10-Minute Neuroscience: Action Potentials

Summary

This neuroscience video delves into the fascinating world of action potentials, the electrical impulses that travel along neurons, triggering neurotransmitter release. It begins by explaining the resting state of a neuron, highlighting the crucial role of ions and ion channels in creating the conditions for action potentials. The video outlines the process of depolarization, where neurotransmitters cause a positive ion influx, leading to the firing of an action potential. It then describes the propagation of this electrical signal down the neuron's axon, facilitated by the opening of sodium and potassium channels. The importance of myelin sheaths and nodes of Ranvier in speeding up signal transmission is also discussed. The video concludes by explaining the refractory periods that prevent immediate reactivation and ensure action potentials relate to the intensity of stimulation, adhering to the all-or-none law.

Takeaways

• 👇 Neurons use action potentials to communicate, which are electrical impulses that travel along the neuron and trigger neurotransmitter release.
• 🔊 At rest, neurons have a specific electrical setup with a membrane potential of about -70 millivolts, due to differing concentrations of ions inside and outside the cell.
• 🔌 Key ions involved are sodium (Na+) and potassium (K+), with Na+ primarily outside and K+ inside the neuron.
• 🛡️ Ion channels and the sodium-potassium pump are crucial in maintaining the ion balance across the neuron's membrane.
• 🔨 Action potentials start with depolarization, where the membrane potential becomes less negative if the neuron's depolarization reaches a certain threshold.
• 💥 Voltage-gated sodium channels play a vital role; opening these channels causes Na+ to rush into the cell, further depolarizing it and propagating the action potential.
• 🚨 The peak of an action potential is when the internal voltage swings to positive, driven by the inflow of sodium ions.
• 💬 Repolarization follows as potassium ions flow out, resetting the membrane potential back towards the resting state.
• 🔬 Myelin sheaths enhance the speed and efficiency of action potential propagation along the axon through a process called saltatory conduction.
• 🔒 After an action potential, neurons experience a refractory period where they cannot fire again immediately, ensuring the action potential only moves in one direction.

Q & A

What are action potentials and why are they important for the nervous system?

Action potentials are electrical impulses that travel down neurons, causing the release of neurotransmitters. They are critical for neural communication and essential to the function of the nervous system.

What is the resting state of a neuron in terms of its electrical properties?

At rest, a neuron has a cell membrane that separates the intracellular environment from the extracellular environment. The resting state involves an unequal distribution of ions, with more positively charged sodium ions outside the neuron and more potassium ions inside.

How do ion channels contribute to the distribution of ions across the neuron's membrane?

Ion channels are tubelike pores in the cell membrane that allow specific ions to pass through. They are selective for certain ions, and some are always open (leak channels), while others open only in response to stimuli such as neurotransmitter binding.

What is the role of the sodium-potassium pump in maintaining ion concentrations?

The sodium-potassium pump is an enzyme that continuously pumps sodium ions out of the cell and potassium ions into the cell, helping to maintain a higher concentration of potassium ions inside and sodium ions outside the cell.

How does the resting membrane potential of a neuron typically measure?

The resting membrane potential of a neuron is typically about -70 millivolts, indicating that the inside of the cell is more negatively charged than the outside.

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

Depolarization is the process where the membrane potential moves closer to 0, becoming less negative. It occurs when neurotransmitters bind to receptors, causing positively charged ions to flow into the neuron. If the depolarization reaches a threshold, an action potential will fire.

How do voltage-gated sodium channels contribute to the rising phase of an action potential?

When a neuron is depolarized to threshold, voltage-gated sodium channels open, allowing positively charged sodium ions to rush into the cell. This influx further depolarizes the cell, causing more sodium channels to open, leading to a rapid change in membrane potential known as the rising phase.

What is the role of potassium ions and channels in ending an action potential?

As the neuron's membrane potential reaches its peak, voltage-gated sodium channels close, and voltage-gated potassium channels open, allowing potassium to rush out of the cell. This helps to repolarize the neuron, moving it back to its resting membrane potential and ending the action potential.

How does myelin increase the speed of action potential propagation along the axon?

Myelin is an insulating material that wraps around the axons of neurons. It prevents current from leaking out and has nodes of Ranvier, which are gaps in the myelin rich in voltage-gated sodium channels. This allows for the action potential to regenerate quickly at each node, increasing the speed of propagation.

