10-Minute Neuroscience: Neurons

Summary

This neuroscience video explores the structure and function of neurons, the basic units of the nervous system. It explains how neurons communicate via electrical and chemical signals, including action potentials and neurotransmitters, and highlights the complexity of their interconnections in the brain. The script also categorizes neurons based on their structure and function, distinguishing between multipolar, bipolar, unipolar, and pseudo-unipolar neurons, as well as motor, sensory, and interneurons.

Takeaways

• 🧠 Neurons are the fundamental units of the nervous system, specialized for transmitting and receiving information.
• 🔌 Neurons communicate with one another and with other types of tissues, such as muscles or glands, through electrical and chemical signals.
• 🌟 There are approximately 86 billion neurons in a typical human brain, forming an incredibly complex network of connections.
• 🔬 Neurons come in various shapes and sizes, with over 1000 different types, each with unique functions and structures.
• 🚀 Action potentials are the main type of electrical signals in neurons, created by the flow of ions that generate an impulse to travel along the neuron.
• 💊 Neurotransmitters are chemical signals that can be released to influence the response of the next neuron in the communication chain.
• 🌱 Dendrites are the input areas of neurons, featuring receptors that interact with neurotransmitters and receive messages from other neurons.
• 🏠 The cell body, or soma, is the metabolic center of the neuron, containing the nucleus and organelles for protein synthesis and other functions.
• 🔋 The axon hillock integrates incoming signals to determine if they are strong enough to initiate an action potential.
• 🚄 The axon conducts action potentials at high speeds, varying in size and length, and is often myelinated to increase signal propagation speed.
• 🔗 Synapses are specialized regions where neurons communicate without direct contact, separated by a synaptic cleft.
• 📊 Neurons can be categorized based on the number of processes extending from the cell body, such as multipolar, bipolar, and unipolar neurons.
• 🔄 Interneurons are the most common type of neuron, acting as intermediaries for information transmission within the nervous system.

Q & A

What are the fundamental units of the nervous system?

The fundamental units of the nervous system are neurons, which are specialized for transmitting and receiving information.

How many neurons are estimated to be in a typical human brain?

Current estimates suggest there are about 86 billion neurons in a typical human brain.

What are the two main types of signals neurons use to communicate?

Neurons primarily use electrical signals called action potentials and chemical signals called neurotransmitters to communicate.

What is the role of dendrites in a neuron?

Dendrites are the part of the neuron that typically receives messages from other neurons and can be thought of as an input area for the neuron.

What is the function of the cell body or soma in a neuron?

The cell body, or soma, is the metabolic center of the neuron, containing the nucleus and organelles involved in various cellular processes.

What is the axon hillock and its role in a neuron?

The axon hillock is a region where changes in the electrical properties of the cell are integrated to determine if incoming signals are strong enough to initiate an action potential.

How does the axon contribute to the neuron's function?

Axons conduct action potentials at very rapid speeds and vary in size, acting as the pathway for signals to travel from one end of a neuron to another.

What is myelin and how does it affect the neuron's signal propagation?

Myelin is a lipid-rich insulatory material that covers axons, speeding up the propagation of electrical signals by preventing current leakage and allowing it to regenerate at nodes of Ranvier.

What are the axon terminals and their significance?

Axon terminals, or synaptic boutons, are the branching endings of an axon where they communicate with other neurons at specialized regions called synapses.

What is a synapse and its importance in neuronal communication?

A synapse is a specialized region where neurons communicate without direct contact, separated by a very small space called the synaptic cleft, allowing neurotransmitters to influence the postsynaptic neuron.

How are neurons categorized based on the number of processes extending from the cell body?

Neurons are categorized into three main groups based on the number of processes: multipolar (one axon and many dendrites), bipolar (one axon and one dendritic structure), and unipolar (a single extension with branches that act as axons and dendrites).

What are the different functional classifications of neurons?

Neurons can be classified based on function into motor neurons, which control movement, sensory neurons, which carry sensory signals, and interneurons, which act as intermediaries for information processing within the nervous system.

Outlines

00:00

🧠 Introduction to Neurons and Their Communication

This paragraph introduces the concept of neurons as the fundamental units of the nervous system, emphasizing their role in transmitting and receiving information through electrical and chemical signals. The video script delves into the structure and function of neurons, highlighting the complexity of their connections, with an estimated 86 billion neurons in the human brain forming a vast network of communication. It also touches on the variety of neuron shapes and types, mentioning over 1000 different kinds, and explains the basic mechanism of neuron communication through action potentials and neurotransmitters. The paragraph sets the stage for a deeper exploration of neuron anatomy and function in subsequent discussions.

