Neuron action potential mechanism | Nervous system physiology | NCLEX-RN | Khan Academy

Summary

This video explores the generation and conduction of action potentials in neurons. It explains how action potentials are initiated at the axon's trigger zone, where voltage-gated sodium channels open in response to reaching the threshold potential. The influx of sodium ions causes a rapid depolarization, leading to the rising phase of the action potential. The falling phase is triggered by the outflow of potassium ions through leak and voltage-gated potassium channels. The video also discusses the refractory period, which prevents immediate reactivation of the action potential, ensuring unidirectional signal transmission along the axon.

Takeaways

• 🧠 The video discusses the generation and conduction of action potentials, focusing on the trigger zone and the axon.
• 🌟 The axon is depicted with a high concentration of voltage-gated ion channels, which are crucial for the initiation of action potentials.
• 🔋 The resting membrane potential is typically around -60 millivolts, and the threshold potential is around -50 millivolts.
• ⚡ Action potentials are initiated when the membrane potential at the trigger zone reaches the threshold potential, causing voltage-gated sodium channels to open.
• 💧 Sodium ions rush into the neuron through the open sodium channels, leading to depolarization and the rising phase of the action potential.
• 🔼 The action potential peaks at around +40 millivolts, not reaching the sodium equilibrium potential due to the inactivation of sodium channels.
• 🔄 The falling phase of the action potential is caused by the efflux of potassium ions through leak channels and voltage-gated potassium channels.
• 🔽 The action potential returns to the resting potential as voltage-gated potassium channels close, and the membrane permeability to potassium normalizes.
• 🚫 The refractory period follows the action potential, during which it is difficult or impossible to trigger another action potential in the same membrane area.
• 🛑 The refractory period consists of an absolute refractory period, where sodium channels are inactivated, and a relative refractory period, where additional stimulation is required to initiate a new action potential.

Q & A

What is the primary focus of the video?

The video primarily focuses on explaining how action potentials are generated, the role of the trigger zone, and how these action potentials are conducted down the axon.

What is the resting membrane potential of a neuron?

The resting membrane potential of a neuron is typically around negative 60 millivolts.

What is the threshold potential and how does it relate to the generation of an action potential?

The threshold potential is around negative 50 millivolts, and it is the value that, when crossed, causes voltage-gated ion channels to open, leading to the generation of an action potential.

What are voltage-gated ion channels and how do they contribute to the action potential?

Voltage-gated ion channels are a type of ion channel that opens in response to changes in membrane potential. They play a crucial role in the action potential by allowing sodium ions to flow into the neuron, causing depolarization, which initiates the action potential.

What is the trigger zone and why is it significant?

The trigger zone is the initial segment of the axon where action potentials usually start. It is significant because it has the greatest density of voltage-gated sodium channels, which allows the action potential to initiate.

Describe the rising phase of an action potential.

The rising phase of an action potential is when the membrane potential rapidly increases due to the influx of sodium ions through the voltage-gated sodium channels, making the inside of the neuron more positive than the outside.

Why does the action potential not reach the sodium equilibrium potential?

The action potential does not reach the sodium equilibrium potential because the voltage-gated sodium channels automatically start to close at higher potential values, preventing further sodium influx.

What causes the falling phase of the action potential?

The falling phase of the action potential is caused by the efflux of potassium ions through leak channels and voltage-gated potassium channels, which helps to repolarize the neuron back towards its resting potential.

What is the refractory period and why is it important?

The refractory period is a time after an action potential during which it is difficult or impossible to trigger another action potential in the same part of the membrane. It is important because it prevents the action potential from traveling back along the axon and ensures unidirectional conduction.

How is the refractory period divided and what are its parts?

The refractory period is divided into two parts: the absolute refractory period, when voltage-gated sodium channels are inactivated and cannot open, and the relative refractory period, when these channels are functional again but the membrane potential is hyperpolarized, requiring more excitatory input to trigger another action potential.

How does the movement of ions across the membrane cause the waveform of the action potential?

The movement of sodium and potassium ions across the membrane, through voltage-gated and leak channels, causes the characteristic waveform of the action potential, including its rising, peak, falling phases, and after-hyperpolarization.

