Hey everyone! It's Nicole from Kenhub, and welcome to our tutorial on the histology of neurons.
In this video, we'll be looking at neurons at a microscopic level and identifying the tiny components that make up these cells. Firstly, what is a neuron?
A neuron or a nerve cell is a specialized cell that conveys electrochemical pulses throughout the body. They can carry information in one or two directions. Efferent neurons carry information from the brain to the tissue it's communicating with. This could be, for example, to skeletal muscle to stimulate movement. In contrast, afferent neurons carry information from various parts of the body to the brain. This feature allows you, for example, to feel the pain of a burn.
Neurons are quite diverse having a wide variety of branching patterns and differences in size. Some neurons are long, wide and myelinated while others are short, narrow and unmyelinated.
In this tutorial, we'll be looking at the structures that make up a neuron that are visible in histological slides as well as different types of neurons based on shape. Specifically, we'll cover the main parts of neurons such as cell body, dendrites and axons and even dive into a bit more detail looking at structures within the cell bodies themselves such as the nucleus and the Nissl bodies. We'll also add a bit of detail into the axons and look at identifying myelinated and unmyelinated axons. Then we'll take a look at the different types of neurons including multipolar, bipolar, unipolar and pseudounipolar neurons. Finally, we'll have a look at a clinical condition and how it shows up on histological slide of nervous tissue. Let's get started.
Neurons are made up of three parts – a cell body, multiple dendrites and a single axon. The dendrites receive information while the axon transfers information to other neurons. Let's start with the cell body. The cell body which is sometimes referred to as the perikaryon or soma houses the nucleus and other organelles within its cytoplasmic membrane. In this image, we can see the cell body highlighted in green. To identify them in histological slides, we can look for the structure that is dilated in shape, so larger and usually rounder than the projections.
Here's another example of a cell body in this image. Another way to identify the cell body is by identifying the nucleus within it. This is the structure we can now see highlighted in green on the left. In this stain, it appears darker than other structures in the cell body. It will be spherical and the largest structure within the cell body. The nuclei are unusually large in neurons. As in all cells, the nucleus houses DNA.
On the left, the many small granular structures we can see highlighted in the cell body of this neuron are called Nissl bodies. Nissl bodies are granular collections of rough endoplasmic reticulum and free ribosomes and can be found throughout the cell body. Since these structures contribute to protein synthesis, this shows that neurons have a high synthetic activity. Depending on the activity of the neuron, the number of Nissl bodies will vary.
The soma or cell body has numerous cytoplasmic projections branching from its surface. We can see two in this image highlighted in green on the right. These are dendrites. Dendrites form connections with other neurons in order to receive information so they're conducting impulses towards the cell body. There are smaller projections called dendritic spines that branch out from each of the dendrites. Unfortunately, we can't see this on the histological slide.
Dendritic spines are the parts of the dendrite that from synapses with other cells. The synapse is where the communication occurs between two cells. Synapses can occur not only between two neurons but also between a neuron and a muscle fiber. If a synapse is occurring within another neuron, the dendritic spines of one neuron are performing that synapse with the axon of another neuron. The axon is the single projection from the cell body that carries information from the cell body to another cell.
To differentiate axons from dendrites, you can look for the following things. One, axons are usually much longer than dendrites. Their length can vary but axons can be over a meter long. Two, axons run a straighter course than dendrites. Dendrites will usually bend and twist to meet an axon, not the other way round. Three, branching of axons is limited to the distal end whereas dendrites might branch earlier. And four, finally, where axons originate from the cell body is quite distinct. The axon originates from the cell body via the axon hillock. This is the pyramid-shaped region that is highlighted in green on the image on the right.
In this new image, we're looking at a cross-section of a peripheral nerve. Within the nerve are many neurons, some of which are shown in the zoomed-in section. When looking at this image, we have to imagine that the nerve is traveling towards us, so the structures highlighted in green are the cut ends of axons traveling towards us through an insulating sheath. This insulating sheath surrounding the axons is what we can now see highlighted in green. This sheath is known as myelin. Its main purpose is to increase the speed of the impulses traveling along the neuron. When we take the green highlight away, we can see that the myelin is, in fact, a white space. That is because during the staining process, the alcohol involved removes the myelin and an empty space is what's left.
Nodal axons are myelinated, however. This new image on the right shows us what unmyelinated axons look like again from a cross-section. They're much thinner since they don't have several layers of myelin wrapped around the axons. We can see the axons of the unmyelinated neurons within the green circles.
Just to give another perspective on the organization of peripheral nerves, we'll take a look at these next images. In the images, we're looking at longitudinal sections of a nerve. There are many neurons running in parallel across the screen. In the image on the left, we can see the axons highlighted in green. Lining the axons are the myelin sheaths which we can see highlighted in green in the image on the right.
