The action potential Flashcards
Equilibrium potential
The electrical potential reached when the concentration gradient of an ion exactly counteracts electrical forces.
Unequal concentrations
For both K+ (potassium) and
A- (some anion), the
concentration inside is 20x
the concentration outside.
Selective permeability
If we add K+ channels, K+ flows out
of the cell down its concentration
gradient. A- can’t cross.
Equilibrium
(+) charge builds up outside, and (-)
inside, which pulls some K+ back
in, exactly counterbalancing the
concentration gradient.
Neurons send electrical signals
The neuron is either at rest or firing an action potential (aka spiker or impulse), at rest, the electrical potential across the cells membrane is negative, and signals from other neurons (or sensory input) can alter the membrane potential, if the membrane potential rises enough, an action potential occurs
Action potential
positive charge spreads rapidly down the axon, causing neurotransmitter release (or direct current transmission) at the synaptic terminal
Action potential summary
- A signal arrives at the dendrites, the membrane potential gets depolarized (rises toward 0), if membrane potential (Vm) is greater than the threshold, voltage gates Na+ channels open, Na+ flows in raising Vm above 0, Voltage gated Na+ channels close and voltage gated K+ channels open, K+ flows out and Vm get hyper polarized (falls below resting level), there is a refractory period until Na+ channels “deinactivate”. No more action potentials can happen during this period, voltage gated K+ channels close, resting Vm reestablished
Depolization
rise in membrane potential from resting level, towards 0, the membrane potential must reach a critical level of depolarization for the neuron to fire (this is the neurons firing threshold) -60 to -30
Hyper polarization
decrease in Vm becoming even more negative than the resting level -30 to -60
When the firing threshold is reached at the axon hillock
the neuron fires an action potential, which sweeps along the axon until reaching the terminal
Initial depolarization
stimulation causes positive ions to flow in
Rising phase
if Vm rises above threshold, voltage-gated Na+ channels open, further depolarizing and opening more Na+ channels
Overshoot
Vm is positive due to all the cation that have entered
Falling phase
Voltage-gated Na+ channels close, voltage gated K= channels open K+ flows out
Undershoot
Voltage gated K+ channels are still open, so positive charge flows out and the membrane is hyperpolarized
Resturn to resting Vm
Voltage gated K+ channels close, Na+/K+ pumps establish concentration gradients, and only passive leak K+ channels are open
Voltage gated sodium channels
Open when Vm rises above a threshold, K+ is too big to fit with an H2O attached
Voltage gated sodium channels more steps
- They are closed at “rest”
- Open when Vm rises above a threshold because the voltage sensor moves and opens the channel
3.Inactivation:close automatically after 1 ms, because the inactivation gate swings into place - Deinactivation: close but ale to open again, when Vm returns to baseline
Voltage gated K+channels
They are also open in response to depolarization, importantly, they are 1 ms slower to open and close, they then cause the falling phase of Vm during the action potential, these are not the same as the passive leak channels for K+, which are always open
Key players embedded in the cell membrane
sodium/potassium pump, passive leak potassium channels, voltage-gated potassium channels, voltage gated sodium channels
sodium/potassium pump
always running, using ATOp to pump ions against their concentration gradients, 3 Na+ out for every 2K+ in, Helps establish and maintain those concentration gradients and contributes to the negative potential at rest
Passive leak potassium channels
always open, allowing mostly just k+ to cross the cell membrane, either in or out, Contributes a lot to the negative resting potential.
Voltage gated sodium channels
open when Vm is depolarized above the threshold level, allowing Na+ to flow in, causing the rising phase. Automatically blocked (inactivated) by the inactivation gate, that inactivation gate is dislodged only when Vm returns to threshold and the channel is close
Voltage gated Potassium Channels
open only in response to depolarization during an action potential, a bot more slowly than the Na+ channels, allow K+ to flow out rapidly, then close when Vm returns to baseline. Contribute to the falling phase and undershoot, which cause the relative refractory period.
