Fundamentals of Neuronal Function/Ion Channels

Question 1

What is plasma membrane potential, Vm?`

Answer 1

The resting membrane potential (Vm), is due to the separation of electrical charges across the cell membrane (inside -70 mV, outside 0mV).

Question 2

How is Vm maintained?

Answer 2

By the Na+/K+ ATPase ‘sodium pump’; actively transporting 2 K+ into the cell for every 3 Na+ pumped out.

Question 3

What is the Na+/K+-ATPase cycle? (6 steps)

Answer 3

1. ) Cytoplasmic (intracellular) Na+ binds to ATPase stimulating phosphorylation by ATP
2. ) Phosphorylation causes conformational change of the ATPase protein
3. ) Conformational change expels 3 x Na+ outside, whilst extracellular K+ (2 x) bind
4. ) K+ binding triggers release of a phosphate group into the cytoplasm
5. ) Loss of phosphate group restores OG ATPase conformation
6. ) K+ is released (into the cell) and Na+ sites are receptive again.

Question 4

How is electrical neutrality achieved inside and outside the cell, and what does this mean?

Answer 4

Outside: Na+ balanced by Cl- (1:1)
Inside: K+ balanced by A- (negatively charged proteins, where other anions e.g. PO4- and Cl- are also present too )
- Equal positive and negative charges.
- 70mV potential difference across but sides are 1:1

Question 5

Describe the passive movement of K+ across the membrane and what it results in.

Answer 5

• Some K+ goes out of the cell through leaky channels (selective for K+) down its concentration gradient
• But the negative charge inside the cell (-70 mV) attracts it back in; an equilibrium is established where electrical potential balances chemical potential (the electrochemical equilibrium).

Question 6

What is the equilibrium potential?

Answer 6

• The voltage at which the electrical force experienced by an ion is equal and opposite to the chemical force produced by the concentration gradient; any chemical gradient can be balanced by an electrical potential.
• Also called the Nernst potential (Ek), described by the Nernst equation.

Question 7

What is the effect of external K+ concentration on membrane potential of skeletal muscle?

Answer 7

Increasing extracellular K+ depolarises the cell.