Cardiac Muscle and Intrinsic Conduction

Question 1

How does cardiac muscle contract

Answer 1

the sliding filament mechanism - sarcomeres

Question 2

Cardiac myocytes

Answer 2

shorter and fatter than skeletal muscle cells - also more branched and interconnected

push and pull on the cardiac skeleton

Question 3

How many cardiac myocyte are centrally located nuclei

Answer 3

1 or 2

Question 4

Large mitochondria accounts for how much cardiac myocyte nuclei

Answer 4

25%-35%

Question 5

Intercalated Discs

Answer 5

the plasma membranes of adjacent cardiac myocytes interlock at junctions

Question 6

Desmosomes

Answer 6

prevent separation during contraction

Question 7

Gap junctions

Answer 7

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

Question 8

Sarcomere

Answer 8

Thick (Myosin)
Thin (Actin)

Unlike skeletal muscles, cardiac muscle cell sarcomeres vary greatly in diameter and branch extensively

Question 9

Calcium delivery

Answer 9

fewer, wider t tubules - 1 per sarcomere - regulate calcium concentration

Question 10

Similarities between skeletal and cardiac

Answer 10

both muscle types are contractile tissues - contractions are preceded by depolarization in the form of action potentials

transmission of an action potential across the t tubules tiriggers the release of calcium from the sarcoplasmic reticulum

calcium binds to toponin, moves tropomyosin, allows cross bridge cycling to begin

Question 11

Differences between skeletal and cardiac muscle (Self Excitability)

Answer 11

some cardiac muscle cells are self-excitable - these are specific, noncontractile cells called pacemaker

self- generated depolarization travel throughout the heart via gap junctions

no neural input is needed for cardiac myocytes

Question 12

Automaticity/Autorhythmicity

Answer 12

the ability to spontaneously depolarize

Question 13

Differences between skeletal and cardiac muscle (Functional Syncytium)

Answer 13

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 contract via motor unit recruitment

Question 14

Differences between skeletal and cardiac muscle (Release of calcium)

Answer 14

in skeletal muscle, depolarization causes release of calcium from the sarcoplasmic reticulum

in cardiac muscle, depolarization opens special slow-flow calcium channels in the cell membrane - the combination of extracellular calcium and calcium from the SR allows contraction

Question 15

Differences between skeletal and cardiac muscle (Tetany)

Answer 15

in skeletal muscle, the refractory period is shorter than contraction allowing for summation

in cardiac muscle, the refractory period is longer than contraction preventing tetany

Question 16

Absolute refractory period

Answer 16

the period during an AP when an additional AP cannot be generated

Question 17

Differences between skeletal and cardiac muscle (Aerobic Respiration)

Answer 17

cardiac myocytes are dense in mitochondria reflecting a great dependence on oxygen

cardiac muscles is more adaptable to using different nutrient sources as fuel

Question 18

Intrinsic Conduction System

Answer 18

noncontractile cells specialized to initiate and distribute impulses throughout the heart

Question 19

Cardiac Pacemaker Cells

Answer 19

the 1% of cardiac myocytes that are autorhythmic - able to depolarize spontaneously - and set the pace of the heart

found in the sinoatria and atrioventricular nodes

Question 20

Initiation of an Action Potential (Pacemaker Potential)

Answer 20

K+ channels are closed, slow Na+ channels are open, the cell’s interior becomes more positive (-60mV to -40mV)

Question 21

Initiation of an Action Potential (Depolarization)

Answer 21

Calcium channels open (around -40mV), calcium influxes leading to an AP

Question 22

Initiation of an Action Potential (Repolarization)

Answer 22

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

Question 23

What is the order that cardiac pacemaker cells pass impulses across the heart

Answer 23

Sinoatrial (SA) Node
Atrioventricular (AV) Node
Atrioventricular Bundle
Right and Left Bundle Branches
Purkinje Fibers

Question 24

The Sinoatrial (SA) Node

Answer 24

“The pacemaker”

Crescent shaped, located in the right atrial wall - just below the entrance of the SVC

generates impulse 75 times/minute and sets the pace for the heart (“Sinus Rhythm”)

Question 25

The Atrioventricular (AV) Node

Answer 25

from the SA Node, the impulse travels down the intranodal pathway to the AV Node located in the inferior portion of the interatrial septum - just above the tricuspid valve at the AV Node, the impulse is delayed .1s allowing the atria to complete their contraction the AV Node has smaller diameter fibers and fewer gap junctions than the other places on the intrinsic conduction pathway

Question 26

The Atrioventricular (AV) Bundle

Answer 26

starts in the superior part of the interventricular septum the AV Bundle is the only electrical connection between the atria and the ventricles the cardiac skeleton is nonconducting

Question 27

Right and Left Bundle Branches

Answer 27

AV Bundles splits into 2 pathways - the Right and Left Bundle Branches these bundles branches course along the interventricular septum towards the heart's apex

Question 28

Purkinje Fibers

Answer 28

long strands of barrel-shaped cells with few myofibrils these fibers complete the pathway through the interventricual septum, penetrate the heart's apex, and turn superiorly into the ventricular walls provide the impulse for the bulk of ventricular depolarization there is a more elaborate network of pukinje fibers on the left side of the heart

Question 29

Ejection of Blood

Answer 29

ventricular contraction almost immediately follows ventricular depolarization the heart contracts with a "wringing" motion starting at the apex the wave of the contraction moves toward the atria blood is "ejected" superiorly in the great vessels

Question 30

What would happen without input from the SA Node

Answer 30

the AV Node would depolarize about 50 times/minute

Question 31

What would happen without the input of the AV Node

Answer 31

the pacemakers of the AV Bundle of the Purkinje Fibers would depolarize about 30 times/minute

Question 32

When do slow pacemakers dominate

Answer 32

when the fast pacemakers stop working

Question 33

Arrythmia

Answer 33

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

Question 34

Fibrillation

Answer 34

rapid or irregular contractions of the heart control of the heart by the SA Node is disrupted fibrillating ventricle are not useful pumps

Question 35

Defibrillation

Answer 35

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

Question 36

Implanatable Cardioverter Defibrillators (ICDs)

Answer 36

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

Question 37

Ectopic Pacemaker

Answer 37

an abnormal pacemaker if the AV Node takes over, the heart rate will be slower (40-60 bpm), still adequate for tissue perfusion/circulation

Question 38

Premature Ventricular Contraction (PVC) (extrasystole)

Answer 38

small regions of the heart become hyper excitable and the heart prematurely contracts after PVC, the heart has a slightly longer time to fill, and the next normal contraction feels like a "thud" typically, an occasional PVC is harmless

Question 39

Damages to what structures will prevent impulses from reaching the ventricles (Heart Block)

Answer 39

AV Node > AV Bundle > R/L Bundle Branches

Question 40

Total Heart Block

Answer 40

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

Question 41

Partial Heart Block

Answer 41

only some impulses get through

Question 42

Artificial Pacemakers

Answer 42

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

Question 43

What can sympathetic branch do?

Answer 43

Accelerate heart rate and increases contractility

Question 44

What can parasympathetic branch do?

Answer 44

decelerate heart rate

Question 45

Where are cardiac centers located

Answer 45

Medulla Oblongata

Question 46

Cardioacceleratory Center

Answer 46

sends impulses through the sympathetic trunk - stimulates the SA Node, AV Node, myocardium, and the coronary arteries to increase heart rate and contractility

Question 47

Cardioinhibitory Center

Answer 47

sends impulses through the vagus nerve to decrease heart rate