15. Nernst and GHK workshop

Question 1

What is membrane potential (Em)?

Answer 1

separation of opposite charges across the membrane

Question 2

What does no membrane potential mean?

Answer 2

no potential difference

Question 3

What does it mean when the membrane has potential?

Answer 3

uneven distribution of charge

Question 4

What does magnitude of potential mean?

Answer 4

membrane B has more potential than A and less potential than C

Question 5

What does Em refer to?

Answer 5

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

Question 6

What do excitable cells have the ability to do?

Answer 6

to produce rapid, transient changes in the cells membrane potential when excited (action potential)

Question 7

What is the resting potential?

Answer 7

constant in non-excitable cells, and in excitable cells at rest

Question 8

Where is Na+ most concentrated?

Answer 8

outside of the cell

Question 9

Where is K+ most concentrated?

Answer 9

inside of the cell

Question 10

Where is A- most concentrated?

Answer 10

inside of the cell

Question 11

Where is Cl- most concentrated?

Answer 11

outside of the cell

Question 12

The electrical gradient for both Na+ and K+ will always be located towards…

Answer 12

the negatively charged side of the membrane

Question 13

At resting potential, how many more times is K+ more permeable than Na+?

Answer 13

100x

Question 14

What is the plasma membrane impermeable to?

Answer 14

large negatively charged intracellular proteins (A-)

Question 15

Effect of movement of K+ ions alone on Em

Answer 15

electrical gradient towards ECF (moving K+ into the cell), concentration gradient towards ICF (moving K+ out of the cell
(Em is negative)

Question 16

What is an equilibrium potential?

Answer 16

when both the electrical gradient and concentration gradient balance each other, so no further movement of ions would occur

Question 17

Calculate the equilibrium potential when K+ ion conc inside cell is 150mV and outside cell is 5mM

Answer 17

61log10(5/150

Question 18

Effect of movement of Na+ ions alone on Em

Answer 18

electrical gradient towards ICF (move Na+ out of the cell, concentration gradient towards ECF (move Na+ into the cell
(Em is positive)

Question 19

What is the resting potential value and how is this established?

Answer 19

• 70 mV
• relatively large net diffusion of K+ outward estabolishes an Ek+ of -90mV
• no diffusion of A- across membrane
• relatively small net diffusion of Na+ inwards neutralises some of the potential created by K+ alone

Question 20

Features of the resting membrane potential

Answer 20

• all cells are inside negative at rest
• measured Em using a micro electrode for a typical nerve cell at rest is -70mV which is much closer to Ek than ENa
• Em is not identical to Ek due to the slight inward leak of Na+ into the cell down the Na+ electrochemical gradient

Question 21

What is the Goldman-Hodgkin-Katz equation?

Answer 21

Em= 61log10 Pk+[K+]o + PNa+[Na+]o/ Pk+[K+]i + PNa+ [Na+]i

Question 22

The effect of Na-K+ pump on Em

Answer 22

• pump helps maintain k+ and Na+ gradients across the membrane
• since 3Na+ and 2K+ out, the pump will generate a hyper polarising current
• the amount that is this current adds to Em is variable
  ( in skeletal muscle, it adds only 2-3 mV to Em but in other cell types, it can add 10mV depending upon resistance of the membrane and pump activity