Neurotransmitter receptors
Open channels for particular ions to enter, initializing depolarization or hyper polarization
Graded inputs
Excitatory stimulation causes a positive charge to flow into the cell, which depolarizes the neuron, by the graded amount (can take on any value, changes continuously), some neurotransmitters are inhibitory and instead hyperpolarize the cell (Vm goes down), the change in potential is transient (quickly fades) and decreases with distance
Summation of graded inputs
The neuron fires and only if one or many graded inputes add up to exceed the firing threshold at the axon hillok (spike initiation zone), the neurons function is determined by how it can be made to fire: By a single neuron dropping one bit of neurotrnasmitter at one synaps
Axon hillock
Where graded potential can add up to induce actions potential
Spatial and temporal summation
EPSP: excitatory post-synaptic potential: a depolarization of post-synaptic membrane potential due to input from another cell.
Any one EPSP may not be enough to trigger an AP, but they can add up across space (multiple synapses) or time (rapid sequence)
Temporal summation
Two little EPSP’s, 4ms apart do not cause an action potential because the first has faded away by the time the second one comes, but if they are only 1 ms apart the second adds up with the first, pushing Vm over the threshold for an action potential
Summation of excitatory and inhibitory inputs
EPSP and IPSP
EPSP
Excitatory post-synaptic potential: depolarization due to input from another cell.
IPSP
inhibitory post-synaptic potential: hyperpolarization due to input from another cell
“all or none” firing pattersn
The action potential is “all or none”, depolarization < threshold evokes no A.P, Any depolarization > threshold evokes a “stereotyped” A.P., But the magnitude of depolarization determines the firing rate. Stronger depolarization = faster rate (up to 1000 Hz)
Refractory periods
Periods of time when it is impossible or difficult to trigger another action potential
Absolute (refractory period)
caused by voltage-gated Na+ channels being inactivated. Lasts 1 ms, another AP is not possible until Vm falls below threshold
Relative (refractory period)
caused by hyperpolarization due to voltage gated K+ channels being slow to close, extra strong input needed to generate another AP during this ‘undershoot’ period, lasts another 1-2ms
Action potential conduction
Thicker axons have faster conduction, less resistance for positive current flow inside, thinner axons need more voltage gated channels, myelin speeds conduction
Action potential conduction pt2
When a neuron fires an actional potential Na+ channels along the axon open one after another form the beginning to the end of the axon like falling dominoes, Na+ enters each Na+ channel as it openes, each entry of Na+ along the axon causes enough depolarization to open the next Na+ channel
Action potential conduction summary
- Na+ channels are open at only one spot at a time where positive current flows in
- Positive current spreads passively inside the axon in both directions
- But the action potenatial only travels away from the cell body (in this cas to the right)
- Positive current that spreads to the right is able to open the next Na+ channels that havent recently been opened thus the AP travels rightward
*But Na+ channels to the left are still inactivated and K+ channels are open. To the left positive current is flowing out to repolarize the membrane, the the action potential cant go backwards
Saltatory conduction
*Myelin insulates the axon in sheaths, each sheath provided by one
Schwann cell or oligodendrocyte.
* Between the sheaths are exposed “Nodes of Ranvier.”
* Voltage-gated K+ and Na+ channels are concentrated in the nodes.
* Positive current can spread passively through the myelinated sections
of axon, without leakage.
* The AP “jumps” from node to node
* This saves time and energy
Action potential summary
A rapid and brief flip of the membrane potential from negative to positive, due
to an influx of cations, that travels from the cell body down to the axon
terminal.
* Triggered by summed inputs crossing the threshold that opens voltage-gated
sodium channels
* Travels down the axon in only one direction, due to inactivation of Na+
channels and the slower opening and closing of voltage-gated K+ channels
* Jumps between nodes of Rangier
* Triggers neurotransmiter release at the synaptic terminal
* It is an all-or-none response, each A.P. being a stereotyped rise and fall of Vm.
* But any one neuron can fire many times in succession at a variable rate
(spikes/second, up to the limit imposed by the absolute refractory period)
* Stronger, sustained inputs can cause lots of APs in a brief interval
Important terms
- Voltage, current, conductance
- Voltage-gated, ion-selective channels
- The membrane potential’s threshold, rising phase, falling
phase, and undershoot - Spike initiation zone at the axon hillock
- Depolarization vs hyperpolarization; EPSPs and IPSPs
- Refractory periods
- Temporal and spatial summation
- Saltatory conduction
