Cardiac Excitation/Contraction Coupling

Question 1

What is the resting membrane potential in phase 4?

Answer 1

-85 mV

Question 2

What sets the resting membrane potential in phase 4?

Answer 2

3 potassium channels (K1, K-ATP, K-Ach)

Question 3

What are the statuses of the sodium gates in phase 4?

Answer 3

inactivation gates are open, voltage gates are closed

Question 4

What occurs in phase 0?

Answer 4

depolarization

Question 5

At what membrane potential do sodium voltage gates open?

Answer 5

-40 mV

Question 6

What causes the inactivation gates to close?

Answer 6

positive membrane potential

Question 7

What channels are closed during depolarizaiton?

Answer 7

potassium

Question 8

What occurs in phase 1?

Answer 8

slight repolarization due to the transient outward potassium channel

Question 9

What occurs in phase 2?

Answer 9

plateau

Question 10

What channels function during the plateau?

Answer 10

calcium channels open and there is an inward current to balance the outward K+ currents of (I-Kur, I-Kr, I-Ks)

Question 11

What happens in phase 3?

Answer 11

repolarization

Question 12

What gates are reactivated in phase 3?

Answer 12

potassium channels (K-ATP, K-Ach)

Question 13

At what voltage do sodium channels reset?

Answer 13

around -85 mV

Question 14

What is the lowest membrane potential that SA node cells reach?

Answer 14

around -65 mV

Question 15

Do SA node cells have fast Na channel activity?

Answer 15

NO, because their membrane potential never gets low enough to “reset” the sodium gates

Question 16

What makes the resting membrane potential of SA node cells unstable?

Answer 16

“funny” Na current

Question 17

What activates the Na “funny current”?

Answer 17

negative membrane potential

Question 18

Why is the slope of the AP in SA node cells less steep?

Answer 18

because L-type calcium channels are slower in conducting current than fast Na channels

Question 19

What parts of the heart have the highest conduction rate?

Answer 19

Bundle of His and Purkinje cells

Question 20

What parts of the heart have the slowest conduction rates?

Answer 20

SA node and AV node

Question 21

What part of the heart has the highest spontaneous discharge rate?

Answer 21

SA node

Question 22

What transmits sympathetic efferent nerves to heart?

Answer 22

sympathetic cardiac nerve

Question 23

What do sympathetic efferent nerves influence?

Answer 23

SA node, AV node and VENTRICULAR CELLS

Question 24

What are the affects of the sympathetic efferent nerves on the heart?

Answer 24

Increase HR
Increase conduction velocity in the AV node
Decreases threshold for calcium channels
Increases excitability of latent pacemakers

Question 25

What transmits parasympathetic efferent nerves?

Answer 25

vagus nerve

Question 26

What do parasympathetic efferent nerves influence?

Answer 26

SA node and AV node only

Question 27

What are the affects of the parasympathetic efferent nerves on the heart?

Answer 27

Decreases HR, decreases conduction velocity, decreases excitability of latent pacemakers

Question 28

Name 3 ways you can decrease HR by altering parameters in the SA node?

Answer 28

1) decrease funny current (so you don’t get depolarization started)
2) increase potassium channel activity (repolarization lasts too long)
3) raise threshold (less negative) for calcium channels to open by decreasing cAMP levels

Question 29

What is the resting HR? What is this without sympathetic and parasympathetic tone?

Answer 29

60-70

without tone it is around 100

Question 30

What is the critical activator of cardiac muscle contraction?

Answer 30

calcium

Question 31

When are calcium levels high enough for contraction?

Answer 31

In phase 2 (plateau) AND Ca must be released from the SR

Question 32

What regulates the strength of muscle contraction?

Answer 32

1) intracellular calcium levels

2) length of cardiac fibers (this determines their sensitivity to calcium)

Question 33

How do catecholamines affect calcium signaling in myocytes?

Answer 33

they are involved in the GPCR pathway that culminates in the phosphroylation of:

1) Troponin I
2) PLB
3) Ica Channel
4) Ryanadine receptor

Question 34

What is the result of phosphorylating troponin I?

Answer 34

decreases its affinity for calcium and releases it leading to cardiac relaxation (lusitrophy)

Question 35

What is the result of phosphorylating PLB?

Answer 35

decreases intracellular calcium quickly and packs it in the SR

Question 36

What is the result of phosphorylating the calcium channel?

Answer 36

increases calcium intake

Question 37

What is the result of phosphorylating the ryanadine receptor?

Answer 37

greater calcium release from the SR so greater cardiac contraction (inotrophy)

Question 38

What is another term for increasing HR?

Answer 38

chronotrophy

Question 39

Does acetylcholine have an affect on ventricular contraction?

Answer 39

NO- parasympathetic activity is only on the SA and AV node

Question 40

What is the equation for SV according to the Frank-Starling effect?

Answer 40

SV= EDV-ESV

Question 41

What does the Frank Starling effect postulate?

Answer 41

increased sarcomere length results in increased active tension

Question 42

What are the 2 ways that increased length corresponds to increased cardiac force?

Answer 42

1) Fast response- increased Ca sensitivity to myofilaments

2) Slow response- activation of stretch-activated calcium channels