12/1/2014 Medical Physiology Cardiac Electrophysiology I Eric Olson

Question 1

Where are the SA and AV nodes located?

Answer 1

The right atrium

Question 2

Conducting cells are found in what areas of the heart?

Answer 2

SA node
atrial internodal tracts
AV node
bundle of His
Purkinje "system"

Question 3

T/F: Conducting cells can generate APs spontaneously.

Answer 3

True

Question 4

Conducting cells are specialized muscle cells that do not significantly contribute to the generation of force in the heart. How then does the heart contract?

Answer 4

Contractile cells.

Question 5

The AP that began at the SA node spreads to the _____ via the atrial internodal tracts.

Answer 5

right and left atria and the AV node

Question 6

What is the important effect of the AV node having a slow conduction velocity?

Answer 6

This ensures that the ventricles have sufficient time to fill with blood before they are activated and contracted.

Question 7

Where is the bundle of His?

Answer 7

Near the AV node, almost in the middle of the atria and ventricles.

Question 8

Once a AP reaches the bundle of His from the AV node, where does it propagate?

Answer 8

common bundle of His –> right and left bundle branches –> smaller bundles in Purkinje system that surround the ventricles –> ventricles.

Question 9

Conduction velocity is not the same in all myocardial tissue. It is slowest in the __1__ and fastest in the __2__.

Answer 9

1. AV node

2. Purkinje fibers

Question 10

T/F: Skeletal and cardiac muscle have similar AP arcs.

Answer 10

False. The skeletal muscle AP happens over 0-20 msec, and it monophasic (up, down, nothing fancy). The cardiac AP can be 300-400 msec and is triphasic (up, down an bit, up, arc down). See page 245 in notes.

Question 11

In Phase 0 of the ventricular AP, the upstroke, there is a transient increase in what ion?

Answer 11

Na+

Question 12

What is dV/dt?

Answer 12

Rate of rise of membrane potential in the upstroke of the ventricular AP.

Question 13

Phase 1 in the ventricular AP is known as:

Answer 13

initial repolarization

Question 14

The plateau in the ventricular AP curve is what phase?

Answer 14

2

Question 15

Repolarization in ventricular AP curve is what phase?

Answer 15

3

Question 16

Phase 4 in the ventricular AP curve is known as:

Answer 16

resting membrane potential or electrical diastole

Question 17

What ions are going where in Phase 1 of the ventricular AP curve?

Answer 17

Na+ gates close, no more Na+ going in;

Outward K+ current cause by large driving force of K+ ions.

Question 18

What ions are going where in Phase 2 of the ventricular AP curve?

Answer 18

Inward Ca2+ current;

Outward K+ current.

Question 19

What ions are going where in Phase 3 of the ventricular AP curve?

Answer 19

Decrease in inward Ca2+ current;
Increase in outward K+ current, followed by a decrease in outward K+ as membrane potential approaches K+ equilibrium potential.

Question 20

What is the resting membrane potential of cardiac muscle?

Answer 20

-85 mV

Question 21

What ions are going where in Phase 4 of the ventricular AP curve?

Answer 21

Na and Ca2+ in, K+ out, all in net equilibrium as cell is repolarized.

Question 22

The effective refractory period in myocardial muscle is?

Answer 22

Right after the absolute refractory period. Still very unlikely to generate an AP.

Question 23

T/F: All parts of the heart, ie SA node, common bundle, Purkinje fibers etc. have a characteristic AP curve.

Answer 23

True. They are all a bit different. See page 247 in notes.

Question 24

The duration of the AP in cardiac muscle determines the duration of what?

Answer 24

The refractory period. 1:1 ratio

Question 25

The AV node and what other tissue can become pacemakers in a pinch, and develop unstable resting membrane potentials to help the heart pump.

Answer 25

Purkinje fibers

Question 26

What is the only part of the heart whose upstroke is not characterized by an inward Na+ current?

Answer 26

SA node, has inward Ca2+ current instead.

Question 27

What tissue has the shortest myocardial AP, and what has the longest?

Answer 27

SA node and atrium both have 150 ms AP duration; | Purkinje fibers have 300 ms AP duration. (Ventricles have 250 ms duration).

Question 28

What is considered a normal heartbeat, corresponding with the normal range of SA node pacemaker firing?

Answer 28

60-100 bpm

Question 29

What is a typical AP of the SA node like?

Answer 29

Phase 4 at about -65mV (repolarized) rising, Phase 0 upstroke to 0 mV, Phase 3 falling steeply but no arc, back to Phase 4, from

Question 30

V-dep and L-type channels are most known for their involvement in what?

Answer 30

SA node calcium channels, open to start upstroke --> massive Ca2+ influx.

Question 31

What causes pacemaker activity in SA node?

