Electrical properties of nerve cells, ions and electrochemical equilibria: Resting membrane potentials Flashcards
Definition of intracellular measuring
Electrode is inside cell to measure electrical activity inside the cell
Definition of extracellular measuring
Electrode is outside cell to measure electrical activity outside the cell
Definition of patch clamping
Electrode is sealed to the cell surface to measure of movement of ions in and out of the cell
Definition of voltage
Measure of electrical work down in the separating charges across membrane
Definition of equilibrium potential
Electrical force=osmotic force
No net movement of ions
Uses of measuring electrical events
ECG
EEG
EMG
3 ways of maintaining the resting membrane potential
ATPase Na+K+ pump moves ions against conc gradients
Channels restrict ion movement through channels, down conc grad
Membrane stores ionic charges on its inner and outer surfaces, capacitor
How does the membrane act as a capacitor
When 1 +ve ion leaves the cell via a channel, 1 -ve ion follows
-ve ion cannot leave the cell but attaches to the inner cell membrane opposite the +ve ion
The greater the separation of the oppositely charged ions, the greater the tendency for the ions to attract each other
What is voltage
Measure of electrical work done in separating charges across the membrane
Describe the osmotic work done by the conc grad
Pump derives energy from ATP hydrolysis, uses this to create conc grad
Gradient=[C]out/[C]in
However this creates electrical drawback in opposite direction
How does conc grad and electrical grad function
As the ions move down their conc grads, there is an increasing tendency for the ions to go down their electrical grad
What is equilibrium potential
When electrical force=osmotic force
How is equilibrium potential determined
If conc grad known
Nernst equation
E=58mV x log[C]out/[C]in
Intracellular concs of Na+ and K+
[Na+] = 10mM [K+] = 140mM
Extracellular concs of Na+ and K+
[Na+] = 140mM [K+] = 4mM
What is the typical resting potential and how is it determined
-70mV
Determined mainly by [K+]
What happens if the inside of the cell is v -ve or +ve
v -ve intracellular, stop K+ leaving [in]/[out]
v +ve intracellular, stop Na+ entering [out]/[in]
Nernst equations at physiological concentrations
Vm much closer to Ek than Ena, membrane is 50x more permeable to K+ than Na+
Constant Vm, net ion flow=0, passive leak of K+ out=leak of Na+ in
What is Ek
-90mV
What is Ena
+50mV
Ek dominantes Vm
If a cell becomes more permeable to an ion, will more down electrochemical gradient
Will drive Vm towards equilibrium potential for ion
What is the driving force
Driving force = Vm - Eeq on ions
Driving force for K+
Vm - Eeq on ion = driving force
-70 - (-90) = +20mV
+20mV K+ forced out
Driving force for Na+
Vm - Eeq on ion = driving force
-70 - (+50) = -120mV
-120mV Na+ forced into cell
Overall driving forces for Na+ and K+
+20mV K+ out
-120mV Na+ in
How is permeability and conductance different from each other
Conductance relates to the no of ions that can move through the cell membrane
Permeability relates to the no of ion channels which the ions can pass through
How does permeability affect the Nernst equation
Nernst deals with 1 ion at a time, does not consider the relative permeabilities
Goldman Hodgkin Katz considers relative permeabilities of monovalent ions
How to use the Goldman Hodgkin Katz
Vm=58mV log [Pk[K+]out + Pna[Na+]out / Pk[K+]in + Pna[Na+]in]
K+ is 50x more permeable than Na+, multiply K+ values by 50
