2 Electrical Activity of the Heart Flashcards
What is the general passage of the cardiac conduction system?
SAN (pacemaker)
electrical spread through atria
AVN (delay)
conduction along HIs bundles and Purkinje fibres
electrical spread from ventricular endocardium to epicardium
How can the SAN spontaneously generate AP’s?
there is no ‘resting membrane potential’, but there is the pacemaker potential.
How is the SAN pacemaker potential formed?
Na+ influx
Ca2+ influx
K+ efflux (low)
net + influx
membrane potential rises to threshold
What are the phases of the action potential in the SAN?
pacemaker potential
rapid depolarisation phase
repolarisation phase
What happens in the SAN rapid depolarisation phase?
fast Ca2+ influx (VGC’s)
What happens in the SAN repolarisation phase?
K+ efflux (VGC’s)
How does the ParaSN control heart rate?
Vagus Nerve ACh
increases K+ efflux (reducing slope of pacemaker potential)
decreases heart rate
How does the SNS control heart rate?
cardiac sympathetic nerves
circulating adrenaline
increases Na+ and Ca2+ influx (elevated slope of pacemaker potential)
increases heart rate
How many AP’s does the SAN and AVN generate?
Why is this important?
SAN - 80 -100 APs/min
AVN - 40 - 60 APs/min
AVN delay ensures atrial depolarisation, contraction and ejection before ventricles depolarise
Do ventricular cardiac myocytes spontaneously fire AP’s?
no, because their resting potential is so low and stable
What are the 5 phases of ventricular myocyte AP’s?
0 - rapid depolarisation 1 - partial rapid repolaristion 2 - plateau 3 - terminal repolarisation 4 - stable resting potential
What facilitates rapid depolarisation in ventricular cardiac myocytes?
Na+ influx through VGC’s
What facilitates partial rapid repolarisation in ventricular cardiac myocytes?
inactivation of Na+ channels
activation of fast K+ VGC’s
What facilitates terminal repolarisation in ventricular myocytes?
K+ efflux (delayed rectifier K+ channels)
What 2 concepts are significant when considering cardiac arrhythmia?
refractory periods
conduction velocity
What is the refractory period determined by?
the speed of the action potential and therefore the number of available and recovered Na+ VGC’s
What is Sodium channel recovery dependent on?
time and voltage
When will sodium channels recover the fastest?
at more negative membrane potentials
What facilitates conduction across cardiac myocytes?
gap junctions (only lets + ions through)
What is the conduction velocity determined by?
charge gradient, governed by the action potential amplitude (APA)
How might conduction velocity across cardiac myocytes be modified?
gap junction expression / function
What is R-R interval used for?
calculating heart rate and identifying irregular rhythms
What are P-R intervals?
What are they used for?
time from beginning of atrial depolarisation to initiation of ventricular depolarisation
used to identify AVN slow conduction
What is QRS duration?
total time of ventricular depolarisation, used to identify ventricular conduction slowing
What is Q-T interval?
What is it used for?
time from start of ventricular depolarisation to the end of ventricular repolarisation
used to indicate prolongation of ventricular action potential duration
What will AVN node disruption do to an ECG?
prolong P-R time
What will bundle of His issues do to an ECG?
prolong QRS duration
What will a problem with the delayed rectifier K+ channels cause?
How would this be expressed on an ECG?
lengthening of Q-T interval
the cell has already depolarised, so the repolarisation phase is elongated
What will excessive cardiac L-type calcium channel blockade cause?
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