05:02

🔬 Anatomy of a Neuron and Its Functional Classification

This paragraph provides a detailed examination of the neuron's anatomy, describing its components such as dendrites, the cell body or soma, the axon hillock, the axon, and axon terminals or synaptic boutons. It explains the role of dendrites in receiving messages, the soma as the metabolic center, and the axon's function in conducting action potentials. The paragraph also discusses the myelin sheath and nodes of Ranvier, which facilitate rapid signal transmission. Furthermore, it explores the concept of synapses, where neurotransmitters are released to communicate with other neurons. The summary also categorizes neurons based on the number of processes extending from the cell body into multipolar, bipolar, and unipolar neurons, as well as functionally into motor, sensory, and interneurons, with further subdivisions into projection, relay, and local interneurons. This comprehensive overview offers insights into the diversity and specialized roles of neurons within the nervous system.

Keywords

💡Neurons

Neurons are the fundamental units of the nervous system, specialized for transmitting and receiving information. They form the basis for the powerful information processing capabilities of the nervous system and can communicate with one another as well as with cells in other tissues. In the video, neurons are described as having unique functions and a complex structure that contributes to the brain's circuitry, with an estimated 86 billion neurons in a typical human brain.

💡Action Potentials

Action potentials are the main type of electrical signals used by most neurons to send messages. They are created when ions flow into neurons, generating an electrical impulse that travels along the neuron. The video explains that these action potentials can lead to the release of chemical signals called neurotransmitters, which are essential for neuron-to-neuron communication.

💡Neurotransmitters

Neurotransmitters are chemical signals that travel from one neuron to another, playing a crucial role in communication between neurons. They can either generate a response or inhibit a response in the receiving neuron. The script mentions that neurotransmitters are released into the synaptic cleft, where they bind to receptors on the postsynaptic neuron, affecting its activity.

💡Dendrites

Dendrites are branch-like extensions of a neuron that typically receive messages from other neurons. They are covered with receptors that interact with neurotransmitters. In the video, dendrites are likened to tree branches due to their shape and are described as the input area for the neuron, essential for receiving information.

💡Cell Body (Soma)

The cell body, also known as the soma, is the metabolic center of the neuron. It contains the nucleus with DNA and various organelles involved in protein synthesis and other cellular functions. The script emphasizes its role in maintaining the neuron's health and facilitating the synthesis of neural components.

💡Axon

The axon is a structure that conducts action potentials away from the cell body at high speeds. It varies in size and can be quite long, with some extending up to a meter in humans. The video describes axons as critical for the rapid spread of signals throughout the brain and mentions the presence of myelin, an insulatory material that speeds up electrical signal propagation.

💡Myelin

Myelin is a lipid-rich insulatory material that covers many axons, helping to speed up the propagation of electrical signals. It prevents current leakage and is punctuated by gaps called nodes of Ranvier, where ions can flow to regenerate the action potential. The script highlights myelin's importance in efficient neural communication.

💡Nodes of Ranvier

Nodes of Ranvier are gaps in the myelin sheath along the axon where ion channels are present. They allow positively charged sodium ions to flow into the neuron, helping to regenerate the action potential and move it down the axon. The video explains their role in the efficient transmission of electrical signals.

💡Synapses

Synapses are specialized regions where neurons communicate with one another. The space separating the neurons at a synapse, known as the synaptic cleft, is very small, allowing for the exchange of neurotransmitters. The script describes synapses as the sites where neurotransmitters bind to receptors on the postsynaptic neuron, influencing its activity.

💡Presynaptic Neuron

The presynaptic neuron is the neuron whose axon terminals end at the synaptic cleft. It is responsible for releasing neurotransmitters into the synaptic cleft when an action potential reaches the axon terminal. The video uses the term to illustrate the neuron's role in initiating communication at the synapse.

💡Postsynaptic Neuron

The postsynaptic neuron is the neuron on the other side of the synaptic cleft, which receives neurotransmitters released by the presynaptic neuron. It has receptors that neurotransmitters can bind to, affecting whether it will fire an action potential. The script explains how this interaction can increase or decrease the likelihood of the postsynaptic neuron's response.

💡Classification of Neurons

The video script discusses different ways to categorize neurons, including based on the number of processes extending from the cell body, resulting in groups such as multipolar, bipolar, and unipolar neurons. Additionally, neurons can be classified by function, such as motor neurons, sensory neurons, and interneurons. This classification helps in understanding the diverse roles and structures of neurons in the nervous system.

Highlights

Neurons are the fundamental units of the nervous system, specialized for transmitting and receiving information.

Neurons communicate with one another and with cells in other tissues, such as muscles or glands.

There are approximately 86 billion neurons in a typical human brain, forming complex circuitry.

Neurons can vary greatly in shape and size, with over 1000 different types.

Most neurons use electrical and chemical signals, such as action potentials and neurotransmitters, for communication.

