Cardiac Muscle and Intrinsic Conduction

Question 1

Cardiac Muscle

Answer 1

Each cardiac myocyte has 1 - or 2 - centrally located nuclei

Large mitochondria account for 25-35% of cardiac myocyte volume

Question 2

What types of junctions exist between cardiac muscles?

Answer 2

Intercalated discs

Question 3

Intercalated discs

Answer 3

contains desmosomes + gap junctions

Question 4

Desmosomes

Answer 4

prevent separation during contraction

Question 5

Gap Junctions

Answer 5

allow ions to pass from cell to cell - transmitting current across the entire heart

Question 6

Functional synctium

Answer 6

Cardiac muscle cells are tied together to form a functional syncytium

Either all cardiac myocytes contract together, or the heart doesn’t contract

Skeletal muscles contact via motor unit recruitment

Question 7

What is autorhythmicity? What types of cardiac cells have it?

Answer 7

the ability to spontaneously depolarize

noncontractile cells called pacemaker cells

Question 8

Depolarization

Answer 8

Ca2+ channels open (around -40mV), Ca2+ influxes leading to an AP

Question 9

Repolarization

Answer 9

K+ channels open, K+ effluxes, cell’s interior becomes more negative

Question 10

Sequence of Excitation

Answer 10

Sinoatrial (SA) Node

Atrioventricular (AV) Node

Atrioventricular Bundle (Bundle of His)

Right and Left Bundle Branches

Purkinje Fibers (Subendocardial Conducting Network)

Question 11

How long does it take for an impulse to pass through the entire heart?

Answer 11

.22s (220ms)

Question 12

Why is there a delay at the AV node? Why is this important?

Answer 12

At the AV Node, the impulse is delayed .1s allowing the atria to complete their contraction

Question 13

What is the ONLY electrical connection between the atria and ventricles?

Answer 13

AV Bundle

Question 14

What is the intrinsic rate set by the SA Node?

Answer 14

75 times/minute

Question 15

What is the intrinsic rate set by the AV Node?

Answer 15

50 times/minute

Question 16

What is the intrinsic rate set by the AV Bundle/Purkinje Fibers?

Answer 16

30 times/minute

Question 17

Ectopic pacemaker

Answer 17

an abnormal pacemaker

Question 18

Arrythmia

Answer 18

an irregular heart rhythm resulting from a defect in the intrinsic conduction system

Question 19

Fibrillation

Answer 19

rapid or irregular contractions of the heart

Question 20

Defibrillation

Answer 20

electrically shocking the heart to depolarize the entire myocardium - ideally, the SA node begins to function normally, and sinus rhythm is restored

Question 21

Implantable Cardioverter Defibrillators (ICDs)

Answer 21

devices that continually monitor heart rhythms; they will slow tachycardia and emit an electrical shock in the event of fibrillation

Question 22

What is a PVC (premature ventricular contraction)? Why can a PVC feel like a thud in the chest?

Answer 22

small regions of the heart become hyperexcitable and the heart prematurely contracts - a Extrasystole

the heart has a slightly longer time to fill, and the next normal contraction feels like a “thud”

Question 23

Total Heart Block

Answer 23

no impulses get through, the ventricles beat at their own intrinsic rate - too slow for adequate tissue perfusion

Question 24

Partial Heart Block

Answer 24

only some impulses get through

Question 25

Artificial pacemaker

Answer 25

medical devices to recouple the atria and ventricles - pacemakers can be programmed to change with changing energy demands and interrogated to with symptoms appear