action potential I Flashcards
passive electrical properties of axons render them
poor conductors of electrical signals over distances greater than a few millimeters.
Axons are poor conductors of electrical signals because they are
essentially very dilute salt solutions.
ECF is essentially the same solution, however; the
cross-sectional area is very small for a nerve axon compared to the ECF.
So for a given length of axon, the intracellular resistance is
much greater than ECF resistance which has more area.
ion channels act like
resistors
Ions continue to leak out after ____
The result of this is that ______
positive charges have ceased flowing into the cell.
less current is delivered down the axon than was originally inserted.
Booster stations act by applying an
energy boost to a decaying electrical signal when the incoming signal is detected.
Action potentials work in essentially the same way.
The detector that senses the decaying message is the ______ and the energy source that propagates the message is the _______
voltage-gated sodium channel
Na “battery”
the Na battery is the
the Na gradient that can inject extra positive charge into the cell.
The larger the membrane resistance becomes.
the farther the voltage can spread down the axon, because increased membrane resistance means there are fewer ion channels for charges to leak out of
Membrane capacitance is the ability.
of charge to accumulate on two sides of a membrane
These charges ______ create an electrical force that that pulls oppositely charged ions toward it.
membrane capacitance and membrane resistence
Reducing membrane capacitance increases the
passive spread of voltage.
When current is injected into the cell, it must follow either
through the ion channels (as resistors) or into the membrane capacitor.
Increased internal resistance will slow the
spread of voltage, as charge is lost to the surrounding fluid.
Resistance increases as cross-sectional area
decreases.
cells communicate infor via
action potentials
how is AP generated?
Vm and current are related to one another, more current results in more Vm, change in potential. If you have enough current, then you end up with an action potential characteric shape
Phases of AP
- resting potential
- depolarization
- repolarization
- after hyperpolarization
What is the basis for membrane potential?
1) Electrochemical gradient for each ion: Eion, the ionic equilibrium potential
[Na+]o»_space; [Na+]i [K+]o «_space;[K+]I Nernst Equation: ENa ≈ +60 mV
Ek ≈ -90 mV
2) Ionic membrane permeability: How easy is it for an ion to cross the cell membrane?
Electrochemical gradients
• K+ “wants” to exit the cell
– If the membrane were permeable to K+ only:
Vm = Ek ≈ -90mV
• Na+ “wants” to enter the cell
– If the membrane were permeable to Na+ only: Vm = ENa ≈ +60mV
ionic permeability
Permeability is determined by ion channels in the membrane- are they open or closed?
resting membrane potential
Vm is determined by Ek, ENa, and the relative membrane permeability to K+ and Na+
(PR = PNa / PK)
• Since the membrane is much more permeable to K+ than Na+, Vm is much closer to Ek: Vm ≈ -70mV
resting membrane potential is
-70mV
What if na+ channels open?
- Relative permeability to NA increase
- Na+ enters the cell (inward current)
- Vm moves close to Ena, ~60mV
in reality, small initial depolarization causes
specialized Na+ channels to open
depolarization means
the membrane potential becomes less negative
some _____ open in response to depolarization
voltage gated Na+ channels
Does Vm exceed a critical threshold: Is inward flow of Na+ > outward flow of K+?
– If no: Vm returns to resting potential
– If yes: More Na+ channels open, causing further depolarization
positive feedback loop leads to
all or none AP
more Na+ channels open–> more depot –> more Na+ channels open
What causes the depolarization in the first place?
initial depolarization: synaptic transmission
Why doesn’t the cell stay depolarized?
- depol is transient,
- voltage gated Na+ channels inactivate and Na+ stops entering the cell
- Also K+ channels open and K+ exits the cell, so Vm is pulled back to Ek
depol is transient
- voltage gated Na+ channels inactivate
- -Na+ stops entering the cell - Voltage gated K+ channels open
- -Channel opening is delayed from when the threshold is reached
- -K+ exits the cell - Vm is pulled bck to Ek
Return to resting potential
- voltage gated channels close
- -Overshoot/ hyperpol/ afterhyperpol - Relative permeability to Na+ and K+ returns to resting baseline
What is the role of the Na+/K+ pump?
it is responsible for concentration gradients of Na and K inside and outside the cel and the potential difference between the two
refractory period is
- short period (3ms) rolling return to baseline which initiation of anther AP is difficult or impossible
- some Na+ channels are inactivated and some K+ channels are still open
absolute refractory period
AP initiation is impossible
Relative refractory period
AP initiation is less likely
function consequence of Refractory period
limits the max possible firing rate of the cell
Effect of hyperkalemia on AP
Ek increases ->
Membrane is depolarized ->
More action potentials are generated(?)
