Transcript 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.
 Is susceptible to damage by heart
disease, especially by ischemia of
the heart tissues
Natural Excitation of the Heart
SA Node = normal pacemaker
 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.
Normal rhythm = 70 – 80 beats/min
Natural Excitation of the Heart
AV Node
 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
Normal rhythm = 40 – 60 beats/min
Natural Excitation of the Heart
Purkinje fibers
Normal rhythm = 15 – 40 beats/min
Cause of the rapid Conduction
mainly by increased numbers of
gap junctions
Natural Excitation of the Heart
Summary 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
Ectopic Pacemaker
Stokes - Adams Syndrome
Overrdrive Suppression
Natural Excitation of the Heart
Purkinje fibers and Reentry Mechanisms
Wolff-ParkinsonWhite syndrome.
Natural Excitation of the Heart
Triggered activities
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.
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
Natural Excitation of the Heart
Triggered activities
 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.