13. APs and Conduction

Question 1

Describe the conduction system of the heart

Answer 1

SA node (60 ms) → AV node (100 ms) → Bundle of His (10 ms) → R/L Bundle Branches → Purkinje Fibers

Question 2

what is the purpose of slow conduction through the AV node

Answer 2

slow conduction to allow time for ventricles to fill

increasing conduction velocity → decreased CO and SV

Question 3

what physiologically makes the SA node the pacemaker of the cell

Answer 3

fastest rate of phase 4 depolarization, shortest AP duration and refractory period)

Question 4

which parts have the greatest conduction velocity

Answer 4

larger diameter fibers are faster

purkinje > atrial and ventricular muscles > AV node

Question 5

what is the purpose of overdrive suppression

Answer 5

to prevent the latent pacemakers (AV node, bundle etc) from driving HR

Question 6

APs of ventricles, atria, and purkinjes: phase 0

• what is it
• what ions are responsible
• why doesnt the membrane potential reach the ion potential
• membrane potential at peak

Answer 6

• upstroke, rapid depolarization
• increase in Na conductance → inward Na current
• inactivation gate closing prevents membrane potential from reaching Na potential
• peak of upstroke, membrane potential = 20 mv

Question 7

APs of ventricles, atria, and purkinjes: phase 1

• what is it
• what ions are responsible

Answer 7

• initial repolarization
• inactivation gates of Na channels close, decreasing inward Na current
• outward K current

Question 8

APs of ventricles, atria, and purkinjes: phase 2

• what is it
• what ions are responsible

Answer 8

• plateau, long period of relatively stable depolarized membrane potential
• increased Ca conductance → inward Ca current
• outward K current balances Ca current

Question 9

APs of ventricles, atria, and purkinjes: phase 3

• what is it
• what ions are responsible

Answer 9

• repolarization
• decrease in Ca conductance → decrease inward Ca current
• increase in K conductance → increase outward K current

Question 10

APs of ventricles, atria, and purkinjes: phase 4

• what is it
• what ions are responsible

Answer 10

• resting membrane potential (-85 mv)

- outward K current balanced by inward Na and C currents

Question 11

which ion current is known as the inward rectifier

Answer 11

I K1

close in response to depolarization, slow to close and remain open for a determined amount of time, then opens to help with phase 3

Question 12

APs of SA Node: phase 0

• what is it
• what ions are responsible

Answer 12

• upstroke

- increased Ca conductance → inward Ca current

Question 13

APs of SA Node: phase 3

• what is it
• what ions are responsible

Answer 13

• repolarization

- increased in K conductance → outward K current

Question 14

APs of SA Node: phase 4

• what is it
• what ions are responsible

Answer 14

• spontaneous depolarization or pacemaker potential

- funny Na channels → inward Na current

Question 15

describe absolute refractory period

Answer 15

• no APs can be generated

- most of Na channels are closed

Question 16

describe relative refractory period

Answer 16

• APs can be generated but requires greater stimulation
• Na channels start to be available to carry inward currents
• second AP will have abnormal configuration, shortened plateau phase, and will be weaker

Question 17

describe supranormal period

Answer 17

• cell is more excitable than normal and easier to generate an AP, may have abnormal conduction and AP will be weaker
• requires less inward current to depolarize cell
• Na channels are recovered (inactivations gates open again)

Question 18

what is a +/- chronotropic effect?

Answer 18

• change in HR, slope of depolarization in phase 4 of SA node

+ : increased HR by increasing phase 4 depolarization, increase If (S)

• : decreased HR by decreasing phase 4 depolarization, decrease If (PS)

Question 19

what is a +/- dromotropic effect?

Answer 19

• change in conduction velocity, slope of phase 0

+ : increased velocity, increased inward Ca current (S)

• : decreased velocity, decrease inward Ca current, increased outward K current (PS)

Question 20

what effect would a +/- dromotropic effect have on the PR interval

Answer 20

+ : decreased PR interval

• : increased PR interval

Question 21

what is an ionotropic effect? lusitropic?

Answer 21

Ionotropic- effects strength of muscular contraction

Lusitropic- effects rate of muscular relaxation

Question 22

what effect does the parasympathetic system have on HR

Answer 22

• chronotropic effect: slower opening of funny Na channels during phase 4
• dromotropic effect: reduced inward Ca current, increase outward K current via special K-ach

PS: vagus nerve, ach → muscarinic (M2/M3)
innervates SA and AV node
no ionotropic or lusitropic influence

Question 23

what effect does the sympathetic system have on HR

Answer 23

+ chronotropic effect: more rapid opening of funny Na channels during phase 4
(beta agonist meds increase rate of phase 4 depolarization by increasing If, beta antagonists decrease)

+ dromotropic effect: increased inward Ca current

S: NE → B1
innervates SA node, AV node, ventricular myocytes (not atrial)
positive ionotropic or lusitropic influence

Question 24

what is the key difference in phase 4 of AP found in SA/AV node vs myocytes

Answer 24

• rmp gradually depolarizes until it reaches threshold

- d/t funny Na channels that open upon complete repolarization of membrane

Question 25

what is the key difference in phase 0 of AP found in SA/AV node vs myocytes

Answer 25

- opening of voltage gated Ca channels rather than voltage gates Na channels - closure of voltage gated K channels (rectifies)

Question 26

what is the key difference in phase 3 of AP found in SA/AV node vs myocytes

Answer 26

- reversal of phase 0 | - closure of voltage gated Ca channels and opening of voltage gated K channels (both rectifiers and traditional)

Question 27

which phase of the AP corresponds with the QRS complex? T wave? ST segment?

Answer 27

QRS: phase 0; rapid depolarization, influx of Na T: phase 1/3, repolarization, efflux of K ST: phase 2, plateau, influx of Ca = efflux of K