Membrane Potential

luigi galvani

discovered electrical impulses in muscle via metal wires and a frog leg

like charges

repel

opposite charges

attract

separating charge needs energy because

they naturally pull together and needs work to pull them apart

attractive forces only act across

short distances

semipermeable barrier

intact membranes don’t allow ions to pass freely (separation of charge)

channels/transporters

proteins act as selective gates to allow certain ions to pass; changes concentration gradiet and electrical potential

membrane potential (Vm)

uneven distribution of electrical charge across membrane creates potential energy; cell uses as a battery to send/receive signals

recording voltage

microelectrode inserted inside cell cytosol and reference electrode remains in extracellular fluid

Vm is measured as

potential inside the cell relative to outside

Vm=

v inside - v outside

standard resting value

cells accumulate a negative charge along inner face of membrane so resting is -70mV

how cells create Vm

plasma membrane has compartmentalization; ions aren’t equally distributed across membrane

key contributions to Vm

membranes are unequally permeable to various ions; more permeable to K than Na at rest

K in out and location

100, 5, inside

Na in out and location

10, 100, outside

Cl in out and location

10, 105, outside

anions in out and location

100, 0, inside

concentration gradient and electrical potential are two

opposing forces

forces driving ion movement and equilibrium potential

chemical concentration and electrical potential

chemical concentration gradient drives

ions from areas of high to low concentration

electrical potential drives

ions towards regions of opposite (charges repel, opposite charges attract)

equilibrium potential (Ex)

membrane potential at which concentration gradient/electrical potential forces are equal and opposite

at equilibrium, K+ is at

equilibrium and no net flux of K across the membrane

the nerst equation

used to calculate equilibrium potential for a single ion when cell is permeable to only that ion

if there is no concentration gradient

equilibrium is 0

Ex for K

-75 mV

Ex for Na

+58 mV

Ex for Cl

-59 mV

equilibrium potential problem

cells are never permeable to one ion; one ion doesn’t set membrane potential so nernst equation can’t be used for Vm

misconception

people think when ions flow across membrane to change Vm, bulk concentration of ions out and in changes a lot

reality of misconception

relative concentrations of ions do not change, takes tiny movement of ions along membrane to make big change in Vm

GHK equation

calculates actual membrane potential as weighted average of each permeable ions eq. potential

takeaways of GHK equation

Vm must always be between most negative Ex and most positive Ex

ion with highest relative permeability pulls Vm closest to its own eq potential

at rest, cells are mostly permeable to K via leaky K channels

Vm is a

steady state, but not an equilibrium

resting Vm is

cell membrane potential at which net flux of all ions together is 0

leaky K channels

one family of K channels that are open at resting membrane potential; K is higher inside and efflux makes resting Vm negative

sodium potassium pump

uses ATP hydrolysis energy to move ions against gradient, 3 Na out 2 K in

2 factors setting Vm

unequal distribution of ions across membrane and unequal membrane permeability

potassium dominates because

resting cells have open leaky channel, resting Vm -70 which is close to eq potential -75

changing Vm

opening or closing channels to alter ion permeability, not altering concentration gradients