What are the absolute and relative refractory periods, and how do they affect the firing of action potentials?

The absolute refractory period is when voltage-gated sodium channels are unresponsive and the neuron cannot fire another action potential. The relative refractory period follows, during which the neuron is briefly hyperpolarized and requires stronger stimulation to fire another action potential. These periods ensure that action potential firing is related to the intensity of stimulation.

What is the all-or-none law regarding the amplitude of action potentials?

The all-or-none law states that action potentials do not vary in size based on the intensity of a stimulus. They either fire or do not fire, and when they do, they are of a consistent amplitude or size.

How can more intense stimuli affect the frequency of action potential firing?

More intense stimuli can cause more frequent firing of action potentials because they can overcome the relative refractory period in neurons, allowing for a higher rate of action potential generation despite the refractory periods.

Outlines

00:00

🧠 Understanding Action Potentials and Neurons

This paragraph introduces the concept of action potentials, which are the electrical impulses that travel along neurons, leading to the release of neurotransmitters. It emphasizes the importance of action potentials in neural communication and the nervous system's function. The paragraph explains the electrical properties of a neuron at rest, including the role of ions such as sodium (Na+) and potassium (K+), and how their distribution across the neuron's membrane is maintained through ion channels and the sodium-potassium pump. The resting membrane potential, typically around -70 millivolts, is also described, setting the stage for the action potential process.

05:03

🚀 The Action Potential Process

The second paragraph delves into the process of how an action potential is triggered and propagates along the neuron. It details the role of voltage-gated sodium channels in depolarization, leading to an influx of sodium ions that further depolarize the neuron. This results in the rising phase of the action potential, where the membrane potential becomes positive. The paragraph then explains how the action potential ends, with the closing of sodium channels and the opening of voltage-gated potassium channels, which leads to repolarization. The speed of action potential propagation is enhanced by myelin sheaths and nodes of Ranvier, a phenomenon known as saltatory conduction. Additionally, the paragraph discusses the absolute and relative refractory periods that prevent immediate reuse of sodium channels and ensure the all-or-none law of action potential firing.

10:05

🎉 Conclusion on Action Potentials

The final paragraph wraps up the discussion on action potentials by highlighting the ability of neurons to fire many action potentials per second, despite the refractory periods. It reiterates that action potentials do not vary in size with stimulus intensity but rather in frequency, which is determined by the strength of stimulation. The all-or-none law is reinforced, stating that action potentials are of consistent amplitude and either occur or do not, with more intense stimuli leading to more frequent firing due to the overcoming of the relative refractory period.

Keywords

💡Action Potential

An action potential is an electrical impulse that travels along the length of a neuron. It is fundamental to neural communication and the functioning of the nervous system. In the video, it is described as the process that leads to the release of neurotransmitters, which are essential for transmitting signals across synapses.

💡Neuron

A neuron is a specialized cell that transmits information through electrical and chemical signals. The video explains that neurons are surrounded by a cell membrane that separates the intracellular and extracellular environments, and that the neuron's electrical properties at rest are crucial for understanding how action potentials occur.

💡Ion Channels

Ion channels are tube-like pores in a neuron's cell membrane that allow specific ions to pass through. They play a critical role in generating action potentials by controlling the movement of ions such as sodium and potassium. The video mentions that ion channels can be either always open (leak channels) or open in response to stimuli, like the binding of a neurotransmitter to a receptor.

💡Sodium-Potassium Pump

The sodium-potassium pump is an enzyme that actively transports sodium ions out of the cell and potassium ions into the cell, maintaining an unequal distribution of these ions across the cell membrane. This pump is essential for establishing the resting membrane potential and is highlighted in the video as a key factor in the neuron's electrical properties.

💡Resting Membrane Potential

The resting membrane potential refers to the electrical charge difference across the neuron's cell membrane when it is not transmitting an action potential. The video specifies that a typical resting membrane potential is about -70 millivolts, indicating that the inside of the cell is more negatively charged than the outside.

💡Depolarization

Depolarization is the process by which the membrane potential of a neuron becomes less negative, moving towards zero. It is a critical step in the initiation of an action potential, often triggered by neurotransmitters causing positively charged ions to flow into the neuron. The video describes how depolarization leads to the opening of voltage-gated sodium channels.

