Neuron action potential - physiology

Summary

This script delves into the intricate workings of neurons, the fundamental cells of our nervous system. It explains the structure of neurons, including dendrites, soma, and axons, and details the process of signal transmission through neurotransmitters and action potentials. The script also explores the resting membrane potential, the role of ions in generating electrical signals, and the mechanisms of excitation and inhibition in synapses. Furthermore, it describes the propagation of action potentials, including the function of voltage-gated channels and the acceleration of signal speed through myelin's saltatory conduction.

Takeaways

• 🧠 Neurons are the fundamental cells of the nervous system, consisting of dendrites, soma, and axon.
• 🌿 Dendrites are branch-like structures that receive signals from other neurons via neurotransmitters.
• 🔬 The soma, or cell body, contains the neuron's main organelles including the nucleus.
• 🚀 The axon transmits electrical signals, facilitated by myelin for rapid conduction.
• 🔄 Neurotransmitters convert chemical signals into electrical signals by opening ion channels.
• ⚡ Action potentials are electrical signals that travel down the axon at high speeds.
• 🔑 The resting membrane potential of a neuron is maintained at about -65 millivolts due to ion concentration differences.
• 🔄 Depolarization occurs when positive ions flow into the cell, making it less negative.
• 🔄 Repolarization is the process of the cell returning to its resting state through potassium ion outflow and the sodium-potassium pump.
• 🔒 The absolute refractory period prevents action potentials from firing too closely together and ensures unidirectional signal propagation.
• 🏃‍♂️ Saltatory conduction is the rapid movement of electrical signals along myelinated axons, 'jumping' from node to node.
• 🔄 The neuron's action potential cycle includes resting, depolarization, inactivation, repolarization, hyperpolarization, and returning to rest.

Q & A

What are the three main parts of a neuron?

The three main parts of a neuron are the dendrites, the soma (or cell body), and the axon.

How do dendrites function in a neuron?

Dendrites are the branches of a neuron that receive signals from other neurons via neurotransmitters.

What is the role of neurotransmitters in neuron communication?

Neurotransmitters act as chemical signals that, when they bind to receptors on the dendrite, open ion channels and convert the chemical signal into an electrical signal.

What is an action potential and how does it propagate within a neuron?

An action potential is an electrical signal that travels down the axon at high speeds, triggered when the combined effect of multiple dendrites changes the overall charge of the cell enough to reach a threshold.

Why are action potentials important for long neurons, such as those from the spinal cord to the toes?

Action potentials are crucial for long neurons because they allow for the rapid transmission of electrical signals over long distances.

What is the resting membrane potential of a neuron and why is it important?

The resting membrane potential of a neuron is a net negative charge of about -65 millivolts relative to the outside environment. It is important because it establishes the baseline charge difference necessary for the neuron to generate action potentials.

How does the opening of ligand-gated ion channels affect the neuron's charge?

The opening of ligand-gated ion channels allows certain ions to flow in or out of the cell, which can either depolarize (make the cell less negative) or hyperpolarize (make the cell more negative) the membrane potential.

What is the difference between an excitatory postsynaptic potential (EPSP) and an inhibitory postsynaptic potential (IPSP)?

An EPSP is a net influx of positive charge resulting from the opening of ligand-gated ion channels, making the cell less negative. An IPSP, on the other hand, is a net influx of negative charge, making the cell more negative or repolarizing it.

What happens when a neuron reaches the threshold value for an action potential?

When a neuron reaches the threshold value, typically about -55mV, voltage-gated Na+ channels at the axon hillock open, initiating an action potential that travels down the axon.

How does the sodium-potassium pump contribute to the repolarization of a neuron?

The sodium-potassium pump is an active transporter that moves three sodium ions out of the cell and two potassium ions into it, helping to restore the neuron's resting membrane potential after an action potential.

What is saltatory conduction and how does it relate to myelin in neurons?

Saltatory conduction is the rapid movement of electrical signals from node to node along a myelinated axon. Myelin, produced by glial cells, insulates the axon and allows the charge to 'jump' between nodes, increasing the speed of signal transmission.

