Membrane Potential Flashcards
What is membrane excitability?
The ability of a cell to generate action potentials
What two things are required to generate a resting membrane potential?
Unequal distributions of the ions across the cell membrane
Open ion channels in the resting membrane of the cell
What ions are present in high concentrations in the ECF?
Na and Cl
Ca is higher relative to inside, but relatively small compared to the other two
What ions are present in high concentrations in the ICF?
K and proteins (generate the negative charge)
What are leak channels?
Selective channels that are open in the resting, unstimulated membrane of the cell
What is the principle of electroneutrality?
The total mEq for cations equals the total mEq for anions in the ICF and ECF
What force is responsible for initially generating a resting membrane potential?
The concentration gradient causes initial diffusion of K out of the cell, but is slowed as the interior acquires a more negative charge
Describe the movement of K ions when the membrane potential is at equilibrium
There is no net movement.
Diffusion out due to concentration gradient is balanced by movement into the cell do to electrical potential
What equation describes the equilibrium potential for one ion?
The Nernst equation
How is the equilibrium potential affected by changes in [K]?
If the change in [K] results in a larger difference across the membrane, then the equilibrium potential will become more negative
What equation describes the equilibrium potential for more than one ion?
Goldman-Hodgkin-Katz equation
When considering multiple ions, how does permeability affect membrane potential?
The equilibrium potential will more closely resemble the equilibrium potential of the ion that is more permeable
How is the K concentration gradient maintained in the case of glial cells?
Since its only permeable to K, the potential can be predicted by the Nernst equation
The quantity of K ions that have to move to establish the resting membrane potential does not significantly alter the original concentrations, so the gradient is maintained
How are Na and K concentration gradients maintained in the case of neurons?
Na-K ATPase
Only a small amount of ion is moved to establish potential
When chloride ions are passively permeable to the membrane, how does Cl affect resting membrane potential?
Chloride ions will develop a concentration gradient that is exactly equal and opposite to the electrical potential across the membrane
In this case, the Cl ions do not contribute tot he resting membrane potential
When chloride ions are actively transported across the membrane, how does does chloride affect the resting membrane potential?
When actively transported, the concentration gradient established for Cl will result in an equilibrium potential that is only a few mV different from the RMP.
Therefore no significant contribution
Which change in ion concentration affects resting membrane potential more, Na or K, and why?
K because it is more permeable
What are the results of changes in extracellular Na concentration?
Significant effects on action potential
Significant effect on tonicity (osmolarity) and hypotonicity can cause cerebral edema
What is membrane surface charge screening?
The extracellular-facing surface of the cell membrane contains fixed, negative charges.
Divalent cations, including Ca, bind to these charges and neutralize them
How do hypocalcemia and hypomagnesemia affect membrane potential?
The charge-neutralizing effect of these ions is lost, and the membrane potential becomes less negative
Why is the CNS partially protected from acute changes in blood electrolyte concentrations?
The presence of the blood-brain and blood-CSF barriers
