2 Electrical Activity of the Heart

Question 1

What is the general passage of the cardiac conduction system?

Answer 1

SAN (pacemaker)
electrical spread through atria
AVN (delay)
conduction along HIs bundles and Purkinje fibres
electrical spread from ventricular endocardium to epicardium

Question 2

How can the SAN spontaneously generate AP’s?

Answer 2

there is no ‘resting membrane potential’, but there is the pacemaker potential.

Question 3

How is the SAN pacemaker potential formed?

Answer 3

Na+ influx
Ca2+ influx
K+ efflux (low)

net + influx
membrane potential rises to threshold

Question 4

What are the phases of the action potential in the SAN?

Answer 4

pacemaker potential
rapid depolarisation phase
repolarisation phase

Question 5

What happens in the SAN rapid depolarisation phase?

Answer 5

fast Ca2+ influx (VGC’s)

Question 6

What happens in the SAN repolarisation phase?

Answer 6

K+ efflux (VGC’s)

Question 7

How does the ParaSN control heart rate?

Answer 7

Vagus Nerve ACh
increases K+ efflux (reducing slope of pacemaker potential)
decreases heart rate

Question 8

How does the SNS control heart rate?

Answer 8

cardiac sympathetic nerves
circulating adrenaline
increases Na+ and Ca2+ influx (elevated slope of pacemaker potential)
increases heart rate

Question 9

How many AP’s does the SAN and AVN generate?

Why is this important?

Answer 9

SAN - 80 -100 APs/min
AVN - 40 - 60 APs/min

AVN delay ensures atrial depolarisation, contraction and ejection before ventricles depolarise

Question 10

Do ventricular cardiac myocytes spontaneously fire AP’s?

Answer 10

no, because their resting potential is so low and stable

Question 11

What are the 5 phases of ventricular myocyte AP’s?

Answer 11

0 - rapid depolarisation
1 - partial rapid repolaristion
2 - plateau
3 - terminal repolarisation
4 - stable resting potential

Question 12

What facilitates rapid depolarisation in ventricular cardiac myocytes?

Answer 12

Na+ influx through VGC’s

Question 13

What facilitates partial rapid repolarisation in ventricular cardiac myocytes?

Answer 13

inactivation of Na+ channels

activation of fast K+ VGC’s

Question 14

What facilitates terminal repolarisation in ventricular myocytes?

Answer 14

K+ efflux (delayed rectifier K+ channels)

Question 15

What 2 concepts are significant when considering cardiac arrhythmia?

Answer 15

refractory periods

conduction velocity

Question 16

What is the refractory period determined by?

Answer 16

the speed of the action potential and therefore the number of available and recovered Na+ VGC’s

Question 17

What is Sodium channel recovery dependent on?

Answer 17

time and voltage

Question 18

When will sodium channels recover the fastest?

Answer 18

at more negative membrane potentials

Question 19

What facilitates conduction across cardiac myocytes?

Answer 19

gap junctions (only lets + ions through)

Question 20

What is the conduction velocity determined by?

Answer 20

charge gradient, governed by the action potential amplitude (APA)

Question 21

How might conduction velocity across cardiac myocytes be modified?

Answer 21

gap junction expression / function

Question 22

What is R-R interval used for?

Answer 22

calculating heart rate and identifying irregular rhythms

Question 23

What are P-R intervals?

What are they used for?

Answer 23

time from beginning of atrial depolarisation to initiation of ventricular depolarisation

used to identify AVN slow conduction

Question 24

What is QRS duration?

Answer 24

total time of ventricular depolarisation, used to identify ventricular conduction slowing

Question 25

What is Q-T interval?

What is it used for?

Answer 25

time from start of ventricular depolarisation to the end of ventricular repolarisation

used to indicate prolongation of ventricular action potential duration

Question 26

What will AVN node disruption do to an ECG?

Answer 26

prolong P-R time

Question 27

What will bundle of His issues do to an ECG?

Answer 27

prolong QRS duration

Question 28

What will a problem with the delayed rectifier K+ channels cause?
How would this be expressed on an ECG?

Answer 28

lengthening of Q-T interval

the cell has already depolarised, so the repolarisation phase is elongated

Question 29

What will excessive cardiac L-type calcium channel blockade cause?

Answer 29

in the plateau period of ventricular depolarisation, there is a balance of calcium efflux and potassium influx

therefore, blocking the calcium channels will allow K+ influx to take over, shortening the plateau period