Neural Conduction, Action Potential, and Synaptic Transmission

Summary

This script delves into the intricate workings of neurons and synapses, explaining how they transmit electrochemical signals. It covers the concept of membrane potential, the role of ions and ion channels, and the impact of neurotransmitters on neuron excitation or inhibition. The process of action potential generation, including depolarization and repolarization, is detailed. The script also touches on the speed of signal conduction, the role of myelinated versus nonmyelinated fibers, and the diversity of neuronal conduction in the brain. Finally, it discusses the structure and function of synapses, including neurotransmitter release and receptor binding.

Takeaways

• 🧠 Neurons transmit electrochemical signals throughout the body, relaying information from sensory organs to the brain and back to the rest of the body.
• 🔋 The concept of membrane potential is crucial for understanding neuron function, representing the electric potential across the neuron's cell membrane.
• ⚡ At rest, a neuron's membrane potential is about -70 millivolts, with more sodium ions outside and potassium ions inside the cell.
• 🚪 Ion channels in the cell membrane control the movement of ions, contributing to the neuron's resting and action potentials.
• 💊 Neurotransmitters, released by pre-synaptic neurons, bind to receptors on post-synaptic neurons, causing changes in membrane potential.
• 📈 Graded potentials, such as excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), vary in magnitude with the intensity of the signal.
• 🚀 If the membrane potential reaches the threshold of excitation (around -55 millivolts), an action potential is generated, propagating along the axon.
• 🔁 The action potential involves a rapid reversal of the membrane potential, from about -70 mV to +50 mV, due to the movement of ions across the membrane.
• ⏱ The action potential's phases include depolarization, repolarization, and hyperpolarization, each occurring within milliseconds.
• 🏃‍♂️ The speed of action potential conduction varies by neuron type, with myelinated fibers conducting signals faster due to saltatory conduction.
• 🔄 Neurotransmitters are cleared from the synaptic cleft through reuptake or enzymatic degradation, resetting the system for new signals.

Q & A

What is the primary function of neurons and synapses in the human body?

Neurons and synapses are responsible for conducting and transmitting electrochemical signals throughout the body, relaying information from sensory organs to the brain and then from the brain to the rest of the body to dictate responses.

What is membrane potential and why is it important for neuron function?

Membrane potential is an electric potential that exists due to the distribution of electrical charge on either side of the cell membrane. It is crucial for neuron function as it determines the neuron's state of polarization and its readiness to transmit signals.

How does the resting potential of a neuron typically measure and what does this indicate?

The resting potential of a neuron is typically measured at about negative 70 millivolts, indicating that the neuron is polarized with a negative charge on the inside and a positive charge on the outside.

What causes the deviation from the resting potential, and what are the two possible outcomes?

The deviation from the resting potential is caused by changes in the membrane permeability of ions like sodium, potassium, calcium, and chloride. The two possible outcomes are depolarization, which makes the neuron more likely to fire, and hyperpolarization, which makes it less likely.

What is the role of neurotransmitters in the process of neuron signaling?

Neurotransmitters play a key role in neuron signaling by binding to ionotropic receptors on the neuron's membrane, causing conformational changes that allow ions to pass through and affecting the membrane potential.

What happens if the membrane potential reaches the threshold of excitation?

If the membrane potential reaches the threshold of excitation, typically around negative 55 millivolts, an action potential is generated, which involves the rapid propagation of electrochemical activity along the axon.

Describe the process of an action potential and how it propagates along the axon.

An action potential involves the sequential opening of voltage-gated sodium and potassium channels, allowing sodium ions to rush in and potassium ions to rush out. This triggers the release of neurotransmitters at the axon terminal, which then propagate the signal to the next neuron.

What is the significance of the refractory period in neurons?

The refractory period, which occurs after an action potential, is significant as it prevents the neuron from firing repeatedly due to continuous low-level stimulation, ensuring that signals are transmitted in a controlled manner.

How does the speed of action potential propagation vary between myelinated and nonmyelinated fibers?

Myelinated fibers, which have increased insulation, propagate action potentials faster due to saltatory conduction, while nonmyelinated fibers are slower as they lack this insulation.

What is the process by which neurotransmitters are released at the synapse?

Neurotransmitters are released at the synapse through a process called exocytosis, which is triggered when the action potential reaches the axon terminal and causes voltage-gated calcium ion channels to open, allowing calcium ions to enter and interact with proteins that facilitate vesicle fusion with the membrane.

