Action Potential in the Neuron

Summary

This educational script delves into the anatomy and function of a neuron, highlighting its four main components: dendrites, cell body, axon, and axon terminal. It explains how neurons transmit signals via action potentials, which are dependent on the movement of ions like sodium and potassium. The script also details the resting membrane potential, the role of ion channels, and the process of repolarization. It further discusses the absolute and relative refractory periods, and how myelin sheaths increase the speed of signal transmission through a mechanism known as saltatory conduction.

Takeaways

• 🧠 A neuron consists of dendrites, cell body, axon, and axon terminal, each with a specific role in receiving, processing, and transmitting information.
• 🌐 Dendrites receive signals, the cell body integrates them, and the axon carries the information to the axon terminal for transmission to the next cell.
• 🚀 The action potential, or 'firing' of a neuron, occurs when stimulation is strong enough to trigger a signal along the axon.
• 🔄 Ions, such as sodium, potassium, and chloride, play a crucial role in the neuron's function, both at rest and during an action potential.
• 💡 The neuron's resting potential is around -70 millivolts, indicating a net negative charge inside the cell compared to the outside.
• 🛡 The electrochemical gradient, resulting from the unequal distribution of ions, is essential for the neuron's ability to transmit signals.
• 🚪 Ion channels, including voltage-gated and ligand-gated types, regulate the flow of ions across the cell membrane.
• ⚡ The sodium-potassium pump actively transports ions against their concentration gradient, maintaining the resting state and readiness to fire.
• 🔁 The process of an action potential involves depolarization, repolarization, and hyperpolarization, returning the neuron to its resting state.
• 🏃‍♂️ Myelin sheaths increase the speed of action potential conduction along axons through a process called saltatory conduction.

Q & A

What are the four main parts of a neuron?

The four main parts of a neuron are the dendrites, the cell body, the axon, and the axon terminal.

What is the function of dendrites in a neuron?

Dendrites receive incoming signals or information from other neurons.

How does the cell body of a neuron contribute to its function?

The cell body, or soma, processes and integrates the information received by the dendrites.

What role does the axon play in the transmission of information within a neuron?

The axon carries the information along its length, from one part of the neuron to another, often over long distances.

How does the axon terminal facilitate communication between neurons?

The axon terminal transmits information to the next cell in the chain, often by releasing neurotransmitters.

What is a nerve, and how is it related to axons?

A nerve is a bundle of axons traveling together, often over long distances to transmit information.

How does the strength of incoming stimulation affect a neuron's decision to pass a signal along?

If the stimulation is strong enough, the neuron will transmit the signal along its axon in an action potential, which is referred to as the neuron firing.

What is the resting membrane potential of a neuron, and what does it indicate?

The resting membrane potential of a neuron is approximately minus 70 millivolts, indicating that the inside of the cell is more negative than the outside when at rest.

What are the two types of gradients that contribute to the electrochemical gradient across a neuron's membrane?

The two types of gradients contributing to the electrochemical gradient are the chemical gradient, due to the unequal distribution of ions, and the electrical gradient, due to the difference in charge across the membrane.

How do ion channels facilitate the movement of ions across the neuron's membrane?

Ion channels, which can be voltage-gated, ligand-gated, or mechanically-gated, facilitate the movement of ions by allowing them to passively diffuse along their concentration gradient.

What is the sodium-potassium pump, and what is its role in maintaining the neuron's resting state?

The sodium-potassium pump is a protein that uses energy from ATP hydrolysis to actively transport ions against their concentration gradient, helping to restore the chemical and electrical gradients necessary for the neuron's resting state.

What happens during an action potential, and how does it relate to the neuron's membrane potential?

During an action potential, the neuron's membrane potential changes from its resting state to a less negative or even positive value due to the influx of sodium ions, followed by an outflux of potassium ions, which is a result of the opening and closing of voltage-gated ion channels.

What is the significance of the absolute refractory period in the context of neuronal signaling?