Dendrites are the input area of a neuron, typically receiving messages from other neurons.

The cell body, or soma, is the metabolic center of the neuron, containing the nucleus and organelles.

The axon hillock integrates signals to determine if the neuron should initiate an action potential.

Axons conduct action potentials at speeds from 1 to 100 m/s.

Myelin is an insulatory material that speeds up electrical signal propagation in axons.

Nodes of Ranvier are gaps in myelin that allow for the regeneration of action potentials.

Axon terminals, or synaptic boutons, are where neurons communicate at synapses.

Synapses are specialized regions where neurons communicate without direct contact.

Neurons can be categorized based on the number of processes extending from the cell body: multipolar, bipolar, and unipolar.

Multipolar neurons are the most common type, with a single axon and many dendrites.

Bipolar neurons have one axon and one dendritic structure, important in sensory systems.

Unipolar neurons have a single extension that branches into an axon and dendrites, common in invertebrates.

Pseudo-unipolar neurons are a variant of bipolar neurons, carrying sensory signals to the spinal cord.

Neurons can also be classified by function, such as motor neurons for movement control and sensory neurons for signal transmission.

Internurons are neurons that pass information between other neurons, serving as intermediaries.

Projection interneurons have long axons for long-distance signal transmission, while local interneurons form short connections for local circuits.

Transcripts

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

I’ll be talking about neurons, the fundamental  units of the nervous system. I’ll cover their  

basic structure and function and then I’ll  talk about some ways we can categorize neurons.

Neurons have some unique functions that form the  basis for the powerful information processing  

capabilities of the nervous system. They’re  specialized for transmitting and receiving  

information, and they can communicate with one  another as well as with cells in other types of  

tissues (like muscles or glands). They can also  carry information (such as sensory information)  

from the rest of the body to the brain. Something  else worth noting about neurons is that there are  

a lot of them. Current estimates suggest there's  about 86 billion neurons in a typical human brain,  

and those neurons create a very complex circuitry  as each neuron forms many connections—usually  

thousands of connections—with other neurons.  This creates anywhere from hundreds of trillions  

to maybe more than a quadrillion areas in a human  brain where neurons communicate with one another.

So if you look at a neuroscience textbook,  you’ll see an image of a neuron that looks  

something like this, and this is what we’ll  use for our discussion of the neuron today.  

But it’s important to note that neurons  come in all shapes and sizes—there are  

over 1000 different types—and most of them  don’t look just like this one–so you can  

see here some examples of different types of  neurons found throughout the nervous system.  

They look quite different, but they're all  variations on a theme, and most of them have  

the same general components—those general  components are what we’ll focus on today.

Before we get into those components let me just  say something about how neurons communicate,  

because it will make the anatomy we’ll talk about  next easier to understand. I’ll have other videos  

that cover these topics more in-depth but  I’ll just provide a quick summary here.  

Most neurons use electrical and chemical  signals to send messages throughout the  

brain. The main type of electrical signals are  called action potentials; they’re created when  

charged particles called ions flow into neurons  and generate an electrical impulse that can travel  

from one end of a neuron to another. These  action potentials can cause the release of  

chemical signals called neurotransmitters,  and neurotransmitters can travel from one  

neuron to the next neuron and either generate a  response or inhibit a response in the next neuron.  

By relying on these mechanisms, neurons  can spread signals throughout the brain  

in a fraction of a second. (I should point  out that there are neurons that don’t use  

neurotransmitters and rely solely on electrical  signals to communicate with one another,  

but we won’t focus on them here because they’re  relatively rare in the adult nervous system.)

OK, so let’s talk a bit about these different  parts of a neuron. So first, on the right side  

of this cell you can see a number of branch-like  extensions jutting out from a circular structure  

that otherwise resembles a typical cell. These  extensions are called dendrites. The word dendrite  

comes from a Greek word that means tree-like, and  the name fits because dendrites bear a resemblance  

to tree branches. Dendrites are the part of the  neuron that typically receives messages from  

other neurons. To accomplish this, on the surface  of dendrites are proteins called receptors that  

neurotransmitters can interact with. Because  dendrites are typically where information  

in a neuron is received, they can generally  be thought of as an input area for the neuron.

The part of the neuron that most resembles a  typical cell is called the cell body or soma  

(and soma is another Greek word that means body).  The cell body is the metabolic center of the cell.  

It contains the nucleus (which contains the DNA),  it contains organelles that are involved in doing  

things like making proteins, and it’s the site of  the synthesis of various other neural components.

Sticking out from one side of the  cell body you can see this little  

hill-like area, that’s called the axon hillock.  