💡Voltage-Gated Ion Channels

Voltage-gated ion channels open or close in response to changes in the neuron's membrane potential. They are crucial for the generation and propagation of action potentials. The video explains that when a neuron reaches a certain threshold of depolarization, these channels open, allowing an influx of sodium ions that further depolarize the cell.

💡Repolarization

Repolarization is the process by which the neuron's membrane potential returns to its resting state after an action potential. It involves the closing of sodium channels and the opening of potassium channels, allowing potassium ions to leave the cell. The video emphasizes that repolarization is a rapid process that restores the neuron's resting membrane potential.

💡Myelin

Myelin is an insulating material that wraps around the axons of many neurons, increasing the speed and efficiency of action potential propagation. The video describes myelin as lipid-rich and highlights its role in preventing current leakage and facilitating saltatory conduction through the nodes of Ranvier.

💡Nodes of Ranvier

Nodes of Ranvier are gaps in the myelin sheath where voltage-gated sodium channels are concentrated. They are important for the propagation of action potentials along myelinated axons. The video explains that when an action potential reaches a node, it causes the inrush of sodium ions, regenerating the action potential and allowing it to 'jump' along the axon.

💡Refractory Periods

Refractory periods are times when a neuron is temporarily unable to generate another action potential. There are two types: the absolute refractory period, when sodium channels are unresponsive, and the relative refractory period, when the neuron is hyperpolarized. The video discusses how these periods relate to the intensity of stimulation and the all-or-none law of action potential firing.

💡All-or-None Law

The all-or-none law states that action potentials do not vary in size based on the intensity of a stimulus; they either occur at a consistent amplitude or do not occur at all. The video explains that while the size of an action potential remains constant, more intense stimuli can lead to more frequent action potential firing due to the overcoming of the relative refractory period.

Highlights

Action potentials are electrical impulses that travel down neurons and cause the release of neurotransmitters, critical for neural communication and nervous system function.

A neuron at rest has a resting membrane potential of about -70 millivolts, with more potassium ions inside and sodium ions outside the cell.

The sodium-potassium pump maintains ion concentration gradients by pumping 2 K+ in for every 3 Na+ out.

Diffusion and electrostatic forces balance out, leading to an equilibrium where the inside of the cell is more negatively charged.

Depolarization occurs when the membrane potential moves closer to 0, becoming less negative, allowing positively charged ions to flow into the neuron.

Postsynaptic potentials are small changes in membrane potential caused by neurotransmitter binding and ion flow into the neuron.

If depolarization reaches the threshold, an action potential fires, initiated by the opening of voltage-gated sodium channels.

The rapid influx of sodium ions during the rising phase of the action potential can cause the membrane potential to become positive, up to +40 mV.

The action potential propagates down the neuron's axon, with adjacent segments also depolarizing due to the opening of voltage-gated sodium channels.

Myelin sheaths around axons increase the speed and efficiency of action potential propagation by preventing current leakage.

Nodes of Ranvier are gaps in the myelin sheath that are rich in voltage-gated sodium channels, allowing for rapid regeneration of the action potential.

Saltatory conduction refers to the action potential appearing to jump down the axon, due to regeneration at nodes of Ranvier and slowing in internodal regions.

There is an absolute refractory period after an action potential during which the neuron cannot fire another action potential.

The relative refractory period follows repolarization, during which a strong stimulus is needed to generate another action potential.

Action potentials do not vary in size based on stimulus intensity - they follow the all-or-none law.

More intense stimuli can cause more frequent action potential firing, overcoming the relative refractory period.

Neurons can fire many action potentials per second, even with the constraints of refractory periods.

Transcripts

Hi everyone, welcome to 10 minute  neuroscience. In this installment,  

I’ll be talking about action potentials, the  electrical impulses that travel down neurons  

and cause the release of neurotransmitters.  Action potentials are a critical part of neural  

communication and essential to the function of  the nervous system. An action potential is an  

electrical impulse, and to understand how it  occurs it’s important to first understand the  

electrical properties of a neuron at rest,  when it’s not firing an action potential.  

A neuron, like any other cell in the body, is  surrounded by a cell membrane that separates the  

intracellular environment from the extracellular  environment. The intracellular and extracellular  

spaces are each filled with fluid, and suspended  in that fluid are charged particles called ions.  

