The brainstem contains the continuations of the long tracts seen in the spinal cord together with nuclei and tracts associated with cranial nerves and the cerebellum. These various tracts and nuclei surround, traverse, or are embedded in the reticular formation (named for its anatomical appearance—the Latin word reticulum means “network”), which forms a central core at all brainstem levels.
This chapter considers the level-to-level arrangements of structures as seen in transverse sections of the brainstem; many of the same structures are revisited in Chapter 8 as parts of functional systems. The sections were made by Pam Eller and stained, as were the spinal cord sections in the previous chapter, by the Klüver-Barrera method, using luxol fast blue for myelin and a neutral red counterstain (which, despite its name, is a basic stain with an affinity for nucleic acids). The result is blue-violet staining of white matter and red staining of large neurons with prominent Nissl substance (e.g., hypoglossal motor neurons in Fig. 3.10 ) and of areas tightly packed with small neurons (e.g., the granular layer of cerebellar cortex in Fig. 3.10 ). A parasagittal section of the brainstem ( Fig. 3.1 ) is used as a reference view throughout the chapter. It includes some of the features characteristic of each brainstem level ( Fig. 3.2 ), such as the superior and inferior colliculi of the midbrain , the basal pons , and a medullary pyramid .
The three major longitudinal pathways (lateral corticospinal tract, posterior columns, and spinothalamic tract) that were followed through the spinal cord in Chapter 2 extend into the brainstem in consistent ways, as indicated in Fig. 3.3 . Corticospinal fibers travel in the most ventral part of the brainstem, traversing the cerebral peduncle , basal pons , and medullary pyramid . At the spinomedullary junction, most of the fibers in each pyramid cross the midline (in the pyramidal decussation ) and form the lateral corticospinal tract. Each posterior column terminates in the posterior column nuclei ( nucleus gracilis and nucleus cuneatus ) of the medulla . Afferent fibers from these nuclei decussate in the medulla to form the medial lemniscus , which travels rostrally and ends in the thalamus. The medial lemniscus starts out near the midline and then moves progressively more laterally as it proceeds rostrally through the brainstem ( Fig. 3.4 ), rotating nearly 180 degrees in the process. The spinothalamic tract at all levels of the brainstem is at or near the lateral edge of the reticular formation. Cranial nerve nuclei are also arranged in reasonably consistent ways, as indicated schematically in Fig. 3.3 and in more detail in subsequent figures.