Physio 5: Membrane Potential

what is membrane potential

• separation of charge that gives rise to voltage across the membrane

what is electrical potential

• the push or pull felt between positive and negative charges

  • measured in volts

what happens to the voltage if the difference in charges increases?

magnitude of the voltage increases

what is membrane gradients

• a difference in electrical charge across a cell membrane

  • created by closing or opening ion channels

what is a membrane current

produced by allowing charges to move across the membrane

what is the direction of current defined as

• direction of net movement of positive charge

  • even if anions were moving, it would be described as movement of cations

the inside of the cell is … charged

• negatively charged

the outside of the cell is … charged

positively

when all ions are accounted for, what is the charge of the whole solution?

• neutral

how is membrane potential measured

• placing 2 electrodes inside and outside of cell

• potential outside of cell is 0

• difference of inside potential from the outside potential

  • more negative inside

what is the resting membrane potential

• between -40 to -90 mV

what are the two types of ion channels

• resting channels

• gated channels

describe resting channels

• carry current across cell membranes

• normally always opened

  • not significantly influenced by extrinsic factors

what is normally responsible for the charge of resting membrane potentials

• resting channels

  • set movement of ions

what are gated channels

• opened in response to changes in membrane potential

what type of channels are responsible for cell depolarization/polarization

• gated channels

what is the resting membrane potential

• voltage difference between the inside and outside of a cell

  • in absence of excitatory or inhibitory stimuli

what are the 2 main things responsible for the resting membrane potential

• open K+ ion channels

• Na+/K+ ATPase

describe open K+ ion channels

• K+ ions are constantly flowing through the cell membrane

  • contribute to resting membrane potential

describe the Na/K ATPase voltage gated ion channels

• responsible for maintining ionic difference

  • more K+ inside (145 mM)

    • less Na+ inside (5 mM)

  • more Na+ outside (145 mM)

    • less K+ outside (5 mM)

what is mainly responsible for the equilibrium potential of a cell

• K+

  • potassium leaks out of the cell down its concentration gradient

  • negativity pulled K+ back in, creating equilibrium voltage

  • K+ moves until membrane voltage equals equilbirum potential (Ek) for K+

why is K+ high inside the cell even though it has leaky channels?

• K+ leaves cell through leaky channels

• leaves behind negative charge

• electrostatic gradient trumps in getting K+ back in for negative charge

  • goes against concentration gradient

• membrane potential = K+ equilibrium potential

what does the equilibrium potential measure?

• equilibrium potential (E) of a specific ion

what is the simplified nernst equation?

Z (valence) for K+ or Na+ = 1

the _____ equation is an expended version of the nernst equation that takes into account several individua ion permeabilities

goldman hodgkin katz

what are the two things the movement of k+ ions are dependent on

• concentration

• charge

describe what would happen in this sceneraio

• more K+ on the right cell than left cell

• right cell K+ will migrate to the left cell

• movement of right cell will leave an excess positive charge in left cell and excess negative charge in right cell

• some K+ will migrate back to right cell

what is the Ek for K+

• -90 mV

  • there is more inside than there is outside

what happens to the membrane potential when the concentration flux of an ion exceeds its electrical flux?

• membrane potential will increase

  • there is an excess positive on one side

  • ions move down their concentration gradient faster than the opposing electrical gradient can stop them (like charges repel one another)

• net movement of charge until electrochemical equilibirum is reached

what are the two opposing forces in equilibrium membrane potential

• concentration flux

  • ions want to go from high to low concentration

• electrical flux

  • as the ions move, they build a charge that repels them

as ions move down their concentration gradient, the potential difference….

increases

• like charges are repelling and want to go to the other side

what is the membrane potential of glial cells

• -90

  • selectively permeable to mainly K+

    • Ek = MP

what is the equilibrium potential for Na+

• 60+ mV

what is the equilibrium potential for Cl-

• -70 mV

if a cell is selectively permeable only for an ion, then the resting membrane potential will equal the…

• equilibrium potential

why is the equilibrium potential for K+ negative and Na+ positive

• K+ is higher inside the cell

• Na+ is higher outside the cell

why is the resting membrane potential closer to the Ek of potassium?

• K+ permeability is about 50x larger than the Na+ permeability

  • membranes have K+ leak channels

What would happen if the Na+/K+ ATPase was impaired?

the resting membrane potential will be changed to less negative from -70 until 0

• No restoration of membrane potential

what establishes membrane potential

leaky potassium channels

describe this

1. High Na+ outside, high K+ inside

2. Leaky potassium channels leak out K+ ions

3. K+ ions leave behind a negative charge

4. Negative charge pulls K+ back into cell

   1. Na+ has both chemical and electrical force pull for inside cell

   2. Na+ is pulled in

5. Na+/K+ ATPase pump restores concentration gradients

   1. steady state with leakage flows and Na+ and K+ gradients