These ions play an important role in creating  the conditions that are just right for an  

action potential to occur, and there are a  couple we need to pay special attention to:  

positively charged sodium ions, which are found at  greater concentrations outside the neuron and are  

represented by these circles with Na+ inside  them, and positively charged potassium ions,  

which are found at greater concentrations inside  the neuron and are represented by the circles  

with K+ inside them. This unequal distribution  of ions is maintained in multiple ways. First,  

the membrane of the cell doesn’t allow ions  to easily pass across it. Instead, they need  

to travel through tubelike pores that span the  membrane. These pores are called ion channels.  

Many ion channels are specific for certain ions.  There are, for example, ion channels that allow  

potassium to cross the cell membrane, and other  ion channels that allow sodium to cross the cell  

membrane. Additionally, some ion channels stay  open all the time, while others open only in  

response to specific stimuli or signals, such as  the binding of a neurotransmitter to a receptor.  

The channels that stay open all the time are  sometimes called leak channels. Neurons have  

a large number of potassium leak channels, and  relatively few sodium leak channels. Because of  

this, potassium is able to cross the membrane of  a typical neuron relatively freely, but sodium  

can’t. This will be important to determining how  potassium and sodium get distributed inside and  

outside the neuron, but we also have to consider  a few other factors. The first is a protein called  

the sodium-potassium pump. The sodium-potassium  pump is an enzyme that continuously pumps sodium  

ions out of the cell and potassium ions into  the cell. It pumps two potassium ions in for  

every three sodium ions it pumps out. Thus, the  pump helps to maintain a higher concentration  

of potassium ions inside the cell, and a higher  concentration of sodium ions outside the cell.  

In addition to the sodium-potassium pump, we  have to consider the influence of diffusion and  

electrostatic forces. Diffusion is the movement  of a substance–in this example the movement of  

ions–from areas of high concentration to areas  of low concentration. Electrostatic forces cause  

like-charged particles to repel one another  and opposite charges to attract one another.  

So, positively-charged ions will be less likely  to move closer to other positively-charged ions,  

but more likely to move towards a negatively  charged ion or a negatively charged environment.  

So we have this situation where  the sodium-potassium pump causes  

an accumulation of potassium ions inside the cell  and a build-up of sodium ions outside the cell.  

Some potassium ions leave the cell through  leak channels, and they move out of the cell  

because they’re moving from an area of high  concentration to an area of low concentration.  

But eventually this tendency to diffuse out of  the cell is balanced out by electrostatic forces,  

because as the positively-charged potassium ions  leave the cell, the inside of the cell becomes  

more negatively charged than the outside, and that  negative charge attracts the positively-charged  

potassium ions, keeping them from leaving. And  so at this point, we’ve reached an equilibrium,  

where the effects of diffusion and electrostatic  forces are balanced out. There is more potassium  

inside the cell and there's more sodium outside,  and there’s a difference in electrical charge  

between the inside and the outside of the  cell. Specifically, the inside of the cell  

is more negatively charged than the outside,  and we call this difference of electrical  

charge a membrane potential. For a neuron at rest  (meaning it’s not firing an action potential),  

its resting membrane potential is typically about  -70 millivolts, although the exact number depends  

on the type of neuron we’re talking about. A  membrane potential of -70 millivolts means the  

inside of the cell is about 70 millivolts more  negative than the outside. Now, the stage is set  

for the action potential. We’ll use this figure  here to look at how the membrane potential changes  

over the course of the action potential. So the  x axis is representing time in milliseconds,  

and the y axis is representing membrane potential  in millivolts. We’re starting at resting membrane  

potential, -70 millivolts, and the first step  leading to an action potential is when the  

membrane potential moves closer to 0, becoming  less negative, a process known as depolarization.  

When we have a separation of charges, we call  that polarization, so depolarization is when  

that separation is reduced. Depolarization can  occur, for example, when neurotransmitters bind  

to receptors and cause positively charged ions to  flow into the neuron. This influx of positively  

charged ions will cause the inside of the  neuron to become less negative, depolarizing it.  

These small changes in membrane potential caused  by neurotransmitter binding and the resultant flow  

of ions into the neuron are called postsynaptic  potentials. The neuron sums together these changes  

in membrane potential, and if the resultant  depolarization reaches a certain point, which  

we refer to as threshold, then an action potential  will fire. The action potential starts because  

when the neuron is depolarized to threshold,  there are sodium ion channels that open.  

These ion channels open in response to changes  in membrane potential, or changes in voltage.  

