natural excitation of the heart
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Natural Excitation of the Heart. Natural Excitation of the Heart. Specialized Excitatory and Conductive System of the Heart. Function: To cause rhythmical contraction of the heart muscle Conducting impulses rapidly through the heart. - PowerPoint PPT PresentationTRANSCRIPT
Natural Excitation ofthe Heart
Function: To cause rhythmical contraction of
the heart muscle
Conducting impulses rapidly through
the heart.
Is susceptible to damage by heart
disease, especially by ischemia of
the heart tissues
Natural Excitation of the HeartSpecialized Excitatory and Conductive System of
the Heart
Have two cell type: Small, round cells having few
organelles and myofibril
(Pacemaker)
Slender, elongated cell
)Conductor(
RMP is much less negative
Lack the iKl, then the ratio of gK to gNa during
phase 4 is much less. Vm deviates much
more from the EK and reach to threshold.
Natural Excitation of the HeartSA Node = normal
pacemaker
Normal rhythm = 70 – 80 beats/min
The AV node is composed of three functional
regions:
1. AN region (transitional zone)
2. N region (mid portion)
3. NH region
The principal delay is in the AN and N regions of the
AV node. The conduction velocity is actually slower in
the N region than in the AN region. However, the path
length is substantially greater in the AN than in the N
region.
Cause of the Slow Conduction Mainly by diminished numbers of gap junctions
Natural Excitation of the HeartAV Node Normal rhythm = 40 – 60 beats/min
Natural Excitation of the HeartPurkinje fibers
Cause of the rapid Conduction
mainly by increased numbers of gap junctions
Normal rhythm = 15 – 40 beats/min
Natural Excitation of the HeartSummary of the Spread of the Cardiac Impulse Through the Heart (second)
The last portions of the ventricles to be excited are the posterior basal epicardial regions and a small zone in the basal portion of the interventricular septum.
Natural Excitation of the Heart
Natural Excitation of the Heart
Overrdrive Suppression
Ectopic Pacemaker
Stokes - Adams Syndrome
Natural Excitation of the HeartPurkinje fibers and Reentry
Mechanisms
Wolff-Parkinson-
White syndrome.
Early afterdepolarizations
More likely to occur when the
prevailing heart rate is slow; a rapid
heart rate suppresses EADs
Induced by the addition of cesium in
an isolated Purkinje fiber
preparation.
More likely to occur in cardiac cells
with prolonged action potentials than in
cells with short action potentials.
Natural Excitation of the HeartTriggered activities
When action potentials are sufficiently prolonged, some Ca++ channels that were activated at the beginning of the plateau have sufficient time to recover from inactivation
Delayed afterdepolarizations The amplitudes of the DADs
are increased by interventions that raise intracellular Ca++
concentrations. Increasing the extracellular [Ca+
+] Administering toxic amounts of
digitalis glycosides.
Natural Excitation of the HeartTriggered activities