Ion channel activity

Question 1

Why is an uneven distribution of ions important for a resting potential

Answer 1

Intra/Extra have similar pH, but ions (Na/K/Cl/Ca) are unevenly distributed. Cell membrane is impermeable to ions - no free flow. Ions must be transported!

Question 2

How do ions cross membranes

Answer 2

Primarily through ion channels

Question 3

Define ‘leak’ channels

Answer 3

Ion channels that are open all the time

Question 4

List some electrophysiological recording approaches

Answer 4

Extra-cellular recording
Intracellular recording
Whole-cell patch recording

Question 5

List the factors responsible for an uneven distribution of ions at resting potential

Answer 5

Selective permeability of membrane
Large organic anions with the cytoplasm
Ion pumps

Question 6

Which ion flow, at low concentration defies the Nernst prediction

Answer 6

[K+} out.

Question 7

The flow of which ions occurs at rest

Answer 7

[Na+]in and [K+]out.

Question 8

What is the role of the Na+-K+ ATPase pump?

Answer 8

Transports 3Na/2K out/in to the cell. Causes a net outward current which causes a small hyperpolarisation.

Question 9

How is an action potential initiated

Answer 9

Positive feedback cycle involving Na+ channels.

Depolarisation ->Na+ channels opened -> Na+ influx -> depolarisation. repeat.

Question 10

What needs to occur before positive feedback can create an action potential

Answer 10

Vm must cross the ‘threshold’ - enough open Na+ channels.

Occurs through chemical/electric signals from elsewhere.

Question 11

What is the cause of the ‘undershoot’ of an action potential

Answer 11

Na+ channels still inactive
K+ channels still open.
Vm goes below resting potential

Question 12

What is refractory period of an action potential

Answer 12

Absolute: Na+ channels inactivated. AP cannot occur.
Relative: Some Na+ channels can open. AP can occur with stronger depolarisation.

Question 13

Describe the structure of voltage gated Na+ channels

Answer 13

Contains pore-loop voltage-sensor. Has 1 large alpha subunit with 4 repeating 6-membrane spanning segments.

Question 14

Describe the sequence of ion channel events that occur during an AP

Answer 14

Resting state: No Na/K flow
Depolarising state: Na channel open, K closed.
Repolarising state: Na closed, K channel open
Undershoot: Na closed, K still open.
-> resting state.

Question 15

What happens to a voltage gated Na+ channel during depolarisation

Answer 15

Channel has +ve sections which are drawn into the cell. Depolarisation weakens this. Pore changes shape, channel opens.

Question 16

How is a voltage gated Na+ channel inactivated

Answer 16

Residues in intracellular loop between domain 3 and 4 are attracted into inner mouth of channel at depolarised potentials. Fills hole. Ions cannot pass through.

Question 17

What are the two types of patch clamp recording

Answer 17

Inside-out patch

Outside-out patch (whole-cell recording)

Question 18

Describe the structure of a calcium channel

Answer 18

2 alpha subunits. 1 beta, 1 delta, 1 gamma. Alpha-1 subunit makes the channel.

Question 19

Name some excitatory ligand-gated ion channels

Answer 19

Glutamate
Nicotinic acetylcholine
Serotonin 5-HT3
Purinergic P2X
Vanilloid

Question 20

Name some inhibitory ligand-gated ion channels

Answer 20

GABAa (Cl-/HCO3-)

Glycine (Cl-)

Question 21

Which type of ligand-gated ion channel is most common in the brain

Answer 21

Glutamate

Question 22

How are glutamate and GABA related?

Answer 22

Both synthesised from glutamate/glutamine (cell metabolism).
GABA is synthesised from Glutamate (by GAD)

Question 23

Describe the structure of the GABAa receptors

Answer 23

Cysteine loop type receptor. 4 membrane spanning domains. (TM2 = selectivity).

Question 24

Describe the structure of the glutamate receptors

Answer 24

Similar to K+ channels.

Tetrameric. Poor loop structure = selectivity