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!