Origin of Membrane Potentials

Question 1

what is the membrane potential

Answer 1

the separation of opposite charges across the membrane

*all cells have membrane potential

Question 2

what is responsible for the potential difference across a membrane

Answer 2

when there is a membrane potential (ie if the ICF has more negative ions than the ECF) the polar ions gather up along each side of the membrane
ie the separated charges are responsible for the potential difference

Question 3

what is Em

Answer 3

the difference in charge between the thin layers of the ECF and ICF located next to the inside and outside of the membrane

Question 4

when would you find a resting membrane potential

Answer 4

in non-excitable cells

and

in excitable cells at rest

Question 5

what direction does sodium travel down the concentration gradient

Answer 5

inward

Question 6

what direction does potassium travel down the concentration gradient

Answer 6

outward

Question 7

what are cations

Answer 7

positively charged ions

ie sodium and potassium (their electrical gradient will always be towards the negatively charged side of the membrane)

Question 8

when is the membrane more permeable to potassium than sodium

Answer 8

when it is at resting potential

Question 9

when considering the equilibrium potential for potassium - what are the two opposing forces acting on potassium

Answer 9

1. the conc. gradient (tending to move K out of the cell)

2. the electrical gradient (tending to move K into the cell)

Question 10

what happens when the two opposing forces acting on K balance each other

Answer 10

there is no further net movement of potassium

Question 11

what is Ek ( E (K))

Answer 11

the equilibrium potential for K

= -90mV (inside the cell)

Question 12

what is the Nernst equation

Answer 12

an equation to calculate the equilibrium potential for any ion

Question 13

what is E (Na)

Answer 13

+61mV

Question 14

what is the resting membrane potential (Em) for a typical nerve cell and what is it due to

Answer 14

relatively large efflux of K making the Em lean toward Ek (-90mV) however there is a small influx of sodium ions which neutralises some of the potential (-70mV)

Question 15

what is the Goldman-Hodgkin-Katz equation and when can we use it

Answer 15

to calculate the Em

if we know: ion concentrations inside and outside the cell and the relative membrane permeability for each ion

Question 16

what effect does the sodium/potassium pump have on the Em

Answer 16

> creates a hyperpolarising current

ie 3Na out and only 2K in, loses a positive charge with every ATP molecule split

> also helps maintain transmembrane sodium and potassium concentration gradients

Question 17

why is the Em important

Answer 17

the Em has specialised use in nerve and muscle cells ie membrane permeabilities are altered rapidly and transiently in repose to certain stimuli > resulting in fluctuation in the Em

*eg changes in the Em is linked to the secretion of insulin from pancreatic B-cells

Question 18

what is the single most important factor in setting the Em

Answer 18

the potassium gradient

Question 19

at rest - what is Em

Answer 19

-83mV