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Question 1

What is the Resting Potential (RP) that is found on the cell membranes of
nerve and muscle cells?

Answer 1

An electrical charge that must be present at all times to allow electrical currents (Action
Potentials) to be produced in muscle and nerve cells.

Question 2

Why is the RP called a transmembrane potential?

Answer 2

It is the difference in charge found across the cell membrane. The RP is a slight
negative charge on the inside of the cell membrane compared to a slight positive charge
on the outside of the cell membrane.

Question 3

What is the voltage of the Resting Potential in millivolts (mV)?

Answer 3

-70 mV (negative 70 millivolts).

Question 4

What is the main cause of the Resting Potential?

Answer 4

The constant leaking of potassium (K+ ) out of the cell through nongated K+ channels
creates most of the voltage of the RP. (Additionally, the Sodium/Potassium pump
also helps make the RP).

Question 5

Why are Action Potentials (AP) so important in the body? (In other words,
what three main body functions do APs control?)

Answer 5

Action Potentials are the electrical currents that cause brain activity, help control organ
activities, and cause muscle contraction.

Question 7

Why is the Action Potential called a reversal of the Resting Potential?

Answer 7

While the Resting Potential is a negative charge on the inside of a cell membrane (with a 
positive charge on the outside), the Action Potential is a positive charge on the inside of a 
cell membrane (with a negative charge on the outside). The Action Potential is therefore 
a reversal of the Resting Potential because the charges on the inside versus the outside 
of the cell membrane have been flip-flopped (changed to their opposites).

Question 8

What is another way to describe the Action Potential?

Answer 8

It is a strong depolarization—because it is a positive electrical charge. The value of the
AP is + 20 mV.

Question 9

In what two other ways is the Action Potential different from the Resting
Potential?

Answer 9

The Resting Potential is present when muscle or nerve cells are “resting,” or not
producing electrical currents. In other words, when a nerve is not firing and when a
muscle is relaxed. The Action Potential causes “action” because it moves through
nerves, helping control the activities of organs and causing muscle contractions. Also,
the Action Potential travels down the axons of neurons instead of “sitting still” on the cell
membrane like the Resting Potential does.

Question 10

What are the three phases of the Action Potential and what causes each
phase?

Answer 10

First phase: Depolarization. Caused when gated sodium channels open, letting Na+
enter the cell. Entry of sodium causes the voltage on the cell to change from -70 mV to
+20 mV.
Second phase: Repolarization. Caused when the gated sodium channels shut, and
gated potassium channels open, letting K+ exit the cell. Exit of potassium causes the
voltage to drop from +20 mV back down to -70 mV.
Third phase: Hyperpolarization. Caused by too much potassium leaving the cell. The
gated potassium channels remain open. This causes the voltage of the cell to drop
slightly below -70 mV.

Question 11

Why do sodium and potassium move in opposite directions when their
gated channels are opened?
How is their movement connected to their concentrations inside versus
outside of cells?

Answer 11

Sodium moves INTO cells when its gated channels are opened because there is a large
concentration (amount) of sodium outside of cells (compared to inside the cell).
Potassium moves OUT of cells through its channels (both gated and
nongated/leakage channels) because there is a large potassium concentration inside
cells (compared to outside cells).
What creates this difference in sodium and potassium concentrations?
Sodium/Potassium pumps, which move 3 sodium out of cells while moving 2
potassium into cells. This means the Sodium/Potassium pumps are responsible for
sodium and potassium moving in opposite directions through their specific channels.