How do neurotransmitters interact with the post-synaptic neuron, and what happens after the signal transmission?

Neurotransmitters interact with the post-synaptic neuron by binding to specific receptors, causing ion channels to open and generating graded potentials. After signal transmission, neurotransmitters are either reabsorbed by the pre-synaptic neuron or degraded by enzymes in the synaptic cleft to prepare for the next signal.

Outlines

00:00

🧠 Neurons and Synapses: The Basics

This paragraph introduces the fundamental concepts of neurons and synapses, focusing on the electrochemical signaling process that allows neurons to transmit information throughout the body. It emphasizes the importance of understanding membrane potential, which is a key concept in the study of how neurons function. The resting potential of a neuron is described, along with the distribution of ions across the cell membrane and the role of ion channels in regulating this potential. The paragraph also touches on how neurotransmitters, acting on ionotropic receptors, can alter the membrane potential and initiate the process of signal transmission.

05:04

🔋 Membrane Potential and Action Potential

This section delves into the dynamics of membrane potential, explaining how depolarization and hyperpolarization can excite or inhibit a neuron, respectively. It discusses the concept of graded responses, where the magnitude of potential change is proportional to the signal intensity. The process of generating an action potential is described, including the sequential opening of voltage-gated sodium and potassium channels, leading to the propagation of electrochemical activity along the axon. The paragraph also explains how neurotransmitters are released at the axon terminals and how they interact with the post-synaptic neuron to continue the signal transmission through a series of synapses.

10:04

⏱️ Time Frame of Neural Signaling

This paragraph explores the time frame of the action potential and the various phases involved in the process, such as the rapid rise and fall of membrane potential due to sodium and potassium ion movements. It details the repolarization phase, hyperpolarization, and the refractory periods that follow an action potential. The paragraph also discusses the role of membrane proteins like sodium-potassium pumps in resetting ion concentrations and the biological significance of these processes in ensuring unidirectional signal propagation and preventing repetitive firing due to continuous stimulation.

15:04

🔬 Synaptic Structure and Neurotransmission

The final paragraph provides a closer look at the synapse, describing the different types of synaptic connections and the process of neurotransmitter release. It explains how action potentials trigger the opening of calcium ion channels, leading to neurotransmitter release via exocytosis. The paragraph also discusses how neurotransmitters act as ligands, binding to specific receptors on the post-synaptic neuron and generating graded potentials. It concludes with a mention of the mechanisms for terminating the signal, such as reuptake or enzymatic degradation of neurotransmitters, and the readiness of the synapse for a new impulse.

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 discussed as the conduits for electrochemical signals that relay information from sensory organs to the brain and then to the rest of the body, illustrating their critical role in communication within the nervous system.

💡Synapses

Synapses are the junctions through which neurons communicate with each other. They consist of a pre-synaptic neuron releasing neurotransmitters that cross the synaptic cleft and bind to receptors on a post-synaptic neuron. The video explains how synapses are essential for the propagation of signals across neurons, highlighting the process of neurotransmitter release and reception.

💡Membrane Potential

Membrane potential refers to the electrical potential difference across the cell membrane, which is crucial for the generation of action potentials in neurons. The video describes how the resting membrane potential of neurons is maintained by the distribution of ions and the selective permeability of the membrane, setting the stage for the neuron's ability to transmit signals.

💡Ion Channels

Ion channels are proteins embedded in the cell membrane that allow the passage of ions, such as sodium, potassium, and calcium, in and out of the cell. The video discusses how the opening and closing of these channels are regulated and how they contribute to changes in membrane potential, which is integral to the neuron's function.

💡Action Potential

An action potential is a rapid, temporary change in the membrane potential that travels along the neuron's axon, allowing for the propagation of a signal. The video explains the process of action potential generation, including the sequential opening of sodium and potassium channels, which is central to the neuron's ability to transmit information.

💡Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across a synapse from one neuron to another. The video details how neurotransmitters, once released, interact with receptors on the post-synaptic neuron, either causing depolarization or hyperpolarization, thus playing a key role in the communication between neurons.

💡Depolarization

Depolarization is the process by which the membrane potential of a neuron becomes less negative, making the neuron more likely to generate an action potential. The video uses the term to describe the excitatory effect on a neuron when neurotransmitters bind to receptors, leading to a change in the membrane potential.