More action potentials are generated in hypekalemia?
No: it is more difficult to generate an action potential - because Na+ channels are inactivated
summary of AP stages
1) Small depolarization to threshold
2) Large depolarization: Voltage-gated Na+ channels open
3) Repolarization: Na+ channels inactivate; K+ channels open
4) K+ channels close; concentrations return to baseline levels
Shape of AP is determined by
ion concentrations and conductances
Action potentials are affected by:
– Ionic concentrations
– Membrane potential
– Relative membrane permeability
– Specialized voltage-sensitive channels
Action potential generation is the output of
a computation that can be communicated with downstream cells
Describe the positions of the activation and inactivation gates in sodium channels during an action potential.
1) At rest: activation gate (m) is shut and inactivation gate (h) is open
2) As the membrane depolarizes: activation gate (m) opens as inactivation gate (h) closes
•activation gate swings faster than the inactivation gate
3) At peak of action potential: activation gate (m) and inactivation gate (h) are open
4) As membrane repolarizes: activation gate (m) is closing and inactivation gate (h) is closed
5) Refractory period: activation gate (m) and inactivation gate (h) are closed
intracellular concentrations of sodium and potassium do not change much after
a single action potential.
During a single action potential,
thousands of sodium ions enter the axon through the sodium channels and thousands of potassium ions exit through potassium channels during repolarization.
In most cells, the influx and efflux of potassium and sodium, respectively,
do not significantly impact the overall concentrations of the ions.
_____ before the Na+/K+ pump is required to restore proper ion balances
many AP’s must be generated
The Na+/K+ pump does not play a direct role in the _______. Instead, the Na+/K+ pump functions to _________
generation of an action potential.
restore ion balances after many action potentials have been fired
The refractory period is the
time after an action potential is fired during which an axon cannot fire another action potential (absolute refractory period) or requires a more extreme stimulus to fire another action potential (relative refractory period).
The number of action potentials that can be fired before Na+/K+ pump activity is required depends on _____
an axon’s surface area relative to its volume.
An axon with a relatively small surface area compared to volume can fire
more action potentials before pump activity is needed than an axon with a large surface area to volume ratio.
A smaller surface area to volume ratio means
less influx or efflux per unit volume, resulting in a smaller percent change in ion concentrations.
The refractory period exists as a result of two mechanisms:
- after an axon depolarizes, both he activation gate (m) and inactivation gate (h) of the sodium channel are closed. The inactivation gate requires time to reopen before another activation potential can fire
- After an axon depolarizes, the K+ channels are still open. K+ is still leaving the axon, which makes it more difficult to depolarize
Accomodation refers to a
nerve cell’s loss of excitability as a result of applying a stimulus gradually, rather than all at once.
During normal depolarization (in the absence of accommodation), the inactivation gate of the sodium channel is ______
This allows sodium to ______.
closing as the activation gate is opening.
rush into the axon and generate the action potential
With accommodation, the stimulus is applied slowly, _______. As a result, sodium cannot enter the cell and an
causing the inactivation gate to close before the activation gate opens
action potential is not generated.
Threshold is the
membrane potential at which sodium and potassium influx and efflux, respectively, are equal.
threshold is the membrane potential at which sodium and potassium currents are equal and opposite.
Threshold is achieved by a
slight initial depolarization of the axon.
The threshold must be reached in order for an
action potential to be fired.
Once threshold is reached, the potential can either be
tipped in the direction of sodium or potassium equilibrium potentials.
Because sodium channels are voltage gated, a momentary, slight influx of sodium will cause
the potential to tip in favor of the sodium equilibrium potential, causing more channels to open.
Sodium will then rush into the cell and an action potential is generated.
An action potential is generated due to ______
This is an explosive, ______ mechanism because sodium channels are voltage gated.
depolarization of an axon caused by sodium influx.
positive-feedback
Once an axon is depolarized to threshold,
- voltage gated sodium channels begin to open
- sodium moves into the cell with further depolarizes the cell
- Further depolarization causes more voltage gated sodium channels to open
This positive feedback (more channels opening in response to increased influx of sodium) repeats, resulting in
a rapid depolarization and the firing of an action potential
