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93 Cards in this Set

  • Front
  • Back
Cardiac cells have multiphasic action potentials; e.g., _______ myocardial cells:
ventricular
Phase 0:
rapid upstroke
Phase 1
early partial repolarization
Phase 2
plateau; prolonged depolarization
Phase 3
final repolarization
Phase 4
interval between depolarizations (resting potential in cells which do not spontaneously depolarize)
There are two major types of action potentials the ________ & ________
fast response & slow response
in “Fast response” APs the resting potential is approx. ______
- 90 mV
in “Fast response” APs Phase 0 is ________
steep (fast).
“Fast response” APs occurrence in ________, ______, & ________.
Normal ventricular and atrial myocardial cells.
& Normal Purkinje fibers.
In "Slow response” cells the
Resting potential is approx. ________
-60 mV.
In "Slow response” cells Phase __ is slower; phase __ is absent & the plateau is less distinct, not as flat.
0
1
In "Slow response” cells amplitude is _________
diminished
In "Slow response” cells ccurr in normal ___ node and ___ node cells & other myocardial cells under abnormal conditions.
SA
AV
Resting potential (in cardiac cells which do not spontaneously depolarize)is largely determined by distribution of ____ and ____ ions across the sarcolemma, and the permeabilities to these ions
K+
Na+
Vm =
membrane potential
* don't memorize
-61.5 log [K+]i+[Na/K] [Na+]i
_____________________________
[K+]o+[Na/K] [Na+]o

Pk=permeability of K
Pna=permeability of Na
Since the ratio (PNa/PK) is only about 0.01 during phase 4, the equation becomes essentially the Nernst equation for the potassium equilibrium potential (EK).

