Membrane Potential

Brinnell caszo

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+

The cell membrane

Recording membrane potential

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

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

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

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

Question:

• Define Equilibrium potential of an ion

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

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

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:

• 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+

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?

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

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

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)

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

Na+ K+ ATPase

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/bio4fv/page/atp_ann.htm

• http://www.cvphysiology.com/Arrhythmias/A007b.htm

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

Questions

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

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

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

Properties

• It decreases in size with distance• It slowly dies out • Proportional with the size of the stimulus

provided

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