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

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Flashcards in Lecture 03: Pacemaker Cells and the Normal Sequence of Cardiac Excitation (Hayward) Deck (40):
0

Where are pacemaker cells located?

AV and SA nodes

2

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

rate of initital membrane depolarization

3

"funny" Na+ channel

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

4

Characteristics of Phase 4 for SLOW type APs

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

5

Pacemaker current

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

6

Do pacemaker cells have fast type Na+ channels?

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

7

Characteristics of Phase 0 for SLOW type APs

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

8

Which phases are absent in SLOW type APs?

Phase 1 and 2

9

Characteristics of Phase 3 for SLOW type APs?

(slow repolarization):
1) Ca++ channels close
2) Low Na+ conductance
3) Delayed rectifier K+ channel activated --> increased K+ conductance to help repolarize

10

Maximum diastolic potential (MDP)

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

11

What does MDP primarily depend on?

K+ and Na+ conductance

12

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

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.

13

conduction velocity

speed at which APs propagate from one area to another

14

Conduction velocity depends greatly on

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

15

Which conducts faster, Purkinje or AV node fibers?

Purkinje fibers

16

Where does excitation and depolarization originate

SA node in RA

17

Bachmann's bundle/interatrial bundle

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

18

Do atrial or ventriclular cells have shorter APs? Why?

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

19

Why must atrial depolarization pass through AV node?

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

20

What is the "latent pacemaker"?

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

21

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

SA node cells

22

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

Bundle of His. Relatively slow, resulting in conduction delay

23

What happens during conduction delay? (2 things)

1)Signal travels from AV node to Bundle of His
2) Atria finish contraction prior to depolarization of ventricles

24

Normal sequence of depolarization

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

25

Purkinje fibers

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

26

Mechanism of cell to cell conduction

low resistance gap junctions

27

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

Allows for apical rigidity and anchors AV valves during ventricular systole

28

T or F: all cardiac cells are electrically coupled

T

29

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

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

30

Why do AV nodal cells have longer refractory period?

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

31

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

Bundle of His

32

What does Bundle of His separate into?

LBB and RBB (L is bigger and depolarizes first)

33

largest diameter fibers in the heart

Purkinje fibers

34

Which heart structure is assoc. with longest plateau phase?

Purkinje fibers

35

"tertiary" pacemaker

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

36

What does papillary muscle contraction prevent?

eversion of AV valves into the atria during ventricular contraction

37

Does RV or LV depolarize first?

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

38

Myocardial cells are refractory when they are contracted or relaxed?

Relaxed

39

Myocardial depolarization causes muscle to _____ and then _______

contract then relax

40

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

Timing of Ca++ conductance

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