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