Answer 31

Interaction between I-Ca (T-type channels), I-K and a special current, deemed I-f (funny current).

Question 32

I-f is also known as:

Answer 32

the pacemaker current.

Question 33

Why is I-f "funny?"

Answer 33

It activates in response to hyperpolarization, not depolarization. It has both an inward Na+ component and an outward K+ component.

Question 34

The effects of the autonomic (S and PS) nervous system on the heart are called:

Answer 34

chronotropic effects - S stimulation --> positive chronotropic effect; PS stimulation --> negative chronotropic effect. The SA node has *neuronal input*

Question 35

What part of the SA node AP changes with sympathetic stimulation?

Answer 35

Phase 4 rate is increased (meaning Phase 4 is shorter) leading to frequency of APs

Question 36

What part of the SA node AP changes with parasymmpathetic stimulation? As with the opposite of stimulation of the sympathetic nervous system, Phase 4 rate is decreased, but somthing else also happens to create the negative chronotropic effect. What is it?

Answer 36

PS activation also hyperpolarized the maximum diastolic potential to decrease the frequency of APs.

Question 37

Describe the vagal brake signaling pathway to reduce the heart rate.

Answer 37

Vagal activity (Cn X) --> ACh released at SA node --> ACh binding to and activation of M2 receptors --> Activation of Gi --> decrease in cAMP --> Phase 4 rate is slower

Question 38

In parasympathetic inhibition of the SA node, Phase 4 steepness is reduced by a number of factors. What are they?

Answer 38

1. Reducing inward I-f 2. Reducing inward I-CaT 3. Increasing outward I-KACh Also, threshold of I-CaL is at a more positive value.

Question 39

Describe the sympathetic stimulation pathway that leads to increased heart rate.

Answer 39

Norepi binds to B-1 adrenegic receptors (metabotropic) --> Gs activated --> AC activated --> cAMP increases --> pacemaker rate increases

Question 40

In sympathetic stimulation of the SA node, does Phase 4 get steeper or less steep?

Answer 40

Steeper, leading to faster rates of depolarization. | Steeper due to increasing inward I-f and increasing inward I-CaT; threshold of I-CaL at more negative value.

Question 41

Cardiac APs are propagated through gap junctions. In this way, the atria and ventricles are both _____ of cells.

Answer 41

Syncytia

Question 42

What proteins make up gap junctions

Answer 42

Connexins

Question 43

Like SA node cells, AV node cells have:

Answer 43

an instrinsic pacemaker activity

Question 44

T/F: Sympathetic and parasympathetic influence can act on the AV node as well as the SA node.

Answer 44

True

Question 45

The intrinsic firing rate of the AV node is:

Answer 45

40 bpm

Question 46

The intrinsic firing rate of the Purkinje fibers is:

Answer 46

< 20 bpm and irregular

Question 47

What is the source of the pacemaker current in the Purkinje fibers?

Answer 47

The funny current

Question 48

What is the order of ventricular muscle activation?

Answer 48

Ventricular apex (endo to epicardium) -->via Purkinje fibers --> ventricular base (top) (endo to epicardium) = coordinated contraction

Question 49

What are the refractory periods of the ventricular AP?

Answer 49

1. Absolute (biggest) 2. Effective (Absolute + a bit longer) 3. Relative (Overlaps slightly with Effective) 4. Supranormal period See page 265 in notes for graph.

Question 50

When does the supranormal period occur?

Answer 50

In ventricular AP, it begins last, at about -70, going to -85 mV. The cell is more excitable than normal during this period.

Question 51

What is the intrinsic firing rate of the Bundle of His?

Answer 51

About 40 impulses/min

Question 52

What happens if the intrinsic firing rate of one of the pacemakers should become faster than that of the SA node?

Answer 52

The latent pacemaker takes over

Question 53

Parasympathetic effects on the heart always involve what kind of receptor?

Answer 53

Muscarinic

Question 54

Alpha-1 and Beta-2 both act on the vasculature during SNS stimulation. What are their effects?

Answer 54

Alpha-1 --> Constriction (vascular smooth muscle ie splanchnic) Beta-2 --> Dilation (skeletal)

Question 55

Name two beta blockers and say what they do:

Answer 55

Slow heart rate by blocking Beta-1 receptors in heart, "cardioselective" Propranolol Atenolol

Question 56

Name an agent that affects muscarinig ACh receptors and when it would be administered.

Answer 56

Atropine, during CPR | Blocks M2 receptors leading to tachycardia

Question 57

A nerve agent which inhibits acetylcholinesterase has what effect on the heart?

Answer 57

Bradycardia, ie sarin gas (also causes convulsions and death, think Japanese terrorist attack)

Question 58

Some antidepressants affect heart rate, how?

Answer 58

They block the reuptake of norepi, ie Prozac and elavil, leading to increased heart rate