Brainstem histology

Under the microscope, the brainstem is not a uniform mass: it holds two materials laid out in an orderly way. One is gray matter, where the cell bodies of nerve cells cluster together, and the other is white matter, made of the fibers that carry signals from one place to another.

Part of that gray matter is called the tegmentum, and it continues without interruption into the gray matter of the rest of the brainstem. This is where the nuclei live, groups of nerve cells packed together, and along the three parts of the brainstem, the midbrain, the pons and the medulla oblongata, those nuclei are the ones that receive and send the information carried by the cranial nerves.

For white matter there are two places to point to. On the front of the brainstem it forms the thick bundle attached to the cerebellum. And in the medulla it is abundant on the outside, where it is continuous with the white matter of the spinal cord: there the two are a single material that keeps going, like two stretches of the same highway with no toll booth in between. And throughout the brainstem there is also diffuse gray matter, the reticular formation, which does not form a compact nucleus but is scattered about.