S6) Changing Membrane Potentials

Question 1

Distinguish between depolarisation and hyperpolarisation

Answer 1

• Depolarisation is a decrease in the membrane potential, so that the inside of the cell becomes less negative
• Hyperpolarisation is an increase in the membrane potential, so that the inside of the cell becomes more more negative

Question 2

There is another term to describe changes to the altered membrane potential which is rarely used.

Identify it and define it.

Answer 2

Repolarisation is an increase in the membrane potential, so that the inside of the cell becomes less positive

Question 3

What is the effect of the ionic distribution of extracellular fluid?

Answer 3

The ionic distribution of extracellular fluid gives positive equilibrium potentials for Na⁺ and Ca²⁺, while those for K⁺ and Cl^- are negative

Question 4

What is the effect of the ionic distribution of fluid on changing membrane potentials?

Answer 4

• Opening Na⁺ and Ca²⁺ channels will depolarise cells
• Opening K⁺ or Cl^- channels will hyperpolarise cells

Question 5

Channel opening is controlled by gating mechanisms.

Compare and contrast ligand gating with voltage gating

Answer 5

• Ligand gating: the channel is opened/closed by the binding of a chemical ligand (extracellular/intracellular)
• Voltage gating: the channel is opened/closed by changes in the membrane potential

Question 6

When channels for more than one ion are open, these ions will contribute to the membrane potential.

How does one deal with cell membranes that are not perfectly selective?

Answer 6

The GHK (Goldman-Hodgkin-Katz) equation is used to calculate the reversal potential across a cell’s membrane, taking into account all of the ions that are permeable through that membrane

Question 7

In three steps, outline fast synaptic transmission

Answer 7

⇒ Neurotransmitter binds directly to ion channels on the postsynaptic membrane

⇒ Permeability of postsynaptic membrane changes

⇒ Electrical potential change

Question 8

In three steps, outline slow synaptic transmission

Answer 8

⇒ Neurotransmitter binds to a receptor protein on the postsynaptic membrane

⇒ Releases an intracellular transmitter in the postsynaptic cell

⇒ Transmitter binds to ion channels on the postsynaptic membrane

E.g. GPCRs

Question 9

Explain the term Excitatory Postsynaptic Potential

Answer 9

• Depolarising transmitters open channels with positive reversal potentials i.e. channels selective for Na⁺ / Ca²⁺
• These transmitters lead to the excitation of cells i.e membrane change is excitatory postsynaptic potential

Question 10

Explain the term Inhibitory Postsynaptic Potential

Answer 10

• Hyperpolarizing transmitters open channels with negative reversal potentials i.e. channels selective for K⁺ / Cl^-

- These transmitters lead to the inhibition of cells i.e. the change in membrane potential they cause is inhibitory postsynaptic potential