Voltage-Dependent Membrane Permeability Flashcards
Resting potential (Vrest)
Membrane potential before current is injected
Is Vrest usually negative or positive?
Negative
Vrest and membrane potential are measuring the charge ___ vs ___.
Inside vs outside
Depolarization
Changes in membrane potential such that Vm > Vrest
Hyperpolarization
Changes in membrane potential such that Vm < Vrest
A passive response occurs when?
Small changes in current cause small changes in membrane potential that are proportional to the change in stimulus
When enough depolarizing current is added to the neuron to cause it to go above its threshold, what occurs?
Action potential
How does an active response differ from a passive one?
A passive response is proportional to the change in stimulus, whereas an active response (action potential) does not
What happens when the neuron reaches threshold that enables an action potential to occur?
Voltage-gated ion channels open, enabling ion permeability
Why is the action potential called “all-or-none?”
With increasing depolarization above threshold, there is no change in action potential amplitude, but instead increased frequency of action potentials
Hyperpolarization always yields a _____ response, whereas depolarization can yield a ____ or ____ response depending on whether or not it raises membrane potential above threshold.
Passive
Passive or active
Inward current is synonymous with _______. Why?
Depolarization
Both bring membrane potential up
Outward current is synonymous with ______. Why?
Hyperpolarization
Both bring membrane potential down
Before the stimulus, the neuron’s membrane potential is considered to be _____.
Vrest
Action potential: rising phase
First phase of action potential
Neuron depolarizes and Vm increases above Vrest
Action potential: overshoot phase
Second phase of action potential
Vm goes above 0 mV (reaches its peak)
Action potential: falling phase
Third phase of action potential
Vm decreases and neuron repolarizes to Vrest
Action potential: undershoot phase
Fourth phase of action potential
Neuron is briefly hyperpolarized (Vm goes below Vrest)
Why did early neuroscientists use giant squid axons?
They could be seen by the naked eye and the neuroscientists could easily create wires the right size to insert into them
A voltage-clamp instrument holds ____ ____ constant in order to measure ___ generated when ion channels open
Membrane potential
Current
How does a voltage-clamp instrument hold membrane potential constant?
Injects equal and opposite current to that caused by opening of ion channels, which prevents membrane potential from changing
In early experiments, neuroscientists measured changes in membrane potential of resting neurons as outside K+ concentration was increased. What occurred as outside K+ concentration was increased, and what did that tell the neuroscientists?
For a resting neuron, when outside K+ concentration is increased, Vm increases
Neurons are permeable to K+ at rest
For a neuron at rest, what happens to membrane potential when you increase outside Na+ concentration? What does that suggest?
For a resting neuron, increasing outside Na+ concentration has little effect on Vm
At rest, the permeability of Na+ is low
If the neuron is only permeable to K+, then why isn’t Vm=E K+?
Slight permeability of Na+ and Cl- have a small effect on the Vm
During an action potential, changing the outside concentration of what ion causes the greatest change in amplitude? What does that suggest?
Na+
Na+ plays a major role in determining the amplitude of the action potential
The action potential max voltage is close to the equilibrium potential of ___.
Na+
During rising phase, there is an increase in ___ permeability.
Na+
During falling phase, there is an increase in ___ permeability and a decrease in ___ permeability.
K+
Na+
Inward current: flow of ____ into or _____ out of cell
Cations (positive ions)
Anions (negative ions)
Outward current: flow of ____ into or ____ out of cell
Anions (negative ions)
Cations (positive ions)
When clamping at a hyperpolarized voltage, ____ current change is seen.
No
When clamping at a depolarized voltage, transient ____ current and sustained _____ current is seen.
Inward
Outward
Transient inward current is called _____ current, whereas sustained outward current is called ____ or _____ current because of when they are seen.
Early
Late/delayed
An ion channel is a ____ protein with a ____ in the center.
Transmembrane
Pore
Can ion channels be ion specific, allow multiple ions through, or both?
Both- some are ion specific, some are more generalized
What type of channels for what 2 ions are responsible for the action potential?
Voltage-gated
Na+ and K+
What does “gated” mean when referring to ion channels?
Opens and closes
Voltage-gated ion channels are usually only open when the neuron is _____.
Depolarized
What about the ion channels causes action potentials to exhibit an all-or-none threshold?
They are voltage-gated
Early transient current peaks around _____ mV.
0
Early current is caused by ____ channels whereas late/delayed current is caused by ____ channels.
