B. Fundamentals of Neurophysiology: Resting membrane potential

Question 1

What are excitable cells? Give examples of excitable cells

Answer 1

Cells that use potential energy to do work:

• Muscle Contraction
• Immune cells migrate (migrate)
• Nerve cells conduct signals (carry information over relatively long distances)

Question 2

What provides potential energy in nerve cells?

Answer 2

The difference in ion concentration across the nerve cell membrane provides the potential energy required to transmit nerve impulses

Question 3

What is the potential energy provided called?

Answer 3

Action potential

Question 4

How could we measure membrane potentials ?

Answer 4

Intracellular recording

Intracellular microelectrodes measure potential difference (mV) across the cell membrane

Question 5

What is plasma membrane potential due to?

Answer 5

Vm is due to the separation of electrical charge across the cell membrane

Question 6

Do all cells have resting membrane potential ?

Answer 6

Yes, however it varies between cells.

The resting membrane potential could be constant throughout the cells lifespan or can change

Question 7

What is the action of the sodium pump (Na+/K+-ATPase)?

Answer 7

Actively transports Na+ and K+ in different directions across the membrane, where 3Na+ out and 2K+ in.
This works against the ions conc gradient

Question 8

Describe the steps of how Na+/K+-ATPase works?

Answer 8

1. 3 x Na+ binds to transport
2. This causes a conformational change which stimulates phosphorylation
3. Further conformational change (outwards facing version of protein)
4. Na+ is expelled
5. K+e binds to transporter and stimulates dephosphorylation
6. Reversion to original conformation (inward conformation)
7. K+ is released into the cytoplasm

Question 9

What is the ion distribution of Na+ outside and inside of the cell ? What is the permeability of Na+ ?

Answer 9

Outside: 145 High
Inside: 10 Low
Low permeability

Question 10

What is the ion distribution of K+ outside and inside of the cell. What is the permeability of K+ ?

Answer 10

Outside: 5 Low
Inside: 145 High
High permeability

Question 11

What does it mean when the membrane potential is described as electrically neutral?

Answer 11

There is equal positive and negative charge outside and inside of the cell.

Na+ and K+ are balanced.
Where Na+ is balanced with CL- outside
And K+ is balanced as A2- inside

Question 12

What are the contradictory influences of the electrochemical equilibrium?

Answer 12

Concentration gradient tends to drive K+ out of the cell
The negative charge inside attracts K+ back in

…so an equilibrium is established where the electrical potential balances the chemical potential

Question 13

What is equilibrium potential?

Answer 13

The voltage at which the electrical force experienced by an ion is equal and opposite to the chemical force produced by the concentration gradient

Question 14

What is the Nernst potential (Ek)?

Answer 14

This is the equilibrium potential

The voltage across the membrane is proportional to the ratio of the ion concentration on either side of the membrane (assuming that the membrane is permeable to the ions)

Question 15

What is the Goldman Equation?

Answer 15

A more realistic approximation of membrane potential

As long as the overall concentration of the ion don’t change, the membrane is controlled by changes in ion permeability due to opening and closing of protein in the membrane

Question 16

Clinical Case

A woman comes to emergency department with symptoms of weakness, fatigue, confusion, muddled, arrhythmias.

She has recently had excessive vomiting and diarrhoea
Blood K+ concentration is 3mM (Normal range= 4-5 mM)

You recognise that the symptoms are due to hypokalaemia

How would you explain the symptoms ?

Answer 16

Excessive vomiting and diarrhoea could have resulted in loss of potassium.

This would have decreased the membrane potential from -60 to -64, therefore more negative.

Neurons are less likely to fire and skeletal muscle less likely to contract.

this could have caused symptoms of confusion and fatigue