L4: action potentials - Levy Flashcards
the farther above threshold the stimulus the greater the frequency of action potential firing. This relationship holds until…
until the fiber reaches its maximum frequency of firing
what limits the frequency of action potential firing?
refractory period
what approximately is resting potential in vertebrates?
-70 mV
describe the “all-or-none” property of an action potential
once threshold is reached, an amplitude will always attain a fixed amplitude that is independent of stimulus strength
describe the “time-limited” property of an action potential
action potential duration does not vary with duration of stimulus – action potential repetitions will change according to duration of stimulus and frequency will change according to strength of stimulus until maximum frequency is reached (due to refractory period)
describe the “non-decremental” property of an action potential
unlike local potentials, an action potential propagates along the cell membrane without decrement, which is valuable for conveying info over long distances
describe the “voltage dependence” of an action potential
when a critical voltage is reached across the membrane, voltage-dependent cahnnels open and allow a series of passive ion movements to produce the action potential
when in the course of an action potential is the absolute refractory period
from the beginning of the action potential to the point when it has repolarized to a value near threshold (before undershoot)
when in the course of an action potential is the relative refractory period
from repolarization to threshold through undershoot to depolarization back to resting potential
what happens to the threshold voltage during the relative refractory period
threshold is raised
what is the orthodromic direction
from receptive region to presynaptic region of neuron
what is the antidromic direction
from presynaptic region to receptive region of neuron
T/F concentration gradients change significantly during the course of an action potential
false - concentration gradients hardly change during the course of an action potential
how is conductance related to current and driving force?
g = I / (Vm - E)
why is the slope of depolarization steep?
g Na+ is increasing because voltage gated Na+ channels are opening
why is the slope of repolarization steep?
g K+ is increasing because voltage gated K+ channels are opening
this technique used a constant Vm and allowed the measurement of ion currents and calculation of voltage dependence of ion conductances
voltage clamp method
the puffer nerve poison tetrodotoxin (TTX) blocks ___
blocks voltage gated Na+ channels and therefore Na+ current due to with no effect on K+ current
tetraethylammonium ions (TEA) block ___
the delayed voltage gated K+ channels and therefore the delayed K+ current but have no effect on Na+ current
this substance blocks Na+ current with no effect on K+ current
TTX puffer nerve poison tetrodotoxin
this substance blocks the delayed K+ current but have no effect on Na+ current
TEA tetraethylammonium ions
describe the entire sequence of events that gives rise to an action potential
- local depolarization activates voltage-gated Na+ channels, allowing influx of Na+ and depolarization
- local depolarization also increases K+ driving force (Vm - EK+), influx K+ and repolarization
- threshold is reached when sufficient voltage Na+ channels are opened and overcome outward K+ current. Na+ influx depolarizes further and opens more Na+ voltage channels in positive feedback
- Na+ channels are inactivated and I Na+ turned off
- (with some overlap to previous step) delayed K+ voltage channels open and efflux, repolarize, hyperpolarize, and then close to depolarize back to normal Vm resting
- absolute refractory is about threshold to threshold when Na+ channels are either open or inactivated
- relative refractory is from re-threshold to Vresting when K+ channels are still open and Na+ are changing from inactivated to closed
what are the voltage sensors that open voltage gates?
groups of charged amino acid residues on transmembrane a-helices
what confers channel selectivity for either Na+ or K+ ions?
particular geometric arrangement of negative amino acids at narrowest part of pores
what are 4 conformations of Na+ voltage gated channel over course of action potential
- rest (closed)
- activated (open)
- inactivated
- inactivated and closed
- back to rest (closed)
this period of an action potential is due to inactivation of Na+ voltage gates
absolute refractory
explain the absolute refractory period
so few Na+ channels have recovered that even if all of them were opened, inward current would not exceed outward K+ current that keeps cell polarized
explain the relative refractory period
fewer than normal Na+ channels are available, so a larger fraction of them must be opened to carry sufficient inward current to offset considerable outward K+ current. hence, a larger depolarizing stimulus (threshold) is necessary
T/F like the Na+ channel, the K+ channel has an inactivation gate
false - contrary to the Na+ channel, the K+ channel has no activation gate
when do the K+ channels open and close?
- delayed open after action potential depolarization
- begin to close during hyperpolarization and finish closing when back to resting membrane potential
when do the Na+ channels open and close?
- open - depolarization
- inactivate - near top of depolarization peak
- back to rest (closed) ~ re-threshold before hyperpolarization
why doesn’t an action potential re-invade a region it has immediately passed through?
region becomes refractory and cannot generate an action potential for a certain time
myelin consists of
fused membranes of satellite cells wrapped around the axon
nodes of Ranvier
regularly spaced gaps in myelin
what are 3 functions of myelin?
- decrease leakage current from axoplasm during conduction
- faster conduction through saltatory conduction
- require less metabolic energy to maintain Na+ K+ concentration gradients (less room for leak channels, less need for Na K pump)
saltatory conduction is
faster conduction jumping from nodes of Ranvier between myelin
