Cardiac electrophysiology

Question 1

What term that describes the heart means it does not need any nerves to beat?

Answer 1

Myogenic

Question 2

What is the difference between an equilibrium potential and a diffusion potential?

Answer 2

Equilibrium potential is the voltage from the gradient of 1 ion at equilibrium across a membrane that is only permeable to that ion. It can be calculated using the Nernst equation.
Diffusion potential is for two or more ions. Calculated using the Goldman equation.

Question 3

What is the approximate equilibrium potential for sodium? Potassium? Calcium?

Answer 3

Na+ = +60 mV
K+ = -90 mV
Ca2+ = +120 mV

Question 4

Depolarization means the membrane potential is (more/less) negative than resting membrane potential. Hyperpolarization means it is (more/less) negative.

Answer 4

Depolarization- less negative, more positive

Hyperpolarization- more negative

Question 5

Raising the concentration of positive ions in a cell makes the cell (de/hyper) polarized. NOTE: simply theoretical. Not during an action potential.

Answer 5

Hyperpolarized. More positive ions inside draw more negative ions inside which makes the membrane potential more negative.

Question 6

What is the approximate membrane potential of a skeletal muscle cell? An atrial or ventricular muscle cell? A cell in the SA node? In the AV node?

Answer 6

Skeletal muscle= -85 mV
Cardiac muscle= -80 mV
SA node= -55 mV
AV node= -65 mV

Question 7

Why do resting potentials vary between cells?

Answer 7

Each cell has a different relative permeability of Na+ to K+. Cells that are more permeable to K+ have more (-) membrane potentials.

Question 8

What is an outward rectifier? Give an example

Answer 8

A channel that conducts more readily at (+) voltages (i.e. when the cell is depolarized). Na+ channel

Question 9

What is an inward rectifier? Give an example

Answer 9

A channel that conducts more rapidly at (-) voltages (i.e. when the cell is hyperpolarized or at resting potential). The Ik1 K+ channel

Question 10

When a cell becomes more permeable to an ion, what is the effect on membrane voltage?

Answer 10

It moves closer to the equilibrium potential of that ion

Question 11

Describe how Na+ channels activate and inactivate during an action potential

Answer 11

Activate- Voltage sensor domain on the protein responds to a depolarization of the membrane and causes a conformational change to open the channel
Inactivation- happens rapidly, spontaneously; like putting a lid on a trash can

Question 12

What occurs during the absolute refractory period of a cardiac action potential?

Answer 12

The heart is in diastole so the previous signal propagates and it fills. Na+ channels are still inactivated.

Question 13

The size of the action potential determines what?

Answer 13

The propagation velocity

Question 14

What are two things that reduce the amplitude of a cardiac action potential?

Answer 14

A lower membrane potential (more negative) and a slower upstroke

Question 15

How does the delayed rectifier potassium channel activate during a cardiac action potential? How does it inactivate?

Answer 15

Activates upon depolarization but with a time delay. Conformational change in the protein
Inactivates spontaneously by a ball-and-chain mechanism.

Question 16

What areas of the heart have similar action potentials? (2 different groups)

Answer 16

SA node and AV node

Also the Atria, Ventricles, Bundle of His and Purkinje network

Question 17

How does a cardiac action potential differ from that of skeletal muscle?

Answer 17

Cardiac action potentials are longer, have a plateau phase and repolarize in 3 phases

Question 18

What are the phases of a Purkinje fiber action potential? What ions/currents are involved?

Answer 18

0- upstroke. Fast inward Na+ current
1- transient repolarization with outward K+ current
2- plateau. Ca2+ into cell, delayed K+ current out of cell
3- repolarization- K+ out of cell
4- resting potential- K+ out of cell (inward rectifiers)

Question 19

Which regions of the heart have pacemaker activity? What are their relative rates?

Answer 19

The SA node (70 potentials/min), the AV node (40/min), the bundle branches (25-40/min), and the Purkinje fibers (<20/min)

Question 20

What ion channel (and what currents) is/are responsible for pacemaker activity?

Answer 20

If or Ih channels. Nonspecific cation channel that opens in response to hyperpolarization (is an inward rectifier). Allows Na+ and K+ to flow down their gradients, creating a slow ramp depolarization.

Question 21

What are the phases of an SA node action potential and what ions are involved?

Answer 21

0- rapid depolarization. Influx of Ca2+
3- slower repolarization. K+ efflux
4- ramp depolarization. (If activated, K+ out, Na+ in)

Question 22

What are three mechanisms by which the SA pacemaker can be slowed?

Answer 22

1. Slow down ramp depolarization
2. Hyperpolarize the membrane
3. Raise the threshold for an action potential

Question 23

How does the parasympathetic NS affect the SA node? The sympathetic NS?

Answer 23

Paras- release acetylcholine which binds M2 receptors to decrease cAMP. This makes Ih channels less effective and slows ramp depolarization. (Also reduce inward Ca2+, increase outward K+ and raise the threshold for IcaL activation)
Symps- Norepinephrine acts on B1 receptors to increase cAMP and reverse the effects of the paras

Question 24

What is the difference between L-type and T-type calcium channels?

Answer 24

L-type have a more positive threshold than Na+ channels and are responsible for the action potential in SA node cells. T-type are transient and open at membrane depolarization. They have less of an effect on membrane voltage

Question 25

How long is the delay stage at the AV node?

Answer 25

About 90 milliseconds (0.09 sec)

Question 26

What effect do sympathetic and parasympathetic neurons have at the AV node?

Answer 26

Both act on the conduction velocity (moreso than the firing rate). Symps increase it by speeding ramp depolarization; paras decrease it

Question 27

What part of the heart’s conduction system has the greatest conduction velocity?

Answer 27

The bundle branches

Question 28

Where do ventricular contractions begin? (both spatially and which layer)

Answer 28

At the apex (the bottom); in the endocardium

Question 29

How does the heart achieve coordinated contraction?

Answer 29

The heart muscle is two electrical syncytia (atria and ventricles). Electrical excitation is spread via gap junctions and contraction is coupled to the excitation.

Question 30

About how long does it take for current to go from the SA node to the AV node? From the AV node to the Bundle branch? To the apex? To the base of the LV?

Answer 30

From SA (0.0s) to AV- 0.03 s
Current delays at AV for 0.09 s
Bundle branch receives current at 0.12 s
Apex receives at 0.2 s. Base at 0.22 s