cardiac conduction

Question 1

SA and AV are capable of

Answer 1

spontaneous depolarization

Question 2

Phase 4 the period of repolarization is

Answer 2

not static as occurs in contractile myocytes

Question 3

during phase 4, there is a

Answer 3

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.

Question 4

cells in AV/SA node conductivity

Answer 4

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

Question 5

The SA node is the usual cardiac pacemaker because

Answer 5

it has the fastest spontaneous rate.

Question 6

The AV node has

Answer 6

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

Question 7

Contractile myocytes

Answer 7

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)

Question 8

Purkinje cell AP appearance

Answer 8

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.

Question 9

purkinje cells allow for

Answer 9

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

Question 10

Once an electrical impulse is initiated by a pacemaker it will

Answer 10

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

Question 11

The sinoatrial (SA) node has

Answer 11

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

Question 12

SA node is located in the

Answer 12

right atrium

Question 13

how the electric impulse travels

Answer 13

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

Question 14

AV node is located

Answer 14

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

Question 15

AV node also called the

Answer 15

“junction”

Question 16

what occurs at the AV node

Answer 16

a delay before the depolarization wave enters the ventricles.

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

Question 17

The right bundle of His is a:

Answer 17

single entity primarily supplying the right ventricle.

Question 18

The left bundle of His

Answer 18

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

Question 19

speed of conduction of cell types

Answer 19

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.

Question 20

In the normal heart depolarization begins in the

Answer 20

atria and then proceeds to the ventricles

Question 21

P wave is due to

Answer 21

depolarization of the atria,

Question 22

QRS is due to

Answer 22

depolarization in the ventricles

Question 23

T wave is due to

Answer 23

repolarization of the ventricles.

Question 24

Repolarization of the atria is not seen because

Answer 24

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

Question 25

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

Answer 25

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

opposite directions.

Question 26

In each cardiac myocyte, repolarization is in the

Answer 26

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

Question 27

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

Answer 27

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.

Question 28

normally there is concordance in direction between the:

Answer 28

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!

Question 29

Discordance between the QRS and T waves in any lead is

Answer 29

pathological, reflecting abnormalities such as ischemia or ventricular hypertrophy.