Now let's discuss the histology of the neuron.
First, the neurons of the nervous system are going to be the electrically excitable cells of the nervous system.
Structures in the neuron include the cell body which is going to contain the nucleus
and also going to contain other types of organelles.
The dendrites which extend from the cell body and are usually going to be involved in receiving information.
The axon which also extends from the cell body and is usually responsible for the output of information.
And some neurons also have a myelin sheath.
This is a layer of cells that are going to help with the conduction of nerve impulses and speed up this process.
Finally, at the end of the axon, we have the axon terminal.
This is the location of the synapses.
The synapses are where the axon is going to interact with either another neuron
or if you're talking about interacting with muscles at the neuromuscular junction.
So now let's look at the different ways that we classify neurons structurally.
Neurons are classified based on the number of processes that extend from the cell body.
Multipolar neurons are going to have multiple extensions from the cell body.
Bipolar neurons are going to usually have two extensions from the cell body.
In a unipolar neuron, there is one extension from the cell body
followed by a divergent in the opposite direction of either the dendrites and the axons.
We can also classify neurons functionally.
Neurons are classified based on the direction of the nerve impulse propagation.
First, we have sensory or afferent neurons which are going to go toward the nervous system
and bring information into the nervous system.
We also have motor or efferent neurons which are going to take that information away from the central nervous system.
And we also have interneurons or association neurons which are going to occur between afferent and efferent,
and can be found in both the central and the peripheral nervous system.
So the nervous system also has a group of non-electrically excitable cells as well.
We refer to these cells as neuroglia.
They make up about half of the volume of the entire nervous system.
Unlike many of the nerve cells, they can actually multiply and divide as well.
There are six different types of neuroglial cells, four found in the central nervous system
while two are found in the peripheral nervous system.
So now let's look at the ones in the central nervous system.
The neuroglia in the central nervous system include astrocytes which have a star shape
which is where it gets its name from
and they are going to be responsible for establishing and maintaining the blood-brain barrier.
The second neuroglial cell in the central nervous system are going to be the ependymal cells.
The ependymal cells are found lining the ventricles of the brain as well as the central canal of the spinal chord.
These cells are gonna be responsible for synthesizing cerebrospinal fluid
and they are also responsible for maintaining the blood cerebrospinal fluid barrier.
The third type of neuroglial cell in the central nervous system is the oligodendrocytes.
These cells are going to make up the myelin sheath in the central nervous system
which allows for faster conduction of nerve impulses.
We will discuss this process in a later chapter.
The final type of neuroglial cell in the central nervous system are the microglial cells.
These are phagocytes which are similar in function to that of microphages.
They undergo phagocytosis of cell debris during normal development
as well as cell debris that is caused by the damage to tissues.
They are also there to get rid of invading microbes.
In the peripheral nervous system, we have two different types of neuroglial cells.
The first one is the Schwann cells.
The Schwann cells are going to create the myelin sheath around peripheral nerves
and are also going to increase the speed of a nerve impulse.
Again, we would discuss this process later in the lecture.
The Schwann cells are similar to and have the same function as the oligodendrocytes in the central nervous system.
The second type of neuroglial cell in the peripheral nervous system are satellite cells.
These cells provide structural support of cell bodies that are found in peripheral nervous system ganglia.
A ganglion is a cluster of cell bodies in the peripheral nervous system.
So now let's talk a little bit about myelination.
So we refer to myelinated neurons as white matter because of their appearance.
We refer to unmyelinated cells as gray matter because of the grayish appearance.
If you look at a transverse section of the spinal chord,
you'll find that white matter is going to be located in the periphery
and is going to surround a centrally located gray matter.
Conversely, in the brain of the central nervous system,
you'll notice that the gray matter is on the periphery and is going to surround the white matter in the center.