Outlines

00:00

🧠 Understanding Action Potentials and Their Initiation

This paragraph delves into the generation of action potentials, focusing on the trigger zone and the conduction along the axon. It introduces the concept of the soma and axon, and graphically represents the membrane potential over time. The discussion includes leak channels that are always open, and the introduction of a new type of channel, the voltage-gated ion channels, which play a crucial role in the action potential. These channels, particularly sodium channels, open when the membrane potential reaches a threshold, leading to an influx of sodium ions and the initiation of an action potential. The paragraph explains the rapid depolarization as sodium ions flow in, creating a chain reaction along the axon, and the subsequent peak of the action potential. It also touches on the automatic closure of these channels at higher potential values, preventing further sodium influx.

05:03

🔋 The Dynamics of the Falling Phase and Refractory Period

The second paragraph continues the discussion on action potentials, focusing on the falling phase and the refractory period. It explains how the membrane potential returns to the resting state through the exit of potassium ions, facilitated by leak channels and voltage-gated potassium channels. The paragraph highlights the differences in the speed of opening and closing between sodium and potassium channels, which influences the shape of the action potential. It introduces the refractory period, which is divided into the absolute and relative refractory periods, explaining why it is difficult to trigger another action potential during these times. The refractory period ensures that action potentials travel unidirectionally from the trigger zone to the axon terminals, preventing them from reversing direction due to the refractory state of the membrane.

Keywords

💡Action Potential

An action potential is an electrical signal that travels along a neuron's membrane, allowing the neuron to transmit information. In the video, the generation and conduction of action potentials are central to understanding how neurons communicate. The script describes how action potentials are initiated at the trigger zone and then conducted down the axon, illustrating the process with a graph of membrane potential over time.

💡Trigger Zone

The trigger zone is the initial segment of the axon where action potentials are typically initiated. It is highlighted in the script as having the greatest density of voltage-gated sodium channels, which is crucial for the initiation of action potentials. The term is used to explain why action potentials usually start at this specific location on the neuron.

💡Membrane Potential

Membrane potential refers to the voltage difference across the cell membrane, which is essential for the generation of action potentials. The script uses a graph to illustrate the membrane potential, showing how it changes from the resting state to the threshold potential, and then during the action potential itself.

💡Voltage-gated Ion Channels

Voltage-gated ion channels are proteins in the neuron's membrane that open or close in response to changes in membrane potential. The script introduces these channels as a new type, crucial for the action potential. They are depicted as opening when the membrane potential reaches a threshold value, initiating the influx of sodium ions and the subsequent depolarization.

💡Resting Membrane Potential

The resting membrane potential is the voltage difference across the neuron's membrane when it is not transmitting signals. The script mentions a typical resting potential of around negative 60 millivolts, which is a state of relative electrical equilibrium before an action potential is triggered.

💡Threshold Potential

The threshold potential is the level at which the membrane potential must reach to trigger the opening of voltage-gated ion channels and the initiation of an action potential. In the script, this is described as around negative 50 millivolts, and it is a critical value that determines whether an action potential will occur.

💡Depolarization

Depolarization is the process by which the membrane potential becomes less negative, moving towards a positive value. The script describes how the opening of voltage-gated sodium channels causes sodium ions to flow into the neuron, leading to depolarization and the rising phase of the action potential.

💡Refractory Period

The refractory period is the time following an action potential during which the neuron cannot generate another action potential. The script explains that this period prevents the action potential from reversing direction and ensures unidirectional signal transmission along the axon. It is divided into the absolute refractory period, where the sodium channels are inactivated, and the relative refractory period, where additional stimulation is required to trigger another action potential.

💡Sodium Channels

Sodium channels are a type of voltage-gated ion channel that allows sodium ions to enter the neuron when they open. The script emphasizes their role in the rising phase of the action potential, where the influx of sodium ions causes the membrane to depolarize rapidly.

💡Potassium Channels

Potassium channels are another type of ion channel that allows potassium ions to exit the neuron. The script describes how these channels contribute to the falling phase of the action potential by allowing potassium to flow out of the neuron, which helps to repolarize the membrane back towards its resting potential.

💡Equilibrium Potential

The equilibrium potential is the membrane potential at which there is no net movement of ions across the membrane due to their concentration gradient. The script mentions the sodium equilibrium potential, which is around positive 50 millivolts, as the value towards which the membrane potential tends to rise during the action potential but does not quite reach due to the closing of sodium channels.