The absolute refractory period is a period when a neuron cannot fire another action potential, regardless of stimulation, which prevents action potentials from occurring too quickly and ensures unidirectional transmission along the axon.

How does the presence of myelin sheaths around axons affect the speed of action potential transmission?

Myelin sheaths increase the speed of action potential transmission by enabling saltatory conduction, where the signal appears to jump from one node of Ranvier to the next, speeding up the process.

Outlines

00:00

🧠 Neuron Structure and Function

This paragraph introduces the basic structure of a neuron, which includes dendrites, cell body, axon, and axon terminals. Dendrites receive signals, the cell body processes them, and the axon carries them to other parts of the neuron or to the next neuron via axon terminals. Nerves, which are bundles of axons, can be long to facilitate signal transmission over distances. The neuron's decision to pass a signal is based on the strength of stimulation. If the stimulation is strong enough, an action potential occurs, and the neuron 'fires.' The movement of ions, like sodium, potassium, and chloride, is crucial for both the resting state and the firing of a neuron. These ions are unevenly distributed across the cell membrane, creating a chemical and electrical gradient, which together form the electrochemical gradient. The neuron's resting potential is typically around -70 millivolts, with the inside of the cell being less positive than the outside.

05:01

🔋 Ion Channels and Action Potentials

This section delves into the specifics of how ions move across the neuron's membrane through ion channels, which can be voltage-gated, ligand-gated, or mechanically-gated. It explains that voltage-gated channels open only when the membrane potential reaches a certain value, allowing specific ions to pass through. The movement of ions through these channels can cause the membrane potential to deviate from its resting state, leading to either a graded potential or an action potential. The sodium-potassium pump is highlighted as a crucial mechanism for resetting the membrane potential and maintaining ionic balance, which is energetically costly for the brain. The paragraph also describes the process of an action potential, including depolarization, inactivation of sodium channels, repolarization due to potassium ion movement, and the absolute refractory period when the neuron cannot fire again immediately.

10:05

⚡️ Action Potential Propagation and Refractory Periods

The final paragraph discusses the propagation of action potentials along the axon and the role of myelin sheaths in increasing the speed of signal transmission through a process called saltatory conduction. It also explains the concept of refractory periods, including the absolute refractory period when the neuron cannot fire another action potential, and the relative refractory period when a larger stimulus is needed to fire the neuron. The paragraph emphasizes that the amplitude of an action potential does not vary with the size of the stimulus, adhering to an 'all-or-nothing' law, but the frequency of action potentials can vary. It concludes with a review of the neuron's response to stimuli, from no stimulus at resting potential to graded potentials and action potentials upon sufficient stimulation.

Keywords

💡Neuron

A neuron is the fundamental unit of the nervous system, responsible for receiving, processing, and transmitting information through electrical and chemical signals. In the video, the neuron is described as having four main parts: dendrites, cell body, axon, and axon terminal, each with a specific role in the neuron's function. The neuron's structure and function are central to the video's theme of how the brain processes and transmits information.

💡Dendrites

Dendrites are the branched extensions of a neuron that receive signals from other neurons. They play a crucial role in the integration of information, as they collect and transmit this information towards the cell body. The video mentions that dendrites are the part of the neuron that receives incoming signals, which is essential for the neuron's decision-making process on whether to pass the signal along.

💡Cell Body

The cell body, also known as the soma, is the main part of the neuron that contains the nucleus and other organelles. It is responsible for integrating the information received from dendrites and preparing it for transmission. The video explains that the cell body processes and integrates information, highlighting its role in the neuron's communication process.

💡Axon

The axon is a long, slender projection of the neuron that carries electrical signals, known as action potentials, from the cell body to other neurons or target cells. The video emphasizes the axon's role in carrying information over long distances, which is vital for the communication between different parts of the nervous system.

