5. Cardiac Electrical Conduction Flashcards
When and how fast do the heart’s pacemakers go off?
- SA node goes off fastest
- AV node is slower and gets overriden by more active SA node
What is the function of the Purkinje fibers?
Specialized conducting system that rapidly propagates the impulse down into the contractile tissues
What happens at the AV node? Significance?
- AV node slows conduction
- Delay is significant for electromechanical coupling –> gives time for atria to empty before contractin the ventricle
What are the conduction velocities of the pacemakers and the Purkinje fibers?
- SA & AV node = 0.5 m/s
- Purkinje fibers = 4 m/s
What is the significance of the atrio-ventricular fibrous skeleton of the heart?
Cartilaginous structure prevents electrical propagation atria to ventricles and vice versa
What makes up the “os of the heart”?
Atrio-ventricular fibrous skeleton
Define automaticity.
- Intrinsic firing rates of conducting fibers
- Spontaneous depolarization
What is the native pacemaker of the heart?
Sinoatrial node
What are the latent pacemakers of the heart?
- AV node & bundle of His
- Purkinje fibers
- Cardiac myocytes
What is the intrinsic firing rate of the SA node?
60-100 bpm
What is the intrinsic firing rate of the AV node and bundle of His?
40-60 bpm
What is the intrinsic firing rate of the Purkinje fibers?
30-40 bpm
What is the intrinsic firing rate of the cardiac myocytes?
<20 bpm
What are escape beats/rhythms?
Beats/rhythms that originate from latent pacemakers if SA Node is slower than AV Node or Purkinje fibers
What are ectopic beats?
Beats that originate outside the normal native rate (not a pacemaker cell)
What makes the membrane potential graph of the SA node different than other myocardial cells?
SA node automatically starts drifting up towards depolarization threshold as opposed to all others that flatline immediately after contraction and don’t creep up until later
Describe the generalized AP and currents for ion influxes and effluxes.
- Na+ influx –> spontaneous depolarization, increase in membrane potential
- Ca2+ influx –> electro-mechanical coupling in heart, sustained contraction
- K+ efflux –> decrease in membrane potential, repolarization
Define depolarization.
- Downward deflection
- Inward flow of (+) ion into cell
Define repolarization.
- Upward deflection
- Outward flow of (+) ion out of cell
What is the funny current (If)?
- Non-selective cation channel
- Mainly Na+ influx
Describe the If of Purkinje cells.
Delayed and slow
What factors may determine the rate of automaticity in pacemaker cells?
- Rate of cation influx during phase 4 depolarization
- Maximum negative diastolic potential (hyperpolarization)
- Threshold potential
What is phase 4 depolarization?
- If going from MDP to TP
- Slow creep back up to threshold potential
How does the autonomic nervous system affect automaticity?
- Autonomic NS modulates the native pacemakers by either speeding up or slowing down their funny currents using sympathetic or parasympathetic hormones
What is the normal AP duration at rest?
~300-350 ms
What is the AP cycle duration at rest?
75 bpm = 0.80 sec (800 ms/beat)
What is the AP cycle duration during exercise?
180 bpm = 0.33 sec (330 ms/beat)
Explain the Interval-Duration relationship?
- Duration of AP varies w/ prior diastole
- When cycle length varies, AP duration is determined in part by the preceding diastolic interval
- Duration of AP and interval gap b/t APs are proportional
- Short preceding interval –> short AP
Sympathetic activity speeds the firing rate of the SA node by increasing what 2 currents?
- Inward Ca2+ current
- Inward Na+ current
Increased vagal tone slows down firing rate of SA node by increasing what current and slowing what current?
- Increasing outward K+ current
- Slowing the inward Na+ current