Highlights

Action potentials are generated at the trigger zone and conducted down the axon.

The axon is depicted with a blown-up size for clarity.

Leak channels are always open and contribute to the neuron's resting potential.

Voltage-gated ion channels are crucial for action potential generation.

Threshold potential is the value at which voltage-gated ion channels open.

Temporal and spatial summation of excitatory potentials is necessary to reach the threshold.

Voltage-gated sodium channels open at the threshold, initiating the action potential.

Sodium influx causes depolarization and triggers a chain reaction down the axon.

The rising phase of the action potential is due to sodium influx through voltage-gated channels.

The action potential peaks at around positive 40 millivolts, not reaching the sodium equilibrium potential.

Voltage-gated sodium channels close automatically, ending sodium influx.

The falling phase of the action potential is due to potassium exiting the neuron.

Leak channels and voltage-gated potassium channels contribute to potassium efflux.

The refractory period prevents immediate re-triggering of action potentials.

The absolute refractory period is when voltage-gated sodium channels are inactivated.

The relative refractory period requires more excitatory input to trigger a new action potential.

The refractory period ensures unidirectional travel of action potentials along the axon.

Transcripts

In this video, I want to talk about how action potentials are

generated the trigger zone and how

they're conducted down the axon.

So I've drawn a soma here in red and one axon in green.

And I've blown up the axon to a very large size

just so I had some room to draw.

Here's our graph of the membrane potential

on the y-axis and time on the x-axis.

And now I've put a couple of different kinds of ion channels

in the membrane of the axon.

The first in this lighter grey are the leak channels that we

talked about when we talked about the neuron resting

potential.

These channels are open all the time.

They're not gated.

And I have not drawn any ligand gated ion channels

like the neurotransmitter receptors that

occur on the soma and the dendrites.

But to talk about the action potential,

I need to introduce an entirely new type of channel that

I've drawn in dark grey with this little v.

And these are voltage gated ion channels.

The membrane of an axon as many voltage

gated ion channels, most of which

open when the membrane potential crosses a threshold value.

So we've talked about the threshold potential before.

And all of these numbers may vary between different types

of neurons, but these would be fairly common values.

So many neurons would have a resting membrane potential

of around negative 60 millivolts and a threshold potential

of around negative 50 millivolts or so

that I've drawn with a dashed line.

And the importance of this threshold potential

is that it determines if these voltage gated

ion channels will open.

So when there is enough temporal and spatial summation

of excitatory grad potentials to get us toward the threshold,

here at the trigger zone, at the initial segment of the axon,

so let me just draw that, that we

have temporal and spatial summation

of excitatory potentials spreading

across the membrane of the soma into the initial segment

of the axon, the trigger zone.

This voltage gated ion channel has a mechanism

to sense this voltage change.

And when the threshold potential is crossed, it's going to open.

And these are going to be sodium channels.

Recall that the electrical and diffusion forces acting

on sodium ions are strongly trying

to drive them into the neuron.

So when this voltage gated sodium channel opens,

sodium is going to flow into the neuron

through the open channel causing that part of the membrane

to depolarize from all these positive charges

now on the inside.

This is going to cause an explosive chain reaction

by triggering the voltage gated sodium

channels in the next piece of the membrane

so that more sodium is going to flow in further depolarizing

the membrane and opening the next voltage gated sodium

channel.

These voltage gated sodium channels open

very quickly triggering each other

in a wave that rapidly spreads down the axon.

The trigger zone has the greatest density

of these voltage gated sodium channels which

is why action potentials usually starts at the trigger zone.

So many of these voltage gated sodium channels

will open that the membrane permeability to sodium

is dramatically increased.

This is going to cause the membrane potential, which

has already gone from the resting potential

to the threshold potential from the grated potentials,

but now that all this sodium is flowing in

through these open channels, the membrane potential

is going to dramatically rise trying

to head toward the equilibrium potential of sodium, which

is usually somewhere around positive 50 millivolts.

This rapid increase in the membrane potential values

is due to these voltage gated sodium channels.

And this is called the rising phase of the action potential.

And in fact, it becomes more positive inside the neuron

membrane during this period that it's

the reverse of the resting potential

because normally it's more negative inside

than outside the neuron membrane.

