Natural Excitation of

The Heart

Guyton 2011 pages 115-

120

Berne 2008 pages 302-

309

Excitatory and Conductive System of the Heart

Natural Excitation of The Heart

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

SA Node = normal pacemaker

Natural Excitation of The Heart

Have two cell type

RMP is much less negative

Lack the iKl, then the ratio of

gK to gNa during phase 4 is

much less

Pacemaker Cells (Small, round cells having few organelles and myofibril)

Conductor Cells (Slender, elongated cells)

SA Node = normal pacemaker

Natural Excitation of The Heart

Change the slope of Phase 4

Change the RMP

Change threshold Potential

Change the Heart Rate

Normal rhythm = 70 – 80 beats/min

SA Node = normal pacemaker

Depolarization and repolarization is slower in slow responses

Na ,

Ca

Ca

K

Natural Excitation of The Heart

Internodal Pathways

Natural Excitation of The Heart

AV Node

Natural Excitation of The Heart

Regions

AN region (transitional zone)

N region (mid portion)

NH region

Cause of the Slow Conduction

Mainly by diminished numbers of

gap junctions

One-way

conduction

Normal rhythm = 40 – 60 beats/min

Purkinje fibers

Natural Excitation of The Heart

Cause of the Rapid

Conduction

mainly by increased numbers

of gap junctions

Normal rhythm = 15– 40beats/min

Summary of the Spread of the Cardiac Impulse Through the Heart (second)

Natural Excitation of The Heart

Natural Excitation of The Heart

Conduction Rate (m/s)

Tissue

0.05 SA node

1 Internodal Pathways

0.05 AV node

1 Bundle of His

2 Left and Right Bundles

4 Purkinje System

1 Ventricular Muscle

Impulse Velocity in Different Part of the Heart

Natural Excitation of The HeartPoint

s

Overrdrive Suppression

Stokes - Adams SyndromeEctopic Pacemaker

Natural Excitation of The Heart

Effect of ANS of cardiac rhythmicity

Effect of ANS of cardiac rhythmicity

Natural Excitation of The Heart

Parasympathetic division

Sympathetic division

Mainly innervates nodes

Acetylcholine

Increased permeability to K

Increases the overall activity of the heart

Norepinephrine

Increased permeability to Na and Ca

Innervates all areas

Decreases Heart Rate

Reentry Phenomenon

Natural Excitation of The Heart

Reentry Mechanism

s

Natural Excitation of The Heart

Pathway around the circle is too long

Like cardiac dilation

Velocity of conduction becomes decreased

Blockage of the Purkinje system, Ischemia of the muscle, High blood potassium levels

The refractory period of the muscle shortened

in response to various drugs, such as epinephrine or after repetitive

electrical stimulation

Global Reentry

Natural Excitation of The Heart

The End