Cardiac Electrophysiology I

Question 1

Distinguish between equilibrium potentials and diffusion potentials

Answer 1

An equilibrium potential is the voltage obtained for a given concentration gradient of a single ion at equilibrium across a semi-permeable membrane. The equilibrium potential refers to a given single ion and is represented by the Nernst equilibrium equation.

Diffusion potentials occur when two or more ions are permeable to a membrane, but the various ions have differing permeabilities. Diffusion potentials can be calculated by the Goldman-Hodgkin-Katz equation. Cell resting potentials and action potentials are examples of diffusion potentials.

Question 2

hyperpolarization

Answer 2

Hyperpolarization means more negative (less positive).

Question 3

depolarization

Answer 3

Depolarization means more positive (less negative).

Question 4

repolarization

Answer 4

refers to the change in membrane potential that returns the membrane potential to a negative value after the depolarization phase of an action potential has just previously changed the membrane potential to a positive value; the repolarization phase usually returns the membrane potential back to the resting membrane potential.

Question 5

describe how sodium channels and the delayed rectifier potassium channel activate and inactivate during the action potential

Answer 5

the delayed rectifier potassium channel is an outwardly rectifying channel (unlike inward-resting) for which the currents are outward and repolarizing. Its conductance is higher at depolarized voltages (this opens up at positive voltages). The delayed outward rectifier K+ channel spontaneously inactivates by a ball and chain mechanism.

Question 6

describe how inward rectifying potassium channels function in maintaining the resting potential of nerve and muscle, and how this channel’s conductance changes during action potentials

Answer 6

At the normal resting potential, the inward rectifier channel iK1 mediates a positive efflux or outflow (leaking out) of potassium (keeping the cell negative). In other words, an inwardly rectifying potassium channel mediates outward potassium current (since current is for positive charges).

The K+ channel (denoted iK1) responsible for the resting potential of nerve and muscle is an inward rectifier. When the cell is hyperpolarized at rest, the conductance of this channel is high; however, upon depolarization during the upstroke of the action potential, the conductance of the iK1 channel becomes considerably less, allowing the inward sodium current to depolarize the membrane potential. Upon repolarization, iK1 re-acquires its high conductance, thereby maintaining the inside negative resting potential.

Question 7

Describe the absolute and relative refractory periods of cardiac action potentials. When do they occur?

Answer 7

Same definitions as normal absoloute & relative refractory periods

The refractory period of the cardiac action potential occurs during diastole, allowing time for the heart to refill with blood before the next contraction. The refractory period prevents the heart chambers from contracting before they have filled with blood.

Question 8

Describe & draw how the various specific ionic currents contribute to the shape of the action potential of Purkinje fibers & Bundle Branches–MUST MEMORIZE THIS

Answer 8

See pg. 197. Note that the different regions of the conducting system have different phases & ions.

See pg. 221, the AP of Bundle Branches is the same as the AP for Purkinje fibers–they have highest conduction velocity in the heart

Phase 0: Upstroke; fast inward sodium current (due to opening of voltage-gated sodium channels) that rapidly inactivates.

Phase 1: Transient repolarization; transient outward potassium currents.

Phase 2: Plateau; slow outward K currents (iKr, iKs, iKCa) and slow inward L-Type calcium current that all slowly inactivate. They negate each-other = plateau.

Phase 3: Repolarization; delayed outward rectifier potassium currents (iKr and iKs delayed rectifiers). iKr is rapidly activating and iKs is slowly activating.

Phase 4: Resting potential; inward rectifier potassium currents (iK1).

Question 9

Describe the main features of action potentials in the various regions of the heart

Answer 9

The action potentials in the atrium, bundle of His, Purkinje network, and ventricle have action potentials involving inward sodium-dependent upstrokes, inward calcium-dependent plateaus, and outward potassium-dependent delayed rectifier repolarizations (in cardiac muscle there is no undershoot region like in nerve cells). LOOK @ FIGURE ON PG 195.

In both SA & AV nodes there is no sodium upstroke, the upstroke is due to calcium (they open more slowly so upstroke curve is less steep). Furthermore, there is no plateau phase & the repolarizations are due to potassium. The SA node pacemaker potential also has a spontaneously depolarizing ramp depolarization responsible for the automaticity of the heart rhythm.

Question 10

Compare action potentials in skeletal and cardiac muscle

Answer 10

In cardiac there is calcium, no undershoot, & an overshoot due to calcium.

Question 11

If they ask: what happens when you increase the sodium concentration in a cell by injecting sodium chloride?

Answer 11

Hyperpolatization

Question 12

Chloride does not contribute to membrane potential because

Answer 12

it is already @ equilibrium

Question 13

Different cells have different membrane diffusion potentials because they have varying values of

Answer 13

alpha = permeability Na/ permeability K

Question 14

Inward rectifiers like iK1 have low conductances @?

Answer 14

Positive voltages

& high conductances @ negative voltages

Question 15

If you inject KCl into a cell what happens?

