Exam 1 - Membrane Potentials Flashcards
what is a Coulomb?
measure of electrical charge of a particle
what is Faraday constant?
total charge on a mole of any monovalent ion
what is potential difference?
work required to move a unit of charge from one point to another = E (voltage)
describe concentrations of the following ions inside and outside cells: Na+, Cl-, Ca2+, K+
Na+: high outside, low inside
Cl-: high outside, low inside
Ca2+: high outside, very low inside
K+: low outside, high inside
what is ionic equilibrium potential?
an electrical potential difference that exactly balances the ionic concentration gradient
what is the principle of electrical neutrality?
can’t have a cell carrying a charge -> product of anions and cations inside cell must equal product of anions and cations outside cell
explain how it is possible to have a charge difference b/w intracellular and extracellular
phospholipid bilayer is very thin, so ions on one side can interact electrostatically with ions on the other side -> the negative charge inside the cell is not distributed evenly in the cytoplasm but rather localized at the inner face of the membrane -> equilibrium is established such that there is no net movement of ions across the membrane, leaving a charge difference between the two sides.
what determines the rate of flow of an ion across the plasma membrane? 3 things
- concentration gradient
- voltage difference across the plasma membrane
- conductance of the ion channels
what does the Nernst equation calculate?
Eion (equilibrium potential) = membrane potential at which there is no net movement of a given ion
what is the Nernst equation?
Eion = (58/z)log([ion]out/[ion]in)
valid only for ions that diffuse - describes the equilibrium state
what does the Donnan equilibrium state?
the products of the concentrations of the permeant ions of opposite signs on each side of the membrane must be equal at electrochemical equilibrium
describe the action of the Na-K pump
- binds Na+ and ATP intracellularly
- cleaves ATP, uses energy to translocate bound Na+ out
- binds K+ extracellularly
- brings K+ in, returns to original configuration
3 Na+ out for 2 K+ in
this pump accounts for most energy consumed in a cell during resting state
how does the Goldman equation differ from the Nernst equation?
Goldman takes into account both the concentration gradients of the permeant ions and the relative permeability of the membrane for each permeant species
what are the relative ionic permeabilities at resting state
Pk: Pcl: Pna = 1: 0.45: 0.02
what are the passive membrane properties?
- transmembrane resistance (resting)
- axial resistance (internal resistance)
- membrane capacitance
if you apply the same amount of current to a small cell and a large cell, which one will get an AP first and why?
small one: E = IR -> for same amount of I, cell w/ higher R will have most change in E and therefore get an AP first
small cells have higher internal resistance than large cells
what is the specific membrane resistance? how does it relate to internal resistance?
resistance of a unit area of membrane measured in units of ohm cm^2
- function of density of ion channels and their conductance
- *input resistance is inversely proportional to the square of the radius (internal resistance is a fxn of cell size)
what does electrotonic potential decay mean?
maximum membrane depolarization occurs at the site of current injection (or synapse) and decays with distance because of membrane resistance
what is the length constant, lambda? how can you increase lambda?
the distance over which an electrotonic potential decrements to 37% of its original peak value
increase lambda by increasing membrane resistance or decreasing internal resistance
what does it mean to have a larger value of lambda?
the larger lambda, the less decay over length (aka the farther the current spreads down the nerve fiber) and therefore the greater chance for two or more synaptic potentials to summate
effect of diameter on electrotonic spread
other properties being equal:
- synaptic potential will be larger in a small process (b/c larger internal resistance)
- spread of potential will be greater in a large process (b/c larger value of lambda)
what is the membrane time constant? what is the equation for it?
the time necessary for the electrotonic potential to decrement to 37% of its original value
tau = (membrane resistance)(membrane capacitance)
when membrane resistance is high, tau is increased b/c injected current first must discharge the membrane capacitor before it can depolarize the membrane
what does a larger value of tau mean?
the larger tau, the less decay over time and therefore the greater chance for summation