💡Axon Terminal

Axon terminals are the end parts of the axon that transmit information to the next neuron or target cell. They release neurotransmitters, which are chemicals that carry the signal across the synaptic gap. The video describes how the axon terminal transmits information to the next cell in the chain, completing the neuron's communication cycle.

💡Nerve

A nerve is a bundle of axons that travel together, often over long distances, to transmit information. Nerves are essential for the nervous system's efficiency, as they allow for the rapid and coordinated transmission of signals. The video mentions nerves as a collection of axons, highlighting their role in long-distance communication within the body.

💡Action Potential

An action potential is an electrical signal that travels along the neuron's axon, triggered when the neuron receives a stimulus strong enough to depolarize its membrane. The video describes the action potential as a phenomenon where the neuron 'fires', indicating the transmission of a signal along the entire length of the axon.

💡Ions

Ions are charged particles, such as sodium, potassium, and chloride, that play a critical role in the neuron's electrical activity. They are distributed unequally across the neuron's membrane, creating a gradient that is essential for the generation of action potentials. The video explains how the movement of ions is responsible for the neuron's electrical properties, both at rest and during signal transmission.

💡Electrochemical Gradient

The electrochemical gradient refers to the imbalance of ions across the neuron's membrane, which includes both chemical and electrical gradients. This gradient is essential for the neuron's ability to generate electrical signals. The video discusses how the resting membrane potential is a result of the electrochemical gradient, which is maintained by the unequal distribution of ions.

💡Resting Potential

The resting potential is the electrical charge across the neuron's membrane when it is not transmitting signals, typically around -70 millivolts. This potential is maintained by the electrochemical gradient and is crucial for the neuron's ability to respond to stimuli. The video explains that the resting potential is a state of electrochemical equilibrium, where the neuron is ready to fire an action potential upon receiving a strong enough stimulus.

💡Ion Channels

Ion channels are protein structures embedded in the neuron's membrane that facilitate the movement of ions in and out of the cell. They play a critical role in the generation and propagation of action potentials. The video describes various types of ion channels, such as voltage-gated and ligand-gated channels, and their roles in the neuron's electrical activity.

💡Sodium-Potassium Pump

The sodium-potassium pump is an enzyme that uses energy from ATP to actively transport sodium ions out of the cell and potassium ions into the cell, against their concentration gradients. This pump is essential for maintaining the resting potential and the electrochemical gradient. The video explains how the pump contributes to the neuron's readiness to fire an action potential by restoring the ionic concentrations and membrane potential after an action potential has occurred.

Highlights

Neurons have four main parts: dendrites, cell body, axon, and axon terminal.

Dendrites receive information, cell bodies process and integrate it, and axons carry it to other parts of the neuron.

Axon terminals transmit information to the next cell in the neural chain.

A bundle of axons is called a nerve, which can be very long to transmit information over distances.

The neuron must decide whether to pass along a signal based on the strength of incoming stimulation.

An action potential occurs when the neuron fires, transmitting a signal along the axon.

Neuronal signal transmission depends on the movement of ions, including sodium, potassium, and chloride.

Ions are unequally distributed across the cell membrane, creating a chemical and electrical gradient.

The resting potential of a neuron is approximately minus 70 millivolts, with more positive charge outside the cell.

Ions move across the membrane through ion channels, which can be voltage-gated, ligand-gated, or mechanically-gated.

The sodium-potassium pump uses ATP to actively transport ions, maintaining the electrochemical gradient.

Action potentials are all-or-nothing events, with a fixed amplitude that does not increase with larger stimuli.

The frequency of action potentials can change in response to different stimuli.

Myelin sheaths around axons increase conduction velocity through a process called saltatory conduction.

The absolute refractory period prevents a neuron from firing another action potential immediately after one has occurred.

The relative refractory period requires a larger stimulus to reach threshold due to hyperpolarization.

The neuron's membrane potential and ion distributions must be maintained at precise levels for it to be ready to fire an action potential.

