Cardiac Electrophysiology Flashcards
What channels are responsible for phase 0 of the cardiac action potential?
v.g. Na+ channels
What channels are responsible for phase 1?
1) K+ (i_to)
2) Cl-
What channels are responsible for phase 2?
1) L-type Ca++ (i_Ca)
2) K+ (i_to)
3) activation of K+ (i_kr)
What channels are responsible for phase 3?
1) K+ (i_Kr)
2) K+ (i_K1)
What channels are responsible for phase 4?
K+ (i_K1)
What occurs in phase 0?
rapid upshoot
what occurs in phase 1?
early repolarization
what occurs in phase 2?
plateau (ERP)
what occurs in phase 3?
repolarization
what occurs in phase 4?
resting membrane potential
What are intercalated disks?
irregular, transverse thickenings of the sarcolemma between muscle cells
What structures make up an intercalated disk?
desmosomes
gap junctions
what is a functional syncytium vs. a true syncytium?
functional syncytium: group of cells that WORK like a long, multi-nucleated cell but is not actually one
what is a desmosome?
spot weld
What are K+ i_to channels responsible for?
phase 1, but active until after phase 2
What are K+ i_Kr channels responsible for?
phase 3, with slow activation in phase 2
What are K+ i_K1 channels responsible for?
Phases 3 & 4
What are K+ i_KAch1 channels responsible for?
parasympathetic nervous system input
how do K+ i_Kach1 channels work?
activated by ACh & G-protein coupled
Describe the gating of cardiac sodium channels?
1) m-activation: -65mV, 0.1msec
2) h-activation: +30mV, 1msec; remains closed until partial repolarization in phase 3
What are the types of cardiac calcium channels?
1) L-type
2) T-type
Describe the L-type cardiac calcium channel’s contribution to the cardiac action potential.
1) activated at -10mV
2) responsible for Ca influx during phase 2
3) slow inactivation
L = long lasting, predominant in heart (CCB targets)
Describe a t-type calcium channel.
t = transient
less abundant in heart
Describe contraction of a cardiac myocyte?
1) sarcolemma depolarized, Ca++ opens @ -10mV
2) cytosol Ca++ binds SR receptor
3) Calcium-induced Calcium-release
4) SR releases calcium –> sarcomere –> contraction
Describe relaxation of a cardiac myocyte
1) Ca++ sequestered in SR (SERCA ATPase)
2) Ca++ sequestered in ECF
- - Na/Ca exchanger uses gradient from Na/K ATPase
Why does extracellular [Ca] affect contraction?
SR does not store enough Ca for strong contraction
T-tubules are large to allow Ca entrance
How does excess K+ affect cardiac function?
1) more positive RMP
2) more excitable myocyte with slower depol, faster repol
3) less Ca++ influx = decreased contractility
What is the target of cardiac glycosides?
Na/K ATPase
What is a cardiac glycoside?
Digitalis (digoxin)
How do cardiac glycosides work?
Na/K ATPase inhibited
- - accumulation of intracellular Na+ - - Na/Ca++ exchanger inhibitied - - increased concentration of intracellular Ca++ - - increased contractility, decreased HR
Where is the SA node located?
in RA below superior vena cava
How does a pacemaker function?
Leaky Na+ and Ca++ channels –> spontaneous phase 4 depol
What is the RMP of a pacemaker cell?
-55 to -60 mV
What is the threshold of a pacemaker cell?
-40mV
RMP of cardiac myocyte?
-75 mV
Where is the AV node located?
base of right atrium
How is the AV nodal delay achieved?
1) smaller fibers (smaller d, slower contraction)
2) fewer gap jxs
3) more negative RMP
4) slower Ca++ channels
Describe the AV bundle.
large, fast fibers in myocardial septum continuous with purkinje fibers
Describe the relative conduction velocity of the AV bundle compared to myocyte & AV node fibers.
6x myocyte conduction
150x AV Node fiber conduction
What are the intrinsic pacemaking cells of the heart?
all cells!
What is the normal pacemaking rate of the SA node?
100/min
What is the normal pacemaking rate of the AV node?
40-60/min
What is the normal pacemaking rate of the purkinje fibers?
15-40/min
What is the normal pacemaking rate of a ventricular myocyte?
5-10/min
Define ectopic pacemaker.
another part of the heart with rhythmic discharge rate > that of SA Node
Consequences of ectopic pacemaker?
circus rhythms
Where are the usual places for ectopic pacemakers?
AV node or Purkinje
Define chronotropism.
change in HR
Define dromotropism
Change in AV Nodal conduction
What is a positive dromotropism?
decreased AV delay
What is a positive chronotropism?
tachycardia
What does the PNS innervate to affect HR?
vagus nerve innervates SA & AV node
How does the PNS affect HR?
ACh opens K+ channels for hyperpolarization & longer time till threshold
What does the SNS innervate to affect HR?
SA for HR,
ventricles for contractility
How does the SNS affect HR?
NE opens Ca++ channels for depolarization
What is the thin filament?
actin
What is the thick filament?
myosin
What does TnC do?
binds Ca++ to induce conformational change
What does TnT do?
binds tropomyosin
What does TnI do?
binds actin
What is tropomyosin?
the filament protein regulating exposure of actin’s active site
