Nerve cells and excitability

Question 1

How does the concentration of potassium ions vary inside and outside the cell?

Answer 1

K+ is high inside the cell but low outside the cell.

Question 2

How can sodium or potassium be transported across the membrane?

Answer 2

Using sodium-potassium ATPase transporters that use ATP to transport the ions against the concentration gradient.

Question 3

What is the resting membrane potential?

Answer 3

The potential difference (mV) between two electrodes placed inside and outside the cell.

Question 4

What is depolarisation?

Answer 4

Increase in membrane potential.

Question 5

Hyperpolarisation?

Answer 5

Decrease in membrane potential (more negative).

Question 6

Where are pyramidal neurones found?

Answer 6

The hippocampus.

Question 7

Where are Purkinje neurones found?

Answer 7

The cerebellum.

Question 8

What does the Nernst equation allow?

Answer 8

The equilibrium potential for any ion to be calculated.

Question 9

What is the Nernst equation?

Answer 9

Eion = 2.30RT/zF x log [ion]outside/[ion]inside. where F = faradays constant, z=charge on ion, T=absolute temperature and R=gas constant.

Question 10

What is the refractory period?

Answer 10

When Na+ channels become inactivated as the membrane depolarizes and cannot be activated again until the membrane is repolarized.

Question 11

What are graded (local) potentials?

Answer 11

Changes in the membrane potential that are confined to a small region of the membrane.

Question 12

Distribution of charged ions

Answer 12

sodium potassium atp transporter maintains gradient, pumps against concentration gradient
3 Na+ out, 2 K+ in
using active transport

Question 13

3 transporters

Answer 13

active transporters
ion channels- selectively permeable, ions diffuse down concentration gradient
voltage gated channels: passive, selective, rapid

Question 14

what drives conformational change of channels

Answer 14

phosphorylation

Question 15

How can Vm (RMP) be calculated

Answer 15

Goldman equation

Question 16

repolarisation

Answer 16

potential moving back to RMP

Question 17

propagation

Answer 17

movement of AP along axon

Question 18

How many states do Na+ VGC’s exist in

Answer 18

3
open, closed, inactivated
once inactivated can’t go back to being open, have to go back to being closed, then open

Question 19

how many states for K+ VGC

Answer 19

2

open and closed

Question 20

Sequence for and action potential ( Na+ VGC’s)

Answer 20

Na+ open rapidly with depolarisation- influx of Na+ ions. Inactivation gate rapidly blocks Na+ permeability during continued depolarisation
Inactivated gates move to closed on repolarisation

Question 21

Sequence for and action potential ( K+ VGC’s)

Answer 21

open slowly on depolarisation, K+ move out of cell, drawing +ve charge out
close slowly on repolarisation
K+ continue to move out till reach equilibrium potential for K+. no net movement
At this point K+ VGC closed and Na+ inactivated
Then pump establishes concentration gradient from the beginning

Question 22

Threshold

Answer 22

point at which AP is generated

determined by extent of depolarisation

Question 23

refractory period- why is it absolute?

Answer 23

because Na+ VGC are inactivated, unable to be opened (unlike in a close state) so cannot generate another membrane because it would require the membrane potential to be at rest and Na+ VGC’s to be open

Question 24

refractory period- why is it relative?

Answer 24

because the membrane is hyperpolarised until K+ channels close, so an AP can only be generated if stimulus is stronger than usual

Question 25

What makes an AP move fast?

Answer 25

axon diameter: large axons offer less resistance to current because they have a larger diameter, so faster propagation. In small axons the signal dissipates and leaks out of the membrane
mylein sheath: electrical insulation, current moves faster. Prevents signal leaking out. Gaps in the myelin sheath are nodes of ranvier which have clusters of Na+channels, so saltatory conduction can occur- AP jumps node to node and depolarisation only occurs where there’s no myelin

Question 26

graded potentials

Answer 26

graded based on changes in strength of stimulus - unlike APs
changes in membrane confined to a small area
depolarisation/hyperpolarisation
decay rapidly
don’t propagate
add to show summation
if threshold for VGSC is reached, an AP is generated

Question 27

How does the concentration of chloride ions differ from the inside to outside of the cell?

Answer 27

Higher concentration outside the cell than inside the cell.

Question 28

What is the equilibrium potential for K+?

Answer 28

-80mV.

Question 29

What is the equilibrium potential for Na+?

Answer 29

+60Mv.

Question 30

The larger the concentration gradient….

Answer 30

…the larger the equilibrium potential.

Question 31

What is resting membrane potential close to and why?

Answer 31

The potassium equilibrium potential (EK) as the permeability of potassium is greater than for sodium.

Question 32

What is the symbol for resting membrane potential?

Answer 32

Vm.

Question 33

What is an action potential?

Answer 33

A large transient change in membrane potential and is an all or none response.

Question 34

What happens to K+ channels during depolarisation?

Answer 34

They open slowly.

Question 35

What happens to K+ channels during repolarisation?

Answer 35

They close slowly.

Question 36

What are the characteristics of an action potential?

Answer 36

Threshold, overshoot, all or nothing event, refractory period, propagate and no decrement.

Question 37

What is the absolute refractory period?

Answer 37

The point in which the Na+ channels are closed and an action potential cannot be regenerated, no matter how large the stimulus.

Question 38

What is the relative refractory period?

Answer 38

The point in which a stimulus needs to be larger than usual to generate and action potential as the membrane is hyperpolarised due to K+ channels remaining open for too long.

Question 39

What is saltatory conduction?

Answer 39

The propagation of an action potential involving nodes of Ranvier.

Question 40

What are the characteristics of graded potentials?

Answer 40

They can be a depolarisation or hyperpolarisation, their size and duration is graded, they decay rapidly, they travel small distances and show summation.