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)
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1
Q

Where are pacemaker cells located?

A

AV and SA nodes

2
Q

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

A

rate of initital membrane depolarization

3
Q

“funny” Na+ channel

A

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

4
Q

Characteristics of Phase 4 for SLOW type APs

A

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
Q

Pacemaker current

A

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

6
Q

Do pacemaker cells have fast type Na+ channels?

A

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

7
Q

Characteristics of Phase 0 for SLOW type APs

A

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

8
Q

Which phases are absent in SLOW type APs?

A

Phase 1 and 2

9
Q

Characteristics of Phase 3 for SLOW type APs?

A

(slow repolarization):

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

10
Q

Maximum diastolic potential (MDP)

A

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

11
Q

What does MDP primarily depend on?

A

K+ and Na+ conductance

12
Q

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

A

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
Q

conduction velocity

A

speed at which APs propagate from one area to another

14
Q

Conduction velocity depends greatly on

A

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

15
Q

Which conducts faster, Purkinje or AV node fibers?

A

Purkinje fibers

16
Q

Where does excitation and depolarization originate

A

SA node in RA

17
Q

Bachmann’s bundle/interatrial bundle

A

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

18
Q

Do atrial or ventriclular cells have shorter APs? Why?

A

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
Q

Why must atrial depolarization pass through AV node?

A

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
Q

What is the “latent pacemaker”?

A

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

21
Q

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

A

SA node cells

22
Q

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

A

Bundle of His. Relatively slow, resulting in conduction delay

23
Q

What happens during conduction delay? (2 things)

A

1) Signal travels from AV node to Bundle of His

2) Atria finish contraction prior to depolarization of ventricles

24
Q

Normal sequence of depolarization

A

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
Q

Purkinje fibers

A

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

26
Q

Mechanism of cell to cell conduction

A

low resistance gap junctions

27
Q

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

A

Allows for apical rigidity and anchors AV valves during ventricular systole

28
Q

T or F: all cardiac cells are electrically coupled

A

T

29
Q

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

A

Fast –> Na+

Slow –> Ca++

30
Q

Why do AV nodal cells have longer refractory period?

A

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

31
Q

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

A

Bundle of His

32
Q

What does Bundle of His separate into?

A

LBB and RBB (L is bigger and depolarizes first)

33
Q

largest diameter fibers in the heart

A

Purkinje fibers

34
Q

Which heart structure is assoc. with longest plateau phase?

A

Purkinje fibers

35
Q

“tertiary” pacemaker

A

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

36
Q

What does papillary muscle contraction prevent?

A

eversion of AV valves into the atria during ventricular contraction

37
Q

Does RV or LV depolarize first?

A

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

38
Q

Myocardial cells are refractory when they are contracted or relaxed?

A

Relaxed

39
Q

Myocardial depolarization causes muscle to _____ and then _______

A

contract then relax

40
Q

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

A

Timing of Ca++ conductance

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