They’re called voltage-gated ion channels.  Now remember we said there are typically more  

positively-charged sodium ions outside the cell  and the inside of the cell is negatively charged  

with respect to the outside. So when these sodium  channels open up, it will cause positively charged  

sodium ions to rush into the cell due to the  influence of diffusion and electrostatic forces.  

This influx of sodium ions will depolarize the  cell even further, and this further depolarization  

causes more voltage-gated sodium channels to  open, leading to even more depolarization.  

This rapid change in membrane potential is  sometimes referred to as the rising phase of the  

action potential. Before you know it, the membrane  potential actually becomes positive and it shoots  

up to somewhere around positive 40 millivolts.  This inrush of positively-charged sodium ions is  

the electrical signal that forms the basis for the  action potential, and the signal will move down  

this long extension of the neuron called the axon.  We’ll talk about how that happens in a moment, but  

first I want to explain how the action potential  comes to an end. When the neuron’s membrane  

potential has reached its peak, the voltage-gated  sodium channels begin to close. During the rising  

phase of the action potential, there are also  voltage-gated potassium channels that open.  

These channels, along with normal potassium leak  channels, allow potassium to rush out of the  

cell because now potassium is trying to move away  from the positively-charged interior of the cell.  

This potassium moving out of the cell helps  to repolarize the neuron, or move it back to  

its resting membrane potential, at which point  voltage-gated potassium channels start to close.  

The whole process, from depolarization to  repolarization typically only takes a couple  

of milliseconds—that’s thousandths of a second,  so it’s incredibly fast. The sodium-potassium  

pump I mentioned earlier also helps to restore the  balance of sodium and potassium inside and outside  

the cell. So, this influx of sodium ions is really  responsible for causing the action potential, but  

what causes it to move down the axon? When this  process of depolarization occurs in one segment  

of the axon, it happens in the segment right  next to it as well because the adjacent segment  

is also rich in voltage-gated sodium channels, so  the depolarization causes them to open and this  

regenerates the action potential in the next  segment of the axon. So the action potential  

is kind of like a spreading fire that moves down  the axon. Many of the axons in the nervous system  

are also covered in an insulating material called  myelin represented by this striped structure here.  

Myelin is a lipid-rich material that  's wrapped around the axons of neurons,  

and it makes the propagation of action potentials  down the axon faster and more efficient.  

One way it does this is by preventing current  from leaking out of the axon, but the main way  

myelin increases speed of propagation is that  it’s interrupted by these areas called nodes of  

Ranvier, where there are gaps in the myelin. The  nodes of Ranvier are rich in voltage-gated sodium  

channels, so when a depolarizing action potential  reaches a node of Ranvier, it causes another  

inrushing of sodium and a regeneration of the  action potential. This causes an action potential  

to be regenerated at each node of Ranvier,  propelling the action potential down the axon.  

This regeneration of an action potential at the  nodes of Ranvier and slowing at the myelinated  

regions between them (which are called internodes)  causes the action potential to appear as if it’s  

jumping down the axon, and we call this process  saltatory conduction, from the Latin saltere,  

which means to jump. So now I’ve described  the generation of the action potential and  

how it’s propagated down the axon, but it’s  also important to mention that for a brief  

period after the initiation of the action  potential, voltage-gated sodium channels  

become unresponsive and can’t be activated. We  call this period the absolute refractory period,  

because a neuron will not be able to fire  another action potential during this phase.  

Additionally, when the neuron is repolarizing due  to the flowing of potassium out of the neuron,  

the potassium channels close gradually and allow  enough potassium to flow out of the neuron that  

it briefly becomes hyperpolarized, which means the  membrane potential is further away from zero than  

it was when it started out. This combined with  the gradual transitioning of sodium channels back  

to an active state creates the relative refractory  period, a time when a neuron will need very strong  

stimulation to produce another action potential  since it’s now further away from threshold.  

One effect of these refractory periods is that  they make the rate of action potential firing  

related to the intensity of stimulation.  Action potentials don’t vary in size  

based on the intensity of a stimulus—they  either fire or they don’t, and when they  

do they’re of a consistent amplitude or size;  this principle is known as the all-or-none law.  

But more intense stimuli will cause more frequent  firing of action potentials because they’ll be  

able to overcome the relative refractory period  in neurons. But even with the roadblocks put  

into place by the refractory periods, neurons can  fire many action potentials (sometimes hundreds or  

even more) per second. And that’s a summary  of action potentials. Thanks for watching!