Lecture 5: Membrane Potentials

Question 1

Why do you want to “excite” a cell?

Answer 1

To allow cells communicate to other cells or its own interior

Question 2

What is resting membrane potential primarily due to?

Answer 2

The permiability of the plasma membrane to potassium ions

Potassium Leaky Channels

Question 3

What is the accepted resting membrane potential for cardiac and skeletal muscle cells?

Answer 3

-80 to -90 mV

Question 4

What is the accepted resting membrane potential for smooth muscle?

Answer 4

-60 mV

Question 5

What is the accepted resting membrane potential for Neurons?

Answer 5

-60 to -70 mV

Question 6

What are the differences between the following?

Voltage Gated

Ligand Gated

Signal Gated

Answer 6

Voltage Gated: Channel is opened in response to change in membrane potential

Ligand Gated: Channel is opened in response to a specific extracellular NT (e.g. acetylcholine)

Signal Gated: Opens in resposne to a specific intracellular molecule

Question 7

In a Na⁺/K⁺ ATPase pump, what goes in and out of the cell?

Answer 7

3 Na⁺ Out and 2 K⁺ In

Requires Energy

Question 8

Which ion are membranes most permeable to?

Answer 8

Potassium

Potassium Leak Channels are open all the time

Question 9

What two forces act on ions to help develop membrane potential, and what determines those forces?

Answer 9

Diffusion/Chemical Gradients: Concentration of a given ion in vs out of the cell

Electrostatic Forces: Like repels like and opposites attract

(Negative ions are less likely to cross over to the side of the membrane with more negative ions, but more likely to cross over to the side with more positive ions)

Question 10

What is equilibrium potential for a given ion?

Answer 10

The membrane potential when electrical and chemical forces on that ion are equal, and no further movement of that ion occurs.

Not the same as resting membrane potential

Question 11

What is the equilibrium potential for Sodium and Potassium?

Answer 11

Sodium: 66 mV

Postassium: -91 mV

Question 12

What is the equilibrium potential for Calcium?

Answer 12

+123 mV

Question 13

What is the equilibrium potential for Chloride?

Answer 13

-66.4 mV

Question 14

What is the Nernst Equation?

Answer 14

E_ion= (61.5/x)(log [out]/[in])

Where [out] is the extracellular concentration of a given ion

Where [in] is the intracellular concentration of a given ion

Question 15

In terms of the Nernst Equation, if the concentration of an ion is greater inside the cell, will the log come out as positive or negative?

Answer 15

Negative

Log [x]out/[x]in

The denominator is larger, requiring the log to be negative.

Question 16

What is the equation to determine the “driving force” of an ion?

Answer 16

[Resting membrane potential] - [Equalibrium constant of a given ion] = Driving Force

Question 17

If a driving force is positive, what is the direction of movement for an ion?

Answer 17

Out of the cell

Example: Potassium

Question 18

If a driving force is negative, what is the direction of movement for an ion?

Answer 18

Into the cell

Example: Sodium

Question 19

Other than the potassium leak channels, what other forces contribute to the resting membrane potential?

Answer 19

Sodium Diffusion

Na⁺/K⁺ ATPase Pump

Question 20

What is the resting membrane potential for Sodium?

Answer 20

+61.5 mV

Question 21

In terms of potassium movement, what is happening when a neuron is at its resting membrane potential?

Answer 21

Potassium movement out of the cell is roughly equal to potsassium movement into the cell

Question 22

Is it easier to depolarize a cell with a higher or lower resting membrane potential?

Answer 22

Higher

The higher the resting membrane potential, the closer it is to threshold, and the less is required to depolarize it.

Question 23

What is a graded potential?

Answer 23

A stimulus that does not reach the threshold to become an action potential

Question 24

What is the function of voltage gated sodium channels?

Answer 24

Depolarization phase of an action potential

Question 25

Do voltage gated sodium channels open quickly or slowly?

Answer 25

Quickly

Question 26

Voltage Gated Sodium Channels

What gate is open upon activation?

What gate blocks out sodium after activation?

Answer 26

Activation Gate

Inactivation Gate

Question 27

What state are voltage gated sodium channels in during repolarization?

Answer 27

Their activation gates are open, but their inactivation gates are closed

Question 28

What is the function of a voltage gated potassium channel?

Answer 28

Repolarization phase of an action potential

(positive potassium leaves the cell)

Question 29

Do voltage gated potassium channels open quickly or slowly?

Answer 29

Open slowly

Question 30

What is a main contributor to causing the hyperpolarization period?

Answer 30

Voltage gated potassium channels stay open a bit too long.

Question 31

What makes up the absolute refractory period?

Answer 31

Depolarization (Overshoot)

Repolarization

Question 32

Why can you not cause sodium channels to open during the absolute refractory period?

Answer 32

Because they are either already open, or the inactivation gates are closed, and have not yet reset.

Question 33

What phase makes up the relative refractory period?

Answer 33

Hyperpolarization

Question 34

What two things are happening relating to voltage gated ion channels to make it harder to generate an action potential during the relative refractory period?

Answer 34

Voltage gated Potassium channels are still open, or too much potassium has left the cell.

Not all voltage gated sodium channels are reset and ready to open.

Question 35

What type of cell has an unusual repolarization graph?

What is a consequence of this fact?

Answer 35

Cardiac Ventricle cells - their repolarization curve is very broad, and they have a long absolute refractory period.

Question 36

What do you see in patients with intermittantly low blood potassium?

Answer 36

Hypokalemia

• Paralysis due to a larger gap between resting membrane potential and threshold.
• Repolarization occurs more quickly

Question 37

What happens with patients with higher levels of potassium in their blood?

Answer 37

Hyperkalemia

• Unable to compensate like regular people
• Prolonged action potential
• Absoulte Refractory Periods lengthened