Vm~Ek=
-61.5 log [K+]i/[K+]o
PNa is not zero, so Vm will always be a bit ________ than EK.
less negative
Since PK is relatively high, resting potential is very dependent upon ____.
[K+]o
Progressive increases in ____ result in progressive depolarization of resting cardiac cell membranes toward 0 mV.
[K+]o
Abnormal plasma _____ levels can have clinically significant effects on myocardial resting potentials.
[K+]
Since PNa is very low, _______ potential is relatively independent of [Na+]o.
resting
The normal concentration gradients would gradually decrease if the slight Na+ ______ and slight K+ _____ were uncompensated.
influx
efflux
Na+ and K+ transport is mediated by a membrane-bound, Na+, K+-activated _____.
ATPase
The pump is electrogenic: the ratio of Na+ to K+ pumped is about _:_ .
3:2
Inhibition of the pump (e.g., by cardiac glycosides) results in partial _______ of the sarcolemma.
depolarization
_________ is opening of “fast” channels for Na+ and increase in gNa (Na+ conductance).
Phase 0
Opening of fast channels is __________: the degree of opening increases as membrane potential approaches threshold voltage.
voltage dependent
When resting potential is normal, fast channels open rapidly and completely after _______. Action potential is large and rapidly rising.
stimulus
Partial depolarization _______ some fast channels. Action potential is smaller and more slowly rising.
inactivates
______ depolarization to threshold produces normal action potential whereas, ______ depolarization to threshold inactivates some fast channels. Action potential is smaller and slower rising, similar to the result when the resting potential becomes less negative.
Rapid
slow
Since depolarization is due to Na+ influx, [Na+]o affects the ______ of the action potential.
amplitude
[Na+]o does not affect the ______ potential significantly, due to low membrane permeability to Na+ in the resting cell.
resting
The recovery of the ability of fast channels to open is delayed until ______ of the fast response.
phase 3
In order for fast channels to open, Vm must approach ____(recovery is voltage dependent).
-90 mV
Full recovery requires several msec after return to resting potential (recovery is _____ dependent).
time
Fast channels begin to close as Vm approaches _____ during phase 0.
0mV
______ in gK begins near the end of Phase 0 (K+ current (IK1) diminishes, but doesn’t stop or reverse).
Decrease
______ in gCa begins during the upstroke (ICa).
Increase
______ is initiated when the closure of fast channels is complete.
Phase 1
During phase I, a transient, small efflux of __ ions contributes (Ito). Small influx of ___ ions may contribute.
K+
Cl-
"Slow” channels carry primarily ___ ions.
Ca2+
"Slow” Ca2+ channels (L-type, ICa) ____ to open during Phase 0.
begin
Calcium conductance, gCa increases as a result of their opening, resulting in a slow ______ current.
inward
Entering Ca2+ triggers release of more Ca2+ by _________.
sarcoplasmic reticulum
Slow channel opening is not affected by less _____ resting potentials as
much as are fast sodium channels.
negative
Ca2+ influx is increased by ___________.
catecholamines
Certain ______ block slow Ca2+ channels (e.g., verapamil, nifedipine).
drugs
In phase 2, ________ is lower than Phase 4, but not zero.
K+ conductance (gK)
in phase 2, a small_____ K+ current balances the Ca2+ _______ , prolonging the plateau at about 0 mV.
outward (efflux)
influx
During phase 3, gK increases, allowing _______ to increase (repolarizing effect, IKr).
K+ efflux
During phase 3, ______ channels close, and gCa decreases; Ca2+ influx is shut off.
Slow
During phase 3, Recovery of ability of ____ channels to open occurs (see above).
fast
In cells with slow responses, fast channels are probably always ______ .
inactivated
Fast responses in fast response cells can be ______ to slow responses by pharmacologically blocking the fast channels (e.g., with tetrodotoxin).
converted
_____ does not affect the amplitude of a slow response action potential.
[Na+]o
Depolarization of phase 0 of a slow response is due to influx of ___ ions through slow _____channels
Ca2+
Ca2+
Depolarization of phase 0 of a slow response cannot be blocked by tetrodotoxin, but can be blocked by ____ channel blockers.
Ca2+
During depolarization of phase 0 of a slow response, Action potential amplitude is directly dependent upon ____.
[Ca2+]o
Conduction velocity is directly proportional to __________and __________
action potential amplitude
rate of depolarization.
Conduction velocity decreases when ________ becomes less negative (via the effects of resting potential on action potential amplitude and rate of depolarization).
resting potential
Since slow response action potentials are smaller and slower in depolarization rate, they are conducted slower than fast responses. Slow responses are also more easily _______.
blocked
_______ slow responses/unit time can be conducted compared to fast responses.
Fewer
Influences which change cardiac conduction velocity are called ________ influences.
dromotropic
During the __________vno propagated depolarizations are possible
effective refractory period (ERP)
Effective refractory period (ERP)lasts from _______ to _______
From beginning of phase 0 to middle of phase 3 (Vm ~ -50mV).
By the end of ERP, some ____ channels can reopen.
fast
During the _________ strong stimuli can result in propagated action potentials.
relative refractory period (RRP)
Duringt the relative refractory period (RRP),the number of fast channels able to reopen depends upon ___
Vm (voltage membrane)
During slow response, ERP may extend beyond phase ___.
3
During slow response, RRP extends into phase ___
4
Slow response, significantly prolongs recovery of ______.
excitability
During slow response, conduction velocity is proportional to level of _________.
excitability
Refractory period duration (and action potential duration) decrease as heart rate ______ (interval-duration relationship).
increases
Pacemaker Cells of the SA Node have several characteristics in common with the slow response:
1. Resting potential is less ________ than ventricular cells.
2. Phase 0 is relatively ____ .
3. A distinct ______is not seen.
4. Phase 3 is relatively ___.
5. Depolarization of phase 0 appears to be due to ___ influx
negative
slow
plateau
slow
Ca2+
Spontaneous phase 4 depolarization is called the ________ .
pacemaker potential
_____________ results from unstable membrane ionic permeabilities and conductances.
Spontaneous phase 4 depolarization
a gradual increase in slow ___ influx begins during phase 3.
Na+
during spontaneous phase 4 depolarization, A slow ____ of Ca2+ begins in phase 4 (T-type channels and possibly a contribution by L-type channels also, ICa).
influx
during spontaneous phase 4 depolarization, ______ of K+ (IK1) gradually decreases throughout phase 4.
efflux
When pacemaker potential reaches the threshold voltage, an action potential occurs. Phase 0 results mainly from ____ channels.
Ca++
Rate of pacemaker potential depolarization and action potential amplitude are decreased by:
a. Decreased external ___ concentration.
b. ____ channel blocking agents.
Ca2+
Ca2+
Mechanisms which can change heart rate by changing SA node depolarization rate:
1. Change the rate of phase __ depolarization
2. Change the ______ voltage.
3. Change the _______ of membrane potential at the end of phase 3
4

threshold

negativity
Influences which change heart rate are called ___________ .
chronotropic influences
Major Types of Ion Channels in Cardiac Cell Membranes:
(give type & role)

Fast Na+ (INa)
Voltage-gated

Opening results in Phase 0 of fast response cells.
Slow Na+ (If)
Voltage-gated &
Ligand-gated

Contribute to Phase 4 of pacemaker cells.
L-type Ca++ (ICa)
Voltage-gated

Long-lasting, slow inward current during Phase 2 of fast response cells and Phases 4 and 0 of slow response and pacemaker cells.
T-type Ca++ (ICa)
Voltage-gated

Channel opening is transient. Contribute to Phase 4 of pacemaker cells.
Inward rectifier K+ (IK1)
Voltage-gated

Open channels are major contributors to Phase 4 potential. Depolarizations close them. Contribute to Phases 2 & 3 of fast response cells. Slow closure contributes to Phase 4 depolarization of pacemaker cells.
Transient outward K+ (Ito)
Voltage-gated

Contribute to Phase 1, of fast response cells., also Phases 2 & 3 to some extent.
Delayed rectifier K+ (IKr)
Voltage-gated

Opening results in Phase 3 repolarization. Also contribute to lesser extent to Phases 2 & 4.
ATP-sensitive K+ (IK, ATP)
Ligand-gated

ATP inhibits opening. Reduced ATP results in greater opening, e.g., during hypoxia.
ACh-activated K+ (IK, ACh)
Ligand-gated

ACh opens channel via Gi protein. Occurs with increased vagal activation