Cardiac Electrophysiology II

Question 1

What does the SA node function to do? What is a special characteristic of it?

Answer 1

It functions as the normal initiation site of cardiac excitation. It exhibits intrinsic pacemaker activity. Rate of AP firing is 60-100/min

Question 2

What are the phases of APs in the SA node cell?

Answer 2

Phases 0, 3, 4

Question 3

Describe phase 0 (of the SA node)

Answer 3

Rapid depolarization; due mostly to I-Ca via V-dep L-type channels

Question 4

Describe phase 3 (of the SA node)

Answer 4

Slower repolarization; due mostly to inactivation of I-Ca plus activation of delayed V-dep I-K

Question 5

What causes pacemaker activity?

Answer 5

The interaction between I-Ca (T-type), I-k, and a special current termed I-f (f for funny). I-f is also known as the pacemaker current.

Question 6

Why is I-f “funny”?

Answer 6

Is is a net inward current that activates in response to hyperpolarization (instead of the usual depolarization) and because it has both an inward Na and an outward K component

Question 7

Describe Phase 4 (of the SA node)

Answer 7

a slow ramping depolarization because of the activation of I-f

Question 8

A major Ca channel in the heart is L-type, describe this channel

Answer 8

Slow inactivation (L for long-lasting). Large channel. Fn’s; SA pacemaker, AV conduction, Excitation-contraction coupling. Low threshold of -40mV

Question 9

A major Ca channel in the heart is T-type, describe this channel

Answer 9

Fast inactivationg (T=transient). Small channel. Fn’s; SA pacemaker, proliferative signaling. High threshold -70mV

Question 10

What are 3 mechanisms that can slow the SA node pacemaker?

Answer 10

1-decreased rate of diastolic hyperpolarization. 2-diastolic hyperpolarization. 3-increased threshold.

Question 11

How does the ANS regulate the SA node?

Answer 11

Neuronal input from both parasympathetic inhibition and sympathetic stimulation.

Question 12

Describe parasympathetic inhibtion; the Vagal brake on the SA node

Answer 12

Vagal activity -> ACh released at heart SA node -> binds to M2 receptors -> activation of Gi and decrease of [cAMP]i. Results; slows heart/ pacemaker rate

Question 13

Describe the sympathetic stimulation on the SA node

Answer 13

Norepinephrine binds to B1-adrenergic receptors: activate Gs -> activate adenylyl cyclase -> increase in [cAMP]i. Increases heart rate/ pacemaker rate

Question 14

What are the mechanisms of action governing Symp stim of SA node

Answer 14

Increases phase 4 steepness by increasing inward I-f. Increases Phase 4 steepness by increaseing inward I-CaT threshold of I-CaL at more negative value

Question 15

What effect does atropine have on sinus pacemaker rate?

Answer 15

Blocks parasympathetic control leading to an elevated HR (60-> 110)

Question 16

What effect does propanolol have on sinus pacemaker rate?

Answer 16

Blocks sympathetic control leading to a decreased HR (60->50)

Question 17

What effect does atropine AND propanolol have on sinus pacemaker rate when combined?

Answer 17

Blocks symp and parasymp contol leading to an increased HR (60->100) but less than atropine alone

Question 18

What are the phases of atrial muscle AP?

Answer 18

0-4. It is different than the SA node phases

Question 19

Describe phase 0 of atrial muscle AP

Answer 19

Rapid depolarization; due mostly to I_Na; overshoots 0mV

Question 20

Describe phase 1 of atrial muscle AP

Answer 20

Small, limited repolarization. Activation of I_TO; decreased I_Ca and I_Na

Question 21

Describe phase 2 of atrial muscle AP

Answer 21

short plateau of depolarization (~200ms); due to prolonged I_Ca PLUS a I_Kur (ultrarapid) the currents balance each other

Question 22

Describe phase 3 of atrial muscle AP

Answer 22

Repolarization; inactivation of I_Ca, increased I_K

Question 23

Describe phase 4 of atrial muscle AP

Answer 23

Resting value of Vm due to increased iK_i; no depolarizing ramp nor I_f

Question 24

What are the consequences of phase 2

Answer 24

Ca++ entry promotes contraction refractory period permits filling

Question 25

Describe the AV node in terms of AP and stimulation from SA node

Answer 25

AP’s are similar to those of the SA node. AP’s from SA node arrive in ~30ms. AV node has an intrinsic pacemaker activity with an intrinsic firing rate of ~40/min. The delay stage between atria and ventricles is ~90ms.

Question 26

Describe the AV conduction system

Answer 26

Ca upstroke AP’s like SA node. AP magnitude is smaller (<0). Slow upstroke. Relatively few gap junctions. Conduction velocity is low

Question 27

What effect does parasympathetic inhibition have on the AV node?

Answer 27

It decreases the firing rate; decreases conduction velocity

Question 28

What effect does parasympathetic stimulation have on the AV node?

Answer 28

It increases firing rate; increases conduction velocity

Question 29

Describe the bundle branches/ purkinje fibers

Answer 29

They have the highest conduction velocity in the heart (~4ms, ~80X nodes, ~4X muscle) AP’s are similar to muscle cells BUT Purkinje fibers also have a pacemaker current I_f. Intrinsic firing rate: <20/min and irregular (tertiary effect)

Question 30

Compare ventricular muscle AP’s to those of atrial muscle

Answer 30

They are similar. No intinsic pacemaker activity. Electrical activation of the ventricles is a very orderly, spatiotemporal process

Question 31

Describe the spatiotemporal characteristics of ventricular activation. What effect does it have?

Answer 31

Vantricular apex (bottom) inside to out (endo to epicardium) —(rapid, via Purkinje fibers)–> ventricular base (top also endo to epi). Effect: coordinated contraction of ventricular muscle cells that produces very efficient ejetion of blood

Question 32

How is coordinated contraction achieved?

Answer 32

1-A wave of electrical excitation. 2-contraction coupled to electical excitation. 3-the heart consists of two electrical syncytia

Question 33

How is the propagation of cardiac AP’s achieved from cell to cell?

Answer 33

By direct electrical coupling via gap junctions. They provide the physical basis for the electrical coupling of cardiac cells.

Question 34

Describe the electrical activation of the heart?

Answer 34

1-depolarize atria. 2-depolarize septum from left to right. 3-depolarize anteroseptal region of myocardium toward the apex. 4-depolarize bulk of ventricular myocardium, from endocardium to percardium. 5-depolarize poseterior portion of base of LV. 6-Ventricles are now depolarized

Question 35

Discuss the normal activation sequence in the heart

Answer 35

SA, Atrial myocardium, AV node, AV bundle, Bundle branches, Purkinje network, Ventricular myocardium