Lecture 03: Pacemaker Cells and the Normal Sequence of Cardiac Excitation (Hayward)

Question 1

Where are pacemaker cells located?

Answer 1

AV and SA nodes

Question 2

Classification of myocardial AP as either fast or slow refers to:

Answer 2

rate of initital membrane depolarization

Question 3

“funny” Na+ channel

Answer 3

Used to depolarize pacemaker cells by SLOW influx of Na+. Activated by hyperpolarization (at end of phase 3) instead of depolarization.

Question 4

Characteristics of Phase 4 for SLOW type APs

Answer 4

1) slow spontaneous depolarization during rest via pacemaker current generated by funny Na+ channels
2) Less negative RMP and maximum hyperpolarizing voltage
3) Decreasing K+ conductance

Question 5

Pacemaker current

Answer 5

slow influx of Na+ through funny Na+ channel to activate depolarization of pacemaker cells

Question 6

Do pacemaker cells have fast type Na+ channels?

Answer 6

Yes, but they are usually inactivated due to persistent less negative RMP generated by funny Na+ channels

Question 7

Characteristics of Phase 0 for SLOW type APs

Answer 7

1) Ca++ influx
2) Inactivation of fast and funny Na+ channels
3) Decreased Na+ conductance

Question 8

Which phases are absent in SLOW type APs?

Answer 8

Phase 1 and 2

Question 9

Characteristics of Phase 3 for SLOW type APs?

Answer 9

(slow repolarization):

1) Ca++ channels close
2) Low Na+ conductance
3) Delayed rectifier K+ channel activated –> increased K+ conductance to help repolarize

Question 10

Maximum diastolic potential (MDP)

Answer 10

maximum negative potential that is reached (during phase 4). Alters pacemaker action potential frequency

Question 11

What does MDP primarily depend on?

Answer 11

K+ and Na+ conductance

Question 12

How do SA nodal pacemaker cells cause depolarization across whole heart?

Answer 12

generate APs with faster rate of depolarization compared to other pacemaker cells (AV). As a result they reach AP threshold first and trigger membrane depolarization throughout the rest of the heart.

Question 13

conduction velocity

Answer 13

speed at which APs propagate from one area to another

Question 14

Conduction velocity depends greatly on

Answer 14

diameter of muscle fiber involved. Larger diameter –> faster conduction velocity

Question 15

Which conducts faster, Purkinje or AV node fibers?

Answer 15

Purkinje fibers

Question 16

Where does excitation and depolarization originate

Answer 16

SA node in RA

Question 17

Bachmann’s bundle/interatrial bundle

Answer 17

conducts impulse from RA to LA so both atria contract almost simultaneously

Question 18

Do atrial or ventriclular cells have shorter APs? Why?

Answer 18

Atria have shorter APs because their Ca++ channels are open for a shorter duration, and K+ channels are closed for a shorter duration during plateau phase

Question 19

Why must atrial depolarization pass through AV node?

Answer 19

AV node is only place where atrial and ventricular cells are electrically coupled, so signal MUST pass through here in order for ventricles to depolarize

Question 20

What is the “latent pacemaker”?

Answer 20

the AV node. Will take over regulating HR if SA node is dysfunctional, but at a slower HR

Question 21

Which has faster rate of RM depolarization? AV or SA node cells?

Answer 21

SA node cells

Question 22

Where does signal travel after AV node? Slow or fast speed?

Answer 22

Bundle of His. Relatively slow, resulting in conduction delay

Question 23

What happens during conduction delay? (2 things)

Answer 23

1) Signal travels from AV node to Bundle of His

2) Atria finish contraction prior to depolarization of ventricles

Question 24

Normal sequence of depolarization

Answer 24

SA node –> atrial tissue –> AV node –> Bundle of His –> L and R Bundle branches –> Purkinje fibers –> cell to cell to ventricles starting with papillary muscles and interventricular septum

Question 25

Purkinje fibers

Answer 25

large diameter/fast conducting cells that distribute depolarization throughout heart.

Question 26

Mechanism of cell to cell conduction

Answer 26

low resistance gap junctions

Question 27

Why do papillary and interventricular septum depolarize first in cell to cell depolarization

Answer 27

Allows for apical rigidity and anchors AV valves during ventricular systole

Question 28

T or F: all cardiac cells are electrically coupled

Answer 28

T

Question 29

Fast type AP is mostly dependent on extracellular _____, while Slow type AP is mostly dependent on extracellular ____

Answer 29

Fast --> Na+ | Slow --> Ca++

Question 30

Why do AV nodal cells have longer refractory period?

Answer 30

Protects ventricles from being stimulated to contract too fast a rate. Prevent retrograde excitation from the ventricles

Question 31

What structure denotes the beginning of the specialized conduction system in the ventricles?

Answer 31

Bundle of His

Question 32

What does Bundle of His separate into?

Answer 32

LBB and RBB (L is bigger and depolarizes first)

Question 33

largest diameter fibers in the heart

Answer 33

Purkinje fibers

Question 34

Which heart structure is assoc. with longest plateau phase?

Answer 34

Purkinje fibers

Question 35

"tertiary" pacemaker

Answer 35

His-Purkinje fiber system, but would be very slow HR (unsustainable for life)

Question 36

What does papillary muscle contraction prevent?

Answer 36

eversion of AV valves into the atria during ventricular contraction

Question 37

Does RV or LV depolarize first?

Answer 37

RV because it is thinner walled and easier for signal to pass through it

Question 38

Myocardial cells are refractory when they are contracted or relaxed?

Answer 38

Relaxed

Question 39

Myocardial depolarization causes muscle to _____ and then _______

Answer 39

contract then relax

Question 40

In slow type AP, shape of AP relates to what?

Answer 40

Timing of Ca++ conductance