Exam1Lec3ActionPotential Flashcards
What is action potential?
rapid, transient change in membbrane potential (Vm)
Na+ channel activation
responsible for transient inward current that depolarizes membrane
K+ channel activation
delayed activation responsible for delayed outward current that repolarizes the membrane
Threshold potential
Minimum depolarization required for an action potential to fire
Local current flow
action potential propagation along un-myelinated axon
Saltatory conduction
action potential propagation along myelinated axons. Much faster conduction
AP refractory period
time lag between action potentials
where ion channels rest b4 another ap can re-fiire
Multiple sclerosis
autoimmune, degenerative disease of axon demyelination
incr passive current flow
decr AP conduction velocity
Action potential usuaslly initiates in ____.
axon hillock
axon hillock= trigger zone
without ____ we don’t have an action potential
ATP
life is a neuron with a charge across membrane, and to get charge we need energy from ATP. So Na2+/K+ ATPase pumps establish and maintain membrane potential
What is EPSPS?
Makes Em more dep, and make is more likely to fire
driven glutamatergic transmission
What is IPSP?
Makes EM more hyperpolarized
driven by GABA
How can Em be changed?
slide 7
What triggers the physiological changes in membrane potential?
Changing internal or external ionic concentrations (usually K+)
changing relative perm of the ions across the plasma membrane
How are ionic permeabilities changed?
- Ligand gated (ex Gaba, glu, 5HT)
- Voltage-gated (Na+, K+)
- Nucleotide-gated (cAMP, cGMP)
- Inward rectifier “leak” channels
- Mechananosensitive-gated (stretch)
What are the 5 types of voltage gated ion channels?
Na+, Ca2+, K+, Cl-, H+
very sensitive to membrane potential
AP depolarization is triggered by transient opening of ____
Na+ channel
this always happens first and then we get a delay of K+ to hyperpolarization
mechanism of the voltage-gated Na+ channels
Na+ channels have 3 conformational states following a dep, what are they?
1st: open (a or m) gate state
2nd: inactive (I or H) gate
3rd: closed state
In the inactivation stage of Na+ what predominates?
Efflux of K+
K+ channel produes the delayed ____ current
outward
How does K+ channel produes the delayed outward current?
- Dep triggers opening of voltage gated K+ channels
- K+ ions rush out of the cell
- K+ ions efflux causes a transient after hyperpolarization
- Pk remains high, causing Vm to be transiently more neg than baseline resting Vm (after hyperpol phase)
this is a built in system to prevent ap from firing too fast to prevent cell death
What is an important component of the molecular structure of Na+ and K+ channels?
They have 4 subunits and the 4th one has a voltage sensing fxn with ion selectivity ( this dictates which ions can flow)
Potassium channels are ____
tetramers (comprised of 4 identical subunits)
____ segments “sense” changes in Vm, and move in response to these changes
S4
amino acids residues change in repsince to small mV differencees in voltage
For a neuronal action potential we see a transient, rapid incr in ____ and a slower more stable increase in ____.
Pna, Pk
Rapid activation of ____ channels causes depolarization
____ channel inactivation stops depolarization
Slower activation of ____ channels contributes to repolarization
Na+, Na+, K+
Action potential waveforms vary in their shape. Why?
Because of differences in types of activated channels and their relative times of activity
depends on type of neuron
Na+ channel activation enters a positive feedback loop, explain this
Na+ channel activation leads to a depolarization that incr probability of activation more channels
K+ channel activation enters a slow neg feedback loop, explain this
Channel activation leads to a hyperpolarization that decr prob of activating more channels
What is action potential threshold?
Minimum dep needed to fire an AP. AP WILL NOT fire if Vm is under threshold. AP is an all or non phenomenon
NOTHING will make is fire if its under threshold.
How does Non-propagated dep spread?
slide 23
Passiviely though local current flow, this is called electronic conduction
we are not hitting threshold at the axon hillock
Why does the dep signal decr over distance as it moves down the axon?
Ionic charges dissipate within an area and charges leak out (leaky membrane)
What is length constant?
the distance that 37% of vmax is reached
vmax is the highest amplitude signal
What factors influence length consant?
Membrane resistance and internal resistance within a cell
internal resistance=interaxonal resistance
If you have a high Rm (membeane resistance and low RI what does this mean for the length constant?
Larger length constant
length constant is ____ to Rm and ____ to Ri
directly proportional, inversely prop
How is AP maintained along the axon?
The peak membrane potential along an axon during an AP remains consistenly dep
What is absolute refractory period?
A 2nd action potential can NOT fire b/c Na+ channels can not yet be re-activated-either in their open or inactivated state
here, the mem potenital is so dep that its imposible for na+ to be reac to fire ap
What is relative refractory period?
A much greater dep is required for an ap to fire. Still active voltage gated K+ channels move membrane away from threshold potential.
mostly contributed by the activation of K+ channels openihng, K+ efflux, and hyperpol. This is after hyperpol so dep is needed for ap to take place
What is needed in order for a charge in membrane potential?
Charges must be re-distributed across the membrane-the membrane must be changed
Time needed to charge membrane to change Vm is the ?
rate limiting evenft for AP propagation (dictates firing propagation)
What is time constant (t)
The time it takes for the change in Vm to reach 63% of its final value.
small neurons=small memrane amt= fast charging= quick time constant
opposite for large neurons
Myelination ____ conduction velocity because Rm ____ length constant and Cm____ time constant.
increases, increases, decreases
Insulating nature of myelin will incr Rm
Cm decr bc or addition of membrane myelin
Saltatory conductions: where propagation along myelinated axons where AP jumps from one node to the next. How does myelination affect Rm, Cm, Vm, Conduction velocity, and length constant
Incr Rm
Decr Cm (at myelination site)
Decr Vm
Incr conduction velocity
Incr Length constant
length constant is incr bc channels can be distributed farther apart bc signal will be maintained over a greater distance
For an unmyelinated axon it has ____ conduction because of ____
slower , local current flow (electrotonic conduction)
areas of de-mye = nodes of ranvier, and this is where Na+ ions collect and where depolarization takes place to trigger dep
For myelinated axons it has ____ conduction because of ____.
Faster, saltatory conduction (action potential jumps from node to node)
What is multiple sclerosis?
An autoimmune degenerative disease of an axon demyelination. This causes conduction velocity to dec. Channels tend to re-distribute along the axon and action potential propagate through passive current flow whihc is much less effective
Conduction velocity is influenced by what two things?
Myelination and axonal size
Ri decr in. proportiion to the square of the axon diameter
