Cardiac Muscle-Excitation and Signaling

Question 1

Why is cardiac contraction classified as myogenic?

Answer 1

it is not initiated by nervous input

Question 2

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

Answer 2

all myocardial cells are electrically connected to each other

Question 3

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

Answer 3

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

Question 4

What mediates phase 0 of the cardiac action potential?

Answer 4

sodium channels (fast) –> sharp upstroke

Question 5

What mediates phase 1 of the cardiac action potential?

Answer 5

efflux via transient K+ channels

Question 6

What mediates phase 2 of the cardiac action potential?

Answer 6

influx of Ca2+ balanced by efflux of K+

Question 7

What mediates phase 3 of the cardiac action potential?

Answer 7

K+ channels

Question 8

What is the resting potential of cardiac myocytes?

Answer 8

approx -80mV (close to Ek)

Question 9

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

Answer 9

no real resting potential but have diastolic depolarization causing pacemaker current

Question 10

What is the myocyte membrane most permeable to at rest?

Answer 10

K+

Question 11

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

Answer 11

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

Question 12

Describe the structure of the potassium channel.

Answer 12

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)

Question 13

What is the selectivity filter for the K+ channel?

Answer 13

GYG sequence –> P loop

Question 14

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

Answer 14

almost all closed

Question 15

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

Answer 15

channels open

Question 16

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

Answer 16

inactivated by ball/chain

Question 17

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

Answer 17

inactivation occurs slower than channels are opened

Question 18

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

Answer 18

tetrodotoxin

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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)

Question 21

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

Answer 21

amiloride

Question 22

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

Answer 22

D-600, Niphedipine

Question 23

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

Answer 23

they inactivate very slowly

Question 24

What is the role of inward rectifier K+ current?

Answer 24

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

Question 25

What is the role of delayed rectifier K+ current?

Answer 25

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

Question 26

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

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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

Question 29

Na or Ca channels? ventricular/atrial myocardium

Answer 29

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

Question 30

Na or Ca channels? SA/AV nodes

Answer 30

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

Question 31

Na or Ca channels? purkinje fibers/bundle of his

Answer 31

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

Question 32

What is the electrocardiographic representation of depolarization of the ventricles?

Answer 32

QRS

Question 33

What is the electrocardiographic representation of repolarization of the ventricles?

Answer 33

t wave

Question 34

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

Answer 34

time lag between Q and T aka QT

Question 35

What can contribute to long QT syndrome?

Answer 35

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

Question 36

What are the dangerous consequences of long QT syndrome?

Answer 36

secondary after depolarizations leading to possible cardiac arrhythmia

Question 37

How do the APs of ventricular and atrial myocardium differ?

Answer 37

atrial myocardium has a faster/steeper plateau

Question 38

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

Answer 38

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

Question 39

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

Answer 39

AV node is slower --> delay allows for ventricular filling

Question 40

Are purkinje fibers nerves or myocytes?

Answer 40

myocytes with mild contractile ability

Question 41

What current contributes to the diastolic depolarization of pacemaker potentials?

Answer 41

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

Question 42

What is the directionality of If?

Answer 42

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

Question 43

How many connexins form a connexon?

Answer 43

6

Question 44

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

Answer 44

potassium

Question 45

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

Answer 45

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