Electrical conduction of the heart

Question 1

describe the pathway of conduction in the heart

Answer 1

• SAN pacemaker (RA)
• electrical spread through atria
• AVN delay -slowest conduction
• conduction along HIS bundles and Purkinje fibres
• spread from endocardium to epicardium

Question 2

describe the action potential of the SAN

Answer 2

• spontaneous generation
• fires around 80-100 aps/min
• exhibits a pacemaker potential
• during the PMP there is NA Influx via HCN channels
• inward movement of ca
• efflux of K (low g)
• na/ca influx exceeds k efflux
• rapid depolarisation stage, fast inward Ca (rising) - VGC
• repolarisation stage - K efflux though VGK - falling phase

Question 3

increased PNS activity via the vagal nerve alters the PMP slope how?

Answer 3

• decreases HR
• increases K efflux (g)
• reduce slope of PMP
• negative chronotropic effect
• ACh at M2r (GIRK)

Question 4

increased SNS activity can alter the PMP slope how?

Answer 4

• increases HR
• increases Na/Ca influx (g)
• elevates slope of PMP
• positive chronaotropic effects
• increase in circulating ad can act in the same way

Question 5

describe the AV node action potential

Answer 5

• very similar to SAN
• PMP less steeper than SAN
• allows blood to enter ventricles before contracting, allows maximum V filling
• slower AP firing rate (slowest) 40-60
• ensures atrial depolarisation, contraction & ejection before ventricles depolarise
• spontaneous generation

Question 6

• Ventricular cardiac myocyte

Answer 6

• no spontaneous generation

- has a stable RMP

Question 7

HR will always be driven by the?

Answer 7

• fastest pacemaker

Question 8

describe the phases of the cardiac myocyte AP

Answer 8

• phase 0- rapid depolarisation (fast VGK open upon depol) and influx of Na via VGN
• phase 1- inactivation of Na channels, activation of fast VGK (partial rapid repol)
• phase 2 -inwards ca Movement and outward K movement - balancing of charge (plateau)
• phase 3 - outwards Movement of k via delayed rectifier (terminal repol)
• phase 4- stable resting set by K channels

Question 9

what is the RP defined as?

Answer 9

the time from initial depolarisation of first AP to the point at which a second AP can be generated

determined by the number of available and recovered (re-primed) VGN channels

Question 10

Na channel recovery time is determined by?

Answer 10

• voltage of the cell (recover at increasingly negative V)
• time (increase Time, increases amplitude)

speed of return to negative RMP determined by APD

Question 11

the overall CV is determined by?

Answer 11

• the charge gradient between the cells
• set by the magnitude of the depol current - AP amplitude
• can be modified by gap junction expression and function
• amplitude depends on number of available Na channels! and their functionality

Question 12

resting potential of a ventricular myocyte ?

Answer 12

• 90mV

Question 13

the resting potential of the SAN/AVN is?

Answer 13

• 65mV