Transcripts

PROFESSOR: This is a neuron, which has four main parts.

The dendrites receive information.

The cell body processes and integrates that information.

The axon carries the information along long distances

from one part of the neuron to another.

And the axon terminal transmits the information

to the next cell in the chain.

A bundle of axons traveling together is called a nerve.

Nerves can be very long, as they often

need to transmit information over long distances.

As we just saw, the dendrites are

the part of the neuron that receives incoming signals.

Based on the strength of this incoming stimulation,

the neuron must decide whether to pass that signal along

or not.

If the stimulation is strong enough,

the signal is transmitted along the entire length of the axon

in a phenomenon called an action potential.

When this happens, we say the neuron fires.

Transmission of a neuronal signal

is entirely dependent on the movement of ions,

or charged particles.

Various ions, including sodium, potassium, and chloride,

are unequally distributed between the inside

and the outside of the cell.

The presence and movement of these ions

is not only important when a neuron fires but also at rest.

To start, let's think about the positively-charged sodium

and potassium ions.

When a neuron is not sending a signal,

it is considered to be at rest.

In a typical neuron in its resting state,

the concentration of sodium ions is higher

outside the cell than inside.

The relative concentration of potassium ions

is the opposite, with more ions inside the cell than outside.

This ionic separation occurs right at the cell membrane

and creates a chemical gradient across the membrane.

Because ions are charged particles,

we also need to consider their charge

when thinking about their distribution

across the membrane.

At rest, there are more positively charged

ions outside the cell relative to the inside.

This creates a difference in charge

across the membrane, which is called an electrical gradient.

Together with the chemical gradient we already mentioned,

we refer to this ionic imbalance as

the electrochemical gradient.

The difference in total charge inside and outside of the cell

is called the membrane potential.

At rest, when no signals are being transmitted,

neuronal membrane has a resting potential

of approximately minus 70 millivolts.

This means that the inside of the cell

is approximately 70 millivolts less positive than the outside.

Both the chemical and electrical gradients we just discussed

contribute to establishing this potential.

While the inside of the cell has a net negative charge

and the outside of the cell has a net positive charge,

the charges line up at the membrane.

And the bulk solution on either side

is actually electrically neutral.

The resting-membrane potential is

the point where the cell has achieved

electrochemical equilibrium.

This means that the concentration gradient

and the electro gradient for each ion is equal and opposite.

Ions cannot simply move across the membrane at will.

Instead, they need a protein embedded in the membrane

to facilitate their movement.

Most ions cross the membrane through a structure

called an ion channel.

Ions move through channels by passive diffusion

along their concentration gradient.

Some ion channels are always open,

but many require signal to tell them to open or close.

For example, voltage-gated channels

only open when the membrane potential

reaches a certain value.

On the other hand, ligand-gated ion channels

are triggered to open when they are

bound by a specific molecule.

Mechanically-gated ion channels open

in response to physical forces, such as changes in length

or changes in pressure.

Most ion channels are selectively permeable,

meaning that they only allow one, or a small subset of ions,

to pass through.

Voltage-gated ion channels, for example,

typically only allow a single ion

to cross the membrane when they open.

This means that we need separate channels for each ion, i.e.

voltage-gated sodium channels, as well as

voltage-gated potassium channels.

As ions move through a channel and cross

from one side of the cell membrane to the other,

they cause the membrane potential

of the cell to move away from its resting potential.

If the resulting change in membrane potential is small,

we call this a graded potential.

Graded potentials can vary in size,

can be either positive or negative,

are transient, and typically do not

result from the opening of voltage-gated ion channels.

When ion channels open and a graded potential occurs,

the neuron moves quickly to reset its membrane potential

to resting values.

This is accomplished primarily by the use

of the sodium-potassium pump, which uses the energy generated

by ATP hydrolysis, to actively transport ions

across the membrane against their concentration gradient.

