The Neuronal Membrane at Rest (Prof. Pedarzani) Flashcards
What is the average Resting Membrane Potential (RMP) ?
-65mV
What are ion channels ?
Channels typically formed of 4-6 similar subunits surrounding a pore, allowing the passive flow of ions across the membrane along their electrochemical gradient. They are often selective and gated.
What are exchangers ?
Proteins that move ions across the membrane against their electrochemical gradient NRGetically fueled by the transport of other ions or solutes along their gradient.
What are pumps ?
Proteins that move ions across the membrane against their electrochemical gradient using the NRG released by ATP breakdown (hydrolysis).
What 2 factors determine current flow ?
How are they measured ?
Electrical potential (voltage) and electrical conductance. They are measured in Volts (V) and Siemens (S).
What is electrical resistance R ?
How is it measured ?
R = 1/g Ω (Ohms)
What is Ohm’s law ?
I = gV = V/R
How can the RMP be measured ?
By inserting a thin glass tube w/ an extremely fine tip (0.5μm) into the membrane. The tip is filled w/ a conductive salt solution and connected to a voltmeter that measures the potential difference between the tip of the micro electrode and a reference electrode placed outside the cell.
What is the equilibrium potential E ?
The electrical potential that exactly balances the concentration gradient
How sensitive is Vm (membrane potential) to ionic concentrations ?
One word: VERY.
Minuscule difference in ionic concentration cause large changes in Vm.
Where does the net difference in electrical charge occur ?
At the inside and outside surfaces of the membrane.
What is the driving force ?
Driving force = Vm - Eion
It shows that ions are driven across the membrane at a rate proportional to the difference between Vm and Eion.
If the concentration difference across the membrane is known for an ion, how can we calculate an equilibrium potential for that particular ion (if we know its charge) ?
By using the Nersnt equation :
Eion = (RT/Fz) * (ln ([ion]outside/[ion]inside))
What does the RMP depend on ?
The distribution of ions across the membrane (electrochemical gradient) and selective permeability for each ion.
Why is their an unequal distribution of ions across the membrane ?
Intracellular ions (mainly proteins) that cannot cross the membrane are electrically counterbalanced by a high concentration of intracellular K+ ions, so that the intracellular compartment is electrically neutral. Conversely, Na+ has a higher concentration outside the cell.
What are the relative concentration of K+ and Na+ inside/outside the nerve cell ?
What are the electrochemical gradients for these 2 ions ?
[K+] intra = 100mM
[K+] extra = 5mM
[Na+] intra = 15mM
[Na+] extra = 150mM
K+ is pushed inside by electrical gradient but outside by concentration gradient
Na+ is pushed inside by both electrical and chemical gradient
How permeable is the neuronal membrane to K+ and Na+ ?
With a Vm of -60mV, the membrane is 40 times more permeable to K+ than Na+.
There is, however, a steady leak of Na+ into the cell.
The small amount of Na+ that does enter and the passive efflux of K+ would, in the long term, dissipate the concentration gradients of these two ions across the membrane.
How is this delt w/ ?
With the Na+/K+ATPase pump maintains the concentration gradient across the membrane.
For every hydrolyzed ATP, 3Na+ are pumped out of the call and 2K+ are pumped in.
Who do we calculate the resting potential across the membrane w/ several different ions ?
By using the Goldman-Hodgkin-Katz equation.
The concentration difference and membrane permeability of each ions must be taken into account.
What is the consequence of the high presence of K+ leak channels ?
The RMP is close to E for K+. Thus, Vm is particularly sensitive to changes in extracellular [K+].
The concentration of K+ is therefore tightly regulated in the brain e.g. by the blood-brain barrier and astrocytes (potassium spatial buffering.