Outlines

00:00

🧠 Neuron Structure and Signal Propagation

This paragraph delves into the fundamental components of a neuron and how they facilitate communication within the nervous system. It explains the roles of dendrites, the soma, and the axon, including the myelin sheath's function in speeding up electrical signal transmission. The process of neurotransmitters converting chemical signals to electrical ones is detailed, along with the generation and propagation of action potentials. The paragraph also discusses the resting membrane potential and how it's influenced by ion concentrations, leading to either excitatory or inhibitory postsynaptic potentials. The importance of the action potential in neuron firing is highlighted, emphasizing the neuron's ability to communicate over long distances.

05:04

🚀 The Dynamics of Action Potentials and Myelin's Role

This section explores the intricacies of how action potentials are generated and propagated along a neuron's axon. It describes the sequence of events starting from the depolarization phase, where sodium ions flow into the cell, to the repolarization phase, where potassium ions flow out, aided by the sodium-potassium pump. The concept of refractory periods, both absolute and relative, is introduced to explain the prevention of action potentials occurring too closely together. The paragraph also explains how myelin sheaths increase the speed of signal transmission through a process known as saltatory conduction, where the electrical charge appears to 'jump' between nodes of Ranvier, thereby enhancing the efficiency of the nervous system.

Keywords

💡Neurons

Neurons are the fundamental units of the nervous system, responsible for receiving, processing, and transmitting information through electrical and chemical signals. In the video, neurons are described as having three main parts: dendrites, soma, and axon, which work together to facilitate communication within the nervous system.

💡Dendrites

Dendrites are the branch-like structures of a neuron that receive signals from other neurons. They play a crucial role in the integration of synaptic inputs. The script mentions that dendrites receive signals via neurotransmitters, which are essential for the initiation of an electrical signal within the neuron.

💡Soma

The soma, or cell body of a neuron, contains the main organelles, including the nucleus. It is the central part of the neuron where the cell's genetic material is housed and metabolic activities occur. The video emphasizes the soma's role in maintaining the neuron's overall function.

💡Axon

The axon is a long, slender projection of a neuron that transmits electrical signals, known as action potentials, to other neurons or target cells. The video script explains that the axon is covered by myelin, which aids in the rapid transmission of these signals.

💡Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across a synapse from one neuron to another. The script describes how neurotransmitters bind to receptors on dendrites, triggering an ion flow that converts chemical signals into electrical signals, which is fundamental to neuronal communication.

💡Action Potential

An action potential is an electrical signal that travels along the axon of a neuron. The video explains that it is triggered when the combined effect of multiple dendrites receiving signals changes the cell's overall charge enough to initiate this rapid electrical signal.

💡Resting Membrane Potential

The resting membrane potential is the difference in electrical charge between the inside and outside of a neuron when it is not transmitting signals. The video script specifies it as being close to -65 millivolts, which is essential for the neuron to be ready to generate an action potential.

💡Depolarization

Depolarization is the process by which the neuron's membrane potential becomes less negative, often due to the influx of positive ions like sodium. The script illustrates this concept when discussing the opening of ligand-gated ion channels by neurotransmitters.

💡EPSP and IPSP

Excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP) are the results of neurotransmitter action on the postsynaptic neuron. EPSPs make the neuron more likely to fire an action potential, while IPSPs have the opposite effect. The video script explains how these potentials contribute to the neuron's decision to fire or not.

💡Voltage-Gated Channels

Voltage-gated channels are ion channels that open and close in response to changes in the membrane potential. The video script describes how these channels, particularly for sodium and potassium, are critical for the generation and propagation of action potentials.

💡Myelin

Myelin is a fatty substance that insulates some axons, increasing the speed at which electrical signals are transmitted. The script explains that myelin is formed by glial cells and facilitates a process called saltatory conduction, where the action potential 'jumps' from one node of Ranvier to the next.

💡Saltatory Conduction

Saltatory conduction is the rapid transmission of electrical signals along myelinated axons, where the signal appears to jump from one node of Ranvier to the next. The video script uses this term to describe the increased efficiency of action potential propagation due to myelin insulation.

Highlights

Neurons consist of dendrites, soma, and axon, with dendrites receiving signals and the soma containing the neuron's organelles.

Neurotransmitters convert chemical signals into electrical signals via ion channels, triggering action potentials.

Action potentials propagate along the axon at high speeds, facilitated by the release of neurotransmitters.

Neurons communicate through neurotransmitters and propagate signals internally via action potentials.

