Origin of membrane potentials

Question 1

what is membrane potential?

Answer 1

• it’s the separation of opposite charges across the membrane. (refers to the difference in charge between the thin layers of ECF and ICF located next to the outside & inside of membrane respectively)
• plasma membranes of all cells are polarised electrically

Question 2

is the membrane charged?

Answer 2

NO - it’s not the membrane itself that is charged

Question 3

when is resting membrane potential in:
a) non-excitable cells?
b) excitable cells?

Answer 3

a) constant
b) resting membrane potential only when they’re at rest

Question 4

where does sodium and chloride ions have the highest concentration?

Answer 4

more Na+ and Cl- outside compared to inside

Question 5

where do potassium ions have the highest concentration?

Answer 5

greater K+ ions inside compared to outside

Question 6

how many more times permeable is K+ compared to Na+?

Answer 6

K+ ions are 100 times more permeable than Na+

Question 7

what direction do K+ ions travel?

Answer 7

K+ move outwards (think opposite of SOPI as SOPI is active transport)

Question 8

what direction do Na+ ions travel?

Answer 8

Na+ move inwards (think opposite of SOPI as SOPI is active transport)

Question 9

what is plasma membrane impermeable to?

Answer 9

the large negatively charged (anionic) intracellular proteins (A-)

Question 10

describe the movement of K+ and it’s effect on Em?

Answer 10

• more K+ inside over outside means potassium ions move from inside → outside meaning positively charged outside and negatively charged inside = membrane potential difference
• however know some K+ ions are attracted back into cell due to negative charge inside the cell
  
  • now we have 2 forces acting across lipid bilayer

Question 11

what is potassium equilibrium potential?

Answer 11

movement (of 2 forces acting against each other) will occur until both forces are at balance

the point of balance = potassium equilibrium potential

value = -90 mV

Question 12

what are the 2 opposing forces acting on K+?

Answer 12

1. The concentration gradient (tending to move K+ out of the cell)
2. The electrical gradient (tending to move K+ into the cell)

Question 13

what does the value of potassium equilibrium membrane potential mean?

Answer 13

-90 mV

= Important: the sign (+ or -) is the polarity of the excess charge on the inside of the membrane
- so at equilibrium the excess charge on inside of membrane is K+

Question 14

what is the nernst equation?

Answer 14

the equation that can be used to calculate the equilibrium potential for any given ion (e.g. -90 mV for K+ and +60 mV for Na+)

E(ion) = RT/zF ln [ion] outside/[ion] inside

it’s is the (gas constant x temp divided by valency x faradays constant) x log of (outside ion concentration divided by inside ion concentration)

simplified to 61 log10 x (outside ion concentration divided by inside ion concentration)

Question 15

what is the movement of Na + effect on membrane potential?

Answer 15

• more outside than inside so some Na+ move inside cell making inside more positive and make outside more negative = concentration gradient
• again means some want to move back out = electrical gradient

Question 16

what is the sodium equilibrium potential?

Answer 16

+ 60 mV when balanced
- means polarity of excess charge on inside membrane is +ve

Question 17

what is equilibrium membrane potential of typical nerve cell at rest and why?

Answer 17

• -70mV (so - tells us that inside of cell more negative than outside of cell)
• this is because membrane more permeable to K+ than Na+ so more K+ going out than Na+ going in which makes overall -ve inside

Question 18

why is K+ equilibrium potential is closer to resting membrane potential?

Answer 18

simply due to plasma membrane being more permeable to K+ over Na+

Question 19

what is goldman-hodgkin-katz equation?

Answer 19

equation used to calculate membrane potential

= 61 log 10 x (relative permeability of K+ x conc of K+ outside cell) + (relative permeability of Na+ x conc of Na+ outside cell) divided by the same but with concentration of inside

Question 20

what is value of Pk+ and PNa+ in GHK equation?

Answer 20

Pk =1
PNa = 0.01

(as PK is 100x PNa)

Question 21

what is the effect of Na/K pump?

Answer 21

• The Na+-K+ pump helps maintain the K+ and Na+ gradients across the membrane.
• Since its stoichiometry is 3Na+ exported for every 2K+ entering, the pump will generate a hyperpolarizing current (SOPI)
• The amount that this current adds to Em is variable as some cells have more Na/K pumps than others

Question 22

what is effect on membrane potential if we make membrane more permeable to a particular ion?

Answer 22

membrane potential will move toward that ions equilibrium potential → essentially what happens in excitable cell

Question 23

what results in fluctuations of membrane potential

Answer 23

• Specialised use of Em in nerve and muscle cells:
  
  • Rapidly and transiently alter their membrane permeabilities in response to appropriate stimulation