Lecture 03: Pacemaker Cells and the Normal Sequence of Cardiac Excitation (Hayward) Flashcards
Where are pacemaker cells located?
AV and SA nodes
Classification of myocardial AP as either fast or slow refers to:
rate of initital membrane depolarization
“funny” Na+ channel
Used to depolarize pacemaker cells by SLOW influx of Na+. Activated by hyperpolarization (at end of phase 3) instead of depolarization.
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
Pacemaker current
slow influx of Na+ through funny Na+ channel to activate depolarization of pacemaker cells
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
Characteristics of Phase 0 for SLOW type APs
1) Ca++ influx
2) Inactivation of fast and funny Na+ channels
3) Decreased Na+ conductance
Which phases are absent in SLOW type APs?
Phase 1 and 2
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
Maximum diastolic potential (MDP)
maximum negative potential that is reached (during phase 4). Alters pacemaker action potential frequency
What does MDP primarily depend on?
K+ and Na+ conductance
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.
conduction velocity
speed at which APs propagate from one area to another
Conduction velocity depends greatly on
diameter of muscle fiber involved. Larger diameter –> faster conduction velocity
Which conducts faster, Purkinje or AV node fibers?
Purkinje fibers
Where does excitation and depolarization originate
SA node in RA
Bachmann’s bundle/interatrial bundle
conducts impulse from RA to LA so both atria contract almost simultaneously
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
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
What is the “latent pacemaker”?
the AV node. Will take over regulating HR if SA node is dysfunctional, but at a slower HR
Which has faster rate of RM depolarization? AV or SA node cells?
SA node cells
Where does signal travel after AV node? Slow or fast speed?
Bundle of His. Relatively slow, resulting in conduction delay
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
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
Purkinje fibers
large diameter/fast conducting cells that distribute depolarization throughout heart.
Mechanism of cell to cell conduction
low resistance gap junctions
Why do papillary and interventricular septum depolarize first in cell to cell depolarization
Allows for apical rigidity and anchors AV valves during ventricular systole
T or F: all cardiac cells are electrically coupled
T
Fast type AP is mostly dependent on extracellular _____, while Slow type AP is mostly dependent on extracellular ____
Fast –> Na+
Slow –> Ca++
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
What structure denotes the beginning of the specialized conduction system in the ventricles?
Bundle of His
What does Bundle of His separate into?
LBB and RBB (L is bigger and depolarizes first)
largest diameter fibers in the heart
Purkinje fibers
Which heart structure is assoc. with longest plateau phase?
Purkinje fibers
“tertiary” pacemaker
His-Purkinje fiber system, but would be very slow HR (unsustainable for life)
What does papillary muscle contraction prevent?
eversion of AV valves into the atria during ventricular contraction
Does RV or LV depolarize first?
RV because it is thinner walled and easier for signal to pass through it
Myocardial cells are refractory when they are contracted or relaxed?
Relaxed
Myocardial depolarization causes muscle to _____ and then _______
contract then relax
In slow type AP, shape of AP relates to what?
Timing of Ca++ conductance