The resting membrane potential of a neuron is maintained by the differential ion concentrations across the cell membrane.

Depolarization occurs when neurotransmitters bind to receptors, allowing positive ions to flow into the cell.

Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) determine the neuron's response to stimuli.

The neuron fires an action potential when the membrane potential reaches a threshold, initiating a chain reaction of voltage-gated sodium channels.

The inactivation gate of sodium channels prevents continuous firing, ensuring unidirectional signal propagation.

Potassium channels and the sodium-potassium pump work together to repolarize the neuron after an action potential.

The absolute refractory period prevents action potentials from occurring too closely in time, maintaining signal integrity.

The relative refractory period requires a stronger stimulus for the neuron to fire, due to the open potassium channels and hyperpolarized state.

Myelin sheaths increase the speed of action potential propagation through a process called saltatory conduction.

The nodes of Ranvier are critical for the rapid transmission of action potentials in myelinated neurons.

The graphical recap illustrates the entire process of neuronal firing, from EPSPs to the return to resting potential.

The video provides a comprehensive understanding of how neurons function and the importance of ion channels and myelin in signal transmission.

Support for the educational content can be found through Patreon donations, channel subscriptions, or social media shares.

Transcripts

Neurons are the cells that make up our nervous system, and they’re made up of three main

parts.

The dendrites, which are little branches off of the neuron that receive signals from other

neurons, the soma, or cell body, which has all of the neuron’s main organelles like

the nucleus, and the axon which is intermittently wrapped in fatty myelin.

Those dendrites receive signals from other neurons via neurotransmitters, which when

they bind to receptors on the dendrite act as a chemical signal.

That binding opens ion channels that allow charged ions to flow in and out of the cell,

converting the chemical signal into an electrical signal.

Since a single neuron can have a ton of dendrites receiving input, if the combined effect of

multiple dendrites changes the overall charge of the cell enough, then it triggers an action

potential- which is an electrical signal that races down the axon up to 100 meters per second,

triggering the release of neurotransmitter on the other end and further relaying the

signal.

So neurons use neurotransmitters as a signal to communicate with each other, but they use

the action potential to propagate that signal within the cell.

Some of these neurons can be very long, especially ones that go from the spinal cord to the toes,

so the movement of this electrical signal is super important!

But why does the cell have an electric charge in the first place?

Well, it’s based on the different concentrations of ions on the inside versus outside of the

cell.

Generally speaking, there are more Na+ or sodium ions, Cl- or chloride ions, and Ca2+

or calcium ions on the outside, and more K+ or potassium ion and A- which we just use

for negatively charged anions, on the inside.

Overall, the distribution of these ions gives the cell a net negative charge of close to

-65 millivolts relative to the outside environment - this is called the neuron’s resting membrane

potential.

When a neurotransmitter binds to a receptor on the dendrite, a ligand-gated ion channel

opens up to allow certain ions to flow in, depending on the channel.

Ligand-gated literally means that the gate responds to a ligand, which in this case is

a neurotransmitter.

So if we take the example of a ligand-gated Na+ ion channel, which, when it opens, lets

Na+ flow into the cell.

The extra positive charge that flows in makes the cell less negative (since remember it’s

usually -65mV), and therefore less “polar” - so that’s why gaining positive charge

is called depolarization.

Neurotransmitters typically open various ligand-gated ion channels all at once, so ions like sodium

and calcium, may flow in, while other ions like potassium, may flow out, which would

actually mean some positive charge leaves the cell.

In the end though - when it’s all added up - if there is a net influx of positive

charge, then it’s called an excitatory postsynaptic potential (EPSP).

In contrast, the opening of only ligand-gated Cl- ion channels would cause a net influx

of negative charge, creating an inhibitory postsynaptic potential (IPSP), making the

cell potential more negative or repolarizing it.

Now, a single EPSP or IPSP causes only a small change on the resting membrane potential,

but, if there are enough EPSPs across multiple sites on the dendrites then collectively they

can push the membrane potential to a specific threshold value- typically about -55mV, although

this can vary by tissue.

When this occurs, it triggers the opening of voltage-gated Na+ channels at the start

of the axon - the axon hillock, voltage-gated channels open in response to a change in voltage,

and when these open sodium to rush into the cell.

