Question 1

Which of the following best explains why cardiac pacemaker cells have a slow depolarization compared to skeletal muscle cells?

Accepted Answer

Pacemaker cells have simultaneous movement of sodium ($Na^+$) in and potassium ($K^+$) out, slowing depolarization.

Question 2

During the plateau phase of a cardiac contractile cell's action potential, which ions are primarily responsible for maintaining the membrane potential?

Accepted Answer

Influx of $Ca^{2+}$ and efflux of $K^+$.

Question 3

A patient is given a drug that increases the permeability of sodium/potassium channels in pacemaker cells. What is the most likely effect on heart rate?

Accepted Answer

Heart rate will increase due to faster depolarization.

Question 4

Which of the following correctly describes the difference in ion usage during action potentials in cardiac contractile cells versus skeletal muscle cells?

Accepted Answer

Cardiac contractile cells use $Na^+$, $K^+$, and $Ca^{2+}$; skeletal muscle cells use only $Na^+$ and $K^+$.

Question 5

What is the primary reason tetanic contractions do not occur in cardiac muscle tissue?

Accepted Answer

Cardiac muscle cells have a prolonged absolute refractory period due to the plateau phase.

Question 6

Which phase of the cardiac contractile cell action potential is associated with the opening of voltage-gated calcium channels?

Accepted Answer

Plateau phase

Question 7

If the permeability of potassium channels in cardiac contractile cells is increased immediately after depolarization, what effect would this have on the absolute refractory period?

Accepted Answer

Absolute refractory period would decrease.

Question 8

Which of the following mechanisms allows cardiac pacemaker cells to be autorhythmic?

Accepted Answer

Sodium channels in pacemaker cells allow both calcium and sodium to pass through.

Question 9

Which type of ion channel opening mechanism is responsible for the influx of $Ca^{2+}$ during the action potential in cardiac muscle cells?

Accepted Answer

Voltage-gated

Question 10

A mutation causes the sodium channels in cardiac pacemaker cells to open at a higher membrane potential than normal. What is the most likely effect on heart rate?

Accepted Answer

Heart rate will decrease due to delayed depolarization.

Question 11

Which statement best describes the channels active during the pacemaker potential compared to those used in other action potentials?

Accepted Answer

They are voltage-gated.

Question 12

Which of the following is responsible for the slow repolarization observed in cardiac contractile cells?

Accepted Answer

Prolonged opening of calcium channels and delayed opening of potassium channels.

Question 13

During which phase of the cardiac contractile cell action potential can the cell NOT respond to another action potential?

Accepted Answer

Absolute refractory period

Question 14

Which of the following best explains why cardiac contractile cells do not exhibit tetanic contractions?

Accepted Answer

They have a prolonged absolute refractory period due to the plateau phase.

Question 15

If a drug blocks the calcium channels in cardiac contractile cells, which phase of the action potential would be most affected?

Accepted Answer

Plateau phase

Question 16

Which of the following is a unique feature of cardiac pacemaker cells compared to cardiac contractile cells?

Accepted Answer

Ability to generate action potentials without external signals (autorhythmicity).

Question 17

Which equation best represents the Nernst potential for potassium ($K^+$) ions in cardiac cells?

Accepted Answer

$E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Question 18

A patient with a genetic mutation has cardiac contractile cells with potassium channels that open more slowly than normal. What is the most likely effect on the action potential?

Accepted Answer

Prolonged plateau phase and increased refractory period.

Question 19

Which of the following best describes the movement of ions during the slow depolarization phase of pacemaker cells?

Accepted Answer

$Na^+$ flows into the cell while $K^+$ flows out.

Action Potentials in Cardiac Cells: Pacemaker and Contractile Cell Physiology

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