NERVOUS TISSUE

The complex morphology of neurons and glial cells, the large size of neurons, the extreme elongation of their ramifications as well as the complexity of their interconnections has resulted in the development of an extensive range of histological techniques for studying nervous tissues.

- Examine the photo of nervous tissue taken with an electron microscope. It is perfectly clear that all the cells are very close to each other.

* You can see the cell bodies (containing the nucleus and most of the organelles) and the extensive cell ramifications. These ramifications belong to the cells whose bodies are visible on this photomicrograph and to cells whose bodies are not visible on the photo because the section didn't include them.

- Study the photos of nervous tissue taken with a light microscope. It is not so obvious that the cells are close together. In fact cells seem far away from each other. The light microscope can be misleading because the cell boundaries cannot be as well defined as with the electron microscope and also because some of the techniques used will reveal some types of cells at the exclusion of all the others.

* In the first photo, the cell bodies of three neurons (A) are well defined because a heavy metal technique has been used, that specifically stains the cytoplasm of the neurons. The outline of the nuclei (N) inside the cell bodies can be seen faintly, but it is very easy to spot their nucleoli (n): the dark spots inside the nuclei. The thicker neuronal ramifications (B) can also be seen clearly: these are the brown lines interspersed in the yellow background. In spite of what it seems, the yellow background is not an extracellular substance separating neurons. It is jam-packed with cells: glial cells (not made visible by this technique) and smaller ramifications of the neurons.

* The second photomicrograph shows four glial cells (A). They are strongly ramified (B). As in the previous photo, the yellow background seems deceptively empty of cells. It is actually packed with neurons and with the smaller ramifications of the glial cells.

Neurons carry their informations from one part of the CNS (central nervous system: brain and spinal cord) to another and also between the CNS and all our different body parts. The long distance transfer of information is done through a neuron's thickest and longest ramification: the axon. The axons that are running along a common path group together into bundles. The bundles connecting different areas of the CNS are called tracts and they make the white matter of the brain and spinal cord. The bundles going in and out of the CNS to the body are called nerves. Sections through nerves and tracts look quite different from sections going through the neurons' cell bodies.

A longitidinal section through a nerve is a section through a multitude of tube-like structures which are the axons (A). The light cocoon around the axons (B) is made by a specific type of glial cells that wrap themselves around the axons.

This section goes through the white matter in the brain. Tracts have been sectioned at different angles. In A a tract sectioned longitudinally can be observed. All the little dark dots (B) are the nuclei of glial cells.