Session 4 ILOs - Membrane Potentials

Question 1

Outline what a membrane potential is

Answer 1

The resting membrane potential is an electrical potential difference across the plasma membrane

Question 2

Outline how the resting potential of a cell may be measured

Answer 2

Using a microelectrode with one sensor extracellularly and the microelectrode inserted into the cell

Question 3

Outline the range of resting membrane potentials found in different tissues

Answer 3

It depends on the tissue type, see below:
Cardiac: -80mV
Neurones: -70mV
Skeletal: -90mV
Smooth muscle: -50mV

Question 4

Outline the values found in a resting membrane potential (ions)

Answer 4

INTRACELLULAR:
NA+ = 12mM
K+ = 140mM
CL- = 4.2mM
A- = 167mM

EXTRACELLULAR:
NA+ = 145mM
K+ = 4.5mM
CL- = 123mM
A- = 40mM

Question 5

Explain how selective permeability of cell membranes arises

Answer 5

Ion channels can be: selective, gated and allows rapid flow of ions down diffusion gradient - each ion channel is selective for 1 (or a few ions) which controls what the membrane is permeable to.
EXAMPLE: Nicotinic acetylcholine receptors have a ring of negatively charged residues which allows channel to be selective for cations

Question 6

Describe how the resting potential is set up given the distribution of ions across cell membranes

Answer 6

Voltage insensitive K+ channel mainly sets the membrane potential close to -95mV but leaky Na+ and Ca2+ voltage sensitive channels (and Cl- channels) raise the resting potential so it’s closer to -70mV

Question 7

State the meaning of the term: equilibrium potential (for an ion)

Answer 7

Refers to the membrane potential at which there is no net movement of an ion across the plasma membrane into or out of the cell (chemical and electrical gradients cancel out but there can still be a positive/negative charge overall)

Question 8

Define depolarisation and mechanisms that can lead to depolarisation

Answer 8

A decrease in the size of the membrane potential from it’s normal value (cell interior becomes LESS negative)
Can occur when ligand gated ion channels open (permeable to Na+ or Ca2+) leading to an excitatory post-synaptic membrane potential (EPSP)

Question 9

Define hyperpolarisation and mechanisms that can lead to hyperpolarisation

Answer 9

An increase in the size of the membrane potential from it’s normal value (cell interior becomes MORE negative)
Can occur when ligand gated ion channels open (permeable to K+ or Cl-) leading to an inhibitory post-synaptic membrane potential (IPSP)

Question 10

Outline how ligand gated ion channels can give rise to synaptic potentials

Answer 10

Opening of ion channels can cause either depolarisation or hyperpolarisation depending on the charge of the ion and the direction of the flow of the ion (depending on electrochemical gradient)