Cardiac Electrophysiology I

Question 1

What is equilibrium potential?

Answer 1

It is the voltage obtained for a given concentration gradient of a single ion at equilibrium across a semi-permeable membrane. The Nernst equilibrium equation represent this.

Question 2

What is Gibbs-Donnan Equilibrium?

Answer 2

Equilibrium involving impermeable polyelectrolyte on one side of a membrane that is permeable to salts but impermeable to the polyelectrolyte. It results in the unequal distribution of salts across the membrane a slight membrane potential that has the same sign as the charge on the polyelectrolyte.

Question 3

When do diffusion potentials occur?

Answer 3

When two or more ions are permeable to a membrane, but the various ions have differing permeabilities. Cell resting potentials and action potentials are examples of diffusion potentials.

Question 4

What are epithelial membrane potentials?

Answer 4

The difference in electrical potential that occurs between two dilute solutions when the membrane itself is a layer of cells. This occurs in the kidney and GI system.

Question 5

What is the effect on the cell if the concentration of external K is increased?

Answer 5

It decreases outward K+ gradient and makes Em less negative which is depolarizing.

Question 6

What effect does raising the concentration of internal K have on the cell?

Answer 6

It increases the outward K+ gradient and makes Em more negative which is hyperpolarizing

Question 7

Raising internal Na will have what effect on the cell?

Answer 7

It decreases inward Na gradient and makes Em less positive indie which is hyperpolarizing

Question 8

Raising external Na will have what effect on the cell?

Answer 8

Increases inward Na gradient and makes Em more positive inside which is depolarizing

Question 9

Why do resting potentials vary among cells?

Answer 9

It is due to the varying relative permeabilities of Na+ to K+

Question 10

What is the implication of a greater relative permeability of Na compared to K?

Answer 10

The more positive the resting potential (or less negative)

Question 11

What do non-Ohmic voltage curve indicate the property of?

Answer 11

Indicated the property of rectification. ie. that the conductance differs for inward and outward currents.

Question 12

Describe an outward rectification

Answer 12

The conductance of outward currents is greater than for inward currents and the current/voltage curve slopes upward nonlinearly.

Question 13

Describe an inward rectification

Answer 13

The conductance of inward currents is greater than for outward currents and the current/voltage plots slope downward nonlinearly. The K+ channel (iK1) responsible for the resting potential of nerve and muscle is an inward rectifier.

Question 14

What are nerve and cardiac resting potentials primarily due to?

Answer 14

They are primarily due to inwardly rectifying K+ channels that mediate a positive efflux of K+. At the normal resting potential the inward rectifier channel iK1 mediates a positive efflux of potassium

Question 15

What is electrodiffusion?

Answer 15

It is the movement of ions through a channel. This occurs single file.

Question 16

In terms of membrane voltage (Vm) what happens when a cell becomes more permeable to an ion?

Answer 16

The effect upon Vm is to brin it closer to the equilibrium potential of that particular ion.

Question 17

What effect would injecting NaCl into the cell have on Vm?

Answer 17

It would be hyper polarizing. The amount of sodium current that flows into the cell during an action potential is so small that the internal concentration of sodium dow not change very much. What does change is the relative permeability of sodium to potassium

Question 18

What does the overshoot give evidence of?

Answer 18

An inwardly directed positive ion gradient of Na rather than an outwardly directed positive ion gradient K is now dominating the membrane voltage.

Question 19

Sodium and potassium channels are outwardly/inwardly rectifying? What does this mean?

Answer 19

Outwardly rectifying. Their conductances are low at hyper polarized voltages, ie. inward positive currents are low at rest but they open up at a threshold level of membrane depolarization and acquire a much larger conductance that enables positive inward currents during the upstroke of the action potential

Question 20

What are 2 examples of time and voltage dependent ion channels? What does this mean?

Answer 20

Calcium and sodium channels. The opening of these channels depends on both voltage and time in that positive voltages open the channes but they spontaneously close with the passage of time.

Question 21

Compared to a normal membrane potential what will a lower membrane potential result in?

Answer 21

It will reduce the rate of depolarization and amplitude of an action potential. The degree of inactivation can influence the size of a propagated action potential. This is because some of the channels will be in the inactivated state the ensuing AP is smaller and has a slower upstroke than normal.

Question 22

Compared with normal what will happen with a stimulus reaching threshold slowly?

Answer 22

If the stimulating voltage to threshold has a slower upstroke than usual some of the channels will inactivate and the ensuing action potential may likewise be smaller than usual and with a slower upstroke

Question 23

Describe delayed outward rectified K channels

Answer 23

They are found in muscle and nerve cells. They are primarily responsible for the repolarization phase of the action potential. They spontaneously inactivate by a ball and chain mechanism

Question 24

What is implied if a channel is an outward rectifier?

Answer 24

A much larger conductance at positive depolarized voltages than at negative hyperpolarized voltages.

Question 25

Describe the model of ion channel gating

Answer 25

There is a selectivity filter to determine which ion will permeate. Other domains are voltage sensores (m-gates). Other domains are inactivation gates (h-gates)

Question 26

Which channels function like a lid on a trash can and which like a ball on a chain?

Answer 26

Na and Ca are lids. K is ball on a chain

Question 27

Describe the action potentials in the atrium, bundle of His, Purkinje network, and ventricle

Answer 27

They have APs that involve Na-dependent upstrokes, Ca-dependent plateaus and K-dependent repolarizations

Question 28

Describe the SA node AP

Answer 28

They have smaller Ca-dependent upstrokes and K-dependent repolarizations with no contributions from Na channels. It alos has a spontaneous depolarizing ramp responsible for the automaticity of the heart rhythm

Question 29

Describe the AV node AP

Answer 29

The Aps have Ca-dependent upstrokes and K-dependent repolarizations

Question 30

Compare the duration of Skeletal muscle APs with cardiac muscle

Answer 30

The duration of skeletal muscle AP is only a few msec whereas cardiac APs have a plateau lasting 300-400msec

Question 31

Describe the repolarization of cardiac muscle compared to skeletal

Answer 31

cardiac repolarization is triphasic meaning that after the initial upstroke the membrane voltage starts to go down, then goes up briefly, then goes back to the resting potetial. Skeletal is monophasic

Question 32

Which AP in the heart is largest?

Answer 32

Purkinje Fiber. -90mV to +30mV

Question 33

What do the long duration and long refractory period prevent in the His-Purkinje fibers?

Answer 33

Prevent adjacent ventricular muscle from re-activating the conduction system

Question 34

Describe the channel opening in Purkinje fibers (0-2)

Answer 34

Phase 0: Upstroke. Fast inward Na current that rapidly inactivates. Phase 1: Tramsoemt repolarization; transient outward K currents. Phase 2: Plateau: Slow outward K currents and slow inward L-type Ca current. They all slowly inactivate

Question 35

Describe the channel opening in Purkinje fibers (3-4)

Answer 35

Phase 3: Repolarization: delayed outward rectifier K currents. iKr is rapidly activating while iKs is slow. Phase 4: Resting potential; inward rectifier K currents

Question 36

Where are T-type calcium channels important?

Answer 36

Contributing the generation of pacemaker currents in the SA node.