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47 Cards in this Set
- Front
- Back
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Membrane Potential |
the (measured) electrical potential difference between the inside and outside of a cell, with ECF being the reference point and having a value of 0 mV |
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What is the resting membrane potential (RMP) for excitable cells (nerve and muscle cells)? |
typically -70 to -90 mV |
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What is the RMP for non-excitable cells? |
range from -10 to -100 mV |
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What cells have a RMP that is positive? |
no cells have a positive RMP |
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Diffusion potential |
A potential difference generated across a membrane created when a charged solute (ion) diffuses down its conc gradient |
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In order for a diffusion potential to occur, what 2 factors must be present? |
1. the presence of a concentration gradient 2. the membrane must be permeable to the ion |
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Vm |
Vm represents membrane potential |
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How does the membrane potential in most cells arise? |
Most of the RMP arises d/t the diffusion of Na+ and K+ down their electrochemical gradients |
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Equilibrium Potential |
The (calculated) value of the membrane potential (Vm) that exactly balances or opposed the tendency for diffusion down the concentration gradient |
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Electrochemical equilibrium |
the chemical and electrical driving forces acting on an ion are equal and opposite, therefore no other net diffusion occurs. |
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Ex |
Equilibrium potential calculated by the Nernst equation |
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What is the equilibrium potential directly related to? |
The size and direction of the ion gradient The charge on the ion |
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What does the Nernst equation calculate? |
The value of the Vm at which a given ion with a given concentration gradient will be at equilibrium (Ex) |
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What does the sign of the diffusion potential vary with? |
1. the charge of the diffusing ion 2. the direction the ion is moving in |
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What is the equilibrium potential for an ion? |
the value of Vm at which the net driving forces acting on the ion are equal to zero and the net movement of the ion = 0 |
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Nernst equation for a cation |
Ex = -61 mV log ([X]i /[X]o) for example ENA = -61 mV log ([Na+]i /[Na+]o) |
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Nernst equation for an anion |
Ex = -61 mV log ([X]o /[X]i)
For example ECl = -61 mV log ([Cl-]o /[Cl-]i) |
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What must be accounted for when using the Nernst equation for Ca++? |
Ca++ has a valence of 2 , therefore we must use half -61 mV within the equation therefore Ex = -30.5 mV log ([Ca++]i /[Ca++]o) |
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Log 10 |
1 |
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Log 100 |
2 |
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Log 0.1 |
-1 |
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In most cells, the actual value of the membrane potential is? |
the net result of the diffusion potentials for Na+, K+, and Cl- |
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What is the concentration of Ki maintained by? |
Na, K-ATPase |
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If [Na+]i = 10 mM and [Na]o = = 100mM what is the equilibrium potential for Na+? |
ENa = -61mV x log(10mM/100mM) ENa = -61 mV x (-1) = + 61 mV |
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Out of K+, Na+ and Cl-, which ion has the largest permeability coefficient (P) in most cells at rest? |
In most cells at rest PK >>PNa and PCl.
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If the permeability of K+ (Pk) is largest in most cells at rest, what is Vm closest to? |
Ek |
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What would happen to Vm if PNa >> Pk + PCl? |
Vm of the GHK equation would become equal to ENa |
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GHK equation? |
Vm = -61mV x ((Pk[k+]i + PNa[Na+]i + PCl [Cl-]o) / (Pk[k+]o + PNa[Na+]o + PCl [Cl-]i)) |
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What does the GHK equation assume? |
1. membrane is only permeable to K+, Na+, and Cl- 2. total net current (Itotal) across membrane is Itotal = Ik +INa + ICl 3. Itotal = 0 |
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Bulk solutions inside and outside of the cell are mainly positive, negative, or neutral? |
Neutral |
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Therefore, the separation in charge between the inside and outside of the cell lies only at the _____? |
membrane interface |
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What two factors must be present for an ion to move an ion across a membrane? |
1. the net driving force must be any value other than 0 2. the membrane must be permeable to that ion (ie there must be an open channel or an available transport carrier) |
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How does Na,K- ATPase contribute to Vm? |
Indirect contribution by maintaining the concentration gradients of Na+ and K+ thru a small exchange of 3 Na+ for every 2 K + |
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Explain why Itotal = 0 |
If membrane potential is constant, net currents must be canceling each other out, therefore Itotal = 0. Do not assume this means no movement, there is movement of individual ions, but their current cancels each other out |
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What does the GHK equation demonstrate? |
GHK equation demonstrates that an action potential can be predicted based on knowing the concentration gradients and permeabilities for the permeable ions of that cell (mainly K+,Na+, and Cl-). |
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Outward current |
Flow of positive charge out of the cell |
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What would outward current do to the Vm of the cell? |
Vm would become more negative |
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Inward current |
Flow of positive charge into the cell |
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Inward current would make Vm ____? |
more positive |
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What would inward flow of Cl- into the cell be defined as, positive or negative current? |
Positive current, b/c same net effect as moving a positive charge out of the cell. |
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What does the GHK fail to account for? |
capacitative current or rather it cannot account for the value of Vm as the membrane potential is changing |
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What is the driving force for ionic currents across the plasma membrane? |
Vm - Ex |
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Gx ? |
conductance for a given ion |
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If a channel for K+ is open, the movement of K+ creates a current as it moves, what is the equation for this current? |
Ik = Gk(Vm-Ek) |
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What is the driving force equation for K+? |
K+ = Vm - EK |
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If Vm = -70 mV and ENa = +60 mV, which direction would the net driving force be in and what would the driving force of Na+ =? |
Into the cell -70mV - 60 mV = 130 mV away from equilibrium |
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What equation predicts the ionic current of an ion? |
IIon = GIon (Vm – EIon)
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