Cardiac Impulse

Question 1

SA node

role, location

Answer 1

Initiates the heartbeat
Sets the pace for the heart
No resting membrane potential but there is spontaneous pacemaker potential
Located in the upper RA

Question 2

Pacemaker potential

Answer 2

Takes the membrane potential to a threshold (slowly towards depolarisation) to generate an AP in SA nodal cells
Due to: funny current (Na+ and K+ influx), transient Ca2+ influx, K+ efflux

Question 3

Pacemaker/Nodal cell AP

Answer 3

Phase 4: pacemaker potential - reaches threshold
Phase 0: Depolarisation - rising phase causes Ca2+ influx
Phase 3: Repolarisation - falling phase causes K+ efflux

Question 4

Spread of cardiac excitation in the heart

Answer 4

SA node 
*cell-cell conduction via gap junctions*
AV node 
Bundle of His (R&L branches) 
Purkinje fibres

Question 5

AV node

Answer 5

Only point of electrical contact between the atria and ventricles
Location: base of right atrium

Question 6

AV nodal delay

Answer 6

Ensures that atria and ventricles don’t contract at the same time

Question 7

Atrial and Ventricular myocyte AP

Answer 7

Phase 0: Depolarisation - rising phase causes fast Na+ influx
Phase 1: Slight repolarisation - transient K+ efflux, closure of Na+ channels
Phase 2: Plateau phase - Ca2+ influx
Phase 3: Repolarisation - falling phase causes K+ efflux
Phase 4: Resting membrane potential

Question 8

Parasympathetic supply of heart

Answer 8

Decreases HR
Increases AV nodal delay
Decreased contractility (-ve inotropic effect)
Neurotransmitter: ACh acting through M2 muscarinic receptors
Vagus nerve - exerts a continuous influence on SA node under resting conditions
Cells take longer to reach AP threshold as the slope of the pacemaker potential is decreased (due to decrease in funny current and calcium current).
Frequency of AP also decreases - -ve chronotropic effect.

Question 9

Sympathetic supply of heart

Answer 9

Increases HR
Decreases AV nodal delay
Increases force of contraction (+ve inotropic effect)
Neurotransmitter: Nor-adrenaline acting through B1 adrenoceptors
Slope of pacemaker potential is increased (due to increase in funny current and calcium current) so cells get to threshold quicker
Frequency of AP increases - +ve chronotropic effect
Decreased duration of systole
Decreased cardiac efficiency

Question 10

Funny current

Answer 10

Plays a role in modulating the pacemaker potential
Mediated by HCN channels
Hyper-polarisation following the AP activates the HCN channels in the SA node which facilitates the pacemaker potential.
If we block the HCN channels; decrease HR due to decrease slope.