1 Resting membrane potential

Question 1

What is membrane potential?

Answer 1

Neurones transmit information by electrical signals, created by quick and controlled altering of membrane permeability

Question 2

Define Resting membrane potential

Answer 2

The resting membrane potential of a cell is defined as the electrical potential difference across the plasma membrane when the cell is in a non-excited state

Question 3

Describe the role of intra- and extracellular ions

and their relative concentrations around the plasma membrane

Answer 3

• Na+ has a higher extracellular concentration
• Cl- has a higher extracellular concentration
• K+ has a higher intracellular concentration
• Ca2+ has a higher extracellular concentration concentration
  (important for synaptic transmission and in muscle contraction near the NMJ)

Question 4

Describe how membrane potential is measured

Answer 4

Membrane potential is referred to as Vm
- It can be measured by using intracellular microelectrodes

This is determined by:

• Distribution of ions intra- and extracellularly
• Variation on the membrane permeability to different ions

It is measured relative to the extracellular, with units of mV

Question 5

Describe ion movements across the plasma membrane (and this is achieved)

Answer 5

Effect of the concentration gradient
- Molecules/ions move by diffusion or active transport

Effect of voltage

• Many molecules are charged, inorganic ions like Na+, Cl-, K+, and Ca2+
• Ions can also be moved by voltage gradient, aka Membrane potential

This works because ions are attracted to oppositely charged regions, and vice versa by being repelled from similarly charged areas

As there is a high [K+] inside, it would naturally diffuse out of the cell, but it can only pass through a protein channel (not through lipid membrane)

Question 6

Describe the movements of K+ and Na+ through the plasma membrane, including the roles of their respective ion channels

AT REST

Answer 6

AT REST

• Channels for K+ are open
• Channels for Na+ are closed
  This means K+ will undergo facilitated diffusion out of the cell, and the flux is from high to low concentration

As potassium diffuses outside, there is an accumulation of +ve charge outside the cell
- outside is more +ve than inside

This charge imbalance means no more K+ can diffuse across, due to repulsion (and they are attracted to the relative -ve charge inside)
- So, the electrical force is opposing the diffusion force; both affect K+ movement

So, there is an electrochemical equilibrium, where:
- The charge difference reaches a high enough level that the electrical force pushing K+ into the cell = diffusion force forcing them out of the cell

Question 7

Describe the Diffusion Potential

Answer 7

This is the voltage at which the electrostatic force is = to and opposite to the chemical force from the concentration gradient

• no net force, no net ion flow
• no further change in ion concentration at the plasma membrane
• no further change in charge distribution or membrane potential

For K+, the diffusion potential = Ek

The diffusion potential = Nernst potential (Ek), and can be described by the Nernst equation

Question 8

Describe the Nernst potential

Answer 8

The Nernst potential of an ion is the membrane potential at which there is no net flow of that ion

When cell membrane is solely permeable to K+
Vm = Ek

Question 9

Describe how many ions move to setup the resting membrane potential

Answer 9

When Vm = Ek
- less than 1/100,000 ions move across the plasma membrane to reach equilibrium (big influence on microenvironment)

When Vm > Ek (more positive than Ek)

• constant flow of ions across the plasma membrane
• Na+ moves in and K+ moves out
• The number of ions moved is very small BUT

THE ROLE OF Na+/K+ ATPase

• to replenish K+ lost from the cell
• to remove Na+ accumulated within the cell

Question 10

Describe the Sodium-Potassium pump

Answer 10

It sits in the outer membrane of the cell
- the sodium-potassium pump maintains the membrane potential, by maintaining the gradient of higher [Na+] outside, higher [K+] inside the cell

It allows these ions to move down their respective concentration gradients when needed (to stimulate an action potential)
- ATP is needed - it is an active process

The sodium potassium pump moves ions against their concentration gradients using ATP

3 Na+ OUT
2K+ IN