Action potentials Flashcards
What determines how a neurone responds to electrical activation?
If a neuron is activated the membrane will depolarise from the RMP.
The level of depolarisation will be proportional to the strength of stimulus applied.
If the membrane is sufficiently depolarised to the threshold then an action potential is generated.
What is depolarisation?
The loss of the difference in charge between the inside and outside of the plasma membrane due to a change in permeability and migration of sodium ions to the interior.
What is the threshold?
The point where Na+ entry is greater than K+ efflux.
What is hyperpolarising stimuli?
If Cl- enters the cell it brings a negative charge and membrane potential is even further from the threshold.
What is the depolarisation phase of action potential?
Stimulus, ligand gated Na+ channels open, the membrane potential becomes more positive.
Depolarises the membrane further and reaches threshold.
VGNaC and VGKC are activated to open, but VGKC are slower than VGNaC.
VGNaC open, Na+ influx, causes depolarisation, approaching ENa to about +30mV.
What is repolarisation phase of action potential?
VGNaC inactivate.
The VGKC are fully open, K+ efflux, repolarisation.
VGKC stay open for longer, the voltage becomes more negative than RMP, and causes hyperpolarisation.
What are the VGNaC gates at RMP?
The activation gate is closed.
The inactivation gate on the cytoplasmic side is open.
What are the VGNaC gates at depolarisation?
The activation gate is open.
The inactivation gate is still open.
The membrane potential is more positive as Na+ influx.
What are the VGNaC gates at 30mV?
The activation gate closes in response to depolarisation.
The inactivation gate closes 0.5ms after.
Na+ influx stops.
Repolarisation, the VGKC opens and K+ efflux.
Membrane potential more negative and VGNaC is reset.
What does the graph of membrane potential during an action potential look like?
see picture
What is ion conductance?
The ease of ion flow through the channel in the plasma membrane.
This varies with the state of the channel - if it is present and open.
What is the absolute refractory period?
There are no further action potentials.
The VGNac are opening then inactivating, or are all inactivated.
What is the relative refractory period?
The membrane potential is becoming more negative so some VGNaC open, Na+ leaves cell and hyperpolarises.
The membrane potential is lower than at rest, so larger stimulus is needed to reach action potential.
Why are refractory periods important?
Set frequency of action potentials are generated.
Limits the rate at which signals can be transmitted.
Ensures one way travel of action potential.
How is conduction in one direction?
The signals intergrate at axon hillock.
Reach threshold, generates action potential and moves down axon.
VGNaC behind the action potential inactivate, the ones in front activate.
How does myelination increase conduction?
Charge travels along axon.
Concentrated VGNaC in Nodes of Ranvier, so can jump across axons.
Saltatory conduction.