💡Hyperpolarization

Hyperpolarization is the process by which the membrane potential of a neuron becomes more negative, making the neuron less likely to generate an action potential. The video contrasts this with depolarization, showing how it can inhibit neuronal firing, which is crucial for regulating neural signaling.

💡Graded Potentials

Graded potentials are changes in membrane potential that vary in magnitude depending on the strength of the stimulus. The video explains how graded potentials, such as excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), are proportional to the intensity of the signal and are part of the neuron's response to neurotransmitters.

💡Saltatory Conduction

Saltatory conduction is a mode of action potential propagation along myelinated axons, where the signal 'jumps' from one node of Ranvier to the next, increasing the speed of signal transmission. The video mentions this as a feature of myelinated fibers, which are important for rapid signal transmission in the peripheral nervous system.

💡Refractory Period

The refractory period is a brief interval following an action potential during which a neuron is unable to generate another action potential. The video discusses the absolute and relative refractory periods, which are crucial for preventing continuous firing and ensuring that signals are transmitted in a controlled manner.

Highlights

Neurons conduct and transmit electrochemical signals throughout the body.

The mechanism of how neurons relay information from sensory organs to the brain and back to the body.

Introduction to membrane potential, an essential concept for understanding neuron function.

Explanation of the resting potential of a neuron and its polarization at about negative 70 millivolts.

The role of ion channels in maintaining the resting potential and their regulation by membrane proteins.

The impact of neurotransmitters on ion channel conformation and membrane potential.

Depolarization and hyperpolarization as graded responses to neurotransmitter binding.

The threshold of excitation for an action potential and the all-or-none response of neurons.

Propagation of the action potential along the axon and the release of neurotransmitters at the terminal end.

The speed of action potential travel and the role of myelinated fibers in saltatory conduction.

The refractory period of a neuron and its importance in preventing continuous firing.

Differences in neuron conduction between the peripheral nervous system and mammalian brains.

The structure of a synapse, including pre-synaptic and post-synaptic neurons and the synaptic cleft.

The release of neurotransmitters via exocytosis and their interaction with post-synaptic receptors.

The process of neurotransmitter reuptake or degradation and the preparation for new impulses.

The importance of understanding the propagation of signals in the nervous system for studying the brain.

Transcripts

Professor Dave again, let’s talk about neurons and synapses.

When we studied nervous tissue in the anatomy and physiology course, we discussed the mechanism

by which neurons conduct and transmit electrochemical signals throughout the body.

These signals relay information about a person’s surroundings from sensory organs to the brain,

and then from the brain back to the rest of the body to tell it how to respond.

That tutorial was a basic introduction to this topic, and it is one which must be viewed

first before moving on with this playlist.

But if you’re up to speed with the basics, let’s go through the whole thing again in

more detail so that we are prepared to understand all of the things we will discuss for the

remainder of this series.

In learning about how neurons work, the most important concept to understand will be the

notion of membrane potential.

This is a kind of electric potential, a concept we discussed from the standpoint of physics

in the classical physics series.

To put it as simply as possible, electric potential describes the amount of work that

must be done to separate oppositely charged particles that are attracted to one another,

just the way that gravitational potential describes the amount of work that must be

done to move a massive object away from the source of a gravitational field.

It can also describe the work that can be produced by the spontaneous motion of charged

particles along their concentration gradients, kind of like water flowing downstream and

pushing a waterwheel.

In a neuron, such a potential exists by virtue of the distribution of electrical charge on

either side of the cell membrane.

There are many positively charged sodium and potassium ions in both the cytoplasm and the

extracellular fluid, and for a resting neuron, there are more sodium ions outside than inside,

and more potassium ions inside than outside.

These ions will tend to diffuse along their concentration gradients, which means they

will have a tendency to disperse as much as possible until their concentrations are the

same everywhere, as entropy demands.

But the cell membrane, with its nonpolar region, makes it very difficult for the ions to do

so, leaving only surface proteins called ion channels as passageways for travel.

This results in some potential energy for the system, with ions that have the potential

to move, just the way an elevated object has the potential to fall down to the ground.

This resting potential is measured as being about negative 70 millivolts, and in this

state the neuron is said to be polarized.

Now the electric potential can deviate from this resting potential, due to the fact that

the membrane permeability of sodium, potassium, calcium, and chloride ions, or their ability

to pass through the membrane, can change depending on the status of certain membrane proteins.

