Electrical Activity of The HEART . Conduction System of the Heart Dr. Amel Eassawi. Objectives. Students should be able to: Identify the components of the conducting system of the heart. Know the sequence of conduction of impulse in the heart.
Conduction System of the
Students should be able to:
Two specialized types of cardiac muscle cells:
Cause of Prepotential
Cause of Depolarization - Ca++ going inside
Cause of Repolarization - K+ going outside
SA Node is the Pacemaker of the Heart
Bundle of His
AV Node delay
2-4 meter/sec, therefore, both ventricle depolarize quickly and at the same time.
Why Conduction is slow at AV-Node?
Slowest Conduction at AV node
Fastest Conduction at purkinje fibers
Sino-atrial block: The sinus node fires but the stimulus does not excite the atria because it is blocked at the junction between the two.
AV junctional block: The blocks at the level of the AV junction are classified according to the severity of the block.
First-degree block: all action potentials are conducted through the AV junction but the velocity of conduction is depressed.
Second-degree block: some of the action potentials are not conducted at all through the junction and therefore do not excite the ventricles.
Third-degree block: all action potentials are completely blocked at the AV junction and the ventricles are not stimulated by the atria. This would lead to cardiac arrest unless a latent pacemaker escapes and drives the ventricles.
Bundle Branch block: Conduction at one of the branches become blocked. The wave of excitation spread from the intact branch to depolarize the whole ventricle, which take longer time than if both branches are intact.
If no conduction occurs from SA Node to the ventricle through AV node, therefore, atrial rate is separate [75/min] from the ventricular rate which follows the Purkinje fibers and is about 30/min.
If ventricular rate is very slow e.g. complete heart block, we need artificial pacemaker [implanted device which generates impulse].
Ventricular Action Potential
Phase 1= absent
Phase 2= Less prominent
Pacemaker Cells Vs. Contractile Cells
Fast Fibers vs. Slow Fibers Action potential
Relationship of an Action Potential and the Refractory Period to the Duration of the Contractile Response in Cardiac Muscle
Why Sympathetic Stimulation causes Prepotential to occur faster?
- more Na+ influx [entry]
- more Ca2+ influx [entry]
- decreased K+ efflux [going outside]
-60mV to -40mV [increases the slope of Prepotential] and when it reaches the threshold level, AP starts.
Why parasympathetic causes Prepotential to
occur after long time?
- decreased Na+ influx [entry]
- decreased Ca2+ influx [entry]
- increased K+ efflux [going outside]
Under resting condition parasympathetic discharge dominates
POINT TO PONDER
In Transplanted Heart, where there is no sympathetic and parasympathetic nerve supply, what will be the rate of SA Node discharge [Heart Rate] ?