When information is received at the dendrites,  it causes changes in the electrical properties  

of the cell, and the axon hillock is a  region where those changes are integrated  

to determine if the incoming signals are  strong enough for the neuron to initiate  

its own action potential. This process of  integrating signals is called summation.

If a neuron fires an action potential, the impulse  will travel down this structure called the axon  

at very rapid speeds ranging from 1 to 100 m/s.  So axons act to conduct action potentials. They  

vary in size and can range from the order  of micrometers, or millionths of a meter,  

to up to about a meter in humans for the axons  that run from the spinal cord to the foot.  

Because there are so many neurons in the brain,  by some estimates if you took all the neurons out  

of the brain and laid them end to end, the axons  would stretch over 100,000 miles—that’s enough  

to wrap around the globe about 4 times. Axons  are usually (although not always) covered in  

a lipid-rich insulatory material called myelin  (and that's what this purple striped structure  

is representing here). Myelin helps to speed up  the propagation of electrical signals down the  

axon. It prevents the current from leaking out  of the axon, but also it has these gaps in the  

myelin called nodes of Ranvier---that's what  this little area is here, a node of Ranvier.  

And Ranvier is just the name of the  scientist who discovered these).  

At these gaps are channels that allow positively  charged sodium ions to flow into the neuron. This  

influx of positively-charged ions helps  to regenerate the action potential and  

move it down the axon (this is something else  that I’ll talk about more in another video).

At the end of the axon, you see that the structure  branches out into endings we call axon terminals  

or synaptic boutons.  

The word bouton is French for button.  

The axon terminals are often situated very close  to the dendrites of another neuron (although they  

can be situated next to any component of  other neurons), and they communicate with  

other neurons at specialized regions called  synapses. So this right here is a synapse.  

The neurons that communicate at a synapse don't  typically come into contact with one another,  

but the space separating them is very small:  often only about 20-40 nanometers wide. I  

know a nanometer is not something we can  easily conceptualize, but for comparison,  

a human hair is about 80,000-100,000 nanometers  wide. So the space between neurons is very, very,  

very tiny. That space is called a synaptic  cleft. The neuron whose axon terminals end  

at the synaptic cleft is called the presynaptic  neuron (and that would be this black neuron here),  

while the neuron on the other side of the synaptic  cleft is called the postsynaptic neuron (that  

would be this blue neuron here). When an action  potential reaches the axon terminal it can cause  

chemicals called neurotransmitters to be released  into that synaptic cleft. The neurotransmitters  

can then bind or attach to receptors on the  postsynaptic neuron, and either increase or  

decrease the likelihood that the postsynaptic  neuron will fire an action potential of its own.

So those are the basic components of a neuron.  Again most neurons have those defined regions but  

they still often differ from one another in other  substantial ways. We can use those differences to  

group neurons into some broad categories. One way  of categorizing neurons is based on the number of  

processes that extend from the cell body. Using  this method of classification, there are three  

main groups of neurons: multipolar, bipolar, and  unipolar. Multipolar neurons are the most common  

type of neuron in the human nervous system and the  nervous systems of other vertebrates. Multipolar  

neurons usually have a single axon and many  dendrites. They come in various shapes and sizes.  

The length of their axons varies as does the  extent of their dendritic branching, so they can  

still look very different from one another, but  all of them are modifications on a similar plan.

Bipolar neurons have a cell body that gives  rise to two extensions: one axon and one  

dendritic structure. Sensory systems, like the  visual system, rely heavily on bipolar neurons.

Unipolar neurons have a single extension  that has multiple branches, one of which  

acts as the axon and others that form dendrites.  Unipolar neurons are the simplest type of neuron,  

and are common in invertebrate nervous systems but  not very common in humans and other vertebrates.

There are also variants of bipolar neurons  called pseudo-unipolar neurons. These cells  

initially form as bipolar neurons, but  the dendrite and the axon fuse together  

to form a single process that extends from  part of the cell body. Pseudo-unipolar  

neurons carry sensory signals, such as  information about touch to the spinal cord.

Neurons can also be classified based on  function. Motor neurons, for example,  

are responsible for controlling movement. So they  have axons that form synapses with muscles to  

cause those muscles to contract. Sensory neurons  carry sensory signals back to the spinal cord  

and brain. This would include information,  for example, about touch, smell, vision,  

etc. But the vast majority of neurons in the  nervous system are considered interneurons,  

which are neurons that receive information from  neurons and then pass it on to other neurons. So  

they act as the intermediaries for neurons.  These interneurons are also often subdivided  

into two types: projection or relay interneurons  and local interneurons. Projection interneurons  

typically have long axons that carry signals  over long distances, such as from one part of  

the brain to another. Local interneurons, on  the other hand, form connections with other  

neurons nearby. So, they have short axons  and are involved in creating local circuits.

So that is your basic summary  of neurons. Thanks for watching!