When looking at a resting neuron, the permeability of sodium is very low, because sodium ion

channels are typically closed, while potassium has a higher permeability because potassium

ion channels are open, though the ions are largely held in place by the negative resting

potential, given that negative charge builds up on the inner surface of the membrane as

potassium ions leave the cell, until they no longer have the tendency to do so.

This results in significant pressure on sodium ions to enter the neuron, due to both the

pressure of random motion to follow the concentration gradient, as well as the electrostatic pressure

from the negative charge build-up, which attracts the positively charged sodium ions.

But as we said, the status of these ion channels will change at specific times, and this is

mediated by signaling molecules called neurotransmitters.

Later in the series we will do a comprehensive survey of all the different neurotransmitters,

but for now let us just consider them in a very general sense.

When a neurotransmitter arrives at a neuron and binds to an ionotropic receptor in the

cell membrane, this will cause a conformational change in the receptor such that ions can

pass through.

This will have an effect on the membrane potential.

The result could be depolarization, which makes the resting potential less negative,

or hyperpolarization, which makes the resting potential more negative.

In either case the change is small, just a couple millivolts.

Depolarization excites the neuron, making it more likely to fire, while hyperpolarization

inhibits the neuron, making it less likely to fire.

These are both forms of graded responses, which means that the magnitude of the change

in potential is proportional to the intensity of the signal, or the amount of neurotransmitters

that bind.

As ions traverse the membrane and the voltage changes, this will cause nearby voltage-gated

ion channels to open, which allows for the diffusion of more ions.

If the sum of this activity is sufficient to depolarize the membrane beyond its threshold

of excitation, which is usually around negative 55 millivolts, an action potential will be

generated.

This involves the propagation of this electrochemical activity all the way along an entire axon.

All the voltage-gated sodium channels in the membrane along the axon open up sequentially,

and sodium ions rush in.

This triggers the opening of voltage-gated potassium channels, and potassium ions rush out.

At the terminal end of the axon, this activity triggers the release of more neurotransmitters,

which travel across the synaptic space to interact with another neuron and continue

the signal.

In this way, communication occurs through a series of synapses.

The pre-synaptic neuron releases neurotransmitters to activate receptors on the post-synaptic

neuron, triggering an action potential that propagates the signal to the next neuron,

and the next, moving throughout the body in this fashion.

This may seem like an incredibly elaborate process, far too elaborate to account for

our rapid reflexes, but don’t forget that chemistry happens on a time scale that is

imperceptible to humans.

The molecular world operates on the order of picoseconds, which are trillionths of a

second, meaning that billions of chemical events can happen quickly enough on this tiny

scale to produce a macroscopic effect, such as the motion of your body parts, which we

merely perceive as being instantaneous.

Now let’s talk a bit more about this action potential.

We will commonly see diagrams like this, which show how the membrane potential changes over time.

The horizontal portions represent the resting potential, which as we said, will be around

negative 70 millivolts.

When neurotransmitters are received by a neuron, if the effect is a depolarization, which as

we said results in excitation, this is called an excitatory postsynaptic potential, or EPSP.

That’s this little hill we can see here.

If the effect is hyperpolarization, we know this results in inhibition, and that’s called

an inhibitory postsynaptic potential, or IPSP.

That’s this little dip here.

If depolarization occurs and it is of sufficient magnitude so as to surpass the threshold of

excitation, an action potential will be produced, and that’s what this is here.

The tiny little bump is the EPSP, and then we see a sharp and pronounced spike, with

the membrane potential jumping up to about positive 50 millivolts.

This complete reversal of the membrane potential is due to ions diffusing across the membrane

all along the entire axon, and it is very sharp because it is not a graded potential.

It does not rely on the intensity of the stimulus.

If the threshold is reached, the action potential is produced, and if it is not reached, it

won’t be produced.

This is what we call an all-or-none response, kind of like squeezing a trigger until a gun fires.

Once squeezed sufficiently, the gun will fire, and squeezing more will not produce any additional effect.

In addition, if a neuron receives multiple signals roughly simultaneously, these will

be integrated.

So multiple IPSPs produce a single IPSP of greater magnitude.

One EPSP and IPSP will cancel each other out.

Multiple EPSPs produce a single EPSP of greater magnitude, and this last scenario is how the

threshold of excitation is reached to produce the action potential.

As we said, this whole process begins with ligand-gated channels that are activated by

neurotransmitters, thus producing a graded potential, causing voltage-gated channels

to open, and if enough ions diffuse so as to surpass the threshold, the action potential

is generated.

