15. Nernst and GHK workshop Flashcards
What is membrane potential (Em)?
separation of opposite charges across the membrane
What does no membrane potential mean?
no potential difference
What does it mean when the membrane has potential?
uneven distribution of charge
What does magnitude of potential mean?
membrane B has more potential than A and less potential than C
What does Em refer to?
the difference in charge between thin layers of ECF and ICF located next to the inside and outside of the membrane
What do excitable cells have the ability to do?
to produce rapid, transient changes in the cells membrane potential when excited (action potential)
What is the resting potential?
constant in non-excitable cells, and in excitable cells at rest
Where is Na+ most concentrated?
outside of the cell
Where is K+ most concentrated?
inside of the cell
Where is A- most concentrated?
inside of the cell
Where is Cl- most concentrated?
outside of the cell
The electrical gradient for both Na+ and K+ will always be located towards…
the negatively charged side of the membrane
At resting potential, how many more times is K+ more permeable than Na+?
100x
What is the plasma membrane impermeable to?
large negatively charged intracellular proteins (A-)
Effect of movement of K+ ions alone on Em
electrical gradient towards ECF (moving K+ into the cell), concentration gradient towards ICF (moving K+ out of the cell
(Em is negative)
What is an equilibrium potential?
when both the electrical gradient and concentration gradient balance each other, so no further movement of ions would occur
Calculate the equilibrium potential when K+ ion conc inside cell is 150mV and outside cell is 5mM
61log10(5/150
Effect of movement of Na+ ions alone on Em
electrical gradient towards ICF (move Na+ out of the cell, concentration gradient towards ECF (move Na+ into the cell
(Em is positive)
What is the resting potential value and how is this established?
- 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
Features of the resting membrane potential
- 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
What is the Goldman-Hodgkin-Katz equation?
Em= 61log10 Pk+[K+]o + PNa+[Na+]o/ Pk+[K+]i + PNa+ [Na+]i
The effect of Na-K+ pump on Em
- 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
