His and then follow the left and right
bundle branches along the
interventricular septum.
These specialized fibers conduct the
impulses at a very rapid velocity (about
2 m/sec).
The bundle branches then divide into an
extensive system of Purkinje fibers that
conduct the impulses at high velocity
(about 4 m/sec) throughout the
ventricles. This results in depolarization
of ventricular myocytes and ventricular
contraction.
12. The Cardiac Rhythm
The normal heartbeat, triggered by the SA node, is called
the sinus rhythm
13. At rest, the adult heart rate is
usually around 70 to 80 beats per
minute (bpm).
Left to itself, the SA node would
fire more often than this, but the
vagus nerves inhibit it and hold it
down to this rate at rest.
14. o If the SA node is damaged, an
ectopic focus may take over the
governance of the heart rhythm.
oThe most common ectopic focus
is the AV node, which produces a
slower heartbeat of 40 to 50 bpm
called a nodal rhythm.
15. If neither the SA nor AV node is
functioning, other ectopic foci fire at
rates of 20 to 40 bpm.
The nodal rhythm is sufficient to
sustain life, but a rate of 20 to 40 bpm
provides too little flow to the brain to
be survivable.
This condition calls for an
artificial pacemaker.
16. Any abnormal cardiac rhythm
is called arrhythmia.
One cause of arrhythmia is a
heart block: the failure of
any part of the cardiac conduction
system to transmit signals,
usually as a result of disease and
degeneration of conduction
system fibers.
17. A bundle branch block, for
example, is due to damage to
one or both bundle branches.
Damage to the AV node
causes total heart block, in
which signals from the atria
fail to reach the ventricles and
the ventricles beat at their
own intrinsic rhythm of 20 to
40 bpm.
18. Physiology of the SA Node
Why does the SA node spontaneously fire 70 or 80
times per minute?
Unlike skeletal muscle or
neurons, the cells of the SA node
do not have a stable resting
membrane potential.
25. SA node membrane
potential starts at about -60
mV and drifts upward,
showing a gradual
depolarization called the
pacemaker potential
This is thought to result
from a slow inflow of Na+
without a compensating
outflow of K.
26. When the
pacemaker potential
reaches a threshold of
-40 mV, voltage-
regulated fast
calcium channels
open and Ca2+ flows
in from the
extracellular fluid
(ECF).
This produces the
rising (depolarizing)
phase of the action
potential, which
peaks slightly above
0 mV.
27. At that point, K+
channels open and
potassium ions leave
the cell. This makes
the inside increasingly
negative and creates
the falling
(repolarizing) phase
of the action potential.
28. When
repolarization is
complete, the K
channels close and
the pacemaker
potential starts over,
on its way to
producing the next
heartbeat.
Each depolarization of the SA node sets off one heartbeat.
29. Clinical Links
• artificial pacemaker - not able to use an ultrasonic
scaler
People who wear
pacemaker can
not get ultrasonic
scaling because
the ultrasonic
sound can
interfere with the
function of the
pacemaker.
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