Answer 15

K+ leaves through channels due to gradient & Cl- stays in = hyperpolarization

Question 16

Discuss the mechanisms for K, Na, & Ca channel inactivation

Answer 16

lid on a trash can = Na+ or Ca++ channels

ball and chain mechanism = K+ channels

Question 17

Order of heart electrical activity

Answer 17

SA node (right atrium), atrial muscle, AV node (delay), bundle branches, Purkinje fibers (rapid conduction), ventricular muslcle

Question 18

Describe the time it takes for an AP to reach the base of the left ventricle from the SA node.

Answer 18

After the S-A node discharges, the action potential travels trough the atria, through the A-V bundle system and finally to the ventricular septum and throughout the ventricle. The last place that the impulse arrives is at the epicardial surface at the base of the left ventricle, which requires a transit time of 0.22 sec from the S-A node.

Question 19

How long does it take an AP to reach the AV bundle of his from the SA node?

Answer 19

The action potential arrives at the A-V bundle at 0.12 sec. It arrives at the A-V node at 0.03 sec and is delayed 0.09 sec in the A-V node, which results in an arrival time at the bundle of His of 0.12 sec.

Question 20

What is the membrane potential (threshold level) at which the S-A node discharges?

Answer 20

-40mV

The normal resting membrane potential of the S-A node is -55 mV. As the sodium leaks into the membrane an upward drift of the membrane potential occurs until it reaches -40 mV. This is the threshold level that initiates the action potential at the S-A node.

Question 21

If the Purkinje fibers take over as pacemakers, the heart rate will be?

Answer 21

Between 15 and 40 beats/min.

Question 22

What is the normal total delay of the cardiac impulse in the A-V node and the A-V bundle system?

Answer 22

0.13 sec

The impulse coming from the S-A node to the A-V node arrives at 0.03 sec. Then there is a total delay of 0.13 sec in the A-V node and bundle system allowing the impulse to arrive at the ventricular septum at 0.16 sec.

Question 23

The resting membrane potential of the sinus nodal fibers is ___?

Answer 23

The resting membrane potential of the sinus nodal fibers is -55 mV.

This is in contrast with the -85 to -90 mV membrane potential of cardiac muscle. Other major differences between the sinus nodal fibers and ventricular muscle fibers are that the sinus fibers exhibit self-excitation from inward leaking of sodium ions.

Question 24

A drug that promotes early activation of the “delayed rectifier” K+ channel (Ik) in cardiac muscle will do which of the following?

Answer 24

The action potential’s duration will be decreased.

Question 25

If parasympathetic nerve activity abruptly increases while sympathetic activity decreases, which of the following is most likely to occur?

Answer 25

Quiz answer was incorrect. New answer:

A decrease in sympathetic input coupled with an increase in parasympathetic input should mainly decrease HR and likely increase the End Systolic Volume not the End Diastolic Volume. This increase in ESV would reduce stroke volume and cardiac output.

Question 26

The primary route of removal of [Ca2+] from the sarcoplasm during relaxation of a cardiac muscle cell is by

Answer 26

active transport into the saroplasmic reticulum

Question 27

State the mechanism by which potassium chloride is used to induce cardiac arrest

Answer 27

KCl eliminates the potassium concentration gradient so the cardiac muscle cells become depolarized and are unable to repolarize for their next beat. The heart stops in diastole.

Question 28

Action potential conduction velocity in cardiac muscle tissue is not influenced by

Answer 28

duration of the plateau phase (phase 2) of the action potential

Question 29

The ___ node has a slowly depolarizing “prepotential”

Answer 29

Sinoatrial node

Question 30

Which structure has the slowest rate of conduction of the cardiac action potential?

Answer 30

A-V bundle fibers

The atrial and ventricular muscles have a relatively rapid rate of conduction of the cardiac action potential, and the anterior internodal pathway also has fairly rapid conduction of the impulse. However, the A-V bundle myofibrils have a slow rate of conduction because their sizes are considerably smaller than the sizes of the normal atrial and ventricular muscle. Also, their slow conduction is partly caused by diminished numbers of gap junctions between successive muscle cells in the conducting pathway, causing a great resistance to conduction of the excitatory ions from one cell to the next.

Question 31

Which of the following best explains how sympathetic stimulation affects the heart?

Answer 31

The rate of upward drift of the resting membrane potential of the S-A node increases.

During sympathetic stimulation, the permeabilities of the S-A node and the A-V node increase. Also, the permeability of cardiac muscle to calcium increases resulting in an increased contractile strength. In addition, there is an upward drift of the resting membrane potential of the S-A node.

Question 32

Ach causes what to occur?

Answer 32

It causes hyperpolarization of the S-A node and the A-V node by increasing permeability to potassium ions, this results in a decreased heart rate.

Question 33

Compare the graph of the different AP’s of the different regions of the heart on pg. 195 & then look @ pg. 226 for the conduction velocities of the heart

Answer 33

-

Question 34

Ramp depolarization is in.

Answer 34

SA node, AV node, Purkinje–anything with pacemaker activity!!

Not ventricles or atria.