#3 Flashcards
Why is an AP important?
It is how axons propagate information from one region to another
How does it work and where does it extend ?
1) first initiated when the membrane potential depolarizes (rises) to a threshold potential
2) starts at the initial segment of the axon and propagate down the length of the axon to the presynaptic terminal
3) it is terminated by the inactivation of the sodium channels and the delayed activation of voltage-gates potassium channels
Is an AP an all or nothing event?
yes it is but, the info is sent by means of the frequency and pattern of the action potentials.
aka more frequent action potentials occur when there is more pressure or inputs ( ex: a neuron that is responding to pressure on the skin will have more frequent action potentials if there is a pressure increase)
how is a threshold potential determined ?
by the properties of ion channels in the axon membrane (especially voltage sodium channels)
What is the voltage at peak of the AP near the initial segment ?
the membrane approaches E_Na+ (+30 m V)
how is the propagation of the action potential caused ?
Caused by the spread of electronic currents from the site of action potential, which then excites the adjacent regions of the axon, causing another action potential (due to the attraction of the new positive charge, to the negative charge of the axon)
this is inevitable because the whole membrane is covered with voltage-gated Na+ channels
how/what is an absolute refractory period?
occurs a few milliseconds after the action potential when the sodium channels are inactivated and the membrane is completely unexcitable
this is why the action potential does not go back down toward the initial segment
the absolute is much shorter than the relative
what is a relative refractory period?
occurs after the membrane potential overshoots its resting level due to voltage gated gated potassium channels being open, and the axon is less excitable and is unlikely to fire an action potential.
These refractory periods set a limit to how many action potentials a neuron can send per second
How do neurons use action potential to send information?
Neurons send info by means of the frequency and patters of action potentials
- these patterns encode information
- they don’t send info by charging the duration or size of the AP ( true for all AP’s)
What can affect sodium channels? and examples
sodium channels are the molecular targets for naturally occurring neurotoxins
ex: puffer fish make tetrodotoxin which is extremely potent inhibitor of sodium channels
or phyllobates frogs secrete batrachotoxin - a powerful sodium channel activator
- the sodium channels are open and they can’t close
- neurons will send signals uncontrollably
do sodium channels block therapeutically important drugs?
yes, including local anesthetic and some antiepileptic agents
which local anesthetics are blocked by sodium channels ?
local anesthetics - AP cant get past the spot where the anesthetic was injected
lidocaine
Benzocaine
Tetracaine
Cocaine
What are Antiepileptics ?
prevent seizures by weakly blocking sodium channels, suppress sodium channels just enough to inhibit abnormal potential firing, does not block normal function of the brain
types of antiepileptics (anti-seizure drugs)
phenytoin (Dilantin)
Carbamazepien (Tegretol)
Lamotrigine
how rapidly can action potentials propagate?
It depends some are as fast as 100 m/s but other as much slower 0.5m/s
fast pain: when you grab smth hot
slow pain: 10-15 seconds after pulling hands away. slow pain is conveyed from fingertip to brain by an axon called C-fiber which propagates info about 0.5 m/s and that is very slow
