HRR: Myocyte Action Potential

Question 1

What is an inward current?

Answer 1

Flow of cations from outside to inside the cell or flow of anions from inside to outside the cell

Inward current is crucial for depolarization in action potentials.

Question 2

What is an outward current?

Answer 2

Flow of cations from inside the cell to outside the cell or flow of anions from outside the cell to inside the cell

Outward currents are important for repolarization.

Question 3

Describe the three states of ion channel cycling.

Answer 3

• Closed: resting state, ready for activation
• Open: activated, allowing ion flow
• Inactive: cannot be activated, cycling back to closed is time-dependent

Understanding these states is essential for grasping how ion channels function in action potentials.

Question 4

What is the basic structure of voltage-gated ion channels?

Answer 4

They have a pore, an ion filter/gate, one or more subunits, and phosphorylation sites

This structure allows for selective ion permeability crucial for action potentials.

Question 5

What is a unique feature of action potential in cardiac muscle?

Answer 5

Hyperpolarization doesn’t happen; it just returns to resting

This characteristic differentiates cardiac action potentials from those in neurons.

Question 6

Is an action potential longer or shorter in working cardiac myocytes compared to neurons?

Answer 6

Longer

The prolonged action potential in cardiac myocytes is essential for coordinated contractions.

Question 7

What happens in phase 0 of an action potential?

Answer 7

Rapid upstroke in charge

This phase is critical for the initiation of the action potential.

Question 8

What happens in phase 1 of an action potential?

Answer 8

Early repolarization

This phase marks the initial return towards resting potential.

Question 9

What happens in phase 2 of an action potential?

Answer 9

Plateau

The plateau phase is important for sustaining the action potential in cardiac muscle.

Question 10

What happens in phase 3 of an action potential?

Answer 10

Repolarization

This phase restores the membrane potential after depolarization.

Question 11

What happens in phase 4 of an action potential?

Answer 11

Return to resting membrane potential

Phase 4 prepares the cell for the next action potential.

Question 12

What is the inward rectifier current (IK1)?

Answer 12

It stabilizes membrane potential; it rectifies itself as needed

IK1 is crucial for maintaining the resting membrane potential in cardiac myocytes.

Question 13

What is a typical membrane potential?

Answer 13

Negative 80-90 mV

This range indicates the resting state of most neurons and myocytes.

Question 14

What is the significance of potassium leak channels?

Answer 14

They help maintain membrane potential; they do not require energy

Potassium leak channels are vital for regulating potassium ion movement across the membrane.

Question 15

Where is potassium found in relation to the cell?

Answer 15

Inside the cell

Question 16

Where is sodium found in relation to the cell?

Answer 16

Outside the cell

Question 17

If sodium equilibrated across the membrane, what would the resting membrane potential (RMP) be?

Answer 17

+64

Question 18

If calcium equilibrated across the membrane, what would the RMP be?

Answer 18

+128

Question 19

If chloride equilibrated across the membrane, what would the RMP be?

Answer 19

-80

Question 20

What ion is the biggest driver of RMP and why?

Answer 20

Potassium; it has a permeability of 1 due to open channels, while other ions have a permeability of 0

Question 21

What happens to the membrane potential when more potassium is added outside the cell?

Answer 21

It causes a depolarization of the cell and raised membrane potential

Question 22

What effect does adding more potassium outside of the cell have on an action potential?

Answer 22

It diminishes it; the amplitude and duration decrease

Question 23

Describe phase 0 of the action potential.

Answer 23

A rapid upstroke in membrane potential due to fast sodium channels

Question 24

What begins to happen toward the end of phase 0?

Answer 24

Chloride channels start to open, leading to the closing of sodium channels

Question 25

Describe phase 1 of the action potential.

Answer 25

Early repolarization; potassium leaves the cell and calcium channels begin opening

Question 26

What is the role of potassium channels in phase 1?

Answer 26

One is calcium independent and the other is calcium dependent

Question 27

Describe phase 2 of the action potential.

Answer 27

L-type calcium channels are fully activated; slow and rapid potassium channels activate to provide a plateau

Question 28

What role does the sodium-calcium exchanger play in phase 2?

Answer 28

It plays a small role

Question 29

Describe phase 3 of the action potential.

Answer 29

Calcium channels are inactivated, and potassium channels are activated, allowing repolarization

Question 30

What is the shape of phase 4 in cardiac myocytes?

Answer 30

It is flat; if not, this indicates potential for arrhythmia

Question 31

What is an effective refractory period?

Answer 31

Another action potential cannot happen due to inactivation of sodium channels

Question 32

What is a relative refractory period?

Answer 32

A period during which a super stimulus could trigger another action potential because some sodium channels are in the closed state.

Question 33

In which phase does a delayed after depolarization occur?

Answer 33

Phase 4

It is caused by spontaneous calcium release from the sarcoplasmic reticulum (SR).

Question 34

In which phase does early afterdepolarization occur?

Answer 34

Phase 2/3

It is caused by sodium and calcium channels or the sodium-calcium exchanger.