Lecture 6 - Ion flux and the resting membrane potential

How is the neuronal cell membrane set up?

• thin clouds of positive and negative ions spread over the outer and inner surface

• at rest, extracellular surface of membrane has excess of positive charge and the cytoplasmic surface has an excess of negative charge

• charge separation allows for difference in electrical potential or voltage across cell membrane

What is membrane potential?

• the difference in voltage across the cell membrane

• resting membrane potential = -60 to -70 mV

How do you record Vm?

1. glass micropipettes filled with saline and placed in the same extracellular solution

2. voltage amplifier and oscilloscope records 0V

3. then when resting potential is recorded, measure -60Vm

   1. see this because the electrode is moving from the extracellular fluid into the intracellular fluid, crossing a selectively permeable membrane that maintains an unequal distribution of ions

What is the Goldman-Hodgkin-Katz equation?

• a modification of the Nernst equation which takes into account that cell membranes are permeable to more than one ion

• more accurate calculation of voltage difference across the membrane

What does GHK equation show us relative to NERNST?

• Driving force of an ion will try to force Vm towards Eion

• thats why Na moves in and K moves out

How do ionic species that cannot cross the membrane (A-) affect Vm?

• since charged macromolecules cannot leave cell, cell at danger of influx of positively charged sodium ions, followed by water

  • can cause swelling and rupture

• electrochemical equilibrium must be considered together with osmotic balance because imbalance of impermeant solute can drive water into cell

What is Gibbs-Donnan Equilibrium

• when diffusible solutes are separated by membrane that is freely permeable to water and electrolytes but impermeable to one ionic species, causing solutes to be unequally distributed between two compartments to establish new equilibrium

• e.g. experiment adding KCl and KA

What are the conditions for Gibbs-Donnan Equilibrium?

1. concentration of cations should be equal to concentration of anions in the ECF to maintain charge balance (same applies for ICF)

2. Product of concentration of permeant ions outside the cell must be equal to inside the cell

3. There is one membrane potential: Em=Ek=ECl

How does Na/K pump prevent osmotic catastrophe of a cell?

• prevents water from inundating interior of cell by pumping out Na+ to counter effect of impermeable anions

• as a result, maintain osmotic equilibrium

• process requires ATP so we refer to cells as being at steady state when there is no net ion flux at equilibrium

How did cyanide result in poisoning in Tylenol scare?

• cyanide interrupts sodium potassium pumps ability to maintain cell volume

• after deaths, led to reform of packaging of over-the-counter medication

• institution of anti-tampering laws

What’s to stop the ionic gradients from dissipating by the diffusion rates of the ions across a cell membrane when the cell is at rest?

• Active transport by the sodium-potassium pump is able to counterbalance the rate of leakage across a cell membrane so that the resting membrane potential is maintained at ~ -70 mV

• This pump is fueled by ATP to enable a steady state of no net ion flux

• so pump prevents dissipation of ionic gradients when cell is at rest AND maintains osmolarity