How nerves work 4 + 5 Flashcards
how are action potentials produced?
they are evoked by voltage gated sodium channels opening which causes a massive depolarisation that brings the cell to threshold.
what is the common threshold potential value?
about -55 mV
so if a graded potential reaches this an action potential will be released
what happens once the threshold is reached?
sodium channels open
sodium rushes into the cell depolarising it
it goes all the way to about +40 mV
to repolarise- potassium channels open up letting potassium move out of the cell in attempt to balance out charge
this all happens very quickly
why would there be a delayed effect if you poisoned the Na+/K+ pump?
it takes time for gradients to run down
why is cell excitability low when Na+ channels are open?
it’s in refractory period- the cell cannot respond to another depolarisation- voltage gated Na+ channels are tired and need to rest before working again.
What are properties of an action potential?
self probagating- ie self spreading/transmission
need G.P to reach threshold for A.P to exist
refractory period
a bigger G.P doesn’t do anything different as A.P is just produced once target threshold is reached and because so much Na+ flows in the threshold is always reached therefore it’s ALL OR NONE
why is refractory period so important?
by not responding it makes sure an action potential can only travel in one direction (not backwards)
current flow can move backwards but it does NOT produce a new A.P that way as they are in refractory state
how does a bigger graded potential lead to more action potentials being produced?
it stays above threshold for longer
why are there no Na+ channels within myelinated parts of axons?
if depolarisation is evoked from Na+ channels at one end then the myelination helps to insulate the neuron and the depolarisation spreads passively quickly along to the next node. Decay is slow at node however it still reaches threshold
what is saltatory conduction?
Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.
de-myelination and multiple sclerosis
an auto-immune, neurological disease which occurs from damage to the myelin sheathes. When one end depolarises the transmission is not good as it decays quickly and is not big enough at next node to reach threshold. Channels therefore never get activated and there is failure of transmission more and more as the disease progresses/gets worse
Why are some nerves unmyelinated?
myelination has a huge metabolic cost so they’re only myelinated if there is an advantage to their potentials being transmitted faster
ie Neurons which pass information about temperature change and prolonged ache don’t need to be fast- these are called C fibres + aren’t myelinated!
However, neurons which have to pass information about sudden pain (touching a burning hot pan) need to be myelinated to protect the body
recording conduction velocity process
stimulating the nerve trunk at one end will cause lots of axons to depolarise at the same time and all fire an A.P at the same time which causes a big wave of depolarisation. They’re all effected at different rates and you can record the velocity of each
what is compound action potential
a series of waves
- The measure of the activity of an individual nerve cell by how frequently it fires A.P
fastest wave
Aalpha - big myelinated
proprioception (detecting a stretch in a muscle)
slowest wave
C- unmyelinated, just fibres- pain from heat
