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Cardiology > Cardiac Muscle-Excitation and Signaling > Flashcards

Cardiac Muscle-Excitation and Signaling Flashcards

1
Q

Why is cardiac contraction classified as myogenic?

A

it is not initiated by nervous input

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2
Q

What do we mean when we say the heart is a functional syncytium?

A

all myocardial cells are electrically connected to each other

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3
Q

T/F the heart can increase strength of contraction by recruiting more myocytes

A

F –> all or nothing tissue response (vs. skeletal muscle)

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4
Q

What mediates phase 0 of the cardiac action potential?

A

sodium channels (fast) –> sharp upstroke

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5
Q

What mediates phase 1 of the cardiac action potential?

A

efflux via transient K+ channels

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6
Q

What mediates phase 2 of the cardiac action potential?

A

influx of Ca2+ balanced by efflux of K+

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7
Q

What mediates phase 3 of the cardiac action potential?

A

K+ channels

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8
Q

What is the resting potential of cardiac myocytes?

A

approx -80mV (close to Ek)

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9
Q

What is the resting potential of cardiac SA/AV node?

A

no real resting potential but have diastolic depolarization causing pacemaker current

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10
Q

What is the myocyte membrane most permeable to at rest?

A

K+

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11
Q

What happens to Vr in a cardiac myocyte in a high [K+] solution?

A

higher resting potential AKA depolarized –> results in more Na+ channels being inactivated –> slower upstroke/phase 0

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12
Q

Describe the structure of the potassium channel.

A

four identical peptides with 6 transmembrane spanning helices each –> 5/6/P form the ion channel, 4 is the voltage sensor, P is the selectivity filter (GYG sequence)

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13
Q

What is the selectivity filter for the K+ channel?

A

GYG sequence –> P loop

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14
Q

What is the state of cardiac myocyte Na+ channels at rest?

A

almost all closed

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15
Q

What is the state of cardiac myocyte Na+ channels between -60 and -40 mV?

A

channels open

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16
Q

What is the state of cardiac myocyte Na+ channels after being open during a long pulse/sustained depolarization?

A

inactivated by ball/chain

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17
Q

If the window of voltage to have Na+ channels open but not inactivated is so small, how do we get physiologic action potentials?

A

inactivation occurs slower than channels are opened

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18
Q

The properties of the Na+ channel is blocked by what toxin?

A

tetrodotoxin

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19
Q

What is the role of T-type Ca2+ channels?

A

threshold/transient type calcium channels activate near the resting potential –> help with early depolarization (esp. in AV/SA nodes)

20
Q

What is the role of L-type Ca2+ channels?

A

responsible for the plateau of phase 2 –> balance the efflux of K+ allowing for sustained depolarization –> activation/inactivation potentials are shifted to higher potentials (between -40 and 20 mV)

21
Q

The properties of the T-type Ca2+ channel is blocked by what toxin?

A

amiloride

22
Q

The properties of the L-type Ca2+ channel is blocked by what toxin?

A

D-600, Niphedipine

23
Q

How do L-type Ca2+ channels extend the life of phase 2 plateau?

A

they inactivate very slowly

24
Q

What is the role of inward rectifier K+ current?

A

responsible for resting potential (high conductivty at low potential: appears steeply sloped on I vs. V graph)–> switches off at -30mV aka when the heart is depolarized

*has a variable resistance hence the name rectifier

25
Q

What is the role of delayed rectifier K+ current?

A

turns on very slowly –> responsible for repolarization after action potential

26
Q

What are the effects that lead to repolarization of a myocyte after action potential?

A

slow inactivation of L-type calcium channels during the slow opening of delayed rectifier potassium channels followed by restoration of the inward rectifier potassium channel

27
Q

What contributes to the slight repolarization during phase 1 of the cardiac action potential?

A

the rapid inactivation of T-type calcium channels during the slow activation of L-type calcium channels

28
Q

T/F you need both Na and Ca channels to have cardiac action potentials

A

F –> either can start an AP but the shape will be different (e.g. without calcium channels there is no plateau)

29
Q

Na or Ca channels? ventricular/atrial myocardium

A

Na current for conduction, CA current for plateau and activating contraction

30
Q

Na or Ca channels? SA/AV nodes

A

Ca current for upstroke and propagation (Na current not active b/c resting potential is high enough to inactivate them)

31
Q

Na or Ca channels? purkinje fibers/bundle of his

A

Na current for conduction, Ca current for plateau and slow responses when Na current is weak

32
Q

What is the electrocardiographic representation of depolarization of the ventricles?

A

QRS

33
Q

What is the electrocardiographic representation of repolarization of the ventricles?

A

t wave

34
Q

What is the electrocardiographic representation relates to the length of the ventricular cardiac action potential?

A

time lag between Q and T aka QT

35
Q

What can contribute to long QT syndrome?

A

genetic or drug induced: blockage of delayed rectifier K+ channels or excess Na channels

36
Q

What are the dangerous consequences of long QT syndrome?

A

secondary after depolarizations leading to possible cardiac arrhythmia

37
Q

How do the APs of ventricular and atrial myocardium differ?

A

atrial myocardium has a faster/steeper plateau

38
Q

How do the APs of SA node and bundle fibers differ?

A

SA node has no resting potential but rather has diastolic depolarization, bundle fibers have a prominent phase 1 repolarization and a negative plateau

39
Q

How do the APs of SA node and AV node differ?

A

AV node is slower –> delay allows for ventricular filling

40
Q

Are purkinje fibers nerves or myocytes?

A

myocytes with mild contractile ability

41
Q

What current contributes to the diastolic depolarization of pacemaker potentials?

A

funny current (If)serves as the bridge between inactivating delayed rectifier K+ channels (Ik) (inactivation leads to an initial depolarization) and the transient calcium current (Ica) that controls the pacemaker upstroke

42
Q

What is the directionality of If?

A

inward current (potassium and sodium in some funky combination) depolarizes the cell

43
Q

How many connexins form a connexon?

A

6

44
Q

What ion is the major source of current flow between cardiac myocytes?

A

potassium

45
Q

What ions are the major source of return current flow as action potentials propagate through cardiac myocytes?

A

sodium and chloride in the extracellular space –> contributes to voltage measured by EKG

Cardiology (41 decks)

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