lecture 4 membrane potential Flashcards
what is equilibrium potential
when flow of ions (current) due to concentration gradient of an ion is exactly counterbalanced by the current due to the electrical gradient across the membrane
what is the nerst equation used to calculate
the membrane potential at which an ion’s fixed concentration gradient is in equilibrium
how do you calculate the nerst potential
60/valence charge of ion
* log (concentration out/concentration in)
what is resting potential
what value a cell’s membrane potential is when it’s at rest
how are glial cells different from neurons relative to ion channels
leak k+ channels are the only ion selective channels
what two potentials are equal in glia cells
membrane potential and potassium equilibrium potential
what does depolarizing mean
membrane potential is getting more positive, going toward positive values
what does hyperpolarizing mean
membrane potential is getting more negative toward negative values
goldman hodgkins katz equation function
how to figure out the resting potential when multiple ions are permeable (such as in neurons)
are na and cl permeable in glial cells
no, the permeability is negligible so the ghk equation isn’t helpful in glia cells
why is there a sodium potassium pump
maintains k+ and na+ concentrations
where does most of energy neurons produce go
na/k pump, because as they move the ions against their concentration gradient they create energy
when is an ion in equilibrium
at its nerst potential, because flow inward and outward are equal
when is an ion in a steady state
with ghk, where the inward and outward currents are equal but may be carried by different ions
what is the pump used in the na k pump made of
atpase, which uses energy by breaking down atp to adp
what is resting potential
what value a cell’s membrane potential is when it’s at rest (-70)
how do glia cells differ from neurons membrane poential
in glia cells, leak k+ channels are the only ion selective channels
what is the membrane potential of glia cells equal to
potassium equilibrium potential
what does ghk stand for
goldman hodkins katz
what is the ghk equation used for
used for calculating resting potential when multiple ions are permeable
what is the ghk equation
60*log {(permeability of ion concentration in/permeability of ion concentration out) + (permeability of ion concentration in/permeability of ion concentration out) + (permeability of ion concentration in/permeability of ion concentration out)
why are there k leak channels
the leak channels bring excitable neurons back to stable state (restores equilibrium and brings back normal potential)
what are the positive cations
k, na, ca
what are the negative anions
cl, a (impermeant cellular proteins)
why is the na k pump electrogenic
it creates a stronger electrical gradient by moving them against their concentration gradient, increasing energy and allowing atp to be made and used
what factors allow ions to move in and out of cells
1) concentration gradient
2) electrical gradient/membrane potential
3) ion selective channels
what gradients are in equilibrium at the equilibrium potential of an ion
chemical and electrical gradient
What two gradients are in balance when an ion is said to be at its equilibrium potential
Draw a graph of neuron’s cell membrane potential as a function of changing extracellular
potassium concentration (and how it differs from the graph of a glial cell)
Recall the equilibrium potentials of potassium, sodium, and chloride and know which way
each would flow in a resting neuron
Be able to apply the Nernst and GHK equations and know when to use which equation
If a membrane is permeable to all ions, ion flow will eventually ______.
reach equilibrium
What makes the cell membrane permeable to specific ions?
ion specific channels
The Nernst equation calculates each ion’s equilibrium potential based on its _____.
both charge and ratio
Glial cells are normally permeable to ______.
just K+
If the concentration of a cation is higher on the outside of the neuron compared to the inside, the equilibrium potential will be _____ .
positive
At a typical neuronal resting potential, the net movement of potassium ions through leak channels _____.
is outward
The equation used to calculate membrane potential when there are multiple permeant ions is called the _____ equation.
Goldman, Hodgkin, Katz
Resting potential
the membrane potential of a cell when it is just
chilling (no synaptic or action potential activity)
Membrane potential
is a measure of the difference in the number of
positive versus negative charges inside a cell
Write out the narrative for what it means for sodium to have an equilibrium
potential of +60 mV
(15mM inside and 150mM outside)
Imagine a cell with no channels and no membrane potential (i.e. it’s at 0 mV) but the outside
of the cell has 10-fold more sodium (Na+) than the intracellular cytosol. Now, let’s allow the
cell to have open sodium channels (i.e. it becomes permeable to Na+). Na+ will move down its
concentration gradient (outside to inside) until the membrane potential becomes +60mV
inside. At that membrane potential (and only that potential) the now positively charged inside
membrane potential will repulse enough of the high concentration of Na+ outside to stop the
net flow inward so that the movement of Na+ inward and outward are equal and opposite: in
equilibrium, that is, the chemical gradient (high outside) is counterbalanced by the electrical
gradient (high pos. inside). (Please remember the number of ions that move is so small that
the concentrations inside and out hardly change at all).
how do capacitors in electric
circuits work
Charge of membrane potential
is “stored” on the membrane
where the excess charges in the
cell (typically too many anions)
are aligned with ions of the
opposite charge on the other
side of the nonconductive lipid
bilayer (the dielectric)
In a neuron at normal resting physiological conditions, the resting potential is a) the same, b)
more hyperpolarized, or c) more depolarized than in a glial cell? Why?
A typical neuron is more depolarized than a glial cell. This is because a neuron at rest is permeable to
sodium and chloride in addition to potassium, while a glial cell is typically permeable only to potassium. The
equilibrium potentials for both sodium and chloride are more positive (depolarized) than the equilibrium
potential for potassium, so the flow of those ions make the membrane depolarized compared to EK. The
exact values can be calculated using the GHK equation
What is the equilibrium potential of Ca++ ions? How does that compare to the equilibrium
potential for potassium? If the extracellular concentration of Ca++ is lower than K+, why is the
equilibrium potential for one positive and the other negative?
ECa = = 30mV*log (2/0.0002)= 120 mV
EK, = -80.6 mV (Calculated in #2). ECa is much more positive (depolarized) than the value for EK
The ratio and not the absolute value of the concentration sets the Nernst potential, and since the outside
concentration is much higher, the reversal potential for Ca is positive
