- RMP is the potential across the cell membrane when at rest, which is usually more negative inside the cell. - It results from an unequal distribution of potassium ions across the membrane, maintained by the sodium-potassium pump, which creates a concentration gradient of potassium into the cell. - A local response is a localized change in membrane potential caused by the opening of ion channels, letting ions enter or leave the cell. This decreases in size with distance from the stimulus. Changes in extracellular potassium and sodium concentrations can affect RMP.

1. Membrane Potential
Brinnell caszo

2. Outcomes
• Explain how RMP is formed
• Describe the equilibrium potential for main
ions
• Explain how a local response is formed
• Explain the effect of changes of EC K+ and Na+

3. The cell membrane

4. Recording membrane potential

5. Resting membrane potential
• Definition: the potential recorded across a cell
membrane when it is at rest
• Usually it is more negative on the inside of a
cell membrane than the extracellular fluid
(outside)
• All cells have RMP
• Our focus on cells that are electrically
excitable: muscle and nerve mainly

6. Equilibrium
• Net movement of an
Ion across a cell
membrane is zero it is
said to be in equilibrium
• What forces an ion to
move across a cell
membrane?
I M X+ 0.1 M X +
-
-
-
-
+
+
+
+
Red: Chemical force (concentration)
Yellow: Electrical force
• When both these forces are
equal and opposite directions,
no net force acts on the ion,
thus no net movement of ions
occurs

7. Equilibrium Potential
• The potential at which there is no net
movement of the ion across a cell membrane
• Can be calculated using the Nernst Equation
• The Nernst potential would calculate the
electrical potential just enough to oppose a
given chemical concentration difference.
• We can use nernst potential to find out what
will happen in a particular experiment

8. Equilibrium potential of Potassium
• Consider K ion as X
• As potassium moves
from A to B, A becomes
more negative.
• Until A is so negatively
charged, prevents K
ions from moving out
I M X+ 0.1 M X +
-
-
-
-
+
+
+
+
Red: Chemical force (concentration)
Yellow: Electrical force
• Now both these forces are
equal and opposite directions,
no net force acts on the ion,
thus no net movement of ions
occurs
A B

9. Question:
• Define Equilibrium potential of an ion

10. Nernst Potential
• What is the use of the Nernst Potential?
• We can calculate the Nernst potential and
compare it to the potential that we actually
measure across a cell membrane

11. Nernst Potential
• Nernst potential has opposite sign to
measured electrical potential  ion is not in
equilibrium
– Both Conc force + electrical force  same
direction
• If same sign but large magnitude ions will
move in the direction of electrical force
• If same sign but smaller magnitude  ions
will move in direction of the conc force

12. Equilibrium Potential of Na+
• (where [Na+]i = 20 mM and [Na+]o = 145
mM; and z=1 because Na+ is monvalent)
• The Nernst potential for K+ can be
calculated as follows:

13. • The Nernst potential for K+ can be calculated
as follows:
• (where [K+]i = 150 mM and [K+]o = 4 mM; and
z=1 because K+ is monovalent)
Equilibrium Potential of K+

14. Question
• What is RMP?
• Explain its basis.
• What is the main ion responsible for RMP.
• What is the effect of changing the
concentration of na and K in ECF on RMP?

15. Resting membrane potential
• Definition: the potential recorded across a cell
membrane when it is at rest
• Usually it is more negative on the inside of a
cell membrane than the extracellular fluid
(outside)
• All cells have RMP
• Our focus on cells that are electrically
excitable: muscle and nerve mainly

16. RMP
• The cell membrane is more permeable to K
ions at rest
• There is an unequal distribution
concentration gradient for K across cell
membrane
• However if K ions were the only ones involved,
K equilibrium potential would be same as RMP
• So K is the main ion plus all other ions moving
across a cell membrane will contribute to RMP

17. RMP ---contd
• In addition: Many particles (esp proteins) are
not free to move across cell membrane
• They are also negatively charged at pH inside
cell
• These ions also contribute to the RMP (Gibbs-
Donnan Effect)

18. Why K?
• Cell membrane has
channels which allow K ions
to pass thru it is
permeable to K +
• Na+ K + ATPase works to
keep K and Na gradients
• ie: [K +] inside cell is high
while [Na +] outside cell is
high

19. Na+ K+ ATPase

20. In summary
• Thus RMP represents an equilibrium situation
where the force due to chemical or
concentration gradient is equal and opposite
to the force due to electrical gradient of the
ions to which the membrane is permeable.
• http://academic.brooklyn.cuny.edu/biology/bi
o4fv/page/atp_ann.htm
• http://www.cvphysiology.com/Arrhythmias/A
007b.htm

21. So what does this mean?
• All cells have RMP
• Neurons and Muscle cells are excitable cells
• Cause of RMP:
– Cell membrane permeable to K
– Gradient for K maintained by Na K ATPase
– Some immobile –ve charged particles inside cells

22. Questions
• What is a local response.
• List a few of its characteristics.

23. Local response
• A localised change in membrane potential is
known as a local response
• The magnitude of the response drops as one
moves away from area
• If it causes membrane potential to be less
negative: DEPOLARISING
• If it causes membrane potential to be less
negative: HYPERPOLARISING

24. How does it happen?
• A “stimulus” supplies energy to certain channels
on the cell membrane
• This energy causes certain ion channels to open
and allow +ve or –ve charged ions to enter the
cell. Usually Na ions
• Results in “localised” response
• If +ve ions enter or –ve ions leave: Depolarisation
• If -ve ions enter or +ve ions leave:
Hyperpolarisation

25. Properties
• It decreases in size with distance
• It slowly dies out
• Proportional with the size of the stimulus
provided

26. Changes in ECF ion concentration and
RMP
• Since cell membrane is not so permeable to
Na at REST…little impact on RMP
• K CONDUCTANCE is high….more impact
• Increase in ECF K  DEPOLARISATION
• Decrease in ECF L HYPERPOLARISATION