Action potential Flashcards
What exactly is an impulse?
A change in the membrane potential (difference in charge inside vs outside) of a neurone which is sent along the length of the whole neurone
In other words, the action potential passing along the length of a neurone
What is the action potential?
The change in potential over the membrane of a neuron that occurs (from rest) when a stimulus is detected in order to send an impulse down the neurone
What do action potentials depend on?
Voltage-gated sodium ion channel proteins
Voltage-gated potassium ion channel proteins causes changes in permeability——–> Depolarisation ——-> Action potential
Are the voltage gated ion channel proteins closed at resting potential?
Yes both of them are
Depolarisation
When a membrane potential becomes less negative (the neg charge inside is no longer as neg relative to the outside of a neurone before)
Or even positive (the inside of a neurone is now more positive relative the the outside)
Depolarising generator potential at receptor
This occurs at start when a stimulus is detected, Na+ ions can enter the neurone (eg down stretch mediated channel protein) increasing pos charge inside the neurone
Threshold value
The minimum amount of Na+ ions that enter neurone from stimulation detected by receptor to generate an action potential
What happens if the threshold value is reached?
Voltage gated sodium ion channel proteins will rapidly open
So Na+ ions (at high conc outside) can rapidly diffuse into neurone down electrochemical grad using these membrane proteins
What happens when Na+ ions can diffuse into neurone us8ng the voltage gated Na+ ion protein?
The neurone is further depolarised- the charge difference between neurone to the outside becomes positive as Na+ ions enter the neurone
Peak membrane potential reading of depolarisation
+40mV
What happens once the peak voltage reading for depolarisation happens?
The voltage gated sodium ion channel proteins close
And triggers opening of potassium ion channel proteins to open
What happens when voltage gated potassium ion channel proteins open?
The potassium ions at high concentration in the neurone can diffuse out of the neurone to lose positive charge in neurone so it becomes negative again
Repolarisation
Facilitated diffusion of potassium ions out of neurone over the neurone cell membrane using voltage gated potassium ion channel proteins to restore negative charge inside the neurone
Hyperpolarisation
So many K+ ions leave neurone during repolarisation through voltage gated K+ channel proteins so membrane potential is more negative than resting potential
What happens when hyperpolarisation is reached?
Voltage gated potassium ion channel proteins close so no more K+ ions can leave neurone membrane
So Na+K+pump protein can pump 3 Na+ ions out/K+ ions to restore resting potential, etc
All or nothing principle
An action potential is only generated if a threshold stimulation is reached
For every given neurone: the generation of an action potential will always be the same size
How can we encode for the strength of a stimulus if the strength of an action potential will always be the same?
By frequency of action potentials in a given time
A very strong stimulus…
First: threshold stimulation is reached and exceeded to cause depolarisation of a neurone
Generated very frequent action potentials so strong stimuli
If no threshold is reached…
Not enough Na+ ions enter neurone to trigger opening of voltage gated Na+ channel proteins
So no depolarisation
No action potential
Why do we have a threshold?
Because if even very weak stimuli could generate an action potential then our nervous system would be very overwhelmed
Threshold allows filtering out of very weak stimuli
How can the brain detect stimulus strength?
Different neurones have different threshold potentials so can be activated by stimulus of different strengths
To determine the strength of a stimulus, the brain can detect the relative number of different neurones to determine the strength of this stimulus
What affects the nerve impulse (action potential) travelling down the neurone?
If neurone is myelinated with Schwann cells or not
Axon diameter
Temperature
