Lecture 7 - Electrical properties of excitable cells: Capacitance & Conductance

How do structural elements determine electrical properties of cell membranes?

• the lipid bilayer is impermeable to ions via simple diffusion, so it behaves like insulator

• the ECF and ICF are highly conductive for ions

• two conductors that are separated by insulator form electrical capacitor which has ability to separate charge

• membrane has capacity to hold charge

• ion channels give membrane electrical conductance

• Capacitance occurs due to the ability of the cell to membrane to separate charge

Determining capacitance?

• C = capacitance

• q = electric charge in coulombs

• V = change in electrical potential

• measured in Farads (F)

• 1F capacitor can store one Coulomb of charge per 1 Volt difference between the conductors

• cell membrane have high capacitance of 10^-6F/cm²

Calculating total capacitance…

How much charge (in Coulombs) must be separated across the membrane of a 50 μm diameter glial cell in order to produce a membrane potential of -90 mV?

part 2 of question

How can you determine how many ions move?

• To convert the amount of electrical charge into the mol of K+ ions that had to move out of the cell to establish Vm in glial cells, use Faraday’s constant

• F = 96 485 coulombs/mol

• use avogadro’s number to determine how many ions it is

7.326 x 10-17 mol of potassium ions have to move out of glial cells to establish EK=Vm of -90mV. Does this efflux significantly lower the K+ content of the cell? (Does ion flux significantly change ion concentrations in cells?)

• final answer = 0.001%

• For every 1 million K+ ions in glial cells, roughly 10 K+ ions are needed to establish Vm = Ek=-90mV

• no, ion movement that occurs to establish a membrane potential does not significantly change the ion concentration inside the cell