Na+
K+
In a voltage clamp experiment, as Vm increases, early inward current first ___, then ___ until it reverses. Why is that?
Increases
Decreases
Ex for Na is around 50 mV- driving force decreases as Vm nears ENa, then stops and switches direction when Vm equals ENa
In a voltage clamp experiment, as Vm increases, delayed/late outward current _____ linearly. Why is that?
Increases
Ex for K is around -80 mV- driving force increases as Vm goes farther away from EK (becomes more positive)
In an experiment where Na+ concentration is 0, which current would be affected, early or late?
Early (Na+ drives early current)
Tetrodotoxin (TTX): which channels blocked, which current affected (early or late)?
Na+ channels
Early
Tetraethyl-ammonium (TEA): which channels blocked, which current affected (early or late)?
K+ channels
Late
Conductance is measured in what unit? What is this conductance specifically measuring?
mSiemens
Amount of current that can go through an ion channel
What value in mV is considered to be the threshold for K+ and Na+?
-40 mV
When plotting membrane potential vs conductance, below -40 mV conductance is ____. Past -40 mV, conductance ____ exponentially until it ____ out.
Low
Increases
Plateaus
Which ion channel, K+ or Na+, opens faster? Because ion channels for Na+ don’t stay open for the whole action potential, what does this mean for Na+’s conductance?
Na+
Conductance for Na+ is transient
Which ion channel, K+ or Na+, opens slower? ___’s conductance just begins to start as ____’s conductance is peaking. What does this mean for K+’s conductance?
K+
K+, Na+
Conductance for K+ is sustained (K+ channels stay open for the duration of the experiment)
At a given ion’s equilibrium potential, the driving force on that ion equals ____.
0
When the membrane potential is less than the equilibrium potential for an ion, current will be ____ and will _____ in magnitude the further away the membrane potential is from equilibrium potential.
Inward
Increase
When the membrane potential is greater than the equilibrium potential for an ion, current will be ____ and will ____ in magnitude the further away the membrane potential is from equilibrium potential.
Outward
Increase
Between driving force, current, and conductance, which will give a linear plot on a graph of membrane potential vs driving force/conductance?
Driving force
When plotting membrane potential vs conductance, the conductance is basically 0 until what value? Past that value, conductance _____ exponentially until it reaches its maximum.
Threshold (-40 mV)
Increases
Current equals _____ x _____.
Conductance
Driving force
Plotting membrane voltage vs current, current is ____ until ____ is reached.
0
Threshold (-40 mV)
When measuring current for sodium, past threshold it ____ in the _____ direction until it reaches the maximum value for ____. Why is this?
Increases in the inward direction
Conductance
I = g x DF: since DF is inward due to Vm being less than ENa, direction of I is inward
When measuring current for sodium, past maximum conductance value in the inward direction, it becomes ___ inward as DF becomes less inward and changes from inward to outward at the ____ ____ for Na. During this time, current is proportional to ____ _____.
Less
Equilibrium potential
Driving force
When measuring current for potassium, past threshold current for conductance, current will ____ consistently. Why is this?
Increase
I = g x DF: Driving force is increasing (outward current- EK is low), so I increases
Why do the plots of conductance for K and Na look almost the same?
The conductance has similar kinetics for both ions: similar thresholds, reach max conductance with similar kinetics
Na channels ___ and ____ fairly rapidly, making its conductance early but transient.
Open
Inactivate
K channels open more ____ than Na channels and stay open much ____, making its conductance delayed and sustained.
Slowly
Longer
Conductance for Na ____ before action potential does.
Ends
Before the action potential begins and the neuron is at rest (Vm = ____), the ion channels for K and Na are ____. It is during this time that synaptic potential can be seen. What is synaptic potential, and what is it due to?
Vrest
Closed
Membrane potential increases while channels are still closed
Due to depolarization of synapses on dendrites
As soon as threshold is reached in the action potential, the ____ channels open. Because the ___ channels are slow to open, they remain closed at this time. Thus, most of the ____ phase corresponds to the conductance of the ____ channels.
Na
K
Rising
Na
Towards the peak of the action potential, what happens to the inactivation domain of the Na channels and why?
Change in charge/length of time passed causes conformational change in inactivation domain, which moves to block the pore of the Na channel so ions cannot flow through
At the time that the Na channels are inactivating, what happens to the K channels?
They open
During falling phase, the ___ channels are open, which ____ the cell because this ion’s current is ____.
