L10: Cardiac electrical activity

Question 1

Which cells have the ‘fast response’?

Answer 1

Atria, ventricles, fast parts of specialized conduction system

Question 2

Draw out the fast response AP graph, labelling the phases and the periods of the graph

Answer 2

-80mV RP
-70mV TO
+20mV phase 0
Phase 3 - ends at 250ms
Phase 1 = early repolarization (transient outward current K+)
FRT = 0-300ms
ARP = Early repolarization + plateau phase
FRT - ARP = RRP + SNP

Question 3

Which cells have ‘slow response’?

Answer 3

SA node and AV node

Question 4

Draw out the slow response AP graph

Answer 4

RP = -60mV
Gradual slope to 10mV
FRT = 250ms

Question 5

What is the function of the refractory period?

Answer 5

To prevent tetanizing of the heart myocardium and ensure the heart has time to relax and thus fill with blood

Question 6

Can an action potential occur during the Relative refractory period?

Answer 6

Yes, because some sodium channels are open

Question 7

Myocardial muscle cells are connected together by:

Answer 7

intercalated disks

Question 8

Draw out the pacemaker graph

Answer 8

RP = -60mV
TP = -50mv
peak = 20mv

Question 9

What are the characteristics of the pacemaker AP?

Answer 9

Constant slow depolarization to threshold potential

Question 10

What is the funny current induced by?

Answer 10

Hyperpolarization (inward Na+)

Question 11

How does the SNS affect pacemaker rate?

Answer 11

NE phosphorylates calcium channels which depolarises the resting potential to threshold potential faster, resulting in faster heart rate

Question 12

How does the PNS affect pacemaker rate?

Answer 12

ACh bind to ACh-gated K+ channels on SA node -> Increase outward K+ = More negative resting potential = More time taken to depolarise to threshold potential.
PNS also slows conduction through AV node fibres

Question 13

What is normal heart rate bpm?

Answer 13

60-100bpm

Question 14

What is bradycardia bpm?

Answer 14

less than 60bpm

Question 15

What is tachycardia bpm?

Answer 15

more than 100bpm

Question 16

What is the sequence of activation of the heart?

Answer 16

SA node -> Atria -> AV node-> His bundle (AV bundles) -> bundle branches -> Purkinje network -> Ventricular myocardium

Question 17

How does the heart allow time for the atria to completely empty its blood into the ventricles?

Answer 17

the AV node conducts very slowly as its fibres have a small radius