Nernst and GHK Flashcards
Membrane potential (Em)
The plasma membranes of all cells are polarised electrically
Membrane potential - the separation of opposite charges across the membrane
Units - mV
The membrane itself is not charged
Em refers to the difference in charge between the layers of intracellular fluid and extracellular fluid located next to the inside and outside of the membrane
how to determine membrane potential
Equal positive and negative charges in the intracellular and extracellular fluid = membrane has no potential
Differing positive and negative charges between the intracellular and extracellular fluid = membrane has potential
Separated charges are responsible for potential - they form a layer along the plasma membrane whilst the remainder of the intra- or extracellular fluid is electrically neutral
The number of charges relates to the magnitude of potential with more charges meaning increased potential
membrane potential and key ions
Em is due to differences in the concentration and permeability of key ions
All cells have a membrane potential
Excitable cells (nerve and muscle cells) have the ability to produce rapid, transient changes in their membrane potential when excited (action potentials)
Resting membrane potential = constant in non-excitable cells, and in excitable cells at rest.
Resting membrane potential is always inside negative
e.g. for a typical nerve cell at rest, Em = -70mV
Unequal distribution and the selective movement of ions through the plasma membrane are responsible for the electrical properties of the membrane
concentration gradients of K+ and Na+
The concentration gradient for K+ is outward
The concentration gradient for Na+ is inward
As K+ and Na+ are cations the electrical gradient for both will always be towards the negatively charged side of the membrane
For a skeletal muscle cell at resting potential the membrane is 100x more permeable to K+ than Na+
The plasma membrane is impermeable to the large negatively charged (anionic) intracellular proteins (A-)
equilibrium potential for K+
the concentration gradient for K+ tends to move this ion out of the cell
The outside of the cell becomes more positive as K+ ions move to the outside down their concentration gradient
The membrane is impermeable to the large intracellular protein anion (A-). The inside of the cell becomes more negative as K+ ions move out, leaving behind A-
The resulting electrical gradient tends to move K+ into the cell
No further net movement of K+ occurs when the inward electrical gradient exactly counterbalances the outward concentration gradient. The membrane potential at this equilibrium point is the equilibrium potential for K+ (Ek+) at -90mV
Ek = approximately - 90mV
equilibrium potential for Na+
The concentration gradient for Na+ tends to move this ion into the cell
The inside of the cell becomes more positive as Na+ ions move to the inside down their concentration gradient
The outside becomes more negative as Na+ ions move in, leaving behind in the extracellular fluid unbalanced negatively charged ions, mostly Cl-
The resulting electrical gradient tends to move Na+ out of the cell
No further net movement of Na+ occurs when the outward electrical gradient exactly counterbalances the inward concentration gradient. The membrane potential at this equilibrium point is the equilibrium potential for Na+ (ENa) at +60mV
ENa = approximately + 60mV
Equilibrium potential
the transmembrane potential difference required to exactly balance a given concentration gradient
The transmembrane potential difference at which there is no NET flux of the ion in question
Each ion has its own specific equilibrium potential
what is The Nernst equation
the equilibrium potential for any given ion can be calculated using the Nernst equation
what is the Nernst equation for monovalent cations at 37ºC
= 61 log ([outside]/[inside])
what is the Nernst equation for monovalent cations at 20ºC
= 58 log ([outside]/[inside])
what is the Nernst equation for a divalent cation at 37ºC
= 30.5 log ([outside]/[inside])
what is the Nernst equation for a divalent cation at 20ºC
= 29 log ([outside]/[inside])
what is the Nernst equation for an anion cations at 37ºC
= -61 log ([outside]/[inside])
what is the Nernst equation for an anion cations at 20ºC
= -58 log ([outside]/[inside])
Concurrent K+ and Na+ effects on Em
for any living cell effects of both K+ and Na+ need to be considered
The greater the permeability for a given ion, the greater the tendency for that ion to drive membrane potential towards the ions own equilibrium potential.
Pk is > PNa (at rest)
Ek = -90mV where ENa = +60mV
Resting membrane potential
All cells are inside negative at rest
Measured membrane potential for a typical nerve cell at rest is = -70mV
Closer to the equilibrium potential of K+ than of Na+
Membrane potential is not identical to equilibrium potential of K+
Due to the slight inward leak of Na+ into the cell
Goldman-Hodgkin-Katz (GHK) equation
can be used to calculate membrane potential
ADD EQUATION
Movement of ions across a membrane
cell membrane is relatively impermeable to ions
What factors need to be considered?
> Ion-specific channels
> Open or closed
> Driving force - something to push the ions through an open channel
> Concentration gradient for that ion
Movement of ions across a membrane will generate a current (I)
Importance of Em
specialised use of Em in nerve and muscle cells
Rapidly and transiently alter their membrane permeabilities in response to appropriate stimulation, resulting in fluctuations in Em
Action potentials
Changes in Em is linked to secretion of insulin from pancreatic β-cells
ions during the action potential
ion fluxes vary during the various stages of the action potential
The relative permeabilities of the relevant ions also vary
Relative Permeability Values
Summary
the magnitude of K+ and Na+ concentration gradients, together with differences in permeability of the membrane to these ions, accounts for the magnitude of the resting membrane potential
At rest the membrane potential is close, but not identical to, the equilibrium potential of K+
Membrane potential is a few mVs less negative that equilibrium potential because of a mall but significance permeability of Na
The K+ gradient is the single most important factor in setting the membrane potential
