L2 - Membrane Potentials (Chapter 5)

Question 1

What is meant by “membrane potential”? What are its units of measure in living cells?

Answer 1

• Charge distribution across a membrane – dependent on membrane transport
• Measured in millivolts
• When neg, there is a more negative charge on the inside of the cell then the outside

Question 2

How is charge distributed throughout living cells?

Answer 2

For the most part, charges are balanced except at the membrane where charges build up

Question 3

Is there a voltage throughout the entire cell, or is the voltage localized to one part of the cell? If so, which part?

Answer 3

• Voltage is localized at the membrane of the cell
• Opposites attract – anions and cations get as close together as they can, but are stopped by the membrane
• Why its called membrane potential and not cell potential

Question 4

What is a “Donnan Equilibrium”?

Answer 4

• Equilibrium that can result in equal distribution of ions across a membrane with no energy input - imbalance of concentration of ions creating a chemical and electrical gradient
• Passive diffusion

Question 5

In what way is it a special type of equilibrium? What happens with this type of equilibrium? (Donnan equilibrium)

Answer 5

Based on the starting condition and presence of non-penetrating ions, there can be an uneven distribution of molecules

Question 6

Most of the membrane potential results from what process? What mechanism contributes to the remainder of the resting membrane potential?

Answer 6

• Passive diffusion - ~95% of resting membrane potential
• Active pumping - ~5% of resting membrane potential

Question 7

Voltage (V)

Answer 7

• diff in electrical charge btwn 2 points – uneven distribution of charge
• In cells its the diff in charge across the membrane – ECF/ICF
• Measured in volts

Question 8

Current (I)

Answer 8

– flow of charge across a membrane

• Vector quantity – has mag and direction
• Measured in amps
• Indirectly proportional to resistance, directly proportional to voltage

Question 9

Membrane Current (Im)

Answer 9

current is flowing across the membrane from ICF to ECF and vice versa
- the flow of charge across the membrane

Question 10

Axial Current (Ia)

Answer 10

current is flowing along the membrane

Question 11

Inward Current (Ii)

Answer 11

• positively charged ions going into the cell
• EX: Na+, Ca+, and glutamate
• Depolarizes the cell membrane, excites the cell – can cause APs in excitable cells

Question 12

Outward Current (Io)

Answer 12

• negatively charged ions going into the cell, or positively charged ions moving out of the cell
• EX: K+, Cl-, and GABA/glycine
• Chloride counts b/c it’s still making the ICF more negative
• Hyperpolarize the cell membrane – drops below membrane potential, inhibiting the cell from having an AP

Question 13

Resistance (R)

Answer 13

– opp of membrane to current flow

• Measured in ohms
• Indirectly proportional to current (Decrease resistance = increase current)
• Influenced by gated channels – when they open, resistance decreases, and when they close, resistance increases

Question 14

Membrane Resistance (Rm)

Answer 14

resistance across the membrane

Question 15

Axial Resistance (Ra)

Answer 15

resistance along the membrane

Question 16

What is the algebraic relationship between V, I and R?

Answer 16

• Ohm’s Law - V=IR or I=gV
• Increase in I or R would increase V, a decrease in I or R would decrease V

Question 17

What does g stand for in the alternate equation for Ohm’s Law?

Answer 17

• conductance
• measured in siemens or mhos
• opposite of resistance

Question 18

What is conductance (C)?

Answer 18

• storage of charge across a thin barrier/dielectric
• important for conduction speed

Question 19

What is membrane capacitance?

Answer 19

• Cm
• referring to the capacitive property of phospholipid bilayer
• important for conduction speed

Question 20

How do cell membranes act as capacitors?

Answer 20

• hydrophobic tails of phospholipids create a nonpolar region in the interior of the bilayer, preventing the passage of polar molecules (such as ions) through the membrane.

Question 21

Sharp microelectrode

Answer 21

– impales the cell, making the cytoplasm of the cell continuous with the electrolyte solution inside of the electrode
- Recording and stimulating electrode
- intracellular recording

Question 22

Patch clamping

Answer 22

– blunt, fire polished electrode is used
- Whole cell - suction is used to tear a hole in the membrane, and the blunt tip can then be inserted through the hole – similar to sharp electrode method
- On cell patch clamp - Form a seal on the membrane without breaking it, such that you have one or more ion channels within the patch you are recording from
- intracellular recording

Question 23

Current clamping

Answer 23

manipulating the current in the cell by injecting current

Question 24

Voltage clamping

Answer 24

– manipulate voltage

• Can study ionic currents, the increase or decrease as a result of voltage change
• Typically clamp the cell at the holding potential, and influence the cell to reach command potential

Question 25

What is equilibrium potential?

Answer 25

• Membrane potential that exactly opposes net diffusion of an ion across the cell membrane
• Calculated for individual ions (Eion) – measures a single ion, diff from membrane potential which is measuring multiple ions

Question 26

What else is equilibrium potential sometimes called?

Answer 26

Electromotive force – EMF

Nernst potential

Diffusion potential

Question 27

How can you calculate equilibrium potential? What equation is used?

Answer 27

• Nernst equation
• R - gas constant
• T - absolute temp (kelvins)
• Z - ion valence
• F - faraday constant

Question 28

If the Nernst equation is being used on an ion that is at body temp, what can it be simplified to?

Answer 28

Question 29

Practice using sample equations from powerpoint:

Answer 29

Question 30

Practice using sample equations from powerpoint:

Answer 30

Question 31

What is reversal potential?

Answer 31

• Voltage at which the direction of current changes
• Current switches from inward to outward
• EX: at rest, K usually flows outside of the cell, making the membrane potential more negative until it reaches –94 mV or greater, which will cause the current to reverse and have K flow back into the cell

Question 32

Have a basic knowledge of the concentration of diff ions:

Answer 32

• K is usually moving out of the cell
• Na is usually moving into the cell
• Ca is usually moving into the cell
• Cl is usually moving into the cell