Lecture 1: Cardiac AP

Question 1

What is the major cause of depolarization (phase 0) of a neural AP?

Answer 1

Increased in Na+ conductance;

Quick to open and quick to close (after a minimal delay).

Question 2

VGCa+ channels are activated when _____________ and opens during what phases?

Answer 2

Activated when the AP is depolarized.

Opens during phase 2 for a determined amount of time, then closes, causing phase 3

Question 3

When are K(a) channels activated?

Answer 3

When the cell depolarizes

Question 4

When are K(b) channels activated and what is their action?

Answer 4

Activated when the cell depolarizes

Slow to close during phase 2 and remain closed for a determined amount of time, then they open to help with phase 3.

Question 5

Which of the following maintains an excitable cell’s RMP?

Answer 5

K+ permeability;

K+ leak helps us to develop a (-) membrane potential. Na+/K+ ATPase helps to bring it back in.

Question 6

What contributes to repolarization of an excitable cells AP?

How so?

Answer 6

Opening of K+ gates

Na+ channels inactivation gate closes, VGK+ open.

The membrane permeability to Na+ decreases and permeability to K+ rises.

Question 7

What is the conduction of the heart?

Answer 7

1. SA node spontaneously depolarizes
2. Depolarization spreds over the atrial muscle: R atria–> L atria
3. AV node- conduction station to the ventricles; causes a delay in conduction to allow the ventricles to fill with blood.
4. Bundle of Hiss
5. L and R bundle branches go towards the apex of the heart
6. Purkinje fibers turn and go to the lateral walls of the ventricle; causing contraction of the ventricles to eject the blood out of the heart.

Question 8

Which layer of the heart receives the AP first?

Answer 8

Endocardium–> epicardium

Goes from internal–> external

Question 9

Which depolarizes first: right ventricle epicardium or the left ventricle epicardium?

Answer 9

Right ventricle epicardium

Question 10

During conduction, which cells repolarize first?

Answer 10

The cells that depolarize last are the FIRST ones to repolarize.

Question 11

Unlike other muscle, conduction occurs directly in between___________in the cardiac muscle.

Answer 11

Gap junctions

Question 12

Which layer of the heart is the first to repolarize?

Answer 12

Epicardium of the heart

Question 13

Conduction velocity is dependent on ___________.

Answer 13

Fiber size

Question 14

______ fibers have a faster AP transmission and thus; a greater velocity.

Answer 14

LARGER

Question 15

What is important to remember what the purkinje fibers?

Answer 15

They have a larger diameter than the myocytes surrounding them, thus, they have a faster AP than the AV/SA.

Question 16

Which fibers of the conduction system have the greatest conduction velocity?

Answer 16

Purkinje fibers (and atrial pathways)> atrial and ventricular muscle> AV node

Question 17

Why is there a delay in the AV conduction?

Answer 17

Allows the atria to empty the blood into the ventricles, before they contract.

Question 18

On its own, would the AV node generate its own spontaneous AP?

Answer 18

Yes. However, it will be slower than the SA node.

Question 19

What does the SA node and AV node (by itself) contract at?

Answer 19

SA node–> 75 bpm

AV node–> 40 bpm

Question 20

Without the SA or AV node, will the purkinje fibers elicit an AP?

Answer 20

Yes. It will elicit an AP and cause contraction of a myocyte, more slowly though.

Question 21

What phase of the SA node is determinant of the HR?

Answer 21

Phase 4 depolarization of the SA node.

Question 22

Describe the SA and AV node action potential

Answer 22

SA and AV node naturally depolarize to reach threshold, without electrical stimulation.

A. Phase 4 (Pacemaker potential):

Opening of funny VGNa+ (f) channels, which open when the membrane is repolarized causes gradual depolarization until threshold is reached.

B. Phase 0:

Slow Ca2+ channels open and special K+ (b) channels close; causing a balancing act between Ca2+ in and K+ out

C. Phase 3 (Repolarization):

-Reversal of phase 0: Slow Ca2+ gates close and special K+ (b) gates open.

Question 23

When the SA node fires, does it fire slower or faster than other regions?

Answer 23

Slower.

Question 24

How is conduction of the AV node different than the SA node?

Answer 24

Depolarization (phase 4) of the AV node is even slower.

Thus, the AV node does not ususally reach threshold until it receives a signal from the SA node.

Question 25

How is the AP of SA and AV node different from cardiac muscle?

Answer 25

There is no phase 1 or phase 2 because there are **not alot** of traditional VGNa+ (m) channels.

Question 26

\_\_\_\_\_\_\_\_\_\_\_ and ___________ will remain depolarized in the SA/AV node AP.

Answer 26

**Bundle of Hiss** **Purkinje fibers**

Question 27

If the Bundle of His and Purkinje fibers are not stimulated for an extended period of time, what will happen?

Answer 27

They will begin to spontaneously depolarize.

Question 28

The rate of the bundle and purkinje fibers is _______ than that of the AV node.

Answer 28

**Slower**

Question 29

Describe the AP in the fast tissue/fibers.

Answer 29

**Phase 4 (RMP)****:** **High K+ (c)** conductance out of the cell sustains the RMP. **Phase 0 (Depolarization):** **+VGNa+ channels (m)** open, causes rapid depolarization **Phase 1 (Brief repolarization):** Inactivation gates of **[VGNa+ channels]** *close* and **[K+ channels (a)]** *open*, causing a small repolarization. **Phase 2 (Plateau phase):** Slow opening of **VGCa2+ channels** and **special VGK+ channels (b)** close. This prolongs contraction. **Phase 3 (Repolarization):** **Slow VGCa2+ channels** close **K+ channels (b)** open

Question 30

The speed of conduction velocity is can be seen in which phase? This is related to differences in the speeds of what?

