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Flashcards in cardiac conduction Deck (29)
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SA and AV are capable of

spontaneous depolarization


Phase 4 the period of repolarization is

not static as occurs in contractile myocytes


during phase 4, there is a

slow depolarizing due to a funny current or pacemaker current, until it reaches a critical voltage where a more rapid Phase 0 occurs due entirely to a calcium current – no fast sodium current is present.


cells in AV/SA node conductivity

very poor conductors of electrical current because of lack of a fast sodium current.


The SA node is the usual cardiac pacemaker because

it has the fastest spontaneous rate.


The AV node has

a slower spontaneous rate but it delays conduction at the junction between the atria and ventricles.


Contractile myocytes

1. stable baseline during repolarization in stage 4
2. exhibit a rapid, robust increase during phase 0
(due to the fast sodium current)
3. good at conducting from cell to cell
(enabling fairly rapid electrical excitation of the atria and ventricles)


Purkinje cell AP appearance

1. action potentials have a similar shape to contractile myocytes
2. AP has slightly higher voltage during phase 0
3. AP has a longer total duration.


purkinje cells allow for

extremely rapid conduction from the AV node to the ventricles through the left and right bundles and their extensions into the myocardium.


Once an electrical impulse is initiated by a pacemaker it will

spread rapidly through the gap junctions, cylindrical structures formed by connexins that allow ions to pass from cell to cell.


The sinoatrial (SA) node has

automaticity and normally is the pacemaker generating the electrical signal that starts a wave of depolarization through the heart.


SA node is located in the

right atrium


how the electric impulse travels

1. SA node
2. through the right and then the left atrium generating the P wave. 3. atrioventricular (A-V) node
4. bundle of His into the left and right bundle branches.
5. Purkinje fibers radiate toward the contractile cardiac myocytes that induce contraction.


AV node is located

between the fibrous tricuspid and mitral valve rings that separate the atria from the ventricles.


AV node also called the



what occurs at the AV node

a delay before the depolarization wave enters the ventricles.

This delay allows contraction of the atria to end before ventricular contraction begins.


The right bundle of His is a:

single entity primarily supplying the right ventricle.


The left bundle of His

divides into anterior and posterior branches or fascicles that supply corresponding regions of the left ventricle.


speed of conduction of cell types

The bundle of His,left and right bundles, and Purkinje fibers all contain specialized cells that conduct depolarization very rapidly.

The vast majority of ventricular cardiac myocytes are primarily specialized to contract (“contractile myocytes”) and conduct depolarization waves much more slowly.


In the normal heart depolarization begins in the

atria and then proceeds to the ventricles


P wave is due to

depolarization of the atria,


QRS is due to

depolarization in the ventricles


T wave is due to

repolarization of the ventricles.


Repolarization of the atria is not seen because

it normally occurs at the same time as ventricular depolarization and is buried in the much larger signal from the ventricles.


the T wave of repolarization in the ventricles is in the ______, whereas in individual myocytes depolarization and repolarization are in ______

same direction in the normal electrocardiogram as the QRS or depolarization signal

opposite directions.


In each cardiac myocyte, repolarization is in the

same direction as depolarization but because of different polarity the two waves are in opposite directions (discordance).


why are the QRS of depolarization and the T wave of repolarization both in the same direction (concordance) in the surface ECG?

The endocardium depolarizes earlier than the epicardium. However, there is a transmural repolarization gradient and epicardial cells repolarize earlier than endocardial cells because they have shorter action potential duration.


normally there is concordance in direction between the:

QRS and the T in every ECG lead:

If the QRS is positive the T wave should be positive!

if the QRS is negative the T wave should be negative as well!


Discordance between the QRS and T waves in any lead is

pathological, reflecting abnormalities such as ischemia or ventricular hypertrophy.