Electrical Properties of nerve cells- membrane potentials Flashcards
How are electrical events in the cells measured
intracellular- electrode inside the cell
Extracellular- electrode outside the cell
Patch clamping- electrode sealed to cell surface
Describe what the membrane potential is like at rest
The inner membrane is negatively charged wrt to the outside
Describe why the membrane can be depolarised
When cells become active the inner membrane becomes positively charged
Why are electrochemical gradients established
- The pump moves ions against their concentration gradient- the enzyme that means that Na+ moves in one direction and K+ moves in the other direction of membrane (ATPase- providing the energy)
- There is restricted ion movement through channels
- The membrane stores ionic charges on its inner and outer surfacers- ie. it is a capacitor
NB. “Capacitance”- the term given to hold electrical charges
Describe the concept of the capacitance
- The concentration of KCl is greater inside than that outside but both solutions are electrochemically neutral- BALANCES OUT
- Opening of the selectively open ion channels
- K+ moves down the concentration gradient from inside to outside
K+ will roll around the mouth of the channel set on the outside of the channel
Means that it will attract a negatively charged Cl- ion - The idea of the membrane becoming associated with ions is the idea of capacitance. The ions pairing up either side of the membrane
The concept of the membrane storing and separating charge on its inner and outer surface is important.
Using the idea of capacitance describe what is mean by the term voltage
Voltage is the work done to separate the charges across the membrane. This is the work done by the pump.
What is meant by the osmotic work done by the concentration gradient?
The idea that the pump obtains energy from the hydrolysis of ATP creating a concentration gradient-
gradient = [C] out/ [C] in
Explain what is meant by the term equilibrium potential
The pump drives ions in the opposite durection of the the concentration gradient.
This creates an electrical drag back in the opposite direction because the negative charged ions can prevent the exit of positively charged ions.
There comes a point where the force of the electrical drag back equals the force of the concentration dragging the ions out of the cell.
At this point the the osmotic force balances the electrical force- this is called the equilibrium potential.
The equilibrium potential hence is the membrane voltage that a cell needs to be at to prevent movement of that ion down its concentration gradient.
ie. if inside is very negative stops K+ from leaving
if very positive prevents Na+ from entering
What is the Nernst Equation
E.z.F = RT . ln [C] out/[C] in
E= RT/z.F. ln [C]out/[C] in
For physiological concentrations:
Ek =
ENa =
-90mV
+50 mV
What is the significance of Vm being closer to Ek than ENa
membrane is 50 times more permeable to K+
When the cell becomes more permeable to that ion, that ion will move down its electrochemical gradient to drive Vm towards the equilibrium potential for that ion
Driving force=
Vm - E eq
Driving force for K+ out
= -70 mV +90 mV
= +20 mV
Driving force for Na +
= -70 mV - 50 mV
= -120 mV
Why is the driving force of K+ less than that of Na+
because membrane is more permeable to K+