Answer 30

**Phase 0.** *Increase conduction velocity= more rapid phase 0 occurs= steeper phase 0.* This is related to the differences in speeds of the VGNa+ channels or VGCa2+ channels.

Question 31

Is the conduction velocity the same throughout the heart?

Answer 31

No. The conduction velocity will differ in different regions of the heart. [which has the highest and lowest?]

Question 32

Defective conduction velocity of the heart

Answer 32

Disruption of conduction can occur on any fiber of the heart and it can have a big impact on heart function.

Question 33

What is AV block?

Answer 33

A defect in conduction velocity due to damage of AV node. This delays conduction

Question 34

**What is an arrhythmia?**

Answer 34

**Damage to the purkinje fibers, which disrupts natural conduction and the contraction of ventricles.**

Question 35

What is a refractory period?

Answer 35

When the electrolyte gates have not 'reset' sufficiently to allow a second AP to be generated.

Question 36

What are refractory periods important for?

Answer 36

to prevent arrhythmias

Question 37

Are refractory periods longer in cardiac cells or neurons?

Answer 37

**Cardiac cells**

Question 38

Absolute refractory period (ARP)

Answer 38

**No depolarization** **from -40 and up**

Question 39

**Relative refractory period (RRP)**

Answer 39

AP can be generated but will have abnormal conduction between -90 and -40

Question 40

**Supranormal period (SNP)**

Answer 40

Cell is more excitable than normal. If a signal is sent, even if it is smaller, it will cause an AP. Just not at the same conduction speed. Phase 0 will be tipped in the direction that will not propogate the AP at the same speed.

Question 41

If stimulated during relative refractory period, will AP be weaker, stronger or not affected?

Answer 41

**WEAKER**

Question 42

What is the **chronotropic effect**? Positive? Negative?

Answer 42

Chronotropic effect changes the rate of depolarization of the SA node and therefore the heart rate. **Positive chronotopic effects** – increase HR **Negative chronotopic effects** – decrease HR

Question 43

**Dromotropic effect?** ## Footnote **Positive dromotropic effects?** **Negative dromotropic effects?**

Answer 43

Dromotropic effect is the **speed of conduction.** **Positive dromotropic effects** – increase conduction velocity through the AV node, increasing HR **Negative dromotopic effects** – decrease conduction velocity through the AV node, decreasing HR

Question 44

What is the **inotropic effect?**

Answer 44

Inotropic effect changes the STRENGTH of muscle contraction +: increase strength of contraction (they will contract more forcefully)

Question 45

Lusitropic effect?

Answer 45

Lusitropic effect changes the rate of muscular relaxtion.

Question 46

How will parasymapthetic stimulation to the SA/AV node affect the chronotropic effect and dromotropic effect?

Answer 46

**Negative chronotopic effects** –Parasympathetic stimulation: ACh binds to M2 muscarinic receptors - Slows opening of Na (f) channels during phase 4 - Increases efflux of K+ via K+/ACh channels, causing hyperpolarizaiton **Negative dromotopic effects** – decreases conduction velocity Parasympathetic stimulation: ACh binds to M2/ M3 receptors - Decreases Ca2+ influx - Increases K+ efflux via K+/K-ACh channels

Question 47

\_\_\_\_\_\_\_\_\_\_\_\_ system innervates the SA and AV node via the _______ nerve by acting on what type of receptors that bind what? How does this happen?

Answer 47

-**Parasympathetic system** innervates the SA and AV node via the **vagus** nerve by acting on muscarinic receptors, which bind **ACh**. When ACh binds, it causes a (-) GCPR response, reducing kinase activity, phosphorylation and post-translational modification of channels.

Question 48

What is the outcome of parasympathetically stimulating the SA and AV node?

Answer 48

**Slows down the heart rate.**

Question 49

Can the SA/AV node and myocytes also be sympathetically stimulates?

Answer 49

Yes. They have muscarinic (B1- adrenergic receptors) that bind NE, which causes an increase in contraction.

Question 50

How will **symapthetic stimulation** to the SA/AV node affect the chronotropic effect and dromotropic effect?

Answer 50

**Positive chronotopic effects** – increased HR by β1 adrenergic receptors increase If. **Positive dromotropic effects** – increase conduction velocity through the AV node increasing HR **Positive iontropic effects**

Question 51

What occurs during a positive chronotropic effect?

Answer 51

+ chronotropic effect--\> increase HR - Sympathetically stimulate the SA node. - NE binds to B1 adrenergic receptors, increasing rate of SA depolarization - Increase in opening of Na+(f) channels in Phase 4 - Increase in Ca2+ influx

Question 52

What occurs during a **positive** **dromotropic** **effect**?

Answer 52

+ dromotropic effect--\> increase conduction velocity--\> increase HR - Sympathetic stimulation occurs when NE binds to B1 adrenergic receptors - Increase Ca2+ influx during phase 2;

Question 53

What are the effects of parasympathetic and sympathetic stimulation on phase 4 in the SA node.

Answer 53

**Sympathetic-** slope of phase 4 **increases** **Parasympathetic**- slope of phase 4 **decreases**

Question 54

What happens if you give a patient atropine or propanolol?

Answer 54

Atropine is a muscarinic receptor antagonist- increases HR alot Propanolol- B adrenergic antagonist- decreases HR a little.

Question 55

what is the difference between atria and ventricule AP?

Answer 55

ATria has a shoter AP than ventricles because phase 2 is not as long