5. Cardiac Electrical Conduction

Question 1

When and how fast do the heart’s pacemakers go off?

Answer 1

• SA node goes off fastest
• AV node is slower and gets overriden by more active SA node

Question 2

What is the function of the Purkinje fibers?

Answer 2

Specialized conducting system that rapidly propagates the impulse down into the contractile tissues

Question 3

What happens at the AV node? Significance?

Answer 3

• AV node slows conduction
• Delay is significant for electromechanical coupling –> gives time for atria to empty before contractin the ventricle

Question 4

What are the conduction velocities of the pacemakers and the Purkinje fibers?

Answer 4

• SA & AV node = 0.5 m/s
• Purkinje fibers = 4 m/s

Question 5

What is the significance of the atrio-ventricular fibrous skeleton of the heart?

Answer 5

Cartilaginous structure prevents electrical propagation atria to ventricles and vice versa

Question 6

What makes up the “os of the heart”?

Answer 6

Atrio-ventricular fibrous skeleton

Question 7

Define automaticity.

Answer 7

• Intrinsic firing rates of conducting fibers
• Spontaneous depolarization

Question 8

What is the native pacemaker of the heart?

Answer 8

Sinoatrial node

Question 9

What are the latent pacemakers of the heart?

Answer 9

• AV node & bundle of His
• Purkinje fibers
• Cardiac myocytes

Question 10

What is the intrinsic firing rate of the SA node?

Answer 10

60-100 bpm

Question 11

What is the intrinsic firing rate of the AV node and bundle of His?

Answer 11

40-60 bpm

Question 12

What is the intrinsic firing rate of the Purkinje fibers?

Answer 12

30-40 bpm

Question 13

What is the intrinsic firing rate of the cardiac myocytes?

Answer 13

<20 bpm

Question 14

What are escape beats/rhythms?

Answer 14

Beats/rhythms that originate from latent pacemakers if SA Node is slower than AV Node or Purkinje fibers

Question 15

What are ectopic beats?

Answer 15

Beats that originate outside the normal native rate (not a pacemaker cell)

Question 16

What makes the membrane potential graph of the SA node different than other myocardial cells?

Answer 16

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

Question 17

Describe the generalized AP and currents for ion influxes and effluxes.

Answer 17

• Na⁺ influx –> spontaneous depolarization, increase in membrane potential
• Ca²⁺ influx –> electro-mechanical coupling in heart, sustained contraction
• K⁺ efflux –> decrease in membrane potential, repolarization

Question 18

Define depolarization.

Answer 18

• Downward deflection
• Inward flow of (+) ion into cell

Question 19

Define repolarization.

Answer 19

• Upward deflection
• Outward flow of (+) ion out of cell

Question 20

What is the funny current (I_f)?

Answer 20

• Non-selective cation channel
• Mainly Na⁺ influx

Question 21

Describe the I_f of Purkinje cells.

Answer 21

Delayed and slow

Question 22

What factors may determine the rate of automaticity in pacemaker cells?

Answer 22

• Rate of cation influx during phase 4 depolarization
• Maximum negative diastolic potential (hyperpolarization)
• Threshold potential

Question 23

What is phase 4 depolarization?

Answer 23

• I_f going from MDP to TP
• Slow creep back up to threshold potential

Question 24

How does the autonomic nervous system affect automaticity?

Answer 24

• Autonomic NS modulates the native pacemakers by either speeding up or slowing down their funny currents using sympathetic or parasympathetic hormones

Question 25

What is the normal AP duration at rest?

Answer 25

~300-350 ms

Question 26

What is the AP cycle duration at rest?

Answer 26

75 bpm = 0.80 sec (800 ms/beat)

Question 27

What is the AP cycle duration during exercise?

Answer 27

180 bpm = 0.33 sec (330 ms/beat)

Question 28

Explain the Interval-Duration relationship?

Answer 28

• 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

Question 29

Sympathetic activity speeds the firing rate of the SA node by increasing what 2 currents?

Answer 29

• Inward Ca²⁺ current
• Inward Na⁺ current

Question 30

Increased vagal tone slows down firing rate of SA node by increasing what current and slowing what current?

Answer 30

• Increasing outward K⁺ current
• Slowing the inward Na⁺ current