Action Potential

Question 1

How can we measure resting potential?

Answer 1

place a microelectrode inside a neurone and a reference electrode into ‘extracellular fluid’
The computer will record a value of around -65 mV after the use of an amplifier (due to working in mV)

Question 2

How does electrical charge work within neurones?

Answer 2

An electrical gradient drives an ion towards an area of oppositely charged ions
E.g. K+ moving to an region of negativity
Ions also move via a concentration gradient

Question 3

How do ions move across the membrane?

Answer 3

Ion pump: The ion binds and then the ion is pumped across the membrane
This can actively move ions against its concentration gradient
Ion channel: The ions diffuse through the channel, down concentration gradients
They are selective, and have a permeability to ions

Question 4

What are some types of ion channels?

Answer 4

Ligand gated - chemicals
Voltage gated - voltage change
Mechanically - stretch activated

Question 5

What are some concentrations of ions at resting potential?

Answer 5

Na+ - has more extracellular than intracellular

K+ has less extracellular than intracellular

Question 6

What are the 4 main factors involved with action potentials? and the other contributing factor?

Answer 6

The Na+/K+ pump: moves 3 Na+ out for every 2 K+ in
The leakage channel: allows K+ to diffuse out of the axon
Na+ channel
K+ channel

The other factor is charge intracellular proteins - Large negatively charged intracellular proteins cannot cross the membrane and leave the cell interior and so contribute to its negativity

Question 7

What is Equilibrium potential (Eion)?

Answer 7

he electrical potential difference that exactly balances an ionic concentration gradient

Question 8

What is the ionic driving force?

Answer 8

The difference between the resting membrane potential (Vm) and the equilibrium potential (Eion): Vm – Eion

Question 9

What is an action potential?

Answer 9

Periods of rapid reversal of the membrane potential that overshoot 0mV, lasting a few milliseconds
An explosion of electrical activity that is created by a depolarizing current, usually from a synaptic input from another neurone

Question 10

How do we record an action potential?

Answer 10

Using three electrodes:

a reference, recording and a stimulus

Question 11

How does magnitude (height) of an action potential vary?

Answer 11

A stronger stimulus = a higher magnitude

Question 12

What are the stages of an action potential?

Answer 12

Resting potential
Depolarisation (overshoot)
Repolarisation
Hyperpolarisation (undershoot)

Question 13

What are the numbers we need to learn for the action potential?

Answer 13

Resting potential: -65 mV
Threshold: -55 mV
Height of depolarisation: +40 mV

Question 14

What is resting potential?

Answer 14

No impulse is being passed along
More positive ions outside the axon
-65 mV

Question 15

Describe resting potential in terms of ions?

Answer 15

Na+/K+ actively transports 3Na+ out of the axon for every 2K+ in
The leakage channel is open and facillitated diffusion of K+ out of the axon down it’s conc gradient takes place
Na+ ion channel is closed
K+ ion channel is closed

-65 mV

Question 16

Describe depolarisation in terms of ions?

Answer 16

Na+/K+ actively transports 3Na+ out of the axon for every 2K+ in
The leakage channel is open and facillitated diffusion of K+ out of the axon down it’s conc gradient takes place
Na+ ion channel opens - Na+ facillitated diffuese into axon down electrochemical gradient
K+ ion channel is closed

-65 mV to +40 mV

Question 17

Describe repolarisation in terms of ions?

Answer 17

Na+/K+ actively transports 3Na+ out of the axon for every 2K+ in
The leakage channel is open and facillitated diffusion of K+ out of the axon down it’s conc gradient takes place
Na+ ion channel closes
K+ ion channel opens - K+ facillitated diffuses out of axon down conc gradient and electrochemical gradient

+40 mV to -65mV

Question 18

Describe hyperpolarisation in terms of ions?

Answer 18

Na+/K+ actively transports 3Na+ out of the axon for every 2K+ in
The leakage channel is open and facillitated diffusion of K+ out of the axon down it’s conc gradient takes place
Na+ ion channel is closed
K+ ion channel is open - too many +ions have diffused out (temporary overshot of the electrochemical gradient)
The Na+/K+ pump, pump K+ ions back into the axon to restore resting potential

Question 19

What is the principal of action potentials?

Answer 19

All or nothing

There is an action potential or not at all
It needs to exceed threshold of -55 mV

Question 20

What are the refractory periods?

Answer 20

Absolute - no further action potentials can be produced to ensure the propagation of the action potential is unidirectional (within de and repolarisation)

Relative - an action potential is inhibited but not impossible if a large stimulus is given (within hyperpolarisation)

Question 21

What are the 3 purposes of the refractory period?

Answer 21

Ensures the action potential moves in only one direction

Produces discrete impulses

Limits the numner of action potentials

Question 22

What happens in myelinated neurones?

Answer 22

Action potentials only form at the Nodes of Ranvier (gaps in the myelin sheath - as myelin acts as an electrical insulator)
Due to localised current, only sections of the whole axon have to be depolarised
The action potential jumps from node to node - called saltatory conduction

Question 23

What are some toxins that inhibit action potentials?

Answer 23

Tetrodotoxin (toxin from a puffer fish):
This blocks voltage gated Na+ channel

A centipede produces SsTx a toxin that blocks the leakage channel at resting potential, which increases neurone firing
This occurs in the heart, blood vessels etc… leading to your heart stopping