Now that we have seen the components of neurons, we'll have a look at the types of neurons and discover the various shape they can come in. Neurons can be classified based on the number of processes or projections that emerge from the cell body. Today, we'll have a look at multipolar, bipolar, unipolar, and pseudounipolar neurons. In this image, we can see a multipolar neuron highlighted in green. When we're speaking about the polarity, we're looking at the number of projections coming from the cell body of the neuron. These cell types have a single axon extending from one end of the cell body and two more dendrites that protrude from the other side of the cell body. Because of the numerous branches, it is called a multipolar neuron.
In this image, we can see three main projections. The single axon is here and there are two dendritic trees – one here and the second here. Multipolar neurons are the most common neuron and the most predominant in the brain and spinal cord. Because they have multiple dendrites, they can receive impulses from several other neurons. This expanded communication means the functional capacity of the nervous system is that much greater.
Bipolar neurons as the name suggests, have two main projections. In this image, we can see the single axon projecting here from the cell body and the lone dendritic tree extending from the other end here. Bipolar cells are only associated with afferent or sensory impulses. They're found in the vestibular cochlea or hearing and balance system, the olfactory or smelling system, and the ocular or visual system. In this image on the right, we can see some bipolar neurons highlighted within an inner nuclear layer of the retina at the back of the eye.
Another subtype of neurons is the unipolar neurons. These neurons have a single projection arising from the soma. In this image, we can see the single projection of the spherical body. This is the axon. There are no dendritic branches projecting from other regions of the cell membrane which means for the other neurons to communicate with unipolar neurons, their axons must form a synapse with the soma of the unipolar neuron. These cells are usually found in peripheral nerves and sensory ganglia.
Finally, we'll have a look at the pseudounipolar neurons. These neurons are similar to unipolar neurons in that they have only one projection leaving the cell body. The difference is the single projection bifurcates into two branches. In this image, the cell body is here. Emerging from it is a single projection and at this point, it bifurcates into its two branches – one here and one here. Pseudounipolar neurons originated as bipolar neurons with two projections. These two projections then merge then fuse to form a common proximal segment from which the two branches then arise.
Pseudounipolar neurons are found in dorsal root ganglia which is within the structure highlighted in green in this image. We're looking at a part of the vertebral column from a posterolateral view. The spinal cord is here running in the vertebral canal and the dorsal root ganglion is within the structure that is exiting through the intervertebral foramen.
In this next image, we're looking at the vertebral column in a cross-sectional view and can see the dorsal root ganglia in isolation a bit better. Here is the vertebral body, the vertebral arch and the spinal cord running in the vertebral foramen. The dorsal root ganglia are on either side highlighted in green. Ganglia are the names of collections of cell bodies that are located in the peripheral nervous system. So the cell bodies of pseudounipolar neurons are housed in the dorsal root ganglia. One branch of the axon travels to the spinal cord while the other travels peripherally.
In this histological image, the structures highlighted in green are the cell bodies of pseudounipolar neurons located in a dorsal root ganglion. Within the cell bodies, the lightly stained structure is the nucleus and the darker stained structure within the nucleus is the nucleolus. In a dorsal root ganglia, the cell bodies are typically arranged in closely packed clusters as we can see in this image. The nerve fibers traveling to and from the cell bodies are seen running between some of these clusters here and here.
Histological slides of nervous tissue can show us a lot about what's going on in someone's nervous system. What we'll briefly look at today is what can be seen in someone with multiple sclerosis or MS is an autoimmune condition that affects part of the central nervous system – so parts of the brain and/or spinal cord. The person's immune cells actually attack the myelin surrounding axon in the CNS. This means that the myelin sheath is lost and so impulses cannot travel along the axons as quickly as they should be able to and this targets different areas in different people leading to a wide range of symptoms such as problems with balance, sensation, vision, movement and more.
In this microscopic image of the central nervous tissue, we can see a couple of characteristic features of MS, the first being this lesion. This lesion is actually a site of myelin loss so areas where the immune cells have been attacking myelin. You can see that compared to the blue area on the right of the image, there are a few round empty spaces. Those spaces are myelin and in the area on the left, they have been destroyed.
The other feature is the large amount of macrophages which are the structures stained in brown. Macrophages are a type of white blood cell that help the body by cleaning up cellular debris. To do this, they engulf or phagocytose this debris and digest it. In the case of MS, the macrophages are degrading what's left of the destroyed myelin sheaths.
So now that you're an expert on the histology of neurons, before I let you go, let's have a quick run-through of what we looked at today.
We started out with the basics of a single neuron. We looked at identifying the structures that make up a neuron including the cell body or soma, dendrites and axon as well as structures within the cell body including the nucleus and Nissl bodies. We then looked at different sections of peripheral nerves, cross-sections and longitudinal sections and identified axons as well as whether or not they were enclosed in an insulating sheath of myelin. Lastly, we looked at four different types of neurons based on shape – multipolar, bipolar, unipolar, and pseudounipolar neurons.
This brings us to the end of our tutorial on the histology of neurons. Hope you enjoyed it. Thanks for joining me!