Cardiac Electrophysiology I Flashcards
resting cardiac muscle cell
- biophysicists look at the cell from the inside
- electrocardiologists look at the cell from the outside
types of biological electrical potentials
- equilibrium potential
- gibbs donnan equilibrium
- diffusion potential
- epithelial membrane potentials
equilibrium potential
- voltage obtained for a given concentration gradient of a single ion at equilibrium across a semi permeable membrane
- given single ion and represent by nernst equilibrium equation
- Na is 60
- K is -90
- Ca is 120
- if membrane is more permeable to one of the ions, the resting potential is closer to it
Gibbs-Donnan equilibrium
- special
- impermeable polyelectrolyte on one side of a membrane that is permeable to salts
- capillary membranes if albumin and other charged plasma proteins are in the blood but not the interstitial fluid
- pH different from isoelectric point of the polyelectrolyte, leads to unequal distribution of salts across the membrane and slight potential that has the same sign as the charge on the polyelectrolyte
diffusion potential
- when two or more ions are permeable to a membrane, but the various ions have different permeabilities
- calculated by goldman hodgkin katz equation using the theory of electrodiffusion
- independent passive movements of ions across membranes under influence of their concentration gradients and electrical forces
- cell resting potential and action potentials are diffusion potentials
epithelial membrane potentials
-differences in electric potential between two dilute solutions when the membrane itself it a layer of cells
kidney and GI
nernst equilibrium for K
- raising external K decreases outward K gradient and makes Ek less negative, which is depolarizing
- raising internal K increases outward flow of K and makes Ek more negative, hyperpolarizing
- at low external K, membrane potential is more positive than predicted by nernst equation due to Na presence inside the cell that brings the charge difference back down (more positive)
raising internal K
increases the gradient out and makes Em more negative inside- hyperpolarizing
raising external K
decreases the gradient out and makes Em more positive inside- depolarizing
raising internal Na
-decreases gradient in and makes the membrane more negative- hyperpolarizing
raising external Na
-increases gradient in and makes the membrane more positive- depolarizing
GHK diffusion potential
- equation includes permeabilities of whatever ions happen to be moving- a is Pna/Pk
- chloride is in equilibrium and doesn’t have any pumps, which is why its not in the equation
- accounts for biological diversity of cell membrane diffusion potentials
- different cells have different permeability constants and therefore different membrane potentials (compared to nernst prediction of all the same since concentrations of K are the same)
- nerve and muscle cells much more permeable to K, Em -80, alpa is 0.05
- RBCs a is 1/4, so resting potential only -11mV
conductance
- predicted by current voltage plot
- outward currents of pos ions are pos, inward are neg
- conductance is 1/R
- depends on number of open channels
- Ohm’s law predicts linear
- biological channels not Ohmic-rectification, conductance differs for inward and outward currents
- iK=Gk (Em-Ek)
outward rectification
- conductance of outward currents is greater than for inward
- current voltage slopes up non-linearly
- closed at hyperpol, open at depol (when membrane potential is positive)
inward rectification
- conductance of inward currents is greater than for outward currents
- current voltage plots slope downward non-linearly
- closed at depol, open at hyperpol (when membrane potential is negative)