Exam #2: Cardiac Excitation-Contraction Coupling

Question 1

What is the sequence of electrical events in the heart?

Answer 1

SA node → AV node → Bundle of His → Bundle branches (L and R) → Purkinje fibers

Question 2

What is the purpose of the AV nodal delay?

Answer 2

Ensures atrial contraction precedes ventricular contraction, allowing for complete ventricular filling between beats

Question 3

What is the only step of the pathway that conducts from the atria to ventricles?

Answer 3

AV node → Bundle of His

Question 4

What type of cells use a fast action potential?

Answer 4

Conductile cells and Contractile cells

Question 5

What type of cells use a slow action potential?

Answer 5

Pacemaker cells (AV and SA node cells)

Question 6

What type of ion drives fast action potentials?

Answer 6

Na+

Question 7

What type of ion drives slow action potentials?

Answer 7

Ca2+

Question 8

What ion/channel does the ANS act on?

Answer 8

Na+ funny channels

Question 9

With fast depolarization, what event is responsible for the rapid depolarization (upstroke)? What type of channel?

Answer 9

Na+ rapidly flowing into cell (ordinary voltage-gated channels)

Question 10

With fast depolarization, what event is responsible for the plateau phase? What type of channel?

Answer 10

Ca2+ slowly flowing into cell (L-type channels)

Question 11

With fast depolarization, what event is responsible for the rapid repolarization? What type of channel?

Answer 11

K+ rapidly flowing out of cell (ordinary voltage-gated channels)

Question 12

With fast depolarization, what event is responsible for the return to resting membrane potential? What type of channel?

Answer 12

K+ slowly flowing out of cell (leaky channels)

Question 13

What is the purpose of the plateau phase?

Answer 13

It increases the duration of the action potential and allows time for the contraction to occur (time for plateau phase = time for full contraction then relaxation)
- Avoid cardiac tetany

Question 14

Why is the RMP for cardiac muscle -90 mV instead of the -70 mV?

Answer 14

K+ permeability is even higher in cardiac cells (MANY:1 when comparing K+:Na+)

Question 15

With slow depolarization, what event is responsible for the slow depolarization? What type of channel?

Answer 15

• Na+ slowly flowing into cell (funny channels)

- Ca2+ slowly flowing into cell (L-type channels)

Question 16

What is another name for the slow depolarization phase in slow APs?

Answer 16

Pacemaker potential because this phase sets the HR

Question 17

With slow depolarization, what event is responsible for the rapid depolarization?

Answer 17

Ca2+ flowing into cell

Question 18

With slow depolarization, what event is responsible for the rapid repolarization?

Answer 18

K+ flowing out of cell

Question 19

What type of muscle is cardiac muscle similar to in terms of excitation-contraction coupling? How is it different?

Answer 19

Cardiac muscle is very similar to skeletal muscle EXCEPT:

- Cardiac cells also receive Ca2+ from the ECF via L-type channels

Question 20

What are the three ways by which intracellular Ca2+ is decreased?

Answer 20

• SERCA (most significant)
• Na+/Ca2+ exchanger - uses Na+/K+ ATPase
• Sarcolemmal Ca2+ ATPase

Question 21

What is the relationship between tension/contraction, sarcomere length and volume?

Answer 21

They are all directly related:

- If sarcomere length increases, tension/contraction increases and volume will also increase

Question 22

How does the SNS affect contractility/inotropy?

Answer 22

Positive contractility/inotropy effect

Question 23

How does the PNS affect contractility/inotropy?

Answer 23

Negative contractility/inotropy effect

Question 24

What type of receptors are utilized by the SNS with relation to contractility/inotropy? What type of protein are they coupled with and what is their action?

Answer 24

SNS = B1 receptors

- Coupled to Gs proteins and AC, which activates cAMP and PKA

Question 25

Which two types of proteins are phosphorylated by cAMP during SNS stimulation? What is the action of each in increasing contractility?

Answer 25

• Sarcolemmal Ca2+ channel: increases Ca2+ release from SR

- Phospholamban: increases Ca2+ uptake and storage into SR (pumps it back into SR)

Question 26

What is the general affect that phospholamban activity has on heart rate

Answer 26

Contraction is briefer aka faster relaxation (increased HR)

Question 27

What type of receptors are utilized by the PNS with relation to contractility/inotropy? What type of protein are they coupled with? WHAT part of the heart do they specifically act on?

Answer 27

PNS = muscarinic receptors
- Coupled to Gi proteins

Decrease contractility in ATRIA

Question 28

With PNS and muscarinic receptors, when ACh is released, what are the two actions it takes to decrease contractility?

OVERALL, what is the general affect of these two actions combined?

Answer 28

• ACh decreases flow of Ca2+ into the cell during plateau phase (less Ca2+ available for contraction)
• ACh increases K+ current which shortens the duration of the AP and indirectly (further) decreases flow of Ca2+ into the cell during plateau phase

OVERALL, ACh decreases the amount of Ca2+ entering atrial cells during an AP AND decreases the amount of Ca2+ being released from the SR

Question 29

What is the mechanism behind Na+-K+ ATPase blockers? How does this happen, and what affect does this have?

Answer 29

Inhibit Na+/K+ ATPase

1. No K+ flows in and no Na+ flows out through Na+/K+ ATPase
2. Na+ accumulates inside cell
3. Na+ is pumped out of cell via Na+/Ca2+ exchanger, so Ca2+ is pumped into the cell
4. Increased Ca2+ inside cell = increased contractility

Question 30

What is the mechanism behind calcium-channel blockers (3)?

Answer 30

• Decreased contractility
• Decreased HR
• Decreased conduction velocity

Question 31

What is the mechanism behind B-Agonists blockers?

Answer 31

1. Increase cAMP production
2. Increase protein phosphorylation
3. Increase Ca2+ influx = Increased contractility