Content Reviewers:Rishi Desai, MD, MPH
The central nervous system or CNS consists of the cerebellum, cerebrum, brain stem, and spinal cord.
The neuron is the basic working unit of the nervous system.
And the neuroglia or glial cells are the non-neuronal cells that support and protect the nervous system.
The central neuroglia includes astrocytes, oligodendrocytes, ependymal cells, and microglia.
The peripheral neuroglia includes Schwann cells, satellite cells, and a number of cells associated with specific organs.
Macroscopically the CNS is made of white matter and grey matter.
The difference in appearance is from the lipid-rich myelin sheaths that cover the axons present in white matter.
Whereas the grey matter consists mostly of neuron cell bodies, dendrites, astrocytes, and microglial cells.
In this high power image of white matter from the spinal cord, the axons are surrounded by clear white space, which is where the myelin was present before the tissue was processed to create this slide.
The outermost portion of the cerebrum and cerebellum consist of grey matter, with their white matter present mainly in the deeper regions of the brain.
One the other hand, the spinal cord has the opposite arrangement, with white matter mainly in the periphery and grey matter mostly located closer to the center, forming an “H” or butterfly-shaped appearance when looking at a cross-section of the spinal cord.
In this low power image, we can see the spinal cord’s two posterior or dorsal horns closer to the top of the image and two anterior or ventral horns at the bottom.
The dorsal horns contain mostly sensory neurons and the anterior horns contain mostly upper motor neurons.
A simple way of remembering the location of the motor neurons is to remember that most cars also have their motors in the front or anterior part of the car.
The entire CNS is also covered by layers of connective tissue called the meninges.
The portion of the meninges that can be seen in this low power image is the dura mater, which is the thickest and outermost layer of the meninges.
The ventricles of the brain and the central canal of the spinal cord are filled with cerebrospinal fluid or CSF and have a lining of cuboidal or columnar cells called ependymal cells.
These cells are responsible for producing CSF.
If we take a closer look at the cells, we can also faintly see that the ependymal cells can have cilia, which helps circulate the CSF within the central canal.
The ependymal cells also have microvilli, but they’re typically too small to be seen with light microscopy.
Now, let’s take a closer look at the border between the spinal cord’s white matter and grey matter.
On the left is the white matter, where we can identify many of the purple-stained axons by the myelin sheaths that leave a white space that surrounds each axon.
On the right of this image is the grey matter.
Although neuron cell bodies can vary a lot in overall appearance, they’re still usually recognizable by their prominent nuclei with distinct nucleoli.
They are also usually much larger than glial cells.
The different glial cells can be difficult to identify and differentiate from each other when using an H&E stain, except for oligodendrocytes.
Oligodendrocytes can be identified by their small, round, and condensed nuclei.
Their cell bodies are not easily seen because their cytoplasm is unstained because it contains a lot of Golgi complexes, which don’t absorb the stain.