In other words, sodium is transported

to the outside of the cell, where its concentration is

higher, and potassium is transported back

into the cell, where its concentration is higher.

One cycle of this pump transports three sodium ions

outside the cell and brings two potassium ions inside the cell.

This unbalanced charge transfer contributes

to the separation of charge across the membrane

and also to the ionic concentrations we see at rest,

thus, restoring the chemical and electrical gradients

to their resting levels.

Maintaining these ionic balance in neurons

is so important that this process

can account for 20% to 40% of the brain's total energy use.

Only when the resting membrane potential and ion distributions

are maintained at precise levels,

will the neuron be poised and ready to fire an action

potential.

When the outside stimulation is large enough

to bring the membrane potential in the neuron

body up from minus 70 millivolts to the threshold

voltage of minus 55 millivolts are higher,

this triggers an action potential

at the axon hillock, which then travels down the axon.

Voltage-gated sodium channels have three states--

open, closed, and inactivated.

At rest, the sodium channel is closed.

Once the cell membrane reaches the threshold voltage,

the channel changes to an open position and sodium

rushes into the cell because of the electrochemical gradient.

As positive-sodium ions enter the cell,

the membrane potential becomes less negative and more positive

as it approaches 0 millivolts.

This is called depolarization.

Eventually, the voltage gradient goes to zero and beyond 0

up to a positive 30 millivolts.

This is called an overshoot.

As the membrane potential becomes positive,

the sodium channel inactivation gate

shuts, making the channel inactivated.

This stops the flow of sodium ions into the cell.

The change in membrane potential also opens

the voltage-gated potassium channels,

though they open and close more slowly.

Because of the potassium-electrochemical

gradient, potassium ions flow out of the cell,

making it less positive and eventually negative.

This process is called repolarization.

Because the potassium channels are a little slow to close,

for a brief period, the membrane potential is hyperpolarized.

It's more negative than the resting potential.

During hyper-polarization, the potassium channels close.

Throughout all of this, the sodium-potassium pump

is still working.

The pump restores the chemical gradients

by putting the sodium and potassium back in place.

And the pump re-establishes the potential gradient

by moving more sodium ions out than potassium ions in.

This returns the membrane potential back

to its resting potential.

During repolarization, the inactivated sodium channels

won't respond to any stimulus at all.

During this time, the neuron is in its absolute refractory

period, the period of time when a nerve cannot fire another

action potential, no matter how strongly it's stimulated.

The absolute refractory period prevents action potentials

from happening again too quickly and prevents action potential

from traveling backwards along the axon.

During hyperpolarization the sodium channels are closed

and the inactivation gate opens.

There is no change in sodium flow,

but now they could be opened again.

This is called the relative-refractory period.

Because, while the sodium channels could open,

it would take a larger than usual stimulus

to reach threshold, because the cell

is hyperpolarized due to the potassium

still leaving the cell.

The amplitude of the action potential

for a particular neuron, that is, the maximum voltage

in one neuron during an action potential, never changes.

An action potential doesn't get bigger with a bigger stimulus.

It's all or nothing.

It either happens, or it doesn't happen.

What can change is the frequency of the action potential.

A neuron might fire many more times per second in response

to, say, an intense pain and less

frequently in response to a gentle breeze.

Some axons transmit action potentials faster than others.

One variable that increases conduction velocity

is the presence of myelin sheaths around axons.

Myelin speeds up transmission through a process

called saltatory conduction, in which the action

potential signal appears to jump along

the part of the axon covered by the sheath.

In the peripheral nervous system,

the sheaths are formed from glial cells

known as Schwann cells.

There are small gaps between Schwann cells

called the nodes of Ranvier.

The action potential appears to jump from node to node,

speeding the transmission.

In the central nervous system, the sheets

are made by cells known as oligodendrocytes.

To review, with no stimulus, the membrane

is at its resting potential.

A small stimulus causes a graded potential.

And a stimulus above the threshold

creates an action potential, and the neuron fires.