Overview of neuron structure (types of neurons)

In this video, I want to provide an overview of neuron structure. Neurons in adults have a soma. It's also called a cell body-- soma. And they have processes called neurites, which are divided into dendrites and axons. Dendrites are usually short, branched processes that are often covered in small spines that increase their surface area and perform some other functions. So these are dendrites. And then the other neurite they have is called an axon, which is usually long and unbranched until it reaches its end. So this is the axon. The area where the axon leaves the soma is called the axon hillock. The axon may be short or it may be very long, up to one meter or more. And it usually is unbranched for most or all of that length, until it gets to the end, in these structures, which are called axon terminals. And at this point, it will often branch and create multiple axon terminals. The first part of the axon is called the axon initial segment. Or it's also called the trigger zone. And we'll get into the reason for that in the next video. Axons can be so long that they are dependent on systems that transport substances from the soma, which contains most of the organelles, to the axon terminals, and vice versa. Things have to be transported both directions. And the axon is dependent on those systems. Large axons are usually wrapped in a sheath of a material called myelin. And axons that have a myelin sheath have little gaps between these segments of myelin called nodes of Ranvier. So the sheath I've drawn in yellow is the myelin, each of these little segments of sheath here. And these gaps that regularly interrupt the sheath are called nodes of Ranvier, these little gaps in the myelin sheath. The axon terminals will come very close to the target cells of the neuron. And I'll just draw it here. So these are the target cells. And these targets cells may be another neuron, they may be a muscle cell, or they may be a gland cell. A few neurons even have axons that terminate on capillaries, to secrete substances called hormones into the bloodstream. The place where an axon terminal comes close to touching the target cell is called a synapse. This is a pretty typical structure for a neuron. But there are multiple structural types of neurons, each of which can be further divided into subtypes. So let's go over some of the big categories of structural types of neurons. In the central nervous system, neurons start as neural stem cells, which turn into most of the cell types of the central nervous system. And these neural stem cells then differentiate into cells called neuroblasts. And don't worry about the details here. Because we'll go into a lot more detail in other videos on development of the nervous system. But neural stem cells and neuroblasts look pretty similar. They're basically just shapeless cells without processes. Neural stem cells can become almost any neural cell of the central nervous system, while neuroblasts can only become neurons. Neuroblasts will then migrate away from the neural stem cells to the location that their somas will have after development. Neuroblasts then extend a process, which is an axon, toward their target cell. And that axon is tipped with this structure called a growth cone-- growth cone. The axon growth cone follows guidance cues in the environment until it reaches the target cell of the neuron. A similar process occurs for neurons in the peripheral nervous system. But the original and the migrating cells for those neurons are neural crest cells, instead of neural stem cells and neuroblasts. Neurons at this stage have only one process, which is an axon. So they are now called unipolar neurons-- unipolar. That's the structural type of this neuron because there's one pole to the cell, one process giving a sense of direction on this otherwise shapeless cell. Unipolar neurons are present in humans, mainly during development. The next structural type of neuron has a soma. And it has one axon. But it also has one dendrite. So since this structural type of neuron has two processes, or two poles, it's called a bipolar neuron-- bipolar. The next structural type of neuron has a soma, just like the others, and one axon. But it has multiple dendrites. And so since it's going to have multiple poles, it's called a multipolar neuron-- multipolar. And this is the most common structural type of neuron in adult humans. The last big category of structural types of neurons is a little different. It has a soma, like all the rest. And then it has one a short process coming out of the soma, that then divides into two long processes going in different directions. And these are both axons. The axon bringing information in from the periphery is called the peripheral axon. And the axon bringing information into the central nervous system is called the central axon. The very end of the peripheral axon acts a lot like dendrites do on the other structural types of neurons. And we'll start to go over the function of dendrites and axons in the next video. And then this part of the peripheral axon near the end is the axon initial segment, where the trigger zone, just like this part is on a multipolar neuron close to the soma. And just like in these neurons where this is the trigger zone, and then the end of the axon has the axon terminals, in this type of neuron this is the trigger zone of the axon. And then the axon terminals are all the way at this end of the central axon. So this type of neuron has a big, long, funny name. It's called a pseudounipolar neuron-- pseudounipolar. And the reason is that it's kind of, sort of like a unipolar neuron, with only one process coming out of the soma. But that little short process immediately splits into these two long axons. So it's really a different shape than the unipolar neurons.