Session 3

Question 0

What is the general range of membrane potentials in animal cells?

Answer 0

-20 to -90mv

Question 1

What is a membrane potential?

How are they expressed?

Answer 1

The electrical potential difference across a resting cell.

They are expressed as the voltage inside the cell relative to the voltage outside the cell

Question 2

How can membrane potentials be measured?

Answer 2

Using a microelectrode that penetrates the cell membrane. Largest in skeletal and cardiac muscle (nerve cells –> -50 to -75mv)

Question 3

How are ion channels characterised?

Answer 3

Selectivity - channel lets through one (or a few) ion species
Gating - channel can be open or closed by a conformational change in protein molecules
Rate of ion flow down the electrochemical gradient of the ion

Question 4

What are the typical ionic concentrations of Na+?

Answer 4

10mM intracellularly

145mM extracellularly

Question 5

What are the typical ionic concentrations of K+?

Answer 5

160 mM intracellularly

4.5 mM extracellularly

Question 6

What are the typical ionic concentrations of Cl-?

Answer 6

3 mM intracellularly

114 mM extracellularly

Question 7

What is the equilibrium potential?

How is it calculated?

Answer 7

The potential difference across the membrane when the electrical and diffusional forces balance, resulting in no net movement of ions.
Calculated using the Nernst equation

Question 8

How is the resting potential difference across a membrane established?

Answer 8

K+ channels are open at rest therefore K+ diffuses out of the cell down the concentration gradient. Since anions cannot follow, the cell becomes negatively charged on the inside. Membrane potential opposes the outward movement of K+, thus the system reaches equilibrium

Question 9

Why is the resting potential of the cell not the same as the Ek of potassium?

Answer 9

The membrane isn’t perfectly selective. Mainly because other types of channels are also open so the resting potential is less negative than Ek.

Question 10

Define depolarisation

Answer 10

Decrease in membrane potential, so that the inside of the cell becomes less negative

Question 11

Define hyperpolarisation

Answer 11

Increase in membrane potential, so that the inside of the cell becomes more negative

Question 12

Which ions have positive equilibrium potentials?

Which ions have negative equilibrium potentials?

Answer 12

Na+ and Ca2+ - positive, therefore opening these channels will depolarise the cell
K+ and Cl- - negative, therefore opening these channels will hyperpolarise the cell

Question 13

How are ligand gated channels controlled?

Answer 13

Channel is opened, or closed, by binding of a chemical ligand. This might be an extracellular transmitter or an intracellular messenger.

Question 14

How are voltage gated channels controlled?

Answer 14

Channel opens or closes in response to changes in the membrane potential

Question 15

Define excitatory postsynaptic potential (EPSP)

Answer 15

The change in membrane potential that arises as a result of depolarising transmitters opening channels with positive reversal potentials which leads to excitation of cells.

Question 16

Define inhibitory postsynaptic potential (EPSP)

Answer 16

Hyperpolarising transmitters open channels with negative reversal potentials, leading to inhibition and the change in membrane potential caused by this is the IPSP.

Question 17

How is fast synaptic transmission achieved?

Answer 17

Using EPSP and IPSP

Receptor itself is also a ligand-gated ion channel

Question 18

How is slow synaptic transmission achieved?

Answer 18

Signals are sent to the channel in one of two ways:
Within the membrane - G protein goes to channel
Intracellular messenger - G protein activates an enzyme-signalling cascade