The action potential

Question 1

Equilibrium potential

Answer 1

The electrical potential reached when the concentration gradient of an ion exactly counteracts electrical forces.

Question 2

Unequal concentrations

Answer 2

For both K+ (potassium) and
A- (some anion), the
concentration inside is 20x
the concentration outside.

Question 3

Selective permeability

Answer 3

If we add K+ channels, K+ flows out
of the cell down its concentration
gradient. A- can’t cross.

Question 4

Equilibrium

Answer 4

(+) charge builds up outside, and (-)
inside, which pulls some K+ back
in, exactly counterbalancing the
concentration gradient.

Question 5

Neurons send electrical signals

Answer 5

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

Question 6

Action potential

Answer 6

positive charge spreads rapidly down the axon, causing neurotransmitter release (or direct current transmission) at the synaptic terminal

Question 7

Action potential summary

Answer 7

• 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

Question 8

Depolization

Answer 8

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

Question 9

Hyper polarization

Answer 9

decrease in Vm becoming even more negative than the resting level -30 to -60

Question 10

When the firing threshold is reached at the axon hillock

Answer 10

the neuron fires an action potential, which sweeps along the axon until reaching the terminal

Question 11

Initial depolarization

Answer 11

stimulation causes positive ions to flow in

Question 12

Rising phase

Answer 12

if Vm rises above threshold, voltage-gated Na+ channels open, further depolarizing and opening more Na+ channels

Question 13

Overshoot

Answer 13

Vm is positive due to all the cation that have entered

Question 14

Falling phase

Answer 14

Voltage-gated Na+ channels close, voltage gated K= channels open K+ flows out

Question 15

Undershoot

Answer 15

Voltage gated K+ channels are still open, so positive charge flows out and the membrane is hyperpolarized

Question 16

Resturn to resting Vm

Answer 16

Voltage gated K+ channels close, Na+/K+ pumps establish concentration gradients, and only passive leak K+ channels are open

Question 17

Voltage gated sodium channels

Answer 17

Open when Vm rises above a threshold, K+ is too big to fit with an H2O attached

Question 18

Voltage gated sodium channels more steps

Answer 18

1. They are closed at “rest”
2. 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
3. Deinactivation: close but ale to open again, when Vm returns to baseline

Question 19

Voltage gated K+channels

Answer 19

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

Question 20

Key players embedded in the cell membrane

Answer 20

sodium/potassium pump, passive leak potassium channels, voltage-gated potassium channels, voltage gated sodium channels

Question 21

sodium/potassium pump

Answer 21

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

Question 22

Passive leak potassium channels

Answer 22

always open, allowing mostly just k+ to cross the cell membrane, either in or out, Contributes a lot to the negative resting potential.

Question 23

Voltage gated sodium channels

Answer 23

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

Question 24

Voltage gated Potassium Channels

Answer 24

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.

Question 25

Neurotransmitter receptors

Answer 25

Open channels for particular ions to enter, initializing depolarization or hyper polarization

Question 26

Graded inputs

Answer 26

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

Question 27

Summation of graded inputs

Answer 27

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

Question 28

Axon hillock

Answer 28

Where graded potential can add up to induce actions potential

Question 29

Spatial and temporal summation

Answer 29

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)

Question 30

Temporal summation

Answer 30

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

Question 31

Summation of excitatory and inhibitory inputs

Answer 31

EPSP and IPSP

Question 32

EPSP

Answer 32

Excitatory post-synaptic potential: depolarization due to input from another cell.

Question 33

IPSP

Answer 33

inhibitory post-synaptic potential: hyperpolarization due to input from another cell

Question 34

“all or none” firing pattersn

Answer 34

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)

Question 35

Refractory periods

Answer 35

Periods of time when it is impossible or difficult to trigger another action potential

Question 36

Absolute (refractory period)

Answer 36

caused by voltage-gated Na+ channels being inactivated. Lasts 1 ms, another AP is not possible until Vm falls below threshold

Question 37

Relative (refractory period)

Answer 37

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

Question 38

Action potential conduction

Answer 38

Thicker axons have faster conduction, less resistance for positive current flow inside, thinner axons need more voltage gated channels, myelin speeds conduction

Question 39

Action potential conduction pt2

Answer 39

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

Question 40

Action potential conduction summary

Answer 40

• 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

Question 41

Saltatory conduction

Answer 41

*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

Question 42

Action potential summary

Answer 42

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

Question 43

Important terms

Answer 43

• 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