Cardiac Electrophysiology II

Question 1

What are the relative rates of pacemaker activity in the regions that have it?

Answer 1

SA node = 60-100/min

AV node = 40/min so slower

Question 2

Describe how the sympathetic and parasympathetic nervous systems regulate SA node activity and heart rate

Answer 2

PNS = Vagal break (CN 10) = Ach @ SA node binds to M2 receptors which decreases cAMP & slows heart/pacemaker rate (decrease funny current so less steep ramp depolarization)

SNS = norepi hits B1-adrenergic receptors & activates adenylyl cyclase which increased cAMP & increases heart/pacemaker rate (increases funny current so steeper ramp depolarization)

Question 3

Describe the specific ionic currents that underlie atrial, AV, and ventricular action potentials

Answer 3

Atrial: has phases 0 - 4, but does not have ramp depolarization

AV: no transient hyperpolarization or plateau

Ventricular: AP goes from Apex to Base & from endocardium to epicardium

Question 4

Describe the main features of the structure of cardiac gap junctions and explain how cardiac action potentials propagate through gap junctions

Answer 4

AP’s from SA node travel through the atrial muscle via gap junction coupling. Gap junction channels permit passage of ions & small molecules between cells. For example, an action potential in Cell A propagates directly to Cell B, and so on. Electrically, the heart is a syncytium of cells.

Question 5

What region(s) of the heart initiates the heartbeat and have pacemaker activity?

Answer 5

SA node exhibits pacemaker activity & starts hearbeat

AV node also has pacemaker activity

Question 6

SA Node’s or AV Node’s AP order:

Answer 6

SA & AV node have same AP

See pg. 213

Phase 0 Depolarization = calcium upstroke through L-type calcium channels

Phase 3 Falling phase = inactivation of L-type calcium channels (long-type) + activation of delayed voltage dependent K outward rectifiers

Phase 4 Spontaneous Slow Ramping Depolarization = Activation of I,f (funny current) inward positive current that activates in response to hyperpolarization = negative voltage. I,f has inward Na & outward K components. This funny current gives the heart pacemaker activity.

Question 7

Describe the factors that slow conduction velocity in the AV node

Answer 7

Parasympathetic decreases heartrate by decreasing slope of ramp depolarization

Sympathetic increases heartrate by increasing slope of ramp depolarization

Question 8

Describe the difference of the roles of L-type and T-type calcium channels

Answer 8

L-type (long): low threshold, long lasting, large conductance, SA pacemaker, AV conduction, excitation-contraction coupling.

T-type (transient): high threshold, short lasting, small conductance. Functions in proliferative signaling–hypertrophy of the heart in disease states.

Question 9

Describe the factors that slow conduction velocity in the SA node

Answer 9

3 ways:

1. Slow down the slope of ramp depolarization (takes longer to reach threshold) = most common
2. Diastolic hyperpolarization: if you go to negative then it also takes longer to get to threshold since you start from lower.
3. Increase threshold

Question 10

B1 blockers do what?

Answer 10

Bradycardia = decrease heartrate

Question 11

M2 blockers do what?

Answer 11

Tachycardia = increase heartrate

Question 12

Nerve agents that inhibit Ach-esterase do what to heartrate?

Answer 12

Bradycardia cause Ach hangs around longer @ synapse

Question 13

Some antidepressants that block the uptake of norepi do what to heartrate?

Answer 13

Tachycardia = increase heartrate

drugs that inhibit the reuptake of a neurotransmitter from the synapse, lead to an increase in the extracellular concentrations of the neurotransmitter!

Question 14

Purkinje fibers

Answer 14

have highest conduction velocity in the heart & the largest AP’s

Question 15

Where are pacemaker activity & conduction velocity fastest?

Answer 15

Pacemaker activity fastest @ SA node

Conduction velocity is equally fastest @ Bundle branches & Purkinje fibers

Question 16

State the phases of the atrial muscle or ventricular muscle AP

Answer 16

The ventricular AP is the same as the atrial muscle.

See figure in pg. 26 in notes 2 or 219 in notes 1

Phase 0 – Depolarization – Rapid Na+ channels are stimulated to open, flooding the cell with
positive sodium ions. This causes a positively directed change in the transmembrane
potential. This shift in voltage is reflected by the initial spike of the action potential = overshoot.

Phase 1 - (dip) is the initial stage of repolarization triggered by closing of the Na+ channels and the brief activation of ItO the transient outward current.

Phase 2 - is the plateau stage where the rate of repolarization by potassium is slowed by the influx of Ca+ ions into the cell. The Ca ions enter the cell slower than the Na ions and help prevent the cell
from repolarizing too quickly, thus extending the refractory period. This mechanism helps
regulate the rate at which cardiac tissue can depolarize. Note that phases 1 & 2 correspond
to the absolute refractory period.

Phase 3 - is the later stages of repolarization where potassium is leaving the cell. Once repolarization is complete, the cell will be able to respond to a new stimulus.

Phase 4 - (no net current) occurs after repolarization is complete.