cardiac action potentials and conduction pathway

Question 1

The heart is myogenic. What does this mean?

Answer 1

The heart generates electrical activity and contraction without external stimulation.

Question 2

What are pacemaker potentials

Answer 2

action potentials generated by SAN

Question 3

What do the firing rate of the action potentials of the SAN equate to?

Answer 3

the heart rate

Question 4

What does the rhythm of the action potentials fired by the SAN equate to?

Answer 4

the heart rhythm

Question 5

What are the 3 phases of pacemaker potentials?

Answer 5

Phase 4: resting membrane potential
Phase 0: upstroke, depolarisation
Phase 3: downstroke, repolarisation

Question 6

How are I-f channels activated?

Answer 6

by hyperpolaristaion
as the cell gets more negative inside, the more I-f channels open
Hyperpolarisation-activated Na+ channels

Question 7

When I-f channels are open, what do they allow in to the cell?

Answer 7

Na+

Question 8

How is threshold reached (Phase 4)

Answer 8

The more I-f channels open
More Na+ enters the cell
Cell becomes more positive
Cell starts to depolarise
Cell soon reaches threshold and phase 0 will commence

Question 9

What occurs during phase 0

Answer 9

Activation of Voltage-Gated Ca2+ Channels not Na+ channels.
Ca2+ influx
Producing depolarisation
Over time theses VGCCs start to switch off
Now we move on to phase 3

Question 10

What occurs during phase 3?

Answer 10

Voltage-Gated K+ channels opens
There is K+ efflux (K+ moving out of cell)
Cell becomes more negative
This repolarisation
However as the cell becomes more negative, I-f channels start to activate (during hyperpolarisation)

Question 11

What is meant by the SAN having an unstable/non-equilibrium resting membrane potential (RMP) and what causes this?

Answer 11

The RMP constantly wants to depolarise and reach a threshold for depolarisation (Phase 0), this is caused by expression of If channels

Question 12

What allows for fast condutcion of electrical activity throughout the heart?

Answer 12

Intercalated discs between cardiac cells

Question 13

What is the journey of electrical activity throughout the heart?

Answer 13

Initiation of a pacemaker potential at the SAN ->
Conduction across both atria to atrial-ventricle node (AVN) ->
AVN slows down electrical conduction to allow time for ventricles to fill ->
Action potential sent to the Bundle of His -> Action potential sent to purkinje fibres (between Bundle of His and Purkinje fibres there is a fast conduction pathway which allows both ventricles to be stimulated and contracted together)

Question 14

What does the P wave represent on an ECG?

Answer 14

Artrial depolarisation

Question 15

What does the QRS complex represent on an ECG?

Answer 15

Ventricle depolarisation

Question 16

What does the ST interval represent on an ECG?

Answer 16

Interval between ventricle depolarisation and repolarisation

Question 17

What does the T wave on an ECG represent?

Answer 17

ventricle repolarisation

Question 18

What information does an ECG provide?

Answer 18

An ECG provides information on electrical conduction through the heart and does not provide information on contraction

Question 19

Where does ventricular excitation and contraction begin and spread to?

Answer 19

begins at apex and spreads to base

Question 20

What is the order of atrial/ventricular action potential phases?

Answer 20

Phase 4 (Stable Resting Potential) -> 
Phase 0 (Depolarisation) -> 
Phase 1 (Brief Repolarisation) -> 
Phase 2 (Plateau Phase) -> 
Phase 3 (Repolarisation)

Question 21

Why is the Plateau phase of the Atrial/Ventricular action potential important? x3

Answer 21

1. The length of the plateau phase means that the ventricular cells are refractory, i.e. they cannot fire any more Action Potentials. 2. Therefore, there is no twitching as one action potential produces one contraction.
2. Therefore, there is proper ejection of blood from the heart

Question 22

Describe electrical-contraction coupling through the heart

Answer 22

• electrical activity generated in SA node (non-contractile tissue) spreads out into the atria to produce contraction → blood is going to be ejected from the atria to the ventricles
• electrical activity then enters the AVN which delays conduction before stimulating ventricles. this allows the ventricles to fill properly during atrial contraction
• excitation conducts rapidly through bundle of His into purkinje fibres
• excitation through purkinje fibres is conducted throughout the ventricular contractile tissue producing contraction
• ventricular excitation and contraction begins at the apex and spreads up to base of the heart.

Question 23

What happens during phase 0 of the atrial/ventricular action potential?

Answer 23

voltage gated Na+ channels open

Na+ influx