The influx of sodium ions and the resulting change in membrane potential causes nearby

voltage-gated sodium channels to open up as well - setting off a chain reaction that continues

down the entire length of the axon—which is our action potential, and when this happens,

we say that the neuron has ‘fired.’

Once a lot of sodium has rushed across the neuronal membrane, the call actually becomes

positively charged relative to the external environment - up to about +40mV.

The depolarization process ends when the sodium channel stops allowing sodium to flow into

the cells- a process known as inactivation.

But this state is different than when the channel’s closed, or open for that matter,

which is what most of the other channels have.

The voltage-gated sodium channel, though, is unique in that it has what’s known as

the inactivation gate, which blocks sodium influx shortly after depolarization, until

the cell repolarizes and the channel enters the closed state again and the inactivation

gate stops blocking influx, although even though the inactivation gate’s not blocking,

the channel’s still closed so no sodium enters the cell.

This middle open state therefore is the only state where sodium gets let into the cell

through the channel, and this is a very short window of time.

Now in addition to these sodium voltage-gated channels, we’ve also got potassium voltage-gated

channels, which are slow to respond and don’t open until the sodium channels have already

opened and become inactivated.

The result is that after the initial sodium rush into the cell, potassium flows out of

the cell down its own electrochemical gradient- removing some positive charge and blunting

the effect of the sodium depolarization.

The potassium channels, do not have a separate inactivation gate and therefore remain open

for slightly longer, which means that there is a period of time when there is a net movement

of positive ions out of the cell, causing the membrane potential to become more negative,

or repolarize.

During this repolarization phase, the cell also relies on the sodium-potassium pump,

an active transporter that moves three sodiums out of the cell and two potassiums into it.

It’s during this repolarization phase that the cell’s in its absolute refractory period,

since the sodium channels are inactivated and won’t respond to any amount of stimuli.

This absolute refractory period keeps the action potentials from happening too close

together in time, but also keeps the action potential moving in one direction.

The combined efforts of this pump and the extended opening of the potassium channels

results in a small period of overcorrection where the neuron becomes hyperpolarized relative

to the resting potential, and at this point the sodium channels go back to their initial

closed state, and for a short period the potassium channels stay open.

Now we’re in the relative refractory period since the sodium channels are closed but can

be activated, but because the potassium channels are still open and we’re in a hyperpolarized

state, so it’s takes a strong stimulus to do so.

Finally, as the potassium channels close, the neuron returns to it’s resting membrane

potential.

Alright, as a quick graphical recap, with membrane potential on the y and time on the

x.

First we start at resting potential of around -65 mV and voltage-gated sodium and potassium

channels are closed, we receive EPSPs enough to hit threshold at about -55 mV, voltage-gated

sodium channels open and we reach a peak of about +40 mV, at which point the sodium channels

become inactivated and we’re in the absolute refractory period.

Voltage-gated potassium channels open, and along with the sodium-potassium pump, start

to repolarize the cell, so much so that it overshoots and hyperpolarizes the cell.

Next the sodium channels enter their closed resting state as potassium channels start

to close we’re in the relative refractory period, until finally they all close and we

reach our resting membrane potential.

Alright, so this process of positive sodium ions moving in and depolarizing the cell transmits

the electrical signal down the length of the axon.

Great.

But really, this process isn’t that fast.

So that’s where the fatty myelin comes in, which comes from glial cells like Schwann

cells or oligodendrocytes.

These myelinated areas don’t have voltage-gated ion channels spanning the membrane, so ions

can’t simply flow into the cell, that only happens in the spots between the myelin, called

nodes of Ranvier.

So instead of propagating via channels, the charge essentially jumps from node to node.

That said though, these ions aren’t just diffusing down the length of the myelin to

the other side...that’d be way to slow.

What actually happens is more like the sodium ions rushing in bumps other positive sodium

ions already inside the cell, which bumps another one, and so on until it reaches the

next node.

The charge moving in this way with the myelinated areas moves really fast, and is called saltatory

conduction, which makes it look like the action potential “jumps” from one one node to

the next.

Okay extremely quick recap - neuron action potentials happen when dendrites receive enough

EPSPs to open voltage-gated sodium channels, which cause rapid depolarization of the neuronal

membrane and propagation of an electrical charge from node to node down the length of

the axon.

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