1. Natural Excitation of the Heart

2. 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

3. 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

4. Natural Excitation of the Heart
AV Node
Normal rhythm = 40 – 60 beats/min
The AV node is composed of three functional regions:
AN region (transitional zone)
N region (mid portion)
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

5. 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

6. 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.

7. Natural Excitation of the Heart

8. Natural Excitation of the Heart
Ectopic Pacemaker
Stokes - Adams Syndrome
Overrdrive Suppression

9. Natural Excitation of the Heart
Purkinje fibers and Reentry Mechanisms
Wolff-Parkinson-White syndrome.

10. 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

11. 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.