Cardiac Cycle: Electrical Events of Cardiac Cycle

Question 1

What is the ‘pacemaker ‘ part of the heart?

Answer 1

Atrioventricular node

Question 2

What is the ‘cardiac cycle’?

Answer 2

the relationship between electrical, mechanical, and valvular events during one complete heartbeat

Question 3

The sum of cardiac potentials at any given time is recorded by what machine?

Answer 3

Electrocardiogram (ECG)

Question 4

is it easier to create an action potential in a pacemaker cell or a normal myocardial cell?

Answer 4

a pacemaker cell (sinoatrial/atrialventricular cell) - has a higher resting membrane potential in pacemaker cells compared to ventricular cells

• they have a tendency to depolarize

Question 5

what ion channel is missing in pacemaker cells?

Answer 5

voltage gated sodium - therefore there is no sodium influence on the nodal cells - depends on slow depolarisation activated Ca2+ channels to produce action potential

Question 6

What are the pacemaker cells called?

Answer 6

atrioventricular cells

sinoatrial cells

Question 7

describe the action potential process in a non- pacemaker cell?

Answer 7

pretty standard electrophysiology - normal muscle cell process

Question 8

where are your highest concentrations of sodium, potassium and calcium?

Answer 8

sodium = highest out of the cell

potassium = highest inside of the cell

calciu = highest outside of the cell

Question 9

name a drug that inhibits fast sodium channels

Answer 9

tetrodotoxin from a pufferfish

Question 10

Describe thet Fast Response Action Potential

Answer 10

• phase 0 = Na+ enters through fast channels
• phase 1 = efflux of K+ through channels (partially repolarize)
• phase 2= influx of Ca2+ through channels is balanced by efflux of K+ through K+ channels
• phase 3= forces favor efflux of K+ through channels over influx of K+ through same channels
• phase 4= restoration of ionic concentraion through 3 enzymes

Question 11

what are the three enzymes that restore ionic concentration in fast response action potential?

Answer 11

• Na/K ATPase
• 3Na+/1Ca++ antiporter
• ATP driven Ca++ pump

Question 12

what is the main ion which contributes to the upstroke of an action potential in non-pacemaker cells?

Answer 12

Sodium

Question 13

Which cells in the heart do you see slow response action potentials?

Answer 13

pacemaker cells of the heart

Question 14

How does depolarization in the nodal cells occur?

Answer 14

it is achieved by influx of Ca2+ throuh the Ca2+ channels (instead of by influx of Na+ through fast Na+ channels)

Question 15

What is the “IF” in the nodal cells?

Answer 15

they are channels for sodium channels that differ from the fast sodium channels present in the non-nodal cells of the heart - it’s called the ‘funny Na current’ because no one expected to find them in these celsl

Question 16

what is automaticity?

Answer 16

the spontaneous generation of action potentials due to unstable RMP

Question 17

nodal cells depend on what for depolarization\?

Answer 17

they depend on slow Calcium channels -

Question 18

are the sodium channels on the nodal cells similar to the sodium channels on the non-pacemaker cels?

Answer 18

no they vary significantly

Question 19

for automaticity to occur, what occurs to the ions of the pacemaker cells?

Answer 19

• calcium - moves inwards to depolarize threshold to -40mv
• sodium and potassium - initial depolarisation to -55mv by inward Na+ current (IF channel) and outward K+ current

Question 20

which of the following ions is not involved in the automaticity of the nodal cells

• calcium
• sodium
• potassium
• magnesium

Answer 20

-magnesium

Question 21

do parasympathetic/sympathetic nerves influence whether or not a heart beat occurs?

Answer 21

no, they only effect the rate aat which it occurs

Question 22

does noradrenaline/sympathetics shift action potential to be more likely or less likely to occur?

Answer 22

it makes the aciton potential more likely to occur

Question 23

how does acetylcholine influence the action potential of the nodal cells?

Answer 23

it slows down the heartbeat - so it makes a longer gap inbetween action potentials -

there are acetylcholine dependent potassium channels

Question 24

What is the refactory period?

Answer 24

frame during which stimulation cannot generate action potentials

= Effective refractory period

or = relative refractory period

Question 25

what is the effective refractory period?

Answer 25

some of the sodium channels are closed, some are open

but you cannot have any sodium conductance

Question 26

what is the relative refractory period?

Answer 26

many but not all of sodium channels re-open between -50mv and complete repolarizaiton?

*sodium channels are starting to open, so they are closed but they can be opened*

Question 27

Why does tetany not happen in cardiac cells?

Answer 27

the purkinje fibres have the longest refractory period - therefore they prolong the absolute refractory period in the cardiac muscle so tetany cannot occur

Question 28

how does the action potential spread from cell to cell in cardiac muscle?

Answer 28

travels from muscle cell to muscle cell through gap junctions in intercalated discs separatig cells - its syncronus

Question 29

describe normal cardiac conduction through the heart

Answer 29

• action potential originates in the sinoatrial nodes - moves across muscle cells through gap junctions and intercalated discs seperating cells
• proceeds sequentially to the atrioventricular node
• moves into ventricles via bundle of his through ventricles (functional sycytium)

Question 30

What are the purkinje cells?

Answer 30

fast conducting muscle cells carrying current from AV node through ventricles

Question 31

Where is the atrioventricular node located?

Answer 31

loctaed on the right side of interatrial septum - imposes a delay of about 0.1 s to the passage of AP

Question 32

what is the rate of conduction at the atrioventricular node, compared to the other conduction fibers of the heart?

Answer 32

Question 33

What occurs in Wolff-parkinson White syndrome?

Answer 33

there is a ‘new ‘ pathway in the heart for the electrical signal called the bundle of Kent or the bypass tract

• so the Sino atrial node flows through the bundle of kent instead of through the AV node, then it goes back into the atrium after passing around the ventricle (this phenomenon is called Global AV reentry) - this leads to supraventricular tachyarrhythmias (above the ventricle fast heartbeat)

*there can also be local reentry sites where the action potentials go around and around in circles - these cause the heart to beat incorrectly *

Question 34

what causes Wolff-Parkinson White Syndrome?

Answer 34

associated with mutations in 5’AMP activated prtoein kinase subunit gamma (PRKAG2 gene)

Question 35

how do you treat Wolff-parkinson White syndrome?

Answer 35

“radiofrequency catheter ablation”- destroy the abnormal heartbeat through a catheter to use heat or cold to bypass the ‘bundle of kent’

or

“electro cardioversion” = electrical charge is produced that slows down the heart beat

Question 36

a denervated heart has a faster or slower heart rate than a noramlly innervated heart?

Answer 36

a dennervated heart would have a faster heart rate than an innervated heart b/c the ANS slows down the heart

Question 37

When you examine a patient, you may describe the normal pulse as being in sinus rhythm. Explain?

*sample short note question*

Answer 37

• Describe location of SA node, then it’s ability to produce action potential
• describe how potential spreads through heart
• describe what occurs when any bit of this process goes wrong (wolf syndrome)

Question 38

what does the term functional syncytium mean?

Answer 38

it is a functional syncytium not a true syncytium where all the cells are fused together

a functional syncytium = electrical impulses propagate freely between communicating cells via gap junctions, so tha thte myocardium functions as a single contractile unit - this allows rapid, synchronized depolarization of the mycardium

Question 39

the AV node delays the signal by what amount of time?

Answer 39

by 0.1 s

Question 40

What are the characteristics of the AV node?

Answer 40

found in the right side of the interatrial septum

• narrow muscle fibre diameter
• low density of gap juncitons
• lack of fast Na+ channels
• low amplitude action potentials
• effect on ANS
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