What is the time frame associated with these events?

Well the rising phase begins when the sodium channels open, shortly after which the potassium

channels open, and the membrane potential rises rapidly for about a half millisecond.

At this point, the sodium channels close, while further efflux of potassium ions causes

the potential to drop again.

This phase is called repolarization, also lasting about a half a millisecond.

Then the potassium channels gradually close, which allows more potassium ions to leave

the cell than are necessary, resulting in a lengthier phase called hyperpolarization.

This takes about two full milliseconds or a hair more.

After all of this activity, a variety of membrane proteins will allow ion concentrations to

reset, such as sodium-potassium pumps, which utilize active transport, spending ATP and

shuttling both sodium and potassium ions back to their respective sides of the membrane,

thus restoring the concentration gradient that provides the resting potential.

Once re-established, there will be a refractory period of around one to two milliseconds where

the neuron is incapable of firing again, which we call the absolute refractory period, after

which there is a brief relative refractory period, where it is possible for the neuron

to fire again, but only through excessive stimulation.

Then the neuron returns to its normal resting state.

This serves an important biological function.

It ensures that signals travel in one specific direction along an axon, and it prevents the

neuron from firing repeatedly due to continuous low-level stimulation, firing a maximum of

about one thousand times per second.

In general, the action potential travels very fast, around 100 meters per second or more,

and the precise speed will depend on the type of neuron.

Myelinated fibers offer some insulation and therefore exhibit an increased rate of conduction,

resulting in something called saltatory conduction.

This is because the signal will travel fastest in these areas, with sodium channels concentrated

at the nodes of Ranvier.

That’s why these neurons make up the peripheral nervous system, where reaction time can be

a matter of life and death.

Nonmyelinated fibers propagate the action potential more slowly, so these are found

in certain internal organs where speed can be sacrificed without detriment.

We should point out that this model we have been discussing most accurately describes

conduction in a motor neuron of the peripheral nervous system, and is thus a simplified model.

In mammalian brains like ours, there are many different types of neurons.

Some have very short axons, or none at all, which means no action potential, and conduction

is passive and decremental, meaning it is initiated and then simply weakens as it moves

along the membrane.

Some fire continually even with no stimulus.

Their action potentials can vary in frequency, amplitude, and duration.

Some show action potentials along dendrites, so this activity is not strictly limited to

an axon.

Cerebral neurons are much more varied and complex than motor neurons, so we must keep

that in mind when integrating the model we have discussed with brain activity, and we

will discuss this in greater detail as we move through the series.

Before moving forward, let’s briefly zoom in on the synapse.

Here we can see one of the many axon terminals of a pre-synaptic neuron, the post-synaptic

neuron, and the synaptic space, or synaptic cleft between them, which is very narrow.

The type of synapse depends on the location where it meets the post-synaptic neuron.

Axodendritic synapses connect at a dendrite, while axosomatic synapses connect at the cell

body of a neuron.

These are the two most common situations, though there are others, like axoaxonal when

connecting at the axon, or even dendrodendritic, when connecting from dendrite to dendrite.

In any case, neurotransmitters are produced in the cytoplasm at each of the thousands

of axon terminals, and then packaged into vesicles, which are stored near the membrane.

When the action potential reaches these terminals, voltage-gated calcium ion channels will open,

and calcium ions will enter.

These act as messengers, interacting with a protein which then causes the vesicles to

fuse with the membrane, thus allowing for neurotransmitter release via exocytosis.

These signaling molecules are released into the synaptic cleft, where they can diffuse

until reaching the receptors on the post-synaptic neuron.

The neurotransmitters act as ligands for these receptors, and each specific neurotransmitter

will be able to bind to a certain type of receptor, thus different neurotransmitters

can relay different types of messages throughout the brain.

As we mentioned, ligand binding will cause the ion channel associated with an ionotropic

receptor to open due to a conformational change, and ions diffuse, generating graded potentials

and eventually an action potential.

Once an action potential is generated, the neurotransmitters must leave the receptors

so as to allow for another signal to be transmitted.

Sometimes they will undergo reuptake by the pre-synaptic neuron, while other times they

will be degraded in the synaptic cleft by enzymes.

Either way, once this is complete, everything is ready to begin again with a new impulse.

Now that we have a better understanding of how these signals propagate through the nervous

system, let’s continue on with our study of the brain.