K
Repolarizes
Outward
Why does the hyperpolarization/undershoot phase of the action potential occur?
The K channels are open past when Vm reaches Vrest (they are slow to close)
Do all the Na channels close at the same moment, and do all the K channels open at the same moment?
No
At deinactivation, the Vm drops below ____ and all ___ channels close.
Threshold
Na
Once both Na and K channels are closed, what can the neuron do?
Fire another action potential
If Na channels took longer to inactivate than normal, what would happen to the shape of the action potential curve?
Action potential would last longer (peak would be broader)
At rest, the Na channel is _____ and its inactivating domain is ____.
Closed
Open
What is the absolute refractory period, and what does it correspond to in the action potential?
Interval during which a new action potential cannot be initiated
Corresponds to inactivation of Na channels
What is the relative refractory period, and what does it correspond to in the action potential?
Interval during which initiation of a new action potential is inhibited, but not impossible
Corresponds to hyperpolarization/undershoot phase
Why is firing an action potential more difficult during the relative refractory period?
Because the neuron is in undershoot phase, it requires more depolarization to bring it up to threshold
What is the minimum inter-spike interval?
The minimum amount of time between which a neuron can fire action potentials
Can a neuron fire an action potential in less time than its minimum inter-spike interval?
No
At or just above the minimum inter-spike interval for a cell, can the cell fire an action potential? How might its action potential be different than if the stimulus was applied a little bit later?
Yes
Action potential amplitude may be smaller- not all of the Na channels have gotten out of their absolute refractory period
The ____ refractory period determines the minimum inter-spike interval.
Absolute
A plot of what versus what reveals the minimum inter-spike interval?
Interval between stimuli in ms
Action potential amplitude
The relative refractory period is due to the properties of the ____ _____.
K channel
Why does the neuron need to have a refractory period?
Action potentials need to propagate away from the stimulus and in one direction
How does passive propagation of an axon work?
Current diffuses along the axon, then gradually decays due to leakage
In an action potential, what actually moves down the axon, ions or electrons/charge?
Electrons/charge
How does active propagation of an axon work?
Current diffuses a short distance, then opens the next ion channels, which causes regeneration of the action potential
An action potential requires both ____ diffusion and ____ propagation.
Passive
Active
When ion channels open on a portion of axon, current passively spreads in how many directions?
2- both toward distal and toward soma
When ion channels open on a portion of axon, why is it that an action potential doesn’t occur on the portion of axon that just had the action potential that caused the channels to open?
The part of the axon that initiated the current action potential is in absolute refractory period
At the axon hillock, all _____ potentials of the ____ get added up. If the sum is above ____, then an action potential occurs.
Synaptic
Dendrites
Threshold
The ____ ____ is the first part of the axon that is attached to the axon hillock and is enriched in what?
Initial segment
Voltage-gated channels
If the summed inputs of the axon hillock bring Vm above threshold, then the action potential starts where?
Initial segment
Inactivation of sodium channels is _____ dependent.
Time (inactivation occurs after a certain period of time- it is not voltage dependent)
The axon initial segment and regions along the axon are enriched in what?
Voltage gated Na and K channels
Why can’t action potentials occur in the soma?
No voltage gated channels in soma
2 ways in which leakage of current can be overcome
Increasing axon diameter
Myelination of axons
Myelination is due to ____ of ____ around segments of the axon. What is the purpose of myelination?
Rolls of fat
Insulate axon to prevent loss of current
In general, are there voltage gated channels in myelinated areas?
No
What are the areas of the axon called where there is no myelination but voltage gated channels are present?
Nodes of Ranvier
Saltatory propagation: how does it work?
Passive current travels down the axon and regenerates action potential at nodes, making action potential appear to “hop” from node to node
What type of axons have the biggest diameter and the most myelination?
Proprioceptive
Increasing diameter increases _____.
Velocity
Capacitance
Ability to store charge
Resistance
Resisting charge from passing through
Increasing axon diameter lowers _____. Why?
Resistance
Greater area for charge to escape through membrane
Increasing axon diameter increases ____. Why?
Capacitance
More membrane is available to store charge
Myelination increases _____ and decreases ______.
Resistance
Capacitance
Resistance and capacitance are _____ related.
Inversely (raise one, lower the other)
Slow inactivation of Na channel affects action potential how?
Prolonged action potential
Short interspike interval is due to quick ____ and ____ of ____ channels.
Deinactivation
Closing
Na
