Action Potential Flashcards
How can we measure resting potential?
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)
How does electrical charge work within neurones?
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
How do ions move across the membrane?
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
What are some types of ion channels?
Ligand gated - chemicals
Voltage gated - voltage change
Mechanically - stretch activated
What are some concentrations of ions at resting potential?
Na+ - has more extracellular than intracellular
K+ has less extracellular than intracellular
What are the 4 main factors involved with action potentials? and the other contributing factor?
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
What is Equilibrium potential (Eion)?
he electrical potential difference that exactly balances an ionic concentration gradient
What is the ionic driving force?
The difference between the resting membrane potential (Vm) and the equilibrium potential (Eion): Vm – Eion
What is an action potential?
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
How do we record an action potential?
Using three electrodes:
a reference, recording and a stimulus
How does magnitude (height) of an action potential vary?
A stronger stimulus = a higher magnitude
What are the stages of an action potential?
Resting potential
Depolarisation (overshoot)
Repolarisation
Hyperpolarisation (undershoot)
What are the numbers we need to learn for the action potential?
Resting potential: -65 mV
Threshold: -55 mV
Height of depolarisation: +40 mV
What is resting potential?
No impulse is being passed along
More positive ions outside the axon
-65 mV
Describe resting potential in terms of ions?
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
Describe depolarisation in terms of ions?
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
Describe repolarisation in terms of ions?
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
Describe hyperpolarisation in terms of ions?
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
What is the principal of action potentials?
All or nothing
There is an action potential or not at all
It needs to exceed threshold of -55 mV
What are the refractory periods?
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)
What are the 3 purposes of the refractory period?
Ensures the action potential moves in only one direction
Produces discrete impulses
Limits the numner of action potentials
What happens in myelinated neurones?
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
What are some toxins that inhibit action potentials?
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
