Nernest Equation

• NERNEST EQUATION

  • describes the behaviour of electrochemical cells
  • cell interior is around -60-770 mV
  • sodium and calcium movies INTO the cell
  • potassium favours electrochemical gradient but the chemical movmement is not it

  
• POTASSIUM

  • at 0mv --> move from outside to inside the cells
  • at -30mv --> rmp is negative and potassium still moves into the cell
  • at -80mv --> equilibrium
• RESTING MEMBRANE POTENTIAL

  • Nernest equation doesn't tell us about a cells membrane potential
  • permeabilities need to be factored in
  • ar rest --> most permeable to K+ ions
• `WHY IS RESTING MEMBRANE POTENTIAL NOT EQUAL TO POTASSIUM EQUILIBRIUM POTENTIAL?

  1. sodium also crosses the membrane
  2. with negative rmp --> sodium will enter the cell down both chemical and electrical gradient
  3. sodium would only be at equilibium at sodium equilibium potential (+61.5 mv)
  4. permeability to sodium = rmp is more positive than equilibrium potential for potassium (much closer to ek than ena)(if pna increases --> rmp becomes more positive)
  5. neither ions are actually at equilibium
  6. chloride permeability is irrelevant as ecl is similar to rmp
• WHY DOES [K+] REMAIN COSTANT AT RMP

  • hardly any ions need to be moved
  • capacitors --.> a device for storing energy via separation of electrical charge
  • charge stored on 2 plates separated by an insulator
  • in cells --> ions line up on the membrane and act as a charge store in plates
• THE GOLDMAN-HODKIN-KATZ EQUATION

  • p= permeability coefficient
  • allows us to calculate the resting membrane potential

  
• VOLTAGE-GATED CHANNEL FUNCTION

  • selectively permeable

  1. done through ring of charges in the selectivity filter (ring of negative charges will attract cations and repel anions)

  • knock-on mechanism

  1. oxygen stabilises ions when crossing the membrane
  2. selectivity filter strips the hydration cell from the ion and the ion moves into the channel
  3. it hits one ion and knocks another one out --> water molecule needs to be stripped off

  • channels opening and inactivating

  1. sodium channels open rapidly bu inactive after 1 ms
  2. 3 states --> resting, open, inactivated
• • the 3 states are dependent on:

  1. voltage
  2. when its depolarised --> moves from open to resting --> open and open --> inactivated
  3. voltage sensor in the 4th transmembrane domain contains charge --> when there is a charge --> it moves the channel open but pulling it up
  4. the inactivation gate will swing up and close the gate (ball like structure)