But now so much sodium has entered,

that it's more positive inside the membrane than outside.

The action potential usually peaks though some where

around positive 40 millivolts.

So it doesn't make it up to the sodium equilibrium potential

that's often around positive 50 millivolts.

And the reason for that is that these voltage

gated sodium channels automatically

start to close at the higher potential values

so that sodium stops flowing into the neuron.

And after they close, they're in a special state called

the inactivated state and they're

unable to open at any membrane potential for a brief time.

The next thing we see happen to the action potential,

basically just as fast as the membrane potential

went from the resting potential to the peak of the action

potential, it then rapidly descends back

toward the resting potential and then actually goes farther.

It goes more negative than the resting potential and then it

levels off.

The reason for this part of the action potential,

which is called the falling phase,

is because potassium starts to exit the neuron

and it does so through a couple of types of channels.

The first are the leak channels that we

talked about when we talked about the resting membrane

potential.

Now a little potassium as exiting through the leak

channels at the resting potential, but even more

potassium than normal starts to exit.

Because during these parts of the action potential,

the membrane potential is positive

so that during this part of the action potential,

both the diffusion force and the electrical force

are strongly trying to drive potassium out of the neuron

so that more leaves through the leak channels that normally

does during the resting potential.

The second type of channel that allows potassium to exit

are voltage gated potassium channels.

These also open when the membrane potential crosses

the threshold, but they're a little slower to open

than the voltage gated sodium channels.

So that at first, all the voltage

gated sodium channels snap open, allowing sodium

to rush in causing the rising phase of the action potential.

And then a little slower the voltage

gated potassium channels open, allowing potassium

to flow out of the neuron contributing to the falling

phase of the action potential.

And then the action potential stops

falling because now it's more negative inside the neuron

again so there's less driving force pushing potassium out

through the leak channels.

And also the voltage gated potassium channels

automatically close at the lower potential values

just like the voltage gated sodium channels automatically

closed.

But just like the voltage gated potassium channels were

a little slower to open than the voltage gated sodium channels,

the voltage gated potassium channels are also

a little slower to close so that it takes a little longer

for this exit of potassium to stop.

And that's why there's this little bit of a longer

period at the end of the action potential

until we kind of slowly settle back

into the resting membrane potential.

Because as these voltage gated potassium channels are slowly

closing, the membrane permeability to potassium

is returning to the normal amount

you get during the resting potential

through the leak channels.

And as that permeability to potassium returns back

to the normal resting potential level,

the membrane potential returns to the resting potential.

This movement of sodium ions and potassium

ions across the membrane causing the wave form of the action

potential starts here at the trigger zone

at the axon initial segment, but then

rapidly spreads in waves down the axon.

First, there's the wave of depolarization from opening up

the voltage gated sodium channels.

So a wave of depolarization rapidly spreads down the axon,

but following right behind it, right on its heels,

is this wave of hyper-polarization caused

by potassium exciting through the voltage gated potassium

channels and the leak channels.

So we have the rising phase of the action potential, the peak

of the action potential, the falling phase of the action

potential, and then this period of hyper-polarization

at the end of the action potential

has a couple of names.

It can be called the after hyper-polarization

because it's the hyper-polarization that

happens after this part of the action potential.

But it's also called the refractory period.

Let me just write that down.

Refractory period right there.

And it's called the refractory period

because during this time, it's difficult or impossible

to trigger another action potential

in that part of the membrane.

The refractory period is divided into two parts.

The first part is called the absolute refractory period.

And it's absolute because the voltage gated sodium channels

when they first close they're in a special state called

the inactivated state.

And they are unable to open at any membrane potential

for a brief time so that no matter how

much excitatory input comes into the neuron,

you can't trigger another action potential

during the absolute refractory period.

The second part is called the relative refractory period.

And during this time, the voltage

gated sodium channels have become functional again.

They can respond to depolarization, however,

the membrane potential is hyper-polarized.

It's not yet back to the resting potential.

Therefore, it would take more excitatory input than normal

to trigger an action potential during the relative refractory

period.

One important effect of the refractory period

is that action potentials travel from the trigger zone

to the axon terminals.

And they don't turn around and head right back

the other direction because the membrane right

behind the action potential is refractory.

It can't